Biological factors affecting distribution of communities

Although inorganic factors exert a major control, there are also biological factors which influence the distribution and composition of benthic communities. The physical and chemical features of the environment determine a range of species which compete, but success or failure in a particular habitat depends ultimately on qualities inherent in the organisms themselves. For example, they are able to some extent to choose their position. Free-living forms can move about to find areas that suit them, but the majority of adult benthic animals remain more or less stationary, confined within burrows or attached to the bottom. Most of these start life as pelagic larvae dispersed by the water (Thorson, 1950). Once they settle and metamorphose they stay in place, and die if conditions are unsuitable. Undoubtedly there are great losses, but the larvae of many species show behavioural features which influence dispersal and favour their chances of reaching situations where survival is possible. Many species of larvae have some control over the depth at which they float, often by virtue of their response to light. Larvae of shallow water species are usually photopositive for a time, collecting near the surface, while those of deeper-dwelling forms mostly prefer dim illumination or darkness, and therefore occupy deeper levels. The depth at which the larvae float must obviously influence the depth at which they settle.

Selective settlement

It has also been demonstrated that larvae of certain species can discriminate between different substrates, and have some powers of selection; for example the larvae of the small polychaete Ophelia bicornis. This worm has a patchy distribution in various bays and estuaries around the British coast, living in a particular type of clean, loose sand. Pelagic larvae are produced which are ready to metamorphose when about five days old. At this stage they begin to enter the deposit. Wilson (1956) discovered that the larvae are able to distinguish between sands from different areas, preferring to complete their metamorphosis in contact with certain 'attractive' sand samples, and avoiding contact with other sands which have a 'repellent' effect. When the larvae settle, they appear to explore the deposit. If the sand is of the 'repellent' type, they leave it and swim away, shortly settling again and repeating their exploration. This behaviour continues over a period of several days, with metamorphosis delayed until a suitable substrate is found. If the larvae find nothing suitable, they eventually attempt to metamorphose none the less, but then usually die. During an early series of experiments, the particle size of the sand seemed to be the main factor to which the larvae were sensitive, but further experiments with artificially constituted sands showed the importance of other factors, notably the coating of organic materials and bacteria on the surface of the sand grains. If sand samples were washed in hot concentrated sulphuric acid they became neutral, losing their attractive or repellent qualities.

Selective examination of the substrate, with metamorphosis delayed as long as possible until a suitable substrate is discovered, has now been demonstrated for a fairly wide range of organisms, including annelids, barnacles, molluscs, bryozoa and echinoderms. Larvae usually become less discriminating with age. Although the prospects of successful metamorphosis become less as they get older, some are able to continue normal development even after extended periods of pelagic life if they eventually find a satisfactory surface for attachment. Many different properties of the substrate influence choice (Bayne, 1964, 1969; Crisp, 1974; Meadows and Campbell, 1972). Particle size selection has now been demonstrated for various burrowing organisms, e.g. the polychaete worms Ophelia, Protodrilus and Pygospio and the horseshoe worm Phoronis. In most cases the attractiveness of sediments is associated with the presence of films of micro-organisms, which may be important as food. This is most evident in burrowers which actually swallow the substrate. Other qualities of surfaces between which benthic larvae discriminate include differences of chemical nature, texture, slope, contour and colour. Sensitivity to aqueous diffusing substances is most typical of species which settle on organic substrates or on sediments containing a high content of organic matter; for instance, the attraction of shipworm (Teredo) larvae to wood, tube worm (Spirorbis borealis) larvae to the seaweed Fucus serratus, the gastropod Nassarius obsoletus to mud. In other species, mainly those settling on rock, e.g. acorn barnacles, spirorbid tube worms and the honeycomb worm Sabellaria, it has been shown that the larvae must usually first make actual contact with the shell, cuticle or cementing substance of their own or a closely related species before settlement is attempted, the stimulus for settlement being dependent on a 'tactile chemical sense'. The majority of rock-settling larvae prefer rough surfaces to smooth, some discriminate between light or dark surfaces and some, e.g. the sponge Ophlitaspongia seriata, settle preferentially on overhanging surfaces. In some rocky shore animals, e.g. Semibalanus balanoides, water turbulence encourages settlement. Barnacle cyprids settle most readily from flowing water, different species preferring different water velocities; for instance, the velocity of maximum settlement for Semibalanus balanoides is higher than for Elminius modestus. In Sabellaria alveolata, a polychaete which forms tubes of cemented sand grains, the larvae settle best from swirling water in which sand grains are present, especially after contact is made with the cementing substance of adult tubes or recently metamorphosed individuals (Wilson, 1968, 1970).

