Major Habitats - Marine Odyssey

Major Habitats

The area and volume that encompasses the marine environment is vast and consequently it contains many different types of habitat. A simple way to appreciate this diversity is to take a theoretical walk from the upper limits of an estuary to the very depths of an ocean trench. An individual would see the slow change from flood plain to salt marsh and brackish waters. Following this, right on the seas doorstep, there could be mangrove, rocky coast or possibly a beach of sand or pebbles. Upon entering the water the observer may find reefs of coral or rock, shallow sandy expanses or seagrass/kelp beds. In some regions of the world, where an oceanic plate is subducted by a continental plate (Chilean coast for example), a trench is formed. More commonly habitats play out across thecontinental shelf for a distance of tens of kilometres until the ocean floor suddenly steepens. Where this occurs marks the beginning of the continental slope, which descends rapidly to depths of typically 4 km. The continental slope levels out to form the abyssal plains, vast expanses of ocean floor covered in sediment or ooze. The rolling expanse of deep sea floor may be interrupted by sea mounts, trenches or submarine ridges. The expanse of water now above the observer represents a huge habitat, which can be broken down into vertical layers defined typically by light penetration, temperature or simply depth. All of the aforementioned environments are generalizations – the worlds marine environments vary greatly from one another depending on locality. This chapter will look, in detail, at these habitats and their ecology.

Divisions of the Marine Environment

The marine environment is so vast that it has to be split in sizeable chunks to make it easier to manage in geographical terms, this has been done in several ways, which will now be covered. The largest division is that of the Oceans, which has been done by allocating five Oceans across the globe, in size order (large to small) these are:

Table 3.1 - Shows the major Oceans of the world along with their surface area in kilometres squared.

All in these waters represent 71% of the worlds surface. It is also important to note the biased distribution of the Oceans in the Southern Hemisphere (80% water coverage) compared to the Northern Hemisphere (61% water coverage). These oceans are further divided into regions by the way they are partially enclosed. Such areas are termed Seas, of which there are approximately 100(depending on definition). A few are listed here:

Table 3.2 - Shows a selection of Seas from across the world along with their size in kilometres.

The worlds Oceans may also be divided in terms of temperature, this approach forms 4 geographic regions – tropical, subtropical, temperate andpolar. The diagram below highlights these regions:

Figure 3.1 - A map of the worlds temperature bands in terms of tropical, subtropical, temperate and polar regions.

The map above may be modified slightly to reflect the prevalent ocean surface temperatures, with this said the map above is rather crude – the extent of each boundary is modified by the translocation of heat via surface currents. For example the temperate zone near Britain is extended by warm waters from the south carried by the Gulf Stream. Similarly the tropical zone of the off the west coast of South America (Peruvian and Chilean coast) is limited due to cold waters carried northward by the Humboldt Current. 

So far we have considered the oceans from a top down perspective, but the marine environment is three dimensional and when considering marine habitats it is essential to become acquainted with some basic geomorphology as well as the way in which the water column is defined. This can be achieved rather eloquently with one comprehensive, if somewhat generalised, diagram:

Figure 3.2 - The vertical and horizontal definition of the marine environment below the surface.

The neritic zone is the water that overlies the continental shelf. Neritic waters are typically highly productive due to their relatively shallow depth which enables easy entrainment of minerals from the sea bed. The pelagic zone is the region of water that lies a substantial distance from land masses and is typically very deep and poor in nutrients. The epipelagic zone is the top most sunlit layer, it is also known as the euphotic or photic zone, and is of particular importance to marine ecosystems as this is where the bulk of marine primary productivity occurs. The epipelagic zone experiences the greatest temperature dynamic of all the water layers. Themesopelagic zone follows and marks the beginning of the aphotic zone – light does not penetrate this region with any strength (photosynthesis is minimal or impossible). There is a marked drop in oxygen in this zone due to the lack of photosynthesis and the respiration of organisms, there is often a defined oxygen minimum zone (typically 500 m), past which there is an increase in oxygenas fauna become more infrequent. Past the mesopelagic is the bathypelagic, abyssopelagic and hadalpelagic zones. The ecology of these areas is still very much unknown, but it has become increasingly evident that in places, despite the crushing pressure (over 400 atms), perpetual darkness and almost freezing water (<4 degrees Celsius), life can proliferate due to the presence of hydrothermal vents, carcass falls, bacterial mats, alongside highly evolved physiological adaptations. These regions will be covered in greater detail later within this chapter.

Estuaries

An estuary may be defined as a partially enclosed embayment that is subject to the tidal influxes of saltwater. The substrate is typically mud formed from the flocculation of silt particles and the settling out of fine mineral and organic particles from the marine environment as well as the freshwater source. The muddy substrate is ideal for small burrowing invertebrates (infauna) which feed upon the wealth of bacteria that thrives within the deeper anoxic sediment as well as upon the surface sediment. Organisms that filter feed may suffer in some estuarine environments due to high deposition rates clogging the feeding apparatus. In some estuaries the mud is exposed with each low tide revealing large expanses of mudflat. This is of particular importance to birds, which are now able to feed upon the mud invertebrates, the bird host includes (to name but a few) waders, geese, gulls, terns and ducks. At low tide the burrowing animals do not dry out due to the retention of water between the fine interstices of the mud particles.

Many types of estuary exist and they are all unique. They may be divided based on the way the saltwater and freshwater mixes within them or they may be classified dependent upon their origin. The first classification is described in Chapter 1. If the estuary is described based upon its origin it will be defined as:

• Coastal Plain – Formed when the sea level rises and coastal valleys are submerged. These are common worldwide and include the Thames or Humber in England and Chesapeake Bay or Delaware in the United States.

• Tectonic – Formed when land that is geologically active subsides and is consequently flooded by the sea. San Francisco Bay in the United States is the classic example.

• Lagoon or Sand Bar – Formed when sand bars are created parallel to a coastal region, consequently a lagoon is produced that is periodically flooded with seawater. An example is the Laguna Madré in Texas.

Photo 3.1 - Estuaries are often breathtakingly beautiful. Photo shows Adyar Estuary in South India. Copyright free from Wikipedia.

