Present day

Six present communities are illustrated here: four of these from around Britain and two from the tropical Indo-Pacific Province. The tropical communities are more diverse than comparable communities in higher latitudes (Thorson, 1957).

The sands, sandy muds and muds in the shelf seas around the British Isles support a diversity of benthic communities, most of which became established after the beginning of the last major glacial advance about 15,000 years ago.

120 Offshore Muddy Sand Community

The community illustrated is based upon samples taken from the Irish Sea (Jones, 1956), but is typical of offshore muddy sand bottoms all around the British Isles at depths between about 25 and 80m. The community is extremely diverse, but is dominated numerically by infaunal polychaetes and bivalves, although gastropods, Crustacea, ophiuroids and echinoids may be common.

Shallow-burrowing, deposit-feeding bivalves such as the siphon-ate Abra prismatica and the palp-feeding Nucula sulcata are abundant. Other common bivalves are mainly suspension feeders, and include the shallow-burrowing Corbula gibba and Parvicar-dium scabrum as well as deeper burrowers with longer siphons such as Cultellus and Dosinia. The scaphopod Dentalium entalis feeds mainly upon benthic foraminifera living within the sediment.

Polychaetes are extremely abundant, and more than 40 species may occur, many of them deposit feeders. Pectinaria is amongst the most common of the larger forms and is interesting, for it forms a tube and has a similar mode of life to the scaphopods. Some of the polychaetes are predators with well developed jaws.

Turritella is a ciliary-feeding gastropod, which lives buried just below the substrate surface and can be very abundant in this community. Other common gastropods are the detritus-feeding Aporr-hais pespelicani, the predators Natica and Trophon, which feed upon other molluscs, and several small species such as Mangelia

Burrow Mollusks

Large burrowers

Fig. 120 Offshore Muddy Sand Community a Abra prismatica (Mollusca: Bivalvia: Veneroida) b Corbula gibba (Mollusca: Bivalvia: Myoida) c Nucula sulcata (Mollusca: Bivalvia: Palaeotaxodonta) d Parvicardium scabrum (Mollusca: Bivalvia: Veneroida) e Cultellus pellucida (Mollusca: Bivalvia: Veneroida) f Turritella communis (Mollusca: Gastropoda: Mesogastropoda) g Dentalium entalis (Mollusca: Scaphopoda) h Nephthys (Annelida) i Amphiura (Echinodermata — Asterozoa)

Large burrowers

Fig. 120 Offshore Muddy Sand Community a Abra prismatica (Mollusca: Bivalvia: Veneroida) b Corbula gibba (Mollusca: Bivalvia: Myoida) c Nucula sulcata (Mollusca: Bivalvia: Palaeotaxodonta) d Parvicardium scabrum (Mollusca: Bivalvia: Veneroida) e Cultellus pellucida (Mollusca: Bivalvia: Veneroida) f Turritella communis (Mollusca: Gastropoda: Mesogastropoda) g Dentalium entalis (Mollusca: Scaphopoda) h Nephthys (Annelida) i Amphiura (Echinodermata — Asterozoa)

Small burrowers j Pectinaria (Annelida)

k Nephrops norvegica (Arthropoda: Crustacea:

Malacostraca) I Dosinia lupinus (Mollusca: Bivalvia: Veneroida) m Echinocardium cordatum (Echinodermata:

Echinoidea) n Brissopsis (Echinodermata: Echinoidea) o pole dab (Vertebrata: Osteichthyes) p norway pout (Vertebrata: Osteichthyes) q Argentina sphyraena (Vertebrata: Osteichthyes) r Anapagurus (Arthropoda: Crustacea: Malacostraca)

which feed upon polychaetes by means of a specialized radula and a toxic salivary secretion.

The burrowing echinoids Echinocardium and Brissopsis are common and the ophiuroid Amphiura, a deposit feeder, may be very abundant, sometimes occurring in such dense aggregations that the whole sea bottom is a mass of ophiuroid arms. The Dublin Bay Prawn Nephrops norvegica is one of the commoner of the larger Crustacea and inhabits extensive burrow systems (not shown here). The hermit crab Anapagurus is very abundant. Fish are important predators on the muddy sand benthos, and analysis of fish diets has shown that the two most common groups in the community, bivalves and polychaetes, are the most common prey of the benthic-feeding fish, which are mainly haddock, cod, whiting and pole dabs.

