Littoral Fauna

the aggregate of animals that populate the littoral. These animals are characterized by an adaptability to periodic intervals out of water, to sharp daily and seasonal fluctuations in temperature and salinity, and to the effects of direct solar radiation and surf. During low tide, many littoral inhabitants hide under rocks, among algae, or under cliff overhangs; some regularly follow the tide as the level of the sea changes. The littoral fauna includes many invertebrates (for example, sponges, coelenterates, worms, crustaceans, mollusks, bryozoans, and echinoderms) and fishes.

The richest and most varied littoral fauna is found in the tropics. From the equator to the poles the number of species progressively decreases. In the arctic and antarctic, where the littoral is covered with ice for most of the year, and on open surf beaches in the tropics, where insolation is very strong, littoral fauna is extremely poor or absent. In the temperate zone the average biomass of the littoral fauna is 2–3 kg/m² (sometimes reaching 30–40 kg/m²); in the tropics the average biomass is somewhat less than 1 kg/m² (sometimes reaching 40–43 kg/m²).

Littoral animals have developed various adaptations that prevent dehydration during low tide, such as the ability to close their shells tightly (in many mollusks and such cirripedes as acorn barnacles), to become firmly attached to rocks by suction (in chitons and limpets), and to compress tightly and cover themselves with mucus (in nudibranchs and sea anemones). Different species are able to remain out of water for relatively long periods of time; this ability determines the vertical zonality in the distribution of littoral fauna. Animals that are well protected from dehydration settle on the upper levels; those less protected stay in the middle and lower levels. Animals that inhabit the sublittoral (some fish and shrimp) appear in the littoral zone only during high tide to use the abundant food resources. In seas of the temperate zone some littoral animals migrate to the sublittoral for the winter; others are anabiotic and can tolerate prolonged freezing. Acorn barnacles, oysters, mussels, other mytilids, sea anemones, chitons, Littorina, and limpets live on rocks; hydroids, bryozoans, and Ascidia are found on and among algae. In soft bottoms live many worms and burrowing bivalve mollusks.

The littoral serves as a transition zone between the sea and dry land. It is in this zone that the transition from a marine to a terrestrial mode of life (for example, crabs and worms) and the secondary transition of terrestrial forms to the sea (for example, lung mollusks) most often occur. In seas that lack tidal fluctuations, the littoral fauna is that which populates the narrow coastal strip washed by waves (pseudolittoral fauna). The animals of the supralittoral zone, the zone of spray and surf, are called supralittoral fauna. Many littoral animals are used as food and for other commercial purposes (oysters, mussels, crabs, sea urchins, and some worms).