Next to the arthropods, Mollusca is he largest phylum of invertebrate animals. It contains many familiar forms. The word "mollusk" means soft, and members are characteristically soft bodied. The only hard part of these animals is the protective shell, and even this is absent in some groups. Mollusks may be more active and conspicuous during the warmer season when they spawn. Some mollusks are active during the day but many are nocturnal. The mantle of the animal is responsible for depositing the layers of calcium carbonate and slowly expanding the shell into many diverse forms. Factors affecting the shell growth rate include available food, temperature and spawning.
The term Gastropoda means "stomach foot" and refers to the soft parts of these animals. Snails can pull back into their shells when disturbed. Some even have a door, or operculum, which closes the aperture when the animal retreats inside the shell. The opercula, which resemble round buttons or "cat's eyes" are often washed up on beaches. Many empty gastropod shells are inhabited by hermit crabs after the snail has died or been removed, although hermit crabs themselves do not kill gastropods to acquire the shell.
(See Nudibranch Section) The members of this order have no shell or mantle cavity and their body shape is elongated and symmetrical. The gills are exposed, thus the name "nudi" (naked), "branchia" (gills). Usually encircle the anus on the upper side. Individuals of this group are among the most brightly colored and attractive animals on the reef. Nudibranchs usually remain under coral heads where waves or currents will not disturb them. Most nudibranchs are carnivores, feeding on coral polyps and other shell animals. some coral polyp eating nudibranchs are able to incorporate live, stinging nematocyst cells (derived from the cnidarians) into their own skins, using these for their own protection.
The bivalves are those mollusks with two similar convex valves attached to each other. Eight families of bivalves are well represented on Philippine reefs. The giant clams (Family Tridacnidae) are the most impressive representatives. Found on almost all parts of the reef, two Tridacna species embed themselves in either a coral or rock substrate while the other specie are free standing.
Cephalopoda, meaning "head foot", is an appropriate name for these specialized swimming mollusks in which the head is closely associated with the foot. The squids and cuttlefish possess ten arms, while the octopods have eight arms or tentacles, with suckers. The intertidal cephalopods are, in general, shy, nocturnal animals, which are not often observed.
Phylum Mollusca (Mollusks)
Class Gastropoda (Snails)
Order Mesogastropoda (Mesogastropods, Cowries and Others) Approximately a dozen families of mesogastropods are common shells on Philippine reefs. The false cowries (Family Ovulidae) are close relatives of the true cowries. They feed exclusively on soft corals and other cnidarians. Two common Philippine species, Ovula Ovum, the Egg shell false cowry, and Calpurnus Verrucosus, are often overlooked on the reef, since their colorful mantles completely cover the egg white shells. A Group of active but inconspicuous shells in coral sand and algae areas are the conch shells (Family Strombidae) that are often encrusted with algae. The most noticeable is the large spider conch (Lambis), with its long, protruding spines.
Family Cypraeidae (Cowries) (Philippine Translation: katipan, poki)
Cypraea Tigris Sp. (Tiger Cowry) The tiger cowry is often in full view during daylight hours and is a popular curio item. Probably the best known of the large cowries. The white shell with black spots of variable size and density is distinctive. Commonly found on elevated intertidal reef platforms, in seagrass beds, in areas of sand and rubble, and on the outside of barrier and fringing reefs. It is generally under coral rubble or in crevices in living reef, but may be found crawling in the open. (1)
Family Eulimidae
Thyca Crystallina Sp. Eulimid gastropods are variable in form. species of Thyca have a cap shaped shell with numerous raised ridges. Color varies from tan to blue. Their color does not always match that of their host. They are parasitic upon echinoderms. Commonly found on the sea star Linckia Multifora and L. Laevigata. It inserts its proboscis into the groove from which the tube feet insert and sucks coelomic fluid and tissue from its host. Frequently, a large female snail and a smaller male are found together on the same host. The male may be present under the shell of the female. (1)
Family Ovulidae (False Cowry)
Calpurnus Verrucosus Sp. (Egg Cowry) This cowry is found on the soft coral Sarcophyton. Its shell has characteristic button shaped protuberances at either end. Has a circular depression at either end of the dorsal surface of the shell that contains an orange or brown line. The tips of both ends of the shell are purplish, The foot and mantle are ornamented with black or brown spots. Usually found right in the open, and is not at all camouflaged, It may be exhibiting warning coloration and may be distasteful to predators. Depth 3-20 meters. (1)
Ovula Ovum Sp. (Common Egg Cowry) Readily distinguished by its black mantle reaching a length of 100mm. the soft corals, Sarcophyton and Sinularia Sp. are the primary food sources. Depth intertidal zone to 20 meters. (1) (2)
Family Strombidae
Lambis (Scorpius or Truncata ?) Sp. (Scorpion or Spider Conch) This species of conch an be recognized by the alternating black and white radial lines on either side of the shell aperture. Distinguished from other conchs by the elongate spines extending from the aperture of the shell. This species inhabits patch reefs and boundaries between rocky and sandy areas. (1) (2) (3)
Subclass Ophistobranchia
Order Nudibranchia (Sea Slugs)
Class Bivalvia (Bivalves)
Family Tridacnidae (Giant Clams) (Philippine Translation: Taklobo, Sali-Ot)
Tridacna Crocea Sp. (Boring Giant Clam) (Philippine Translation: Dagatan, Sali-Ot) This is the smallest giant clam species and its' mantle is brightly colored, often with iridescent blues and greens. Reaching a maximum length of about 100mm. Lives in small cracks on shallow reef flats. (1) (2) (3) (4)
Tridacna Gigas Sp. (True Giant Clam) (Philippine Translation: Taklobo, Tilang Dako) The largest of all bivalves, it can reach over 200 kilograms (animal and shell), but has become locally extinct in many areas and very rare in the Indo-Pacific. smooth shell with no concentric sculpture. Usually found in isolated offshore reefs. In many places this species is rare owing to over harvesting by humans. (1) (2) (3) (4) (5)
Class Cephalopoda (Cephalopods)
Family Octopodidae
Octopus Sp. (Common Reef Octopus) (Philippine Translation: Kugita, Tabugok) This finless cephalopod has a short, rounded body, the head having eight tentacles often of equal length. Like most octopi, it is able to quickly change its color to match its surroundings. (1)
Family Sepiidae
Sepia Latimanus Sp. (Broadclub Cuttlefish) (Philippine Translation: Kubotan, Kolambutan) This species, like all others, is able to change its color and texture, is good to eat and is keenly sought by fishermen. Is the only large cuttlefish in most of the tropical Indo-Pacific. It may reach 500 mm in length. Found on shallow water reefs to depths o at least 30m, where single individuals or pairs are often observed where it is active during the day. (1) (2) (3) (4) (5) (6) (7)
*External Resource: CephBase for Cephelapods*
*Reference: Philippine Coral Reefs A Natural History Guide By Alan T. White & Coral Reef Animals Of The Indo-Pacific By Terrence M. Gosliner, David W. Beherens, Garry C. Williams
*Photos: Are Full Moon Divers. All Rights Reserved.