121 Intertidal Mud Community

This community, often known as the 'Macoma balthica' community, is typical of intertidal mud flats around the shores of north-west Europe and examples can be seen off the east coast of Britain, at places such as the outer Thames estuary and the Wash (Mistakidis, 1951).

The community is developed on inshore muddy sands and silts stretching out from the intertidal areas to a depth of about 10m; as these facies are frequently found in estuarine areas, salinities are often fluctuating or reduced. The community is dominated numerically by deposit-feeding animals which occur in huge numbers and include the bivalves Macoma balthica and Scrobicularia plana, mobile species with two very long siphons. The lug worm Arenicola marina is a well known deposit-feeding polychaete, inhabiting 'U' burrows which leave the characteristic surface traces of casts and depressions so conspicuous on intertidal flats.

Mya arenaria, a sedentary suspension feeder with long, thick, fused siphons is often common in this community; where the sediment is more sandy, the common cockle Cerastoderma edule, a shallow-burrowing suspension feeder, can be extremely abundant. Aggregations of the mussel Mytilus edulis may be found wherever small pieces of hard substrate provide it with an anchorage. The winkle Littorina littorea is the most abundant of the larger gastropods, leaving feeding trails in the mud. The tiny gastropod Hydrobia ulvae is abundant on the sediment surface and leaves fine trails of looped furrows. Hydrobia occurs in immense numbers; there are usually between 3,000 and 10,000 per square metre, but densities as high as 60,000 per square metre have been

Mya Arenaria

Fig. 121 Intertidal Mud Community a Macoma balthica (Mollusca: Bivalvia: Veneroida)

b Cerastoderma edule (Mollusca: Bivalvia: Veneroida)

c Mya arenaria (Mollusca: Bivalvia: Myoida)

d Hydrobia ulvae (Mollusca: Gastropoda: Mesogastropoda)

e Corophium volutator (Arthropoda: Crustacea: Malacostraca)

f Littorina littorea (Mollusca: Gastropoda: Mesogastropoda)

g Scrobicularia plana (Mollusca: Bivalvia: Veneroida) h flounder — Pleuronectes flesus (Vertebrata: Osteichthyes)

recorded. Out of water it is a deposit feeder, consuming mainly the bacterial coatings around the sediment grains and detritus, but in water it has the ability to float on a raft of mucus which holds planktonic material on which the animal can feed. It thus exploits the food resources of both the sediment surface and the inshore plankton. Another deposit feeder is the small amphipod Corophium volutator which occurs in very large numbers and inhabits small 'U' burrows up to about 10cm deep.

These intertidal muddy sand communities are important feeding grounds for vertebrates which at high water are mainly flatfish, but at low water include a wide variety of largely migrant wading birds. Cockles and mussels are an important food for the oyster-catcher, and Hydrobia and Corophium are major items in the diets of the redshank and turnstone.

122 Shell Gravel Community

At the western end of the English Channel, much of the bottom between 20 and 100m consists of coarse shelly sands or shell gravels with many shells; the bottom is unstable and swept by tidal currents.

The community developed on such a bottom is neither very diverse nor abundant (Ford, 1923); it is dominated by suspensionfeeding bivalves, particularly Glycymeris glycymeris and Venus fasciata; other smaller venerids such as Timoclea ovata and Venus spp. may also be common. Glycymeris has notably unspecialized burrowing, feeding and respiratory mechanisms, but is typical of coarse current-swept bottoms in many parts of the world and appears to have been restricted mainly to this type of habitat throughout its history. It lacks siphons, and therefore lives just covered by the sediment, often with the posterior end of the shell exposed; in coarse sediments it burrows rather more deeply but maintains feeding and respiratory currents through the gravel.

Deposit feeders are less common, but include the siphonate bivalves Arcopagia crassa, Gari tellinella and Tellina pusilla. The cephalochordate Amphioxus lanceolata is also a typical animal of this habitat, living buried just beneath the surface of the gravel. Polychaetes, such as Glycera and Lumbriconereis, and the echin-oids Spatangus purpureus, Echinocardium and Echinocyamus may also be common.