Whatever the form, such environments are characterised by fairly anoxic sediments (past the first few centimetres), periodic fluctuations in salinity, turbid water and large amounts of flowing water. These factors are stressful to animals and plants that wish to settle and consequently species richness tends to be fairly small in such regions, although it should be noted that biomass there may be very great. The salinity dynamic is perhaps the most stressful factor. An animal submerged in a medium that fluctuates in salinity loses or gains water via osmosis through permeable tissues, if too much water is lost the animal dehydrates, if too much water is gained than the biochemistry of cells will be affected or the cells themselves may burst. For an animal to survive it must have the correct physiological apparatus or it must adopt some behavioural strategy, both of which are energy expensive. For example the common shore crab of Europe (Carcinus maenas) is physiologically able to tolerate fluctuations in salt water. Bivalves are able to close their shells, creating an impermeable barrier between themselves and the external medium. Polychaete worms, such as Arenicola marina, are fairly immune within their burrows where the overlying salinity change is buffered via the mud. Chapter 5 will cover these strategies in greater detail.

Photos 3.2 - 3.4 - Three common estuarine animals. Left, the european bass Dicentrarchus labrax; Middle, sand casts of the lugwormArenicola marina; Right, the edible mussel Mytilus edulis. These creatures have evolved mechanisms to survive the rigors of estuarine life.All photos are copyright free from Wikipedia.

Estuarine environments are typically inhabited by mud snails which feed upon detritus and algal mats (Hydrobia ulvae), polychaete worms that may be active predators (Nereis diversicolor) or deposit feeders (Arenicola marina), isopod crustacea that may becarnivorous (Eurydice pulchra) or amphipod crustacea that may be epistrate feeders (Corophium pugalator), numerous crab species (Carcinus maenas and Eriocheir sinensis), bivalve molluscs belonging to the clam and mussel varieties (Macoma spp orMytilus spp respectively), shrimps (Crangon spp) as well as others. Fish also visit estuarine environments for breeding or feedingpurposes. Flatfish (such as Family Pleuronectidae) visit mudflats to feed on the siphons of bivalves as well as polychaete worms andsoft crabs. Other fish species such as mullet (Family Mugilidae) or bass (Order Perciformes) are also regular visitors. Migratory species such as the eels (Order Anguilliformes) and the salmonids (Family Salmonidae) are also present during certain times of the year. Macroalgae are often rare due to the lack of anchoring points and the turbidity of the water i.e. low light levels. Some seaweed may prosper however, these include the green algae Ulva lactuca and Enteromorpha intestinalis. The bulk of primary production within estuaries is derived from diatoms. Planktonic diatoms arrive periodically with the rising tide whilst resident benthic diatoms are present at all times, coming to the surface of the sediment when illumination is adequate. There is also a significant contribution of plant material from the riparian annuals that make up the salt marshes in temperate regions. In the tropics mangroves are commonly found fringing estuaries. The rich organic content present is metabolised by a host of meiofaunal organisms (0.5 mm – 62 µm in diameter) as well as bacteria. The diagram below illustrates some of the species that may be found in estuaries:

Figure 3.3 - An idealised estuary illustrating the various fauna and flora the my be present. Typically estuarine biodiversity is low but biomass is high due to the large amount of rich organic matter that enters from the freshwater source, marine environment and riparian fringe.

Salt Marshes

Often in association with estuaries are salt marshes (aka saltings) typified by anoxic mud, creeks and brackish water. The semi-terrestrial land bordering an estuary is often periodically flooded by saltwater and consequently it takes a hardy group of plants and animals to survive here – biodiversity is typically low although biomass is often great. This region is much different from an exposed intertidal environment as the sheltered nature of the estuary allows for increased deposition rates, ultimately leading to anaccumulation nutrient rich mud. Upon this mud pioneering halophytic (salt loving) plants, Salicornia spp – aka samphire or poor mans asparagus and Spartina spp – cord grass, take root and increase deposition rates further, soon a pseudosoil develops allowing for the colonization of more delicate plant species such as Aster tripolium (sea aster) or Puccinellia maritima (salt marsh grass). This transition or zonation is often observed over distance but may be seen within one location over time if the prevailing conditions are correct. Once the soil has been sufficiently stabilised true terrestrial plants may establish.

Photos 3.5 - 3.7 - Three common salt marsh plants. Left, Norfolk samphire Salicornia europaea; Middle, sea aster Aster tripolium, Right,Spartina maritima. All photos are copyright free from Wikipedia.

Salt marshes are extremely important environments as they provide a habitat and food source for many marine and terrestrial animals. Productivity within a marsh is often very high, especially amongst the emergent vascular plants, yet there is quite visibly a lack of herbivorous invertebrates. This is thought to be down to the low nutritional content and high salt content of the marsh plants. Various wildfowl, such as geese and ducks, may graze the plants directly, but it is more likely that birds obtain their nutrition predominantly from the invertebrates that feed upon the living and decomposing plant matter. It is important to note that the marsh plant community consists of angiosperms primarily – they are seed producing annuals. These seeds are of great importance to the numerous bird species that frequent the marsh environment – they may well travel vast distances to feed as the plants mature and shed their nutritional packets.

Photos 3.8 - 3.10 - Three common salt marsh animals. Left, eider duck Somateria mollissima; Middle, greylag geese Anser anser; Right, oyster catcher Haemotopus unicolor. All photos are copyright free from Wikipedia.

Certain fish species, such as the European bass (Dicentrarchus labrax), are reliant upon the many channels and creeks that are carved in the mud by the tide that passes through the marsh. Bass spawn within the nooks and crannies of the marsh, the juvenile fish then mature within this environment feeding upon the many invertebrates that inhabit the muddy substrate. Once of a certain size the bass my then head to the sea proper in search of larger and more nutritional meals.

Salt marshes are undoubtedly very important, yet time and time again they are disturbed by irresponsible human activities, more of which will be mentioned in more detail within the Mans Impacts chapter.

Rocky, Sandy and Muddy Shores

Estuaries and salt marshes are only brief interruptions along coastlines, the majority of which are composed of rocky, sandy or muddy substrate. All coastlines are unique to their location – a direct product of their proximal biotic and abiotic factors. The next three sections will define these common regions and attempt to cover the salient points with respect to the basic physical processes and ecology.