Selective settlement offers several advantages. It reduces larval losses by hindering metamorphosis in unsuitable locations. By encouraging gregariousness it facilitates fertilization, especially cross-fertilization between hermaphrodite forms requiring internal fertilization, such as acorn barnacles. Settlement in areas where others of the species have already survived ensures that conditions are likely to be congenial. It is also a form of behaviour favouring close adaptation to particular habitats, with the advantages of specialization. Conversely, although this may confer great efficiency in specific conditions, it increases the risks of extermination if the environment changes and suitable sites are lost.

Competition and predation

Once the larvae have settled, many other biological factors begin to influence their chances of survival. A community is a society of organisms with many interactions between the individuals. There is often competition for living space, and the outcome of this aspect of the struggle for existence is determined by a complex of factors. After settlement many larvae exhibit some exploratory behaviour, moving about over the surface and often spacing themselves to some extent to avoid crowding. Delayed settlement is usually followed by reduced spacing movements. Some larvae tend to space themselves from their own species but not from others, on which they may actually settle and attach. Such behaviour reduces intraspecific but increases interspecific competition. The relative success of different species in competing for space is also influenced by differences of breeding periods, reproductive capacities and growth rates. One species may gain advantage by early settlement following a seasonal decline in numbers of the community, or a fast-growing species may oust a slower-growing competitor by overgrowing it, or by claiming an increasing proportion of a shared food source.

Interaction between predator and prey must regulate the numbers of both. Mortality due to predation is usually highest during the period following settlement while the individuals are still small. Certain predators, notably ophiuroids, are sometimes so numerous that they virtually carpet the bottom and it seems surprising that any small creatures suitable for food can escape. In these conditions, survival may depend upon the time of settlement. It has been observed (Thorson, 1960) that some benthic carnivores have phases when feeding diminishes or ceases, usually in association with breeding. This passive period may last several weeks, and some species which settle during this time may be able to reach a sufficient size to become relatively safe from predation before their enemies start active feeding again. The composition of a community may therefore reflect coincidences between passive periods of predators and settlement periods of other species.

Interrelationships

The individuals of a community are in various ways interdependent, and some organisms thrive only in the presence of particular associated forms. Each type of animal is dependent upon other organisms for food, and the quantity and quality of food sources obviously exert a profound control over numbers and composition of communities. Certain organisms depend upon others to provide surfaces of attachment upon which they can grow, and in some cases are seldom found elsewhere; for example, the barnacle Pyrgoma anglicum on the cup coral Caryophyllia smithii, the anemone Adamsia palliata on the hermit crab Pagurus prideauxi. Some animals share the burrows formed by others; for example the polychaete Lepidasthenia argus shares with another polychaete, Amphitrite edwardsi. The polychaete Harmothoe lunulata may be free-living but often shares the burrows of the polychaetes Arenicola marina, Amphitrite johnstoni, and A. edwardsi, or the echinoderms Acrocnida brachiata, Leptosynapta inhaerens, Labidoplax digitata and others. One species may even live inside another; for example, the shrimp Typton spongicola within sponges including Desmacidon fruticosum, and the barnacle Acasta spongites embedded in the sponge Dysidea fragilis.

Relationships between species certainly involve numerous associations of an epizoic, commensal, symbiotic or parasitic nature. A striking example of a group of species commonly found living in close proximity is associated with the hermit crab Pagurus bernhardus, widely distributed on gravelly deposits in shallow water around the British Isles. The adult crab inhabits shells of the whelk Buccinum undatum. Several animals are commonly found on the surface of the shell, the most conspicuous being the anemone Calliactis parasitica. Other inhabitants of the outer shell surface, sometimes extending into the opening, include saddle oysters Anomia ephippium, the hydroid Hydractinia echinata, the serpulid worms Spirorbis spirillum, Pomatoceros triqueter and Hydroides norvegica, the barnacle Balanus crenatus and the sponge Suberites domuncula. The barnacle Trypetesa lampas apparently lives only in Buccinum shells inhabited by hermit crabs, where it burrows into the shell substance just inside the shell aperture. The shell is also sometimes bored by the worm Polydora ciliata. Living within the shell alongside the hermit crab there is frequently a large worm, Nereis fucata, which feeds on fragments of food dropped by the crab. Also inside the shell the tiny crab Porcellana platycheles is sometimes found. The hermit crab may carry parasites; for example, the isopods Pseudione spp. in the branchial chamber, Athelges paguri attached to the abdomen or occasionally in the branchial chamber, and the parasitic barnacle Peltogaster paguri extruding from the abdomen.

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