Mollusca Bivalvia

Fig. 122 Shell Gravel Community a Glycymeris glycymeris (Mollusca: Bivalvia: Arcoida) b Venus fas data (Mollusca: Bivalvia: Veneroida)

c Timoclea ovata (Mollusca: Bivalvia: Veneroida) d Venerupis rhomboides (Mollusca: Bivalvia: Veneroida)

e Parvicardium scabrum (Mollusca: Bivalvia: Veneroida)

f Laevicardium crassum (Mollusca: Bivalvia: Veneroida)

g Gari tellinella (Mollusca: Bivalvia: Veneroida)

h Echinocardium flavescens (Echinodermata: Echinoidea)

i Portunus pusillus (Arthropoda: Crustacea: Malacostraca)

j Amphioxus (Chordata: Cephalochordata)

k Glycera (Annelida)

I Lumbriconereis (Annelida)

m whiting — Gadus merlangus (Vertebrata: Osteichthyes)

n Arcopagia crassa (Mollusca: Bivalvia: Veneroida)

Fig. 122 Shell Gravel Community a Glycymeris glycymeris (Mollusca: Bivalvia: Arcoida) b Venus fas data (Mollusca: Bivalvia: Veneroida)

c Timoclea ovata (Mollusca: Bivalvia: Veneroida) d Venerupis rhomboides (Mollusca: Bivalvia: Veneroida)

e Parvicardium scabrum (Mollusca: Bivalvia: Veneroida)

f Laevicardium crassum (Mollusca: Bivalvia: Veneroida)

g Gari tellinella (Mollusca: Bivalvia: Veneroida)

h Echinocardium flavescens (Echinodermata: Echinoidea)

i Portunus pusillus (Arthropoda: Crustacea: Malacostraca)

j Amphioxus (Chordata: Cephalochordata)

k Glycera (Annelida)

I Lumbriconereis (Annelida)

m whiting — Gadus merlangus (Vertebrata: Osteichthyes)

n Arcopagia crassa (Mollusca: Bivalvia: Veneroida)

123 Intertidal Rocky Shore Community

This is the most accessible and most well known of marine communities. The intertidal rocky shore is a steep environmental gradient between the land and the sea. Plants and animals inhabiting the intertidal zone must be able to tolerate often severe wave action, alternate emersion and submersion because of tides, and exposure to the climatic stresses of heat, cold, desiccation, and freshwater incursions. Despite these deterrents the intertidal zone is highly productive, and organisms which can tolerate the conditions are often extremely abundant. Intertidal organisms are often sharply zoned down the environmental gradient, the position of their zone depending upon the physiological tolerances of each organism, and the competitive interactions between the species (Lewis, 1964).

The community illustrated is typical of the middle parts of the rocky shore where algae, particularly Fucus, are abundant. In turbulent environments, many of the animals present show obvious adaptations which help them to avoid being dislodged, or to guard against damage from wave impact. Thus most of the animals are thick shelled (such as limpets) or elastic (such as sea-anemones) and adhere closely to the rock surface, either by cementation {Balanus), byssal attachment {Mytilus) or muscular adhesion {Patella). Free-moving animals, such as the gastropods Littorina and Gibbula, retreat into cracks and crevices at times of stress.

Most animals on the shore are either suspension feeders living on plant or animal material [Mytilus, Balanus, Actinia) or herbivorous grazers feeding upon algae {Patella, Littorinia, and Gibbula). Predators include Nucella lapillus, the dog whelk, which feeds on Mytilus by boring holes into the shell, or on barnacles by penetrating the opercular plates. The shore crab Carcinus maenas feeds mainly upon Littorina by cracking the shells. On limestone, sandstone or shale substrates, mechanical rock-boring bivalves, such as the siphonate Zirphaea and Hiatella, may be abundant. The rock is also penetrated by polychaetes and clionid sponges.

The intertidal area is a rich feeding ground for fish at high water and birds such as oyster-catchers, gulls and migrant waders at low water.