Rocky Shores

Rocky shores have been intensely studied over the centuries and as such many empirical texts exist on their ecology. This is in large due to the ease of access that scientists may access the shore. So, although rocky coastlines make up a small part of the marine environment, they have had a massive amount of study. Rocky shores are a haven for organisms. The solid substrate affords a site for animals and plants to securely attach themselves and where the rock form is varied (heterogenous) there may be a wealth ofmicrohabitats for invertebrates and small vertebrates to live, breed and feed. The daily inundation of the tides allows for true marine animals to encroach toward the terrestrial environment, whilst mobile terrestrial fauna may navigate the lower shore as the tide retreats. At this point it is important to understand the various divisions of the intertidal environment or littoral zone (from the Latinlitoris meaning shore). The littoral zone is classically split into three subdivisions; the upper, mid and lower shore, which are termed the supralittoral fringe, midlittoral zone and infralittoral zone respectively. Other names exist for these regions yet this scheme is perhaps the most universal amongst scientists. The divisions can be seen diagrammatically below:

Figure 3.4 - An idealised rocky incline showing classic zonation and typical inhabiting organisms. The dashed lines show the mean high and low tides. This example is based upon observations along the West coast of the UK.

The tide very rarely submerses the supralittoral zone and can only do so during spring tides. Moisture is delivered to this region via wave splash. Common organisms include crustose algae, certain species of small acorn barnacle and lichen.

Tide traverses the midlittoral region daily. Organisms are numerous and space is limiting therefore competition is fierce. Organism abundance is determined by physiological and behavioural adaptations to survive the harsh abiotic regime. Common organisms includemedium to large acorn barnacles, shore crabs, limpets, macroalgae (predominantly green), herbivorous littorinid snails, musselsand chitons.

This region is submersed for the majority of time, only rarely is it exposed to the air (spring tides). More delicate organisms may survive here due to the more benign environment compared to that of the higher shore. Organism abundance is determined by predator-prey interaction as physical factors are far less critical. Common organisms include those mentioned in the midlittoral plus predatory gastropod snails, sea urchins, anemones, brittle stars, star fish, small fish, and macroalgae (predominantly red and brown).

Photos 3.11 - 3.16 - Rocky coast fauna. Far left acorn barnacles (Order Sessilia); Second from left chiton Acanthopleura sp.; Third from left, european mussel Mytilus edulis; Third from right, limpets (Family Patellidae); Second from right, starfish (Class Asteroidea); Far right, rock pool not far from Sydney, Australia. All photos are the authors work.

So overall as an individual walks down the rocky littoral zone at low tide they may expect to see a move from a few hardy individuals to many more different species of a more delicate nature. Often distinct bands of colour and texture may be observed from the supralittoral down to the infralittoral, this is down to the success of individual species at differing sections to the exclusion of other species. The collective term for this is zonation. For example only certain species of lichen can survive the upper most shore where desiccation is severe. As water and nutrients become more readily available other species may survive. These new species may have an improved tolerance to salt water compared to its successor allowing it prevail further down the shore. The lichens will eventually give way to barnacles or perhaps encrusting algae, and are therefore displaced entirely. As each organism is unique they will form a distinct band of colour within their site of attachment. This is seen clearly in photo 2.12 of the previous chapter.

Sandy Shores

Sandy shores have also been studied fairly comprehensively, although not to the same extent as rocky shores. Sandy beaches may seem to be devoid of life, but this is not usually the case. Where conditions are suitable life may proliferate within the sand substrate. As the surface of the sand is often in constant motion, due to tides and currents, fauna and flora find it hard to settle. The beach itself will be formed of fragmented shells and rocks, the grains varying between <0.1 mm and >2 mm.The predominant size is determined by the overlying wave action of the region in question – where water is turbulent only the coarsest material will settle. The ratio of grain sizes within a beach is of upmost importance to the flora and fauna, this will be explained in more detail later in this section.

Sand is forever shifting. The lack of solid anchorage means the common fauna and flora of rocky shores will not be found on a sandy shore, for example barnacles, limpets, chitons, urchins, anemones – these are all epifaunal (they live upon the substrate). The most visible absence is that of the macroalgae. Due to the lack of attachments sites, and the often high degree of exposure, macroalgae are uncommon. Where a sandy beach is relatively sheltered, perhaps adjacent to a headland, some seaweeds may exist ephemerally attached to small pebbles or bivalve shells.

Sandy environs are dominated by infaunal species – organisms that live within the substrate itself. A burrowing mode of life is morphologically demanding. A species body form must be of the right shape and consist of the correct bio-mechanics in order to live a subterranean existence (see the Animal Physiology chapter for further specifics). Not only does an infaunal mode of life affordsubstantial protection from wave action, the layer between the animal and the medium above buffers against environmental fluctuations, such as variations in salinity and temperature. This buffering effect allows animals to direct a larger proportion of theirenergy towards activities such as feeding or reproducing, rather than spending considerable amounts on physiological survival mechanisms such as osmoregulation.

Common infauna include a myriad of molluscan bivalves, as well as numerous crustacea belonging to the Orders amphipoda andisopoda. Polychaete worms are also common. It is interesting to note that the highest biodiversity exists where exposure is low, yet the highest biomass exists where exposure is high.

A sandy shore may be split into sections in a similar manner to that seen for the rocky shore, consisting of a supralittoral, midlittoral and infralittoral zone, the figure below illustrates this:

Figure 3.5 - An idealised sandy incline showing the typical inhabiting organisms. The dashed lines show the mean high and low tides. This example is based upon observations along the West coast of the UK.

The supralittoral section of a sandy shore is exposed to wind shear and suffers high rates of water loss. Common fauna include numerous brachyurans (crabs – ghost crabs/shore crabs/sand bubblers), as well as amphipods (beach hoppers). At the upper fringe of the littoral zone dessication resistant marram grass may be present. Where marram grass takes hold it aids in the stabilisation of material invariably incresing the level of the beach over time. Over time a dune ecosystem may develop.

The midlittoral zone consists of isopod crustacea (sea lice) as well as sand crabs (aka mole crabs) – these crustaceans bury themselves almost entirely with only their foremost appendages protruding to capture suspended food from the backwash of waves. The midlittoral will experience migrations of animals as the tide turns, with upper shore animals invading with a dropping tide and lower shore animals invading with a rising tide. The animals within the midlittoral (as well as the supralittoral) tend to be much more active at night. Birds(osyter catchers, teal, sand pipers, plovers etc)are common in the midlittoral, feeding upon the infauna with specially modified bills.

The infralittoral zone consists of the numerous infaunal bivalve molluscs (razor shells, large and small clams, scallops, cockles etc), sand eels, as well as migratory fish (especially flat fish such as dab, sole, flounder and plaice), predatory and carrion feeding gastropod molluscs (olive shells and moon snails), burrowing polychaete worms (lugworms and ragworms) and various echinoderms (heart urchins, sand dollars and burrowing sea cucumbers).

