Page 283 - (DK) Ocean - The Definitive Visual Guide

MOLLUSCS 281

CLASS BIVALVIA This mollusc has a pronounced “beak” CLASS BIVALVIA CLASS BIVALVIA
covered in tooth-like projections at
Common Piddock the front end of its shell. It uses this Common Edible Atlantic Jackknife
feature for boring holes into relatively
Pholas dactylus soft substrates, such as mud, chalk, Cockle Clam
LENGTH peat, and shale. Like the Shipworm
Up to 15cm (6in) across (opposite), this piddock relies on its Cerastoderma edule Ensis directus
HABITAT burrows for protection from predation, LENGTH LENGTH
Up to 5cm (2in)
Lower shore to shallow because the shell does not encase all 16cm (6in)
sublittoral of its body – its two fused siphons HABITAT HABITAT
Middle and lower Sandy and muddy
(tubes for eating, breathing, and shore, 5cm (2in) below shores and shallows
DISTRIBUTION South and east coasts of UK, Severn excretion) trail out behind it. The surface of sand or mud
estuary in UK, west coast of France, Mediterranean shell is fragile, elliptical, and covered
in a pattern of concentric ridges DISTRIBUTION Barents Sea, eastern north Atlantic DISTRIBUTION Atlantic coast of North America,
from Norway to Senegal, West Africa introduced to North Sea
anterior beak and radiating lines. If disturbed, the
of elliptical Common Piddock has an unusual
shell
defence strategy: it squirts a luminous This edible bivalve has a robust, ribbed Atlantic Jackknife Clams live in deep,
blue secretion from its outgoing, shell and burrows in dense populations vertical burrows on muddy and soft,
or exhalant, siphon. Such just below the surface of sand or mud, sandy shores. Native to the northeast
bioluminescence is very filtering organic matter such as coast of North America, the free-
rare in molluscs, plankton from the water. Cockles swimming larval stage is thought to
seen in only a are an important have been introduced to the North Sea
few species. commercial species in 1978 when a ship emptied its ballast
but also a vital tanks outside the port of Hamburg.
food source This clam has spread along the
for wading continental coast. In places, it affects
birds such as local polychaete worm populations,
fused siphons oystercatchers. but it is not considered a pest.

CLASS BIVALVIA from the water using its ingoing, or of their shell halves are mirror images
inhalant, siphon, which is fringed with of one another, larger individuals may SPAWNING
Giant Clam small tentacles. However, it differs in be unable to close their shells fully, so
obtaining most of its nourishment their brightly coloured mantle and Reproduction in Giant Clams is
Tridacna gigas from zooxanthellae (unicellular algae siphons remain constantly exposed. triggered by chemical signals that
LENGTH that live within its tissues) – a type of Many Giant Clams appear synchronize the release of sperm
Up to 1.5m (5ft) relationship also associated with coral irridescent due to an almost continuous and eggs into the water. Giant
HABITAT polyps. The algae have a constant and covering of purple and blue spots on Clams start life as males and later
Reefs, reef flats and safe environment in which to live; in their mantles, while others look more
shallow lagoons to become hermaphroditic, but
20m (65ft) return they provide the clam with green or gold, but all have a number of during any one spawning event,
essential nutrients, the carbon-based clear spots, or “windows”, that allow they release either sperm or eggs
DISTRIBUTION Tropical Indo-Pacific from south China products of photosynthesis. In fact, so sunlight to filter into the mantle cavity. in order to avoid self-fertilization.
seas to northern coasts of Australia, and Nicobar dependent is the Giant Clam on these Fertilization is external and the eggs A large clam can release as many
Islands in the west to Fiji in the east algae that it will die without them. hatch into free-swimming larvae before as 50 million eggs in 20 minutes.
The adult is sessile (immmobile) settling onto the sea bed. The exhalant
The largest and heaviest of all molluscs and its inhalant and exhalant (outgoing) siphon expels water and at spawning
is the Giant Clam. Like other bivalves, siphons are the only openings in its time provides an exit point for the
it feeds by filtering small food particles mantle. Although the scalloped edges eggs or sperm. OCEAN LIFE