The marine biotic communities can be studied separately for different life zones of the ocean as follows:
1. Biotic communities of oceanic region:
The oceanic region or pelagic zone is less rich in species and numbers than the coastal areas, but it has its characteristic species. Many of these are transparent or bluish and since the sediment free water of the open sea is marvelously transparent, these animals are nearly invisible.
Animals that are too thick to be transparent frequently have smooth shiny and silvery bodies which make them invisible by mirroring the water in which they swim. Further, the animals of the pelagic zone of ocean encounter stable and uniform environmental conditions due to the continuity of the waters of the sea.
This continuity of sea waters causes the individuals of the open ocean to stay away temporarily or permanently from any solid objects, and they are provided with different locomotor organs. However, the plank tonic forms lack a locomotor organ and float freely in the vast expanse of the sea.
Biotic communities of pelagic zone:
The pelagic plankton also called epiplankton are exceedingly diverse. Phytoplankton includes diatoms and dinoflagellates, which together produce most of the organic carbon in the sea (and most of the oxygen in the atmosphere), as well as other forms of golden-brown algae and flagellated green algae. Some seaweeds, such as the large brown algae Saragassum, may have a floating stage.
The zooplankton of pelagic zone include representatives of every major phylum and most minor phyla, either as permanent members of the plankton community (holoplanktonic forms) or as transients during their larval stages (meroplanktonic forms).
Common among permanent planktonic forms are the Forminifera and Radiolaria, arrow worms (Sagitta), certain annelid worms, swimming snails, jelly fishes and most abundant of all are the crustaceans such as shrimps, copepods and cladocerans.
Among the temporary zooplanktons are the larva of larger animals from all marine environments and even some fresh-water environments. For example, in the zooplankions of inshore waters of Mandapam (South India) Prasad (1956) reported following animal species—flagellate protozoan Noctiluca miliaris; larval forms of coelenterate like Planula, Ephyra, Semper’s larva and Cerianthus larva; medusae of certain coelenterates such as Bougainvillia, Obelia, Charybdea, Rhizostoma and Rhopalina; ctenopho Teslike Pleurobraeliia and Beroe; larvae of many polychaet annelids and annelids such as Tomopteris, Alciopa and Autolytus, several species of chaetognaths like Sagitta and Spadella ; heteropod and pteropod molluscs like Carinaria, Pterotracliea, Cliona, Creseis; larvae of echinoderms such as bipinnaria, ophiopluteus, echinopluteus, auricularia; tornaria of Bal wglossus; species of copepods like Acartia, Calanopia, Labido- cera. Centropages, Paracalanus, Canthocalanus, Acrocalanus, Eucala- nus and Microsetella larvae of decapod Lucifer; protochordate species of Larvacea, Thaliaceae and Ascidiaceae such as Thalia democrat ica, Jasis zonaria, Salpa cylindrical appendicularian— Oiko- pleura diocia, O. parava and O. cephalocera and also cephaicchordate Amphioxus.
The largest animals in the pelagic region are the nekton. These include cephalopods such as squid and nautili among the invertebrates, as well as many marine vertebrates, such as bony fishes, sharks sea turtles and whales. In addition, sea birds also feed on many of the same food types as these large carnivores. Air-breathing nekton, such as the turtles, and whales, are found mainly in the photic zone, but fishes extend from the sea surface to the bottom.
Certain fishes such as tuna, shark, sardine, mackerel, herring, bonito, and anchovy live near the surface. Those fishes which occur at greater depths are often grotesque and unlike any fish found at the surface. They tend to be small—15 to 20 m is large for the ocean deeps— and they are exceedingly dispersed.
Many have luminescent appendages with which to lure prey, or mouth that look several sizes too large for the rest of the body. Food is not plentiful in the deep waters of the sea, so these fishes must go for long periods of time without food, and then consume as much as they can when they have a chance.