Photos 3.17 - 3.22 - Sandy coast fauna. Far left, scallop Pecten sp.; Second from left, brachyuran sand bubbler excavation balls and burrow;Third from left, unidentified clam with gastropod radula wound (Class Bivalvia); Third from right, sand mason case Lanice conchilega; Second from right, unidentified gammarid amphipods (Family Gammaridae); Far right, ghost crab Ocypode sp. All photos are the authors work.

The bulk of primary productivity is carried out by interstitial diatoms which have the ability to migrate vertically through sand grains. The diatoms form part of the meiofauna – organisms between 62 µm – 500 µm in body size. Meiofauna may exist on sand grains (epibenthic), between sand grains (mesobenthic) or amongst sand grains (endobenthic). Typical meiofauna include gastrotrichs,polychaetes, bryozoans, hydroids, gastropod molluscs, holothurians (sea cucumbers), flatworms, nematode “worms” andcrustaceans.

There is much more to a sandy beach then meets the eye. Often, if conditions are suitable, there will be a wealth of animals within the sand, all of them feeding upon the organic matter delivered frequently by the tide, as well as feeding upon one another. As mentioned earlier grain size is important. This is true as the animals that live within and feed upon the substrate have evolved to favour particular size ratios of sand grains. The smaller the sand grains the larger the total available surface area for the attachment of microorganisms and the larger the size grains the more habitable space is afforded. It is essential that these grains are of a mixed composition. If all the grains are predominantly fine then oxygen willdeplete rapidly with depth forming an close to the sediment surface, the anoxic layer habitable only by anoxic layeranaerobic bacteria or specially adapted infauna. If grain sizes arepredominantly large then there will be insufficient surface area for the attachment of microorganisms, also deposit feeders have a maximum size limit to the grains they may engulf.

Muddy Shores

Where a region of coast is particularly sheltered from wave action, and where there is a substantial load of suspended sediment, muddy regions may occur. Typically these “mud flats” are associated with estuaries, lagoons or past sheltered spits or headlands. The topmost layer of mud is yellow or brown in colour, with depth the colouration changes to grey and eventually black. The grey layer is the transitional stage between the oxygenated upper layer and the anoxic lower layer, and is known as the redox potential discontinuity layer (RDP). Due to the very fine nature of mud, oxygen struggles to penetrate past the upper few centimetres, aerobic organisms in the upper layer use up the slowly diffusing oxygen before it has the chance to penetrate deeply. This loss of oxygen with depth can be measured quantitatively by recording theredox potential of oxygen with an electrode. The figure below shows a typicaloxygen redox potential curve:

Figure 3.6 - A typical oxygen redox potential curve. Note the relatively thin transitional layer has the greatest range (approx 500 mV).

The muddy substrate is teeming with life. The upper layer of mud, which is laden with organic detritus, often seems barren, but at times large numbers of grazing gastropod snails may be present, such as the mud snail Hydrobia ulvae. The bulk of animals are infaunalhowever. These mud dwelling animals feed via the ingestion of the mud or by sucking up the organic detritus that is continually being laid upon the substrate surface - they are predominantly deposit feeders. Similar groups are successful here as seen in the sandy shore section above – polychaetes, amphipods and bivalve molluscs are particularly successful and include the soft shelled clam Mya arenaria, the lugworm Arenicola marina, the gammaridean amphipod Corophium volutator, the omnivorous polychaete Nereis diversicolor as well as many others. The sediment is fairly firm (consisting of a significant number of clay particles) and the environment is more favourable to a burrowing mode of existence then the sandy or rocky shore. The lugworm will form a U shaped burrow that is open at one end. The closed end has a constant supply of infalling sediment which is rich in organic material, which the animal ingests. The polychaete will then defecate at the opposite opening forming, overtime, quite visible casts. The bivalve molluscs, such as Mya sp., may be located via the key shaped openings they produce at the mud surface. These openings are formed by the clams inhalant and exhalent siphons which it may protrude to vacuum up organic material as well as entrain well oxygenated water.

Photo 3.23 - Mudflats are incredibly scenic environments. Photo shows Kilnsea Mudlats, Yorkshire. Copyright free from Wikipedia. 

Muddy shores are highly productive. Not only is material deposited via the daily inundation of the tides, it is also delivered from the riparian fringe, a significant contribution is received from the saltmarsh annuals that die back during the winter months. Mudflats are also areas of primary productivity, but not soley in the common sense. Within the anoxic layer of the sediment lies a significant biomass of autochemosynthetic bacteria that reducesulphur compounds in order to obtain the energy required to fix carbon into more complex carbon compounds. The waste products from these bacteria (namely methane, hydrogen sulphide and ammonia) travel upward to the RPD layer where they are metabolised further by the aerobic bacterial population. The aerobic bacteria produce further waste products in the form of carbon dioxide, sulphates and nitrates which pass downwards and are utilised by the anaerobes. Overall there is a close cycling of nutrients occurring within the substrate which ensures optimal use of the proximal resources.

In addition to the infaunal primary production it is not uncommon to encounter diatom populations blanketing the surface of muddy environments. The presence of the unicellular plants may be discerned via a slight iridescent or yellow-gold hue to the sediment surface. The diatoms are mainly pinnate (pen or ovoid) in form and wend their between the mud particles to optimise their photosynthetic orientation to the sun. So, with the presence of primary producers both on and within the substrate, muddy shores are truly unique – no where else can there be found photoautotrophs and chemoautotrophs in such close proximity.

Mangroves

In association with muddy environments a group of salt tolerant trees and shrubs may prosper. These plants are known as mangroveor mangal and can only take root along sheltered coasts, this is mainly due to the mode with which the young tree establishes itself rather than a requirement for a muddy sediment – mangroves can establish themselves on many different substrates. There are 12 genera of trees and shrubs that belong to 8 families. The most dominant genera include Avicennia, Bruguiera, Rhizophora andSonneratia. The mangrove forests are limited to latitudes no greater then 25 degrees, and are consequently only located tropically orsubtropically. Mangroves proliferate in regions of significant tidal range – tides are crucial (almost paradoxically) to flush excess salt water from the substrate. Where tides only rarely inundate the lands salt concentrations, overtime, reach lethal levels.

Photos 3.24 - 3.27 - Four common genera of mangrove. Left, Avicennia sp.; Middle left, Bruguiera sp.; Middle right, Rhizophora sp.; Right, Sonneratia sp. All photos are copyright free from Wikipedia.