124 Tropical Marine Grass Beds

Inshore sand areas of the tropics and sub-tropics are frequently colonized by beds of grass-like marine angiosperms, particularly forms such as Thalassia, Cymodocea and Thalassodendron. The

Lithotrya Structure

Fig. 123 Intertidal Rocky Shore Community a Patella vulgata (Mollusca: Gastropoda: Archaeogastropoda) b Gibbula umbilicalis (Mollusca: Gastropoda: Archaeogastropoda)

c Nucella lapillus (Mollusca: Gastropoda: Neogastropoda — dog whelk) d Mytilus edulis (Mollusca: Bivalvia: Mytiloida) e Zirphaea crispata (Mollusca: Bivalvia: Myoida)

f Balanus (Arthropoda: Crustacea: Cirripedia — barnacles)

g Hiatella arctica (Mollusca: Bivalvia: Myoida) h Littorina littorea (Mollusca: Gastropoda: Mesogastropoda)

i Actinia equina( Coelenterata: Anthozoa — sea anemone)

j Fucus vesiculosus (Algae — bladderwrack) k Carcinus maenus (Arthropoda: Crustacea — crab)

Fig. 123 Intertidal Rocky Shore Community a Patella vulgata (Mollusca: Gastropoda: Archaeogastropoda) b Gibbula umbilicalis (Mollusca: Gastropoda: Archaeogastropoda)

c Nucella lapillus (Mollusca: Gastropoda: Neogastropoda — dog whelk) d Mytilus edulis (Mollusca: Bivalvia: Mytiloida) e Zirphaea crispata (Mollusca: Bivalvia: Myoida)

f Balanus (Arthropoda: Crustacea: Cirripedia — barnacles)

g Hiatella arctica (Mollusca: Bivalvia: Myoida) h Littorina littorea (Mollusca: Gastropoda: Mesogastropoda)

i Actinia equina( Coelenterata: Anthozoa — sea anemone)

j Fucus vesiculosus (Algae — bladderwrack) k Carcinus maenus (Arthropoda: Crustacea — crab)

Codakia Animal

Fig. 124 Tropical Marine Grass Beds a Codakia tigerina

(Mollusca: Bivalvia: Veneroida) b Gafrarium pectinatum

(Mollusca: Bivalvia: Veneroida)

Pilar affinis (Mollusca: Bivalvia: Veneroida) Tellinella virgata (Mollusca: Bivalvia: Veneroida) Pinna muricata (Mollusca: Bivalvia: Pterioida) Conus litteratus (Mollusca: Gastropoda: Neogastropoda) Strombus gibberulus (Mollusca: Gastropoda: Mesogastropoda) Holothuria atra (Echinodermata: Echinozoa) Porites (Coelenterata: Anthozoa: Scleractinia) Naso unicornis (Vertebrata: Osteichthyes)

Smaragdia rangiana (Mollusca: Gastropoda: Archaeogastropoda) 1 Lambis lambis (Mollusca: Gastropoda: Mesogastropoda) m Cypraea annulus (Mollusca: Gastropoda: Mesogastropoda) n Natica marochiensis (Mollusca: Gastropoda: Mesogastropoda) o Tripneustes (Echinodermata: Echinoidea) p Callianassa burrow (Crustacea)

q Menaethius monoceros (Arthropoda: Crustacea: Malacostraca)

k communities developed in association with the grass beds are important components of coral reef ecosystems in the Indo-Pacific and Caribbean marine provinces.

The leaves of the grass act as baffles, trapping sediment particles, which are then stabilized by the extensive rhizome and root systems. In this way, sediment can be stabilized in sites of relatively high wave action. Grass bed communities are among the most productive in the marine environment; they support a very diverse biota and organic detritus is abundant.

The community illustrated is based upon a western Indian Ocean grass bed, but the types of animals shown are generally similar to those found throughout the tropics. The stable sediment is an ideal habitat for infaunal animals because there is little risk of major disturbance. Lucinoid bivalves, which, although siphonless, construct mucus-lined inhalent tubes leading to the surface, are particularly characteristic of this habitat, and include such genera as Codakia and Anodontia. Other infaunal suspension feeders are the byssate sedentary Pinna muricata and the siphonate venerids Pitar and Gafrarium. Deposit feeders may be locally abundant, particularly near the shore, and include Tellinella staurella and Quidnipagus palatum. Burrowing shrimps (callianassids and alpheids) are abundant, and construct large and complex burrow systems among the roots and rhizomes. Other infaunal animals, not illustrated, include a large variety of burrowing worms.