Biotic communities of abyssal benthic zone:
The abyssal benthic zone or deep sea of oceanic region is pitch dark and universal absence of light in this environment excludes the possibility of any growth of vegetation or other photosynthetic organisms. Consequently, most of the deep sea dwellers depend upon the detritus (dead body excreta of surface forms) which sink to the sea bottom while a large number of species are active predators.
However the deep ocean benthos is surprisingly diverse. Food is a little more plentiful on the bottom than it is in the deep waters of the ocean, because the bottom sediments are the ultimate resting place for all detritus raining out of the upper layers (oceanic surface and pelagic zones).
The bottom of the sea is a soft ooze, made of the organic remains and shells of Foraminifera, Radiolaria, and other animals and plants. Sea cucumbers, brittle stars, crinoios (sea lilies), sea urchins, certain benthic fishes, and several types of crustaceans as well as sea anemones, clams, and similar animals are all found on the bottom, not only on the abyssal plains, but some even at the bottoms of the deepest ocean trenches. Many of them are detritus feeders but a large number of them are carnivores.
In fact, the biomass of carnivorous brittle stars is often higher than that of the detritus feeders that serve as their food source; however, their metabolic rate is much lower. The great diversity of sea bottom fauna has been related with great constancy in the physical environment of the deep sea.
2. Biotic Communities of Continental Shelf:
The communities of continental shelf are both richer and more diverse than those of Open Ocean. Diatoms and dinoflagellates are still the most productive phytoplanktons, but in the shallower regions green, brown, and red algae anchored to the bottom may be of great importance. Several kinds of these seaweeds are harvested from rocky shores as human food or for some commercial purpose in eastern countries, USA and India.
The zooplankton of continental shelf is generally the same as in the pelagic region, but some purely pelagic species have been replaced by neritic species, and the overall diversity is somewhat higher. The temporary zooplanktons are much more abundant over the shelf region than in the open ocean.
The nekton of the neritic sea is both diverse and well known. The significant nekton species of the region are large squid, whales, seals, sea otters, and sea snakes. The most numerous nektonic forms, of course, are the fishes, which include many species of shark as well as herring-like species such as menhaden, herring, sardine and anchovy; cods and their relatives, such as haddock and Pollack; salmon and sea trout; flounder and other flatfish such as sole, plaice and halibut; and mackerels, including tuna and bonito.
Communities of sea shore or intertidal zone:
The littoral, intertidal or eulittoral zone is the region of sea shore which exists between the high and low tide lines. The region of the high tide mark is called supratidal or supralittoral zone. The region of low tide is called sub tidal zone.
Davenport (1903) has divided sea shore or intertidal zone into a submerged zone which is the portion of the shore below the low tide exposed by very low tides, a lower beach occupying the area between the mean low tide and mean high tide and an upper beach which is reached by the highest of high tides.
The intertidal zone is the most variable zone in the entire sea. It is completely covered at high tide, and is completely uncovered at low tide except for tide pools. Any organism that is to survive in the intertidal must be either resistant to periodic desiccation or able to burrow to water level.
For example intertidal polychaets escape desiccation by resorting to a tubicolous mode of life whereby they can resist desiccation by retaining sufficient moisture till the advent of the next wetting. Barnacles can survive desiccation by remaining in a state of suspended activity during dry periods.
Further, the majority of shore animals utilize oxygen dissolved in water and when the low tide exposes them during the periods of drought, they utilize the oxygen stored in the blood plasma, e.g., Nereid like, Arenicola. Barnacles achieve this by enclosing an air bubble between the shells.
One of the outstanding characteristics of this region is the ever present action of the waves, and the organisms living on a sandy or rocky beach have had to evolve ways of resisting wave action. The many seaweeds have tough pliable bodies, able to blend with the waves without breaking, while the animals either are encased in hard calcareous shells, such as those of mollusks, Bryozoa, starfish, barnacles and crabs, or are covered by a strong lethery skin that can bend without breaking, like that of the sea anemone and octopus.
Many sedentary shore animals (e.g., sponges, tunicates, etc.) have special modes of adhesion which keep them firmly attached to the substratum. However, animals of sandy beaches escape wave action by taking burrowing mode of life, because, these lack a hard substratum for the attachment of the animals.