A mangrove disperses in a similar way to that of the coconut palm. A seed germinates on the parent tree and drops to the waters edge. As the tide rises the seed is lifted, being buoyant, and is carried away with the outgoing tide. Eventually the young tree enters shallow water and sets down roots and develops leaves. The young plant, along with others, will modify the environment it has settled in by further slowing water flow, this in turn increases deposition rates and will lead to the establishment of a predominantly muddy substrate. Through this process mangroves have the ability to generate land, often at an astonishing rate.

Figure 3.7 - Mangrove plantlets, or propagules, disperse in a manner similar to that of the coconut from a coconut palm.

Mangroves have several distinguishing features which are the product of adapting physiologically to survive within a saline, andpredominantly muddy, environment. Perhaps the most striking feature is the presence of numerous pneumatophores – aerial rootsthat jut up from beneath the mud to obtain oxygen. Such an adaptation is vital in a mud environment as past the first few centimetres intense microbial respiration creates anoxic conditions (see fig. 3.6). In addition to these pneumatophores a mangrove will have numerous arching and slender “prop roots” which anchor and stabilise the plant within the mud. The leaves of mangroves are often very succulent and possess special salt secretion glands, upon close inspection salt crystals may be observed on the leaf surface.

Photos 3.27 - 3.30 - Interesting features of mangroves, Left, pneumatophore roots; Middle left, salt crystals fromed via the excretion of salt;Middle right, mangrove plantlet or propagule; Right, prop roots for stability in mud. All photos are copyright free fromWikipedia.

Ecologically mangrove forests are important and fairly unique. They create a transitional step between the marine and terrestrial environments. True marine animals, namely fish such as such as mullet, snappers and snooks, will arrive with the incoming tide and browse amongst the roots of the trees as well as forage within the mud. Infaunal invertebrates such as shrimps, mud crabs, deposit feeding polychaetes and clams are abundant within the mud. Opportunistic fiddler crabs (Uca sp.) will scuttle amongst the roots and burrow into the sediment. Upon the prop roots sessile acorn barnacles and mussels will be common as well as boring amphipod crustacea. Animals, such as the mudskipper (Periophthalmus sp.) and the crab Goniopsis cruentata, have evolved to survive lengthy periods of time immersed. The mudskipper has highly vascularised (rich blood supply) gill chambers and strengthened muscular pectoral fins as well as eyes that focus better in air then water. Goniopsis also has highly vascularised gill chambers. True terrestrial animals may inhabit the boughs of the mangrove trees including numerous insect and bird species, as well as a few species of lizard.

Photos 3.31 - 3.36 - Mangrove fauna. Far left, fiddler crab burrows Uca sp.; Second from left, Malaysian tiger prawn Macrobranchium rosenbergii; Third from left, the mangrove crab Aratus pisonii; Third from right, mangrove jack Lutjanus argentimaculatus; Second from right, the mudskipper Periophthalmus modestus; Far right, roseate spoonbill Platalea ajaja. All photos are copyright free fromWikipedia.

Mangroves are fascinating in their regenerative ability. Although it is estimated that at least 50% of the worlds mangrove forests have been destroyed with careful management it would not be unreasonable to regain much of this within our lifetime. The plight of mangroves will be covered in more detail in the Mans Impacts chapter.

Seagrass Meadows

Seagrasses are marine flowering plants that often form vast swathes of foliage over the sea bed, referred to as meadows. They are completely seperate from marine algae as they have true leaves, fully vascularised tissues and develop flowers – from an evolutionary perspective they are secondary invaders of the sea. Leaves are typically thin and strap like, being anchored to an extensive and meandering rhizome system. Morphology between species varies little within this fairly small ecological group, although the overall size of different species may be dramatic, for instance smallest variety is in the order of centimetres whilst the largest is measured in metres. Seagrasses belong to four families Posidoniaceae, Zosteraceae, and HydrocharitaceaeCymodoceaceae, with sixty species in total. There are two predominant species world wide, eelgrass (Zostera marina) and turtle grass (Thalassia testudinum), inhabiting the temperate and tropical regions respectively. Reproduction may be sexual or asexual, depending on species, and runs along the same parallels as terrestrial pollination bar the necessity of insects as vectors - pollen is delivered via the saltwater medium.

Figure 3.8 - Stylised diagram of seagrass showing key morphological features. Seagrass is based on Thasallia testudium.

Seagrass may form small islands of vegetation or encompass larger regions – this is dependent upon overlying hydrographic conditions. Where currents flow slowly, waves are of low amplitude and frequency and water is relatively deep meadows may cover several square kilometres. In regions where currents flow rapidly, waves are of high amplitude and frequency and water is shallowseagrasses will form patches. This flowering plant is a common resident in waters close to estuaries and may inhabit the intertidal zone, although it is far more prevalent in the sublittoral. Where the plant has established a significant population it may affect the environment in a manner, not entirely dissimilar to that seen by mangroves, by and slowing water flow increasing deposition rates. The preferred substrate for seagrass is mud, although this is highly dependent upon the species with some able to colonise granite bedrock. Seagrasses increase in species number with decreasing latitude with many more located in tropical waters then temperate. Seagrass does not seem to be limited significantly by temperature, although this is most certainly a factor, the common seagrassZostera marina may be found at most latitudes. The dominating factor that limits growth is light availability – water depth andturbidity are therefore crucial.

Photos 3.37 - 3.39 - Left, turtle grass Thasallia testudinum; Middle, young Thasallia sp. shoots attached to rhizome; Right, Eelgrass Zostera marina. Images copyright free from Algaebase and attributed to Diane Littler.

Seagrass meadows are very important marine habitats, they are a source of food, attachment site and nursery ground for numerous animals and algae. The productivity of meadows is significant, not only do the plants act as primary producers, they are a site for the attachment of epiphytic algae. Diatoms covering the blades of a seagrass add to the overall primary productivity and are an additional food source to grazing fauna. Primary grazers of seagrass include wildfowl, dugongs, some fishes such as grey mullet and juvenile pollack, urchins and a few turtle species. Due to the high cellulose content of seagrasses few animals feed directly upon the plant – this is similar to the plants located within saltmarsh habitats. Much of the seagrass biomass therefore finds itself entering the detrital system where it is broken down by bacterial and fungal populations, this frees up valuable organic compounds and helps in the general cycling of nutrients. Due to the high percentage of microorganisms, the fine interstitial nature of the substrate and low overlying water movement the sediment becomes anoxic with depth. Faunal groups similar to those found in estuaries thus prevail subsurface, yet these species may differ from those in the estuarine habitat as they do not have to be adapted to survive severeeuryhaline conditions.

