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 The tentacles are rapidly withdrawn if disturbed.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 a shark). Gill structure—elasmobranch means "plate-gilled"— and other physical features differ between bony fish and sharks and rays, which are considered a more primitive form.
Sharks. There are many species of sharks in Indonesian waters, but those most commonly seen by divers are the reef white-tip shark (Triaenodon obesus), the gray reef shark (Carcharhinus amblyrhynchos), and the reef black-tip shark (C. melanopterus).
The reef white-tip shark grows to 1.7 meters, and is a thin, gray fish with white tips to its dorsal and tail fins. This is the most commonly seen shark on the Indonesian reefs. This small shark can be often be found hiding under overhangs.
The gray reef shark grows up to 2.3 meters, and has a very dark trailing edge to its tail. Although this animal is known to be aggressive in some areas, it is not considered dangerous in Indonesia.
The reef black-tip shark grows to 1.8 meters, and is pale gray or brown with distinct black tips on all its fins. This shark sometimes comes up into very shallow water on reef fiats and in lagoons to look for food.
The largest fish extant is the whale shark (Rhincodon typus), a harmless animal that strains krill and small fish from the water. Growing to more than 12 meters in length (although specimens of 5-7 meters are more common), the whale shark is not a reef fish, although it can be found seasonally off some reefs in Indonesia.
The only really dangerous sharks a diver might encounter on an Indonesian reef are the tiger shark (Galeocerdo cuvieri), a large—up to 5.5 meters—scavenger that sometimes comes up onto the reefs at night or in the late afternoon/These sharks, however, are very rarely seen.
Rays. Structurally, rays are essentially flattened sharks. The stingrays have one or two stout spines at the base of their tail, which are their main means of defence. They will not normally be used against divers, although you should always take care to avoid stingrays while walking in shallow water.
Stingrays are bottomfish, and have strong teeth which they use to crush shellfish. In areas where stingrays are common you may see large craters in the bottom, caused by their feeding activities. Perhaps the most common stingrays in Indonesia are two species of blue-spotted stingray: Taeniura lymma, which frequents coral rich areas, and Dasyatis kuhlii, which lives in sandy areas of disturbed reef, or between patch reefs. A much larger animal is the grey reef ray (Taeniura melanospilos).
Not all rays are bottom-dwellers. The spotted eagle ray (Aetobatis narinari) cruises the reef edge looking for crustaceans. Eagle rays can reach 2.3 meters across.
The largest ray, however, is the manta (Manta birostris). Like the whale shark, mantas are essentially open water fish, but they are regularly seen by divers in Indonesia. The manta
