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 structure perhaps most familiar for its use as a dietary supplement for cage birds. Although it provides some stiffness, the most important use of the porous "bone" is for buoyancy control, balancing the animal's vertical movements across the reef face.

      Nautilus. These animals, with their distinctive spiralled shell, are the most unusual of the living cephalopods. The chambered shell serves as a form of buoyancy control, like the cuttlefish bone, but much more sophisticated. This control is necessary as the animals undergo a considerable daily vertical migration. During the daylight hours, the nautilus stay at 1,000-1,500 meters, and only rise into relatively shallow water at night. Only very rarely are they found in depths a sport diver could reach. In this way they avoid predators, and perhaps also are able to more easily find their food—carrion and, particularly, the molts of crustaceans.

      A mantis shrimp, Odontodactylus scyllaris. These animals are fierce predators, using their modified front claws to seize or bludgeon prey in the manner of their namesake, the praying mantis. Odontodactylus is the most colorful and one of the larger mantis shrimps — it is said to be able to smash a four-inch crab with one strike. Some divers call these animals "thumb-splitters" and with good reason. Do not try to touch one! Tulamben, Bali.

      There are several species, but the most common on Indonesian reefs is the pearly nautilus (Nautilus pompilius).

      A dispute developed between these two prawn gobies, Mahidolia mystacina, when the yellow goby and its shrimp wandered into the grey goby's territory. When the grey fish came out of its burrow, the sand started to fly. Both of these gobies are females. Tulamben, Bali.

      Crustaceans

      The jointed-foot animals— Arthropoda—is the single most successful phylum of animals. On land, the insects and spiders dominate; in the water, the sub-phylum Crustacea is king, with almost 40,000 species. Crustaceans—crabs, shrimp and lobsters—are very abundant on Indonesian coral reefs, but many keep themselves well-hidden, particularly during the day. They are most likely to be seen by night divers.

      The largest commonly seen crustaceans are the spiny lobsters, Panulirus. By day spiny lobsters hide in caves and crevices, often in small groups, with only their long antennae protruding. But at night they venture out of their retreats in search of food. If surprised out in the open, spiny lobsters can swim backwards with great speed using powerful flicks of their tail.

      These lobsters, of course, make very fine eating, but visiting divers should resist the temptation of trying to catch a lobster for the table. Removal of animals from a dive site is short-sighted, and lobster catching is quite a skilled operation. An unpracticed diver who attempts it is likely to be left only with painful cuts and a handful of antennae.

      Shrimps

      On night dives large shrimps can sometimes be spotted out in the open where their reflective eyes catch the light and stand out as two bright red spots. But even by day the careful observer should be able to spot several species of small shrimp.

      Commensals. A variety of sometimes colorful shrimp associate with anemones, coral and echinoderms for protection, making them easy to spot. The tiny bumble-bee shrimps (Gnathophyllum) associate with sea urchins. Various species of Periclimenes, some quite colorful, associate with anemones, gorgonians, and echinoderms. One,

       P. imperator, lives in the folds of the Spanish dancer nudibranch.

      Cleaner shrimps. Also easy to see are the cleaner shrimps, protected from predation by the services they offer. These cleaners pick parasites and bits of dead tissue from fish, and can all be recognized by long, white antennae.

      The candy shrimps (Lysmata) axe beautiful red striped or spotted cleaners. The coral shrimps (Stenopus) live in pairs in small caves or holes extending

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