The biota of intertidal zone also has to overcome the wide fluctuations in salinity and temperature. Thus, like the estuary, this variable environment is a zone in which an organism must be adapted to a broad range of environmental conditions.
Like the estuary also, it is an area of very high productivity with a simple community many of whose members may be exceedingly abundant. The diversity of biota of intertidal zone is determined by nature of substratum whether loose sand or mud or rocky coast.
Ecologists have recognized three major, types of intertidal zones or seashores, namely, rock, sandy and muddy. Eltringham (1971) has included a fourth type of sea shore, the pebble shore in this classification. Each type of sea shore possesses specific biota each of which remains specifically adapted for its peculiar habitats. Here we will discuss only rocky shore and sandy beach habitats.
i. Biotic communities of rocky shore:
The rocky shore presents solid substratum for the attachment of many sessile animals which often remain abundant here. The animals which are sedentary and inhabit rocky shore are limpets like Patella, Haliotis, Fissurella, and oysters, barnacles, tunicates and bryozoans.
Sessile organisms like the sponges, the colonial hydrozoans, anthozoans like Gammaria and Zoanthus attached to rocks. Certain animals such as some sponges, annelids and mollusks either bore the soft rocks like limestone or seek protection in the crevices of hard rocks. All these sedentary sessile animals are adopted for filter feeding.
Further, all these animals of rocky shore occur in successive zones (Fig 21.6). At the uppermost end is a zone of bare rock marking the transition between land and sea? Next is a spray zone with dark patches of algae on which the periwinkles (Littoria) graze Below this is the zone called barnacle zone regularly covered by the high tide; rocks in this zone are encrusted with acorn barnacles, limpets and mussels (e.g., Mytilus). This zone is a vulnerable zone being exposed to the brunt of wave action and active predation by starfishes and gastropods.
The mussels act like hard substratum for tunicates, sponges and small mussels. The next one is the zone of oysters is less vulnerable and includes green algae (Enieromorpha and Ulva) and barnacles. The next zone includes mussels, i.e., boring bivalves, which remain within crevices with exposed siphons only to be protected from waves and predation.
The chief species of borer bivalves are Pholas, Martesia, Hiatella, Tridacna, etc. Besides bivalves a variety of filter feeder bryozoans, and brachiopods and tubicolous polychaets like serpulids, terebellids, sabellids and cirratulids which live in tubes are the chief inhabitants of this zone. The subtidal zone includes red algae, brown akae (Laminarians and fucoids), sea anemones, sea urchins, corals, etc.
For example, the Okha coast of Gujrat in India is a typical rocky sea coast which has well defined fauna and flora (Gopala- krishuan, 1970). Dominant weeds of sub tidal zone of Okha coast are Sargassum, Dictyopteris, Graciltaria, Padina, Ulva, Polysiphonia, etc.
This subtidal zone also contains sea urchins, sermilid tubes of species Pomatoleius crosslandi, and mollusc species like Trochus radiatus Cvpraea arabica, Thais alveolata. The upper limit of the dense weed growth of subtidal zone marks the lower end of eulittoral or intertidal zone, where occurs a dense cluster of Crassnitrea cucullata and Littorina sp. The intertidal zone includes three sub- zones, i.e.
1. The suhzone of patchy weed growth;
2. The subzone of limpets like Sellana radiata and barnacles and
3. The subzones of oysters and barnacles
The supratidal zone is inhabited by green algae and molluscs. Chlorophyceae like Enteromorpha, Ulva, Bryopsis. Codium, Halimeda, Caulerpa, Dictyosphaeria and Cladophora, Phaeophyceae like Padina, Sargassum, Colpomenia, Iyengarif, Spathoglossum, Dictyopteris and Turbinaria; Rhodophyceae like Gracilaria, Hypnea, Acanthophora, Polysiphonia, and Cryptonemia are the chief algae of the area.