Photos 3.40 - 3.45 - Seagrass grazers and inhabitants. Far left, green turtle Chelonia mydas; Second from left, Long spined sea urchinDiadema antillarum; Third from left, canada goose Branta canadensis; Third from right, grey thin lipped mullet Liza ramadas; Second from right, hard clam Mercinaria mercinaria; Far right, sea cow Dugong dugon.All photos are copyright free from Wikipedia.

Due to the great ability for meadows to clear water of suspended material they inadvertently aid other proximal commuites, this is perhaps most marked in coral reefs which require high water clarity for photosynthesis and suffer clogging of feeding structures if depositional rates are too high. Sadly, much of the seagrass communities are threatened by human activities. Human populations typically prosper in coastal regions, this often results in massive outpourings of sewage and agricultural runoff,as well as harmfulrecreational, industrial and commercial activities. This will be discussed in greater detail in the Mans Impact chapter.

Kelp Forests

Within the Oceans cold and tide churned water there is often an abundance of kelp. Kelp, as seen in the previous chapter, is a brown seaweed belonging to family Fucophyceae.Kelp are anchored to rocky substrate via a root like holdfast. Leading from the holdfast is the slender stipe, which is analogous to a terrestrial plants stem, and branching from this may be several blades which are analogous to a plant leaf. The holdfast only resembles terrestrial roots in morphology and does not aid in water or mineral uptake – the kelp absorbs required nutrients directly from the seawater via the blades. Interestingly the stipe may be cross-sectioned and observed under an optical microscope where light and dark bands may be visible. These bands can be used to age the kelp with light bands laid down during fast growth (summer) and dark bands during slow growth (winter). Some genera may have airbladders or pneumatocysts which aid the alga by lifting the blades into a vertical orientation enabling optimal exposure to light. As mentioned these brown seaweeds are abundant in temperate regions where water is cold year round (no greater than 20 degrees Celsius) and the water is oftenturbulent, this combination allows for rapid growth of the alga, especially in summer where it may form dense forests or beds. Several key genera predominate this algal group – Nereocystis, Laminaria and Macrocystis. Laminaria is common in Atlantic waters whilst Nereocystis and Macrocystis prolifer in the Pacific. The largest kelp is Macrocystis pyrifera, the giant kelp, which is also the fastest growing at 50 cm per day in ideal conditions. Although very tough kelp may suffer significant loss of blades during winter stormswith foliage ripped and shredded against the rock substrate. The plants are resilient however and as long as the holdfast is in good health the algae may grow new blades. Kelp are limited to depths of up to approximately 30 meters, past this visibility is ofteninadequate to support recruitment. Kelp arealso limited to subtidal regions where or for emersion is rareshort periods only.

Photos 3.46 - 3.48 - Three common genera of kelp. Left, Macrocystis pyrifera forest; Middle, Laminaria digitata on the Yorkshire coast; Right,Nereocystis luetkeana. Photo 3.48 is copyright free from Wikipedia. Photos 3.46 and 3.47 from Algaebase and attributed to D. Schories and M.D. Guiry respectively.

Kelp beds and forests are of significant ecological importance. For a few species the algae themselves act as a food source, this is most notable in the urchins which graze the holdfasts voraciously. Perhaps more importantly the kelps form a distinct habitat, acting as asite of attachment for invertebrates and a refuge for larger vertebrates. Amongst the holdfasts there are often isopod and amphipodcrustacea which feed upon the kelp detritus. As kelp biomass in its raw state is relatively indigestable a large part enters the detrital food chain. Tiny iridescent blue-rayed limpets cling to hold fasts, as well as the stipe, and graze the surface continually for diatomswhile juvenile, but will feed upon the kelp itself when mature. Also amongst the holdfasts may be juvenile starfish, brittle stars,urchins as well as polychaete worms. Small tube worms belonging to the Spirorbis genus may be found encrusted to the surface in their coiled cement like tubes. Bryozoans are also common – small (0.5 mm width) colonial invertebrate suspension feeders which form banks of mineralised grids upon the surface of alga as well as other structures. Larger animals, such as cartilaginous and bony fish,cephalopods and marine mammals and birds may also rely on kelp, which harbours their food, shelters their young and mitigates destructive wave action to some extent.

Photos 3.49 - 3.54 - Common kelp inhabitants. Far left, large predatory fish, such as the black rockfish Sebastes melanops, will smash urchin tests with their powerful jaws; Second from left, bryozoans, such as this species of Membranopra, commonly encrust kelp; Third from left, urchins are important residents of kelp forests and beds and may consume large quantities; Third from right, the sea otter Enhydra lutrissmashes urchins on stones to gain access to the soft innards - they must consume approximately thirty urchins to meet daily metabolic demand; Second from right, the blue rayed limpet, Helcion pellucidum, is a common resident on Atlantic kelp species - it feeds upon the kelp only when mature (5 mm size); Far right, sedentary polychaete worms of the Family Spirorbidae may be found attached to kelp as well as other macroalgae. Photos 3.51 and3.53 are courtesy of Algaebase and are the work of M. D. Guiry. All other photos are from Wikipedia and are copyright free.

It should be noted that there are keystone fauna within kelp communities, most notably predatory starfish, large predatory fish and the sea otter. These animals control the abundance of resident urchins which graze indiscriminately upon kelp holdfasts. When urchin predators are removed from a kelp system it can be devastating - with no biological control the urchin population can become hugeand kelp may suffer excessive mortality with swathes of kelp bed being removed. This leads to kelp barrens – regions where the loss of the alga has impoverished the environment in terms of physical complexity and thusly biological diversity. In New Zealand intensive fishing of large snapper led to urchin barrens. When marine reserves were implemented, limiting the fishing, these regionsimproved dramatically as the average size of the snapper increased. Small snapper do not feed on urchins as they lack the necessary tough mouths and powerful musculature which comes with larger body size. The most famous case study of kelp keystone species is that of the Aleutian Islands in Alaska. The majority of these islands lost their resident sea otters (Enhydra lutris) to the fur trade in the seventeenth century, there was consequently a loss of kelp. Otters are very efficient at controlling urchin numbers, they collect the spiny creatures from the seafloor and bring them to the surface with a flat stone which they rest upon their bellies. The urchin is then smashed upon the “anvil”. The otter must consume approximately 30 urchins to fulfill its energy requirements. A few islands had a reintroduction of sea otters which in turn established healthier kelp communities. It should be noted that there is a limit to urchin numbers even in predator free environments, partly due to available food, but also due to increased likelihood of disease. 

