Wednesday, December 19, 2012

Molluscs (Phylum Mollusca) of Singapore

The molluscs (Phylum Mollusca), sometimes spelled "mollusks", is the second largest phylum of animals after the arthropods, and the phylum with the most named marine species. The word "mollusca" has its roots in the Latin word "mollis", which means "soft". "Mollusca" is also the name of a kind of soft nut with a thin shell.

Most molluscs have shells, though in many species they can be very reduced or completely lost. They have a soft body which is unsegmented and bilaterally symmetrical, and most have a muscular foot. Many molluscs have a special feeding structure called "radula", which is a tongue-like structure with rows of teeth on it. They usually have mantle with a cavity used for breathing and excretion. Examples of molluscs include snails, slugs, squids, cuttlefish, chitons and clams.

The major molluscan classes that are be found in Singapore are as follow:

A) Chitons (Class Polyplacophora)

Polyplacophoran (Phylum Mollusca, Class Polyplacophora), or Chiton
A polyplacophoran (class Polyplacophora), or chiton, is a soft-bodied animal with a shell made up of eight separate plates on its back, embedded in and surrounded by a leathery structure known as a girdle. The muscular girdle may be covered with scales or spicules, and usually has a somewhat oval outline.  The name "polyplacophora" means "to carry many tablets" - "poly" means "many" in Latin, "placo" means "tablet" in ancient Greek, while "phora" means "to carry, to bear" or "bearer".

B) Bivalves (Class Bivalvia)

Bivalve (Phylum Mollusca, Class Bivalvia)
A bivalve (class Bivalvia) is a soft-bodied animal with a two-part shell that is usually large enough to enclose the whole animal. The two parts (or valves) of the shell are connected by a hinge, which is essentially a flexible ligament, and interlocking "teeth" on each valve. Like other shelled molluscs, the shells of bivalves are composed of calcium carbonate. Unlike most other molluscs, bivalves have no head or radula (a tongue-like structure for feeding). They are mostly filter-feeders, using their modified gills to filter plankton from the water. These gills are used for breathing as well.

C) Gastropods (Class Gastropoda)

A gastropod (class Gastropoda) is a soft-bodied animal, often with a shell on its back. "Gastropoda" means “stomach-foot", as gastropods often appear as if they are crawling on their stomach, which is actually their foot. They usually have one to two pairs of tentacles, and most have a radula, which is a rasping tongue-like structure for feeding. Gastropods can be snails or slugs - snails have a single coiled shell that is usually large enough for the animal to retract into, while slugs either have a much reduced shell, an internal shell, or no shells at all. As this is a rather large group, I have decided to break them into three different sections:

Terrestrial and Freshwater Snails and Slugs (Phylum Mollusca, Class Gastropoda)
Terrestrial and Freshwater Gastropods - These are the snails and slugs that can be found on land, trees and in freshwater bodies such as ponds and streams.

Marine Snails (Phylum Mollusca, Class Gastropoda)
Marine Snails - These are the snails that can be found in the marine environment, such as in coral reefs, sandy beaches, rocky shores or mangrove forest.

Marine Slugs (Phylum Mollusca, Class Gastropoda)
Marine Slugs - These are the slugs (with very reduced shells or none at all) that can be found in the marine environment, such as in coral reefs, sandy beaches, rocky shores or mangrove forest.

D) Cephalopods (Class Cephalopoda)

Cephalopod (Phylum Mollusca, Class Cephalopoda)
A cephalopod (class Cephalopoda) is a soft-bodied animal with a prominent head and a set of arms or tentacles attached to the head, and hence the name "cephalopoda" which means "head-foot" in Greek. The arms and tentacles are modified from the primitive molluscan foot, and may come with suckers or ridges at the tip, allowing the cephalopod to hold on to the substrate or prey tightly - they are all carnivorous. Examples of cephalopods include cuttlefish, squids, octopuses and nautiluses. They also have a mantle, which is basically a tube-like body to store the internal organs.

E) Tusk Shells (Class Scaphopoda)

Scaphopod (Phylum Mollusca, Class Scaphopoda) or Tusk Shell
A scaphopod (class Scaphopoda) is a soft-bodied animal with a a bilaterally symmetrical, tubular, tapering and curved shell which resembles a miniature elephant's tusk, and hence they are also often called "tusk shells". However, unlike a tusk, both ends of the shell are open.  The name "scaphopoda" means "shovel-footed", refering to the long, pointed foot of most tusk shells. The animal is attached to the shell with the muscles near the posterior end (the narrower tip). They have no eyes, and the mouth is located the base of the foot. They have filaments for seizing food (usually microscopic organisms) at the corners of the mouth. Like most other molluscs, they have a radula in the mouth.

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