The charming little boxer crab, Lybia tesselata. The boxer crab clutches two tiny sea anemones, which it uses for defense, and even to immobilize small prey. The sea anemones (Triactis producta) are so important to this crab that the only time it releases them is when it sheds its integument. And then, observers say, it seems very nervous until it picks them up again. You easily can find these crabs under coral rocks on the drop-off at Tulamben, Bali from about 3-12 meters depth. Take care not to break up living corals, however. Ambon, Maluku.

      Food filtered from the current by the fine hairs on the crinoid's arms are passed down a channel to the central mouth. Crinoids sit "upside-down" compared to the starfish, and the mouth is on top the animal.

      Some feather stars are nocturnal and hide by day in reef crevices. As night falls they come out of hiding and climb up onto prominent blocks of coral or other high points where they are exposed to the strongest current flow. Crinoids are particularly abundant in plankton-rich areas.

      Sea urchins. Sea urchins are important and abundant grazers on Indonesian coral reefs. Even the spiniest of urchins may be attacked if they venture out into the open by day, so they tend to confine their activities to the night. By day they wedge themselves into crevices or hollows to avoid the attentions of predatory fish. Sea urchins have a very sophisticated feeding apparatus which they use to scrape at the reef, removing not only algae but also quantities of coral rock. In fact, some small species actually excavate their own daytime hiding places out of the soft coral rock by the constant scraping of their jaws and spines.

      On shallow, quiet reefs in Indonesia one can often see the black, long-spined urchin Diadema, so-named for the cluster of glistening "jewels" set into its upper body. This urchin has very long and brittle spines, and stepping on one would be a real disaster. In harbors and other disturbed areas of reef, very large numbers of these animals can be found. Shrimpfish and urchin clingfish hide among their spines.

      The rarely seen slate pencil urchin (Heterocentrotus mamillatus) is a distinctive species, with thick, pink spines. No longer used as chalk, the unfortunate animals' attractive spines are in some areas now being made into wind chimes.

      The bodies of most sea urchins seem roughly spherical, but in fact they are made up of five radial segments, in typical echinoderm fashion. Sea urchins develop a calcareous skeleton or test, which contains the feeding apparatus, the intestines, and the gonads. Prior to reproduction the gonads expand to fill the whole shell, and it is this rich substance that make sea urchins so attractive to hungry fish despite their spiny defenses.

      The ripe gonads of the sea urchin Hemicentrotus pulcherrimus are prized in Japan for sushi; the taste of this uni is strong, but delicious.

      Sea cucumbers. Though at first they look just like loose sacks, or large worms, sea cucumbers (class Holothuria) are constructed with the same five-sided symmetry typical of the echinoderms. Because they are so elongate, they have a "head" and a "tail," unlike the starfish or urchins. The head of a sea cucumber is not, however, particularly well developed, consisting of little more than a ring of tentacles around the mouth. Sea cucumbers are an important trade item in Indonesia. (See "Trepang Fishing," page 27.)

      Most species are detritus feeders, the tentacles being used to pick up sand and pass it into the mouth. Organic matter is digested and the undigested remains are passed out through the anus. Sea cucumbers have to eat a lot of sand in order to obtain enough food, so they often leave a continuous trail of sandy feces behind them. A few species are filter feeders. They hide their bodies in reef crevices and hold their tentacles up in to the water current to feed. The tentacles are rapidly withdrawn if disturbed.

      Sea cucumbers appear as elongated and somewhat flaccid forms lying among coral rubble or sea grass, moving slowly in a worm-like way by contractions of their bodies. These are usually black or dull-colored. A few species, such as the sea apple (Pseudocolochirus), are very colorful, however.

      Many sea cucumbers are active by day. Since they are not attacked by predatory fish it would seem that they must have some efficient means of defense. Some species can discharge sticky white threads if molested, and most tropical sea cucumbers contain toxins.

      Tunicates

      The tunicates or sea squirts are an entirely marine group of animals, and are unfamiliar to many people. Despite their unimpressive appearance, they are chordates, and—technically—are more closely related to human beings than to any of the invertebrates listed above. They have a notochord, a primitive backbone, only in their larval form. Once they settle out of the plankton and become sessile filter-feeders, the backbone is unceremoniously shed. (So much for the vaunted evolutionary superiority of "higher order" forms.)

      The tunicates seen on Indonesian reefs are all in the class Ascidiacea, a name derived from the ancient Greek word for leather bottle. They are rather like little bottles, with (usually) two openings rather than just one. Water is drawn in through the uppermost of these siphons, filtered through a basket-like arrangement internally, and then passed out through the lower siphon. Peer into the opening of a large tunicate and you may be able to make out the fine sieving apparatus within. Many tunicates have stout spikes projecting from the inner wall of their siphons, to thwart small fish or other unwanted intruders.

      One of the most common and conspicuous tunicates on Indonesian reefs is the beautiful white, purple and yellow Polycarpa aurata. These creatures are about the size of a man's thumb, and have a tough leathery outer coating, or tunic. Polycarpa is a solitary and very distinctive animal and easy to identify underwater. But many tunicates are colonial, and can easily be mistaken for sponges. If the siphons of a sea-squirt are touched (gently so as not to harm the animal) they will squeeze shut. Sponges do not react to touch. If a tunicate is lifted out of the sea this same contraction will cause water to be squirted out of its siphon—hence the common name sea squirt.

      A cluster of tunicates, Rhopalaea crassa. Water enters through the uppermost opening, is filtered of plankton and nutrients, and then passed out the lower opening. Bunaken group, Sulawesi.

      The blue ribbon eel, Rhinomuraena quaesita, is one of the most attractive moray eels. Young eels are black, and don't turn electric blue until they reach a bit over a half-meter in length. Bunaken group, Sulawesi.

      Most colonial tunicates are overlooked because they tend to be tucked away in dark corners. An exception are the marblesized, white-and-green grape ascidians, Diademnum molle, a common compound tunicate on shallow reefs in Indonesia. Diademnid tunicates have a single large inhalent opening, and many small exhalent openings around their globular tunics.

      Their green color comes from a symbiotic algae living within its tissues, much like the zooxanthellae of stony corals. The relationship between this tunicate and its algae is one of mutual dependency, neither party being able to survive alone. Diadem-num larvae even carry samples of the algae with them to ensure that the relationship is continued in the next generation.

      The Fishes

      Corals and other invertebrate animals can provide a lifetime of interest for a diver in Indonesia, but the fish are what really grabs one's attention. On most reefs, brightly colored and beautifully patterned fish are everywhere, darting among the corals or lying sedately in mid-water. It would be impossible in the space available here to offer a complete description of the thousands of fish species found on Indonesian reefs, so all that will be attempted is a brief survey. Consult "Further Readings" page 321 for more complete resources.

      Elasmobranchs

      Sharks and rays are elasmobranchs, and differ from true bony fishes by having a cartilaginous skeleton, only parts of which are calcified (e.g., the jaws of

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