The octopus has highly developed eyes and a very sophisticated nervous system. It is thus considered "intelligent," and people find it hard to believe that it is a mollusc. The white color of this specimen suggests exhaustion, and perhaps the photographer was a bit overzealous in getting this shot.
Trochus. Top shells (Trochus spp.) are relatively large (6-8 cm.), and conical. Before the advent of plastics they were widely collected for the manufacture of buttons. Removing the grubby outer layer of shell reveals the lustrous nacre, or mother-of-pearl beneath. Until the invention of Bakelite, and the many plastics that followed, shell nacre for buttons was an important business in Indonesia. Today they are still collected, most to be used in souvenirs and to supply the small market for "real" buttons.
Clams and Oysters
The bivalves include such familiar forms as clams, oysters, mussels and scallops. All have two articulated shell halves that can be closed with a large muscle. It is this muscle that makes bivalves so prized as seafood. With a very few exceptions, bivalves cannot move, like gastropods, and thus most have adapted to filter-feeding. They draw water in through one tube or "siphon" and pass it out through another. This stream of water passes through the animal's gills, which serve the dual purpose of respiration and filtering out food particles.
All bivalves must hold their shell halves at least slightly ajar to maintain water circulation through their bodies. But when danger threatens they are clamped shut. Some Indonesian bivalves gain further protection by boring into corals and reef rock, so that predators cannot reach them. The boring is achieved by a combination of chemical action and rasping with the two shell haves. Eventually, reef bivalves become so encrusted with sponges, coralline algae, bryozoans and cnidarians that they are barely visible.
Giant clams. The giant reef clams, Tridacna spp., have a different means of feeding. Like reef-building corals, Tridacna clams harbor zooxanthellae in their fleshy mantles, and can thus "manufacture" most—or perhaps all—of their own food. Like corals, they require lots of light, and tend to be found in the shallows. They grow with the hinge of their shells down, and their rippled gape facing the sun.
There are seven species of Tridacna, of which the giant clam, T. gigas, is the most dramatic. These animals can reach a meter and a half in diameter. An animal that big could be a century old. Although smaller than T gigas, T squamosa has a beautiful ruffled shell. The fleshy mantles of Tridacna clams are beautiful, varying in color from brown to yellow to green to blue, with contrasting spots or mottling.
Tridacna clams are a great delicacy in Asia, particularly in Taiwan, and their shells are made into terrazzo in factories in Surabaya. Over-harvesting has greatly reduced their numbers throughout Indonesia. Shallow reefs in Eastern Indonesia that used to support literally fields of giant clams have been stripped in just the past few years. There is fear that the population in many areas is no longer at a self-sustaining level.
Recently, however, researchers at the Micronesian Mariculture Demonstration Center in Palau, headed by Gerald Heslinga, have discovered a method of "farming" giant clams by inoculating the veliger larvae with zooxanthellae. Once the symbiotic algae is in place, the clams need only a good supply of sea-water and plenty of light to thrive. The farming operation requires little room, and the clams reach 10 centimeters across in just two years. Because of the commercial potential for these clams, a number of pilot farms have recently been established in the Pacific region.
The thorny oyster ("Spondylus sp.) is often so encrusted with sponges, algae, tunicates and other organisms that only when it is agape with its bright mantle showing (as here) can it be seen. Halmahera, Maluku.
Oysters. A number of oysters can be found on the reef, in many cases so well camouflaged with encrusting growths that they are at first invisible. The cock's comb oyster (Lopha cristagalli) has a distinctive sharp, zig-zag opening, and is often covered by encrusting sponges.
The colorful mantle of the thorny oyster (Spondylus spp.) stands out, although its rough shell is usually overgrown with algae, sponges, and small cnidarians.
In many parts of Indonesia, Japanese operators seed pearl oysters (Pinctata) and hang them in the shallows to grow pearls. The oysters are purchased from local collecters, and the "seed" comes from a freshwater mussel found in the Mississippi basin. Security on these "farms" is high, and divers are unwelcome.
Cephalopods
These animals, despite their close relationship to the snails and clams, are active, "intelligent" predators with highly developed eyes and sophisticated behaviors. The octopus has eight suckered arms, while squid and cuttlefish have an additional two grasping tentacles. Both octopi and squids have a hard, chitinous beak. The nautilus—of which only one genus is extant— differs markedly from the other cephalopods. It has 90 arms, without suckers, and a well-developed shell. Unlike other cephalopods, the nautilus has very primitive eyes, lacking a lens and open to the water.
Octopi. These familiar animals can be found on the reef, although they normally hide in small caves or crevices. They have no internal skeleton so are able to squeeze into surprisingly small spaces.
Chromatophores on their skin give octopi remarkable abilities to change color, which they do either to blend in with their surroundings or to display emotion. Some species can also change their surface texture, from smooth to lumpy and back, producing very believable imitations of shells, and even lionfish.
Octopuses are particularly fond of eating crabs and other crustaceans, and a pile of shells often marks a hole where one is resident. Normally an octopus crawls rather slowly across the reef, but it can also swim by contractions of its legs, much like an umbrella opening and closing. If disturbed, it can produce a short burst of speed by squirting water out of its large gill cavity through a muscular siphon.
Beware of the common, small blue-ringed octopi (Hapalochlaena) which can be found under rocks on the reef flats in Indonesia. Do not pick one up. They possess a very virulent poison.
Squid. Squid are free-swimming animals, usually seen in groups in shallow lagoon areas or along the reef edge. They have perfected the mode of jet-propelled movement. While stationary they maintain position with gentle undulations of their lateral fins. Movement, either forward or backward, is achieved by the highly maneuverable water jet. Like the octopus, squids can change their coloration, adopting a sparkling array of brilliant colors and patterns.
Squid have a rudimentary internal "shell," actually a noncalcareous strengthening device called a pen. It is of a clear, flexible substance that looks and feels like a piece of plastic.
Although you will never see one on the reef, the largest cephalopods by far are the giant squids (Architecteuthis), which can reach a length of 18 meters. These animals frequent very deep water, and little is known of their habits. They are the preferred prey of the sperm whale.
Cuttlefish. Cuttlefish (Sepia) superficially resemble squid, but can be distinguished by their generally larger size and more robust shape. Unlike squid, which often travel in large groups, lone cuttlefish can often be seen foraging on the reef slope, and are the most frequently encountered cephalopods.
Like the other cephalopods, cuttlefish can squirt out a blob of ink if threatened. The shape of this blob, roughly the size of the animal that ejected it, and its strong smell, distracts the would-be predator while the cuttlefish jets away. In earlier times, this ink was used for writing, as is suggested by the cuttlefish's genus name, Sepia.
Instead of the squid's flexible pen, cuttlefish have a "cuttle-bone," a calcareous