In some parts of the world kelp is harvested in vast quantities for its inherent alginate as well as iodine salts. The kelp may also be rendered down into fertiliser for agricultural use. Kelp is incredibly valuable in its ability to create spatial heterogeneity, its exploitation must be managed effectively if ecological damage is to be avoided.

Coral Reefs

Coral reefs are vast calcium carbonate structures that form in close proximity to coastal regions. The reefs are fairly unique in the natural world as they are formed predominantly by one group of organisms – the cnidaria. The phylum cnidaria contains the jellyfish(schyphozoans), anemones (anthozoans) and hydroids (hydrozoans), all of which are united by their symmetrical three layer body plan. Corals are closely related to the anemones and consequently belong to the same class. There are two major coral types, the reef building corals referred to as being hermatypic, and the non-reef building corals referred to as being ahermatypic. The hermatypic corals contain symbiotic unicellular algae which are vital to their success in warm well lit waters, the ahermatypic corals may also contain these zooxanthallae but are far less dependent on the symbiotic relationship. Both coral types are important ecologically, yet it is the hermatypic corals that form the vast reef structures which in turn support massive tropical communities, the largest being theGreat Barrier Reef off eastern Australia. Reef building corals are limited to clear, high salinity and warm waters (no less than 20 degrees Celsius) and are consequently restricted to tropical waters. The ahermatypic corals are found in all the worlds oceans. Hermatypic corals will not establish in waters adjacent to estuaries due to the high output of suspended sediment and freshwater – for example reefs are located on the east coast of South America away from the Orinoco and Amazon Rivers and are absent from thewest coast where cold upwelling waters dominate. Corals therefore flourish in wave swept regions where sediment cannot settle and there is a constant supply of nutrient laden and oxygenated water.

Photos 3.55 - 3.57 - Left, a soft coral (ahermatypic) belonging to Family Clavulariidae; Middle, satellite view of the Great Barrier Reef, Australia; Right, a hard coral (hermatypic) Platygyra lamellina. Images copyright free from Wikipedia - coral photos are the work of Nick Hobgood and the sattelite image is from NASA.

Coral Anatomy

Corals are typically colonial organisms which profit from the ability to modify their surrounding environment – most notably they have the ability to produce their own substrate. The most apt way to describe a coral is to imagine a tiny anemone (typically only several millimetres in width) sitting within a cup of lime. The coral may withdraw its tentacles into the cup or corralite as it is known by some. This is of significant importance as it allows a basic defence against would be predators and affords some protection to desiccation during exposure. The animal is radially symmetrical and composed of three tissue layers. The outermost layer is theepidermis which is covered in mucus secreting cells, stinging cells arranged in banks, as well as rows of cilia. These cells act together to capture planktonic prey – the stinging cells (nematocysts) fire out harpoon like structures when the cnidocil trigger is provoked, the harpoon structures contain neurotoxins that incapacitate the prey. The victim becomes trapped within the mucus and is wafted toward the central mouth via the unified beating of the cilia. The opening to the gullet is in fact the only opening to the animal and acts doubly as both mouth and anus. The second layer of the animal, which separates the epidermis from the gullet, is the mesoglea – this is a fibrous and homogenous supporting tissue. The gullet is surrounded by the third layer of tissue – aptly named thegastrodermis. The dinoflagellate symbionts are located within this inner most tissue. Corals may also possess mesenterial filamentswhich can be extruded from the gastrovascular cavity, these threadlike structures are actually the digestive organs of the coral and have the ability to produce powerful enzymes – they are employed in combating infringing coral species. Individual polyps are connected to the colony via extensions either side of the animal, these are the coenosarc channels.  

Figure 3.9 - (a) elkhorn coral colony (b) individual coral polyp cross section (c) neumatocyst battery showing coiled and sprung stages.

Coral Symbiosis

As mentioned, the hermatypic corals contain symbiotic algae known as zooxanthellae, which are a vegetative (non-motile) form of dinoflagellates belonging to the genus Symbiodinium. The algae live within the gastrodermal layer of the coral animal and relinquish valuable oxygen, amino acids and carbohydrate (glucose) via photosynthesis. This is imperative to the success of the coral as the surrounding waters are typically oligotrophic (low nutrient value), the close cycling of nutrients reduces loss to the surrounding medium. In return Symbiodinium has a secure environment to inhabit and a ready supply of ammonia, carbon dioxide, nitrates andphosphates – all of which are metabolic products of the coral. The algae also increase the corals ability to secrete calcium carbonate, which has been proven via experimental removal of the algae under lab conditions. The dinoflagellates do not provide the entirety of the corals diet as there is an inevitable loss of energy and essential compounds overtime, it is therefore convenient that the coral is in fact heterotrophic and can capture planktonic organisms to supplement its diet. Such a mutualistic relationship may also be observed in various anemones, jellyfish and giant clams.

Photos 3.58 - 3.63 - The symbiotic relationship between animal and algae is not exclusive to corals. Far left, the upside-down jellyfishCassiopea xamachana contains similar zooxanthallae to coral, as does the aggregating anemone Anthopleura elegantissima, Second from left, and the giant clam Tridacna gigas, Third from right; Third from left, zooxanthellae are in fact vegetative dinoflagellates much like regular free-living species such as Nocticula sp. Second from right, if environmental conditions become stressful a coral may expel its zooznthellae, losing its vivid colouration - this is 'bleaching'; Far right, the nudibranch Elysia chlorotica feeds upon the macroalgae Vaucheria litorea and has the amazing abilty to translocate the ingested chloroplasts to its own tissues unscathed, the sea slug is therefore capable of obtaining metabolic derivatives from the photosythesising organelles. Photos 3.58 - 3.62 are from Wikipedia and are copyright free. Photo 3.63 is fromPNAS.

Coral Reproduction, Dispersal and Expansion

Corals are typically hermaphroditic although some are gonochoristic (single sex). Like many invertebrate taxa the corals are capable ofsexual or asexual reproduction, with both strategies having advantages and disadvantages. Asexual reproduction allows a coral animal to rapidly expand its colony via simple budding – this is vital as coral reefs are space limited and competition is fierce. Sexual reproduction results in the production of free swimming planktonic planulae. The planula is the corals solution to dispersal. Firstly, the corals synchronise gamete release via lunar clues, this maximises encounter rates and minimises predation, the fertilised eggs then develop into planulae. Depending on species, fertilisation may occur pelagically (broadcast strategy) or within a brood chamber(brooding strategy). The brooding strategy allows for lower planula mortality and ensures maximal local dispersion. The broadcast strategy has a higher planula mortality rate but has the benefit of wider dispersal. Corals may also establish new colonies viafragmentation, this is where a piece of the parent coral breaks off and settles within a favourable environment, it may then form a new colony identical (genetically) to that of the parent coral head. The aforementioned strategies are depicted in below:

Figure 3.10 - Corals have differing strategies with respect to dispersal, reproduction and expansion - asexual reproduction, fragmentation and sexual reproduction. Most coral species can carry out all three strategies and are often hermaphroditic.


