The five-sided symmetry of the echinoderms is clearly displayed in the starfishes. Most Indonesian species have five arms, although some individuals may have one arm more or less. The common cobalt-blue starfish Linckia laevigata is particularly variable in this regard. Some of the larger starfishes may have a great number of arms.
Starfishes are predators, feeding on a wide variety of bottom-dwelling animals, or detritivores. A feeding starfish envelops its prey with its arms, then actually pushes part of its stomach out through its mouth over the victim, digesting it externally. Starfishes are able to hang on to even actively struggling prey with their myriad tube feet, tiny suckers that cover the under-sides of their arms. The gripping power of these animals is considerable, and over time they can even overpower the strong muscle of a bivalve. The tube feet are also used for locomotion.
Some starfish have very thick arms, particularly the pincushion starfish (Culcita spp.) common on Indonesian reefs. These animals can inflate their bodies to the point where they become almost spherical. Culcita normally have tiny symbiotic shrimps living on their lower surfaces.
The most notorious starfish in Indonesian waters is the crown-of-thorns, Acanthaster plancii, which is found throughout the tropical Indian and Pacific Oceans. This animal feeds exclusively on coral polyps.
Normally the crown-of-thorns, large, multi-armed and bristly, occurs in very low numbers on coral reefs—divers usually see perhaps one per dive. But population densities have occasionally reached plague proportions, and at these times whole reefs can be destroyed. Some of the greatest damage has been on the Great Barrier Reef of Australia and on the reefs of southern Japan, but Acanthaster outbreaks have occurred throughout its range, including Indonesia.
These plagues have been the subject of a long and heated debate by reef scientists. Some argue that over-fishing, over-harvesting of predators like the triton conch, and agricultural runoff have contributed