The common animals of this area are coelenterate species Stoichactis, Ixalactis, and Gyrostama; Nereid like Eunice, Tubifex; mollusks like Chiton, Nerita rumphii, Cypraea ocellata, Cypraea arabica seaweed grazer (nudibranch) mollusks, Aplysia benedicti, Onchidium, Elysia gradifolia, Eolis, Doris and floating mollusc Janthina, etc.
The midlittoral reef community of Okha coast consists of fishes like Epinephelus, Petrachus, the crab Pilumnus vespertilio, the annelid Eurythoe complanaia and molluscs like Murex, Bursa, etc.
Rock crevices of intertidal zone harbour within them crabs like Atergatis, Grapsus, Pilumnus, Matuta, Charybdea, Gelasimus, squillids like Sesarma and Gonodactylus; coelenterates like Stoichactis, Ixalactis, Zoanthus, Lobophytum; the hydroid species like Lytocarpus, Sertularia and Plumularia; and annelids like Serpula, Polynoe, Eurythoe, Sabella, etc.
ii. Biotic communities of sandy shore:
The sandy shore may be even harsher than the rocky shore. It is subjected to all the extremes of the latter (temperature, salinity, turbidity, wave action, etc.) plus inconvenience of a constantly shifting substratum. The last makes life on the surface almost impossible; life has retreated below the surface. Generally sandy beaches are characterized by gentler wave action. Because of the prolonged time taken for drying up, these beaches are suitable for animal life.
Decaying seaweeds and dead remains of animals result in the addition of organic matter to the sand the coastal subsoil water is the environment of a special interstitial brackish water fauna. Most of the interstitial organisms are small, elongate, and vermiform with transparent bodies. They have no eye but possess well developed adhesive organs and sense organs. They are negatively photo tactic and gregarious in habit. They may be herbivore, carnivore, and omnivore or detritus feeder.
Large animal forms which are few in number prefer coarse sand grains and smaller one which are very abundant, prefer finer sand. The principal animal groups of interstitial water are the Nematoda, Turbellaria, Annelida, Gastrotricha and Acrania. Certain animal groups such as Protozoa occur sporadically.
The top 20 cm of sand between low and mid-tide levels of the beach is occupied by the inhabitants such as Hydrozoa, Turbellaria, Nemertina, Rotifera, Archiannelida, Polychaeta, Ostracoda, Halacaridae and Nudibranchiata. Species of Gastrotricha, Kinorhyncha, and Isopoda occur at mid-tide levels preferably in deep layers of sand.
Oligochaets are sporadic and are found at different levels. Harpactcoid copepods remain restricted to tide level. Tardigrades occupy the region between mid-tide and high-tide. Thus, sandy beaches also possess characteristic successive zones or zonations of animals (Fig. 21.9).
For example, according to the studies of Ganapathy and Rao o sandy beach fauna of Visakhapatnam coast though lacks or contains small number of tubicolous polychaets and Crustacea due to very unstable substratum, but include following animal species—spinoid Prionospio krusadiensis, Aricia and Lumbriconeries, all of which possess the capacity to secrete mucus and formation of cover of sand grain over the body for protection; Glycera lancadivae, G. alba, Nerina bonnieri, Pijionidens indica, Pisione complexa and archiannelids Saccocirrus minor and Protodrilus also occur.
The crustacean fauna of this zone includes Emerita asiatica, Albunea symnista (burrowing), amphipod species Harpinia sp., isopod Sphaeroma walkeri, mysid shrimp Gastrosaccui spinifer, hermit crabs Clibanarius aretheustus living within the shells of Thais and Cerithium, the eight-oared swimming crab Maiuta victor, the spider crab, Philyra scabruescula, Ocypoda platytarsis and U. macrocera.
Lamellibranchs like Donax cuneatus and Pamphia textile, burrowing snails like Sinum neritoideum, common gastropods like Oliva gibbosa and Terebra sp., Bullia vittata are also found along the coast of Visakhapatnam. This beach also contains eel Ophichtys and siphonophore coelenterates like Physalia, Velella and Porpita; and medusae-like Aequoria and Chiropsaltnus.