Reef Types and Formation

There are three categories of reef – fringing, barrier and atoll. Fringing reefs, as their name suggests, form adjacent to landmasseswhere the water is clear and shallow. Barrier reefs are very similar in that they form in proximity to landmasses, they may be differentiated from fringing reefs by an increased stretch and depth of water between the reef and the proximal landmass. Coral atolls are ring shapedstructures that surround a central lagoon – formation is by subsidence of the landmass (volcanic island) around which they would have originally formed. Over time the island will subside, yet the coral reef will remain at an optimum depth due to secretion of calcium carbonate. The corals must form reef at a rate that meets or exceeds that of the subsidence, if they fail to do so the platform upon which the animals rest will gradually descend into depths that cannot support coral growth. This theory was first put forward by Charles Darwin and is often overlooked by his later theory of natural selection. Some scientists offered alternate hypotheses to atoll formation, Darwin’s theory was reinforced in the mid twentieth century when it was proven, via drilling, that coral reefs sit upon volcanic rock. Coral atoll formation may be highlighted by referring to a diagram from Darwin’s book “The Structure and Distribution of Coral Reefs”.

Figure 3.11 - A - edges of barrier reef before subsidence; L, initial lagoons before subsidence; A' - edges of barrier reef with loss of island; L'- central lagoon forms with loss of island. Diagram from Charles Darwin's "The Structure and Distribution of Coral Reefs" (3rd ed., 1889).

Reef Zonation

Coral reefs are regions of high spatial heterogeneity. Over a fairly small region there may be observed several well defined habitats, which could include calm sandy flats to rocky exposed faces and all the grades between. The large number of habitats are commandeered by an equally large number of species. The driving factors behind the variety of habitats are predominantly light penetration and wave action. Light availability and wave action act together to determine the distribution and composition of the coral fauna - where corals form water becomes calmer in the lee of the structures and depositional rates increase forming sandy patches, where corals do not form encrusting algae becomes dominant and the surface of the reef remains rugged and craggy.Biological interactions are also important in zonation yet play out over a smaller scale than physical factors. A general pattern for zonation is hard to define – reefs vary greatly worldwide due to proximal oceanographic and climatic conditions. When attempting to describe a reef it is common practice to begin from the seaward edge and to move landward. In the simplest terms a reef may be described as having a steep seaward slope which rises rapidly over a short distance to the oceans surface, this exposed slope is dominated by the massive corals (i.e. brain coral) which are more able to withstand severe wave action than the more fragilebranching corals (i.e. staghorn). The seaward slope often has deep channels and gullies which have been carved by the hydraulic action of waves. Wave action prevents the settlement of corals at the uppermost section, instead encrusting algae are often found in high densities, their success afforded by their prostrate and foliose form, this gives rise to the algal ridge. The seaward slope peaks at the reef crest, behind which water is sheltered from the prevailing wind and waves. This calm region is referred to as the reef flatand is typified by coral rubble, sand and sediment. The depth of the reef flat rarely exceeds more than a metre or so and may bewarmer and more saline than the adjacent sea. Past the reef flat there may be a lagoon – a relatively deep body of water that is sheltered from the sea via the reef and reef flat and which is inundated intermittently by high tides. Small reef systems may form within the lagoon itself.

Figure 3.12 - An organic reef has many habitats across its breadth, this physical heterogeneity results in astounding biodiversity. 

Reef Biodiversity

Coral reefs harbour a bewildering array of species and are often paralleled with rainforests in terms of biodiversity. The indo-pacific reefs have the largest biodiversity of any known and can typically have several thousand species of fish alone. Invertebrates, other than corals, may seem rare at first yet can be found in vast numbers within the coral structures, this is probably due to the massive feeding pressure of reef systems – an exposed invertebrate is a dead one. Pelagic migratory fishes, mammals and turtles are frequent visitors of reefs, tapping into the reef food web or utilising the services of crustacea and small fishes to pick off parasites. The astounding biodiversity of reefs has been well noted for many hundreds of years but there is still a vacancy for a unifying theoryto explain this high number of species. One of the most convincing arguments is that the great variety of habitats (sand, rocky faces, caves, channels, coral rubble, shallow to deep water) simply affords habitable space. The large number of habitats is not thought to be enough to explain the biodiversity, additional theories are required, perhaps the high level of specialisation allows for minimal competition or maybe the fact that many reef species operate diurnally or nocturnally allows for reduced competition for space. There are many theories and it is more likely than not that high biodiversity is the result of many subtle factors.

Photos 3.64 - 3.75 - Coral reefs are home to almost every major animal group. Top far left, a pair of anemonefish Amphiprion ocellaris make themselves comfortable within their host anemone; Top second from left, two common residents, the angelfish Centropyge potteri and the surgeonfish Ctenochaetus strigosus; Top Third from left, a tunicate colony Clavelina moluccensis filter the water for particulates; Top third from right, a blacktip reef shark Carcharhinus melanopterus patrols a reefs seaward slope; Top second from right, the green turtle Chelonia mydas forages coastal waters for invertebrates while juvenile; Top far right, a dragon wrasse Novaculichthys taeniourus has parasites and dead scales plucked away via two gregarious rainbow wrasse Labroides phthirophagus; Bottom far left, this colourful goby relaxes on a gorgon (se whip) arm;Bottom second from left, the nudibranch Notodoris gardineri glides over the reef in search of calcareous sponges upon which it feeds; Bottom third from left, a large grouper rests alongside a vivid red gorgon; Bottom third from right, the cleaner shrimp Lysmata amboinensis picks the teeth of a moray eel; Bottom second from right, a colourful porcelain crab guards its territory; Bottom far right; space is very limited in reef systems as highlighted by this dense assembly of corals. All image are copyright free from Wikipedia and attributed to Nick Hobgood, Albert Kok, Jenny Huang and Matthias Kleine, Dr. Dwayne Meadows, Mila Zinkova.

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