The epifaunal component of the grass beds community is distinctly stratified, with many foraminifera, amphipods and gastropods such as the small, green, well camouflaged Smaragdia living amongst the algae attached to the grass leaves. Other animals, for. instance bryozoans, sponges and calcareous hydroids such as Millepora, frequently encrust the stems, whilst the substrate surface between the stem and leaf bases is colonized by a wide variety of larger organisms. Deposit-feeding holothurians are abundant; the black Holothuria occurs in densities of as many as 3 — 5 per square metre and ingests, by means of its tentacles, huge quantities of sediment, from which it extracts the organic detritus. Large, thick-shelled herbivorous gastropods are common, and include such forms as Strombus and Lambis with a digitate lip, and predatory gastropods such as Conus litteratus which feeds upon in-faunal polychaetes. Other herbivores include the echinoid Tripneustes pileolus which covers itself with dead shells and coral debris as camouflage.

Corals are not important members of grass bed communities, and tend to be small and restricted to a few genera. Porites, a genus particularly tolerant of marginal conditions, often occurs in small unattached ball-like colonies which are rolled by wave action.

The marine grass beds are often intertidal, in which case at high tides they are important feeding grounds for fish such as

Naso and turtles, while at low water egrets and migrant palearctic waders exploit the habitat for small Crustacea and gastropods.

125 Tropical Coral Reef Assemblage

Coral reef ecosystems comprise a variety of communities reflecting the range of environmental conditions found across the reef system. The Indo-Pacific community illustrated is found on a coral-dominated hard substrate in a shallow sublittoral area of good circulation but not heavy wave action.

The substrate is a coral-rich limestone which may be a product of late Pleistocene rather than recent reef growth. The epifauna is dominated by scleractinian corals, particularly species of the genus Acropora, which is very important in forming the framework of the reef. Acropora is particularly characteristic of sites of good circulation and low turbidity. Corals, although containing symbiotic algae, feed mainly upon zooplankton which they catch by means of stinging cells or mucus strings. Also containing symbiotic algae, but suspension-feeding, is the large, thick-shelled bivalve Tridacna, firmly anchored by an enormous byssus. These giant clams are believed to have evolved from forms resembling cockles.

Many other epifaunal animals live beneath the corals, including echinoids, Crustacea, gastropods, ophiuroids and a host of encrusting sponges, tunicates and bryozoans. Such habitats are characterized by richness in species but by small populations. Because of the antagonistic reactions few animals are able to live on the surfaces of the living corals. The calcareous rock surface is penetrated by a wide variety of boring and nestling animals including the sponge Cliona, the boring bivalves Lithophaga and Gastrochaena, polychaetes and sipunculid worms and the echinoid Echinometra.

The huge and diverse fish populations of the reefs exert important influence on the habitat, behaviour and distribution of the reef invertebrates. Some fish graze upon the living coral, some on calcareous algae, while many others feed on particular types of invertebrates. The habitats and diet of reef animals are highly specialized and this is thought to be the reason why so many species can be accommodated in reef communities. This kind of specialization can only take place where environmental conditions and food supply are stable and predictable.

Fig. 125 Tropical Coral Reef Assemblage i Tridacna gigas (Mollusca: Bivalvia: Veneroida) 3 Heterocentrotus mamillathus (Echinodermata: Echinoidea)

c Echinometra (Echinodermata: Echinoidea) d Stichopus chloronotus (Echinodermata: Echinoidea) e Ophiocoma (Echinodermata: Asterozoa) f Conus textile (Mollusca: Gastropoda: Mesogastropoda) g Aspidosiphon (Sipunculida) h Lithophaga (Mollusca: Bivalvia: Mytiloida)

i eunicid and nereid worms (Annelida)

j Rocellaria lamellosa (Mollusca: Bivalvia: Myoida)

k Botula silicula (Mollusca: Bivalvia: Mytiloida)

1 Lithotrya (Arthropoda: Crustacea: Cirripedia) m Phascolosoma (Sipunculida)

n Batistes (Vertebrata: Osteichthyes — trigger fish)

o Pomacanthus (Vertebrata: Osteichthyes)

p Dascyllus (Vertebrata: Osteichthyes) q Holocentrus spinifer (Vertebrata: Osteichthyes)

r Atergatis floridus (Arthropoda: Crustacea: Malacostraca)

Dosinia Lupinus
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