Page 281 - (DK) Ocean - The Definitive Visual Guide

MOLLUSCS 279

Reproduction LIMPET CHAIN
Slipper Limpets
In many molluscs, reproduction simply involves releasing sperm or eggs change from
(gametes) into the water. Fertilization is external and there is no parental male to female
as they grow.
care. Individuals may be of separate sexes or hermaphrodites (having This chain
both male and female reproductive organs). Hermaphrodites may of four such
function as male or female at different times or, as in nudibranchs, limpets has
a female at the
produce both eggs and sperm, although eggs can be fertilized bottom and smaller
only by cross-fertilization. Some species, such as slipper limpets, males above her.
change sex with age, while oysters can change sex several
DEVELOPING EMBRYOS
In 4 months, Australian Giant times in a breeding season. Among cephalopods, males
Cuttlefish eggs develop into court females, fertilization is internal, and in some species,
mini-replicas of the adults. the eggs are protected by the females until they hatch.

Lifecycles
HUMAN IMPACT
OYSTER DEMAND Most molluscs produce eggs that
either float or are deposited in
Oysters have long been harvested clusters, anchored to the substrate.
as a food source. Their high market
value and increasing demand have led Most forms have eggs that hatch
to overexploitation of wild stocks. into shell-less larvae, which live in
In the North Sea, the European the plankton. The larvae are called
flat oyster has vanished from ciliated trochophores due to their
much of its former range, and bands of hair-like cilia, used in
today most oysters are swimming. In gastropods, bivalves, PLANKTONIC LARVA
commercially farmed. The visible bands of this veliger larva of the
and scaphopods, the trochophore Common Limpet beat with tiny hair-like cilia,
SLOW RECOVERY PERIOD larvae change into veliger larvae, which are used in locomotion and feeding.
Relatively long-lived and reproducing only which have larger ciliated bands,
sporadically, the European flat oyster (right) takes
a long time to recover from overexploitation. and sometimes adult features such as a mantle or a rudimentary shell or
both. As they approach maturity, the larvae float down from the surface
and, on reaching the sea bed, change into adults. Only those that land
in a suitable environment survive to reach sexual maturity. Cephalopod
READY AND WAITING FOR PREY eggs hatch into active predators. Some resemble mini-adults; others live
This cuttlefish hovers with its arms outstretched.
When prey comes within reach, the two feeding arms, in the plankton and initially look and behave differently from the adults.
currently contracted and set above the two lower
arms, will shoot forward to grab the prey. SECURING EGG CLUSTERS
This female Bigfin Reef Squid produces each finger-shaped
egg capsule holds
up to 400 egg capsules containing about
up to seven eggs
2,500 eggs. Here, she is securing egg
capsules to a solid substrate.

MOLLUSC CLASSIFICATION

The phylum Mollusca is the second largest animal phylum, comprising
more than 73,000 species, and their diverse form has led to the BIVALVES CEPHALOPODS
Class Bivalvia
Class Cephalopoda
identification of eight different classes. The majority of species live in
marine habitats, but freshwater and terrestrial species are also numerous. 9,209 species 816 species
Bivalves, or clams and their relatives, have a Squid, octopuses, and cuttlefish are all
hinged shell of two halves, but no radula. Most cephalopods – fast-moving and intelligent,
CAUDOFOVEATES MONOPLACOPHORANS are sedentary and marine. Siphons create a with a complex nervous system and large
Class Caudofoveata Class Monoplacophora
water current through the shell, aiding feeding eyes. The shell is internal or absent, the
and respiration. Sexes are usually separate. head surrounded by arms, with or without
131 species About 30 species
These are marine, shell-less, worm-like These deep-sea molluscs lack eyes but have a suckers. The central mouth has a parrot-like
organisms of deep-water sediments. Their radula and a cone-like shell. They are more beak and a radula. The sexes are separate.
GASTROPODS
horny outer layer is covered with spines. abundant as fossils than as living species. Class Gastropoda
CHITONS
Class Polyplacophora
61,682 species
SOLENOGASTERS TUSK SHELLS Familiar as slugs and snails, these molluscs are
Class Scaphopoda
Class Solenogaster marine, freshwater, and terrestrial. They have 970 species
a spiral shell and a large, muscular foot. The Chitons have a repeating structure with a
273 species 571 species OCEAN LIFE
Another marine class of shell-less, worm- These animals have a tubular, tapering shell, body is twisted 180º so the mantle cavity lies series of plates (usually 8) on their backs
like organisms, solenogasters live in or on open at both ends. The head and foot project over the head. Many species can retract into enclosed by an extension of the mantle.
the ocean floor. Some lack a radula. from the wider end and dig in soft sediments. their shell; hermaphrodite species are common. The underside is dominated by the foot.