The phylum Mollusc comprises snails and slugs, shells and cephalopods
like octopus and squid. With more than 150,000 species they are
one of the largest phylums in the animal kingdom.
Molluscs are characterized
by a muscular mantle, a tooth-covered rasping tongue and a muscular
foot, which may be modified for diverse uses. Most types possess
a shell of calcium carbonate that is secreted throughout the lifespan.
However, many molluscs and some cephalopods have lost their shells
entirely or it is much reduced. A well-developed set of gills used
for respiration is another characteristic of most marine molluscs.
Sea shells are usually burrowers or are attached to the substrate.
Because they are unable to actively search for food, most rely on
their elaborate gill structure, to filter out tiny plants and animals.
Many species possess microscopic alga cells in their exposed mantles.
The alga actually produces food for its host and apparently very
little extra nutrients are required. The mantle also contains light
and pressure sensitive spots that cause the shell to quickly close
its valves if disturbed.
The sex is separate in most molluscs. Sea
shells release clouds of eggs and sperm into open water. Somehow,
perhaps by sensing changes of temperature or salinity, the spawning
of the entire population of a local area is synchronized. The fertilized
eggs hatch out as larvae, swimming or drifting in the sea for a
period ranging from about 2-40 days, before finally settling on
in shallow reefs around the Similan Islands and Phuket to about 10 m. Embedded hinge-down in corals,
shell with rows of flanges.
This species carries both sexes
in its body; eggs and sperm get released at different times
to avoid self-fertilization.
species has a heavy, almost circular shell. One shell is cemented
to the substrate, the other shell is moveable as lid.
shells have radial ribs and spines on the outer surface. It
rapidly closes when shadowed.
Normally heavy overgrown by
SNAILS & NUDIBRANCHS
Snails and nudibranchs are by far the most common type of molluscs
encountered on coral reefs. Most of the body of a typical sea snail
is hidden within its shell, which offers protection from predators. Aside from the shell the only other body part usually seen is the
muscular foot. The foot secretes mucus to cut down friction.
feeding habits are just as variable as their appearance. In contrast
to sea snails the evolutionary trend in nudibranchs has been towards
a reduction or even complete loss of the protective shell.
of these bright colored creatures is frequently toxic or distasteful
due to various chemical secretions. The bright coloration serves
to warn predators of their inedible qualities. The majority of species
feed on algae, sponges and cnidarians.
distinct body shape of the shells of this genus is due to
the muddy areas on which it lives.
It feeds on mussels, hidden
in sand and mud.
sea slugs are masters of chemical defense: they are capable
of polluting an entire aquarium if stressed in any way.
produce a strong toxin that can kill all life in the aquarium.
They have no known enemies.
large nudibranch is mostly found crawling in pairs.
alternately raises and lowers its head as it crawls. It feeds
Members of this family have the most advanced nervous system of
all invertebrate animals. Many live in the open ocean or in abyssal
depths. Relatively few species are encountered on coral reefs.
family members have in common an ability to eject ink, produced
by a gland within the fleshy mantle, to deter and escape from potential
Although outwardly cephalopods seem very unique, these
animals have the same basic molluscan body plan, but with special
modifications. Beside the well-known octopus the other members of
this group are cuttlefish, squid and the nautilus.
The eyes of cephalopods
are human-like and accurately register shapes, textures and colors.
The keen eyesight and well developed brain make them deft hunters,
feeding on fish, crustaceans and shelled molluscs.
species can be found down to a depth of at least 40 m duiring dives at the Similan Islands. The
nocturnal hunter feeds on crustaceans and shells.
have a large brain and are regarded as the most highly developed
nocturnal species. During mating and spawning they can also be
seen in daytime.
Cuttlefish are well-known for rapid color
changes, when disturbed. They feed on crabs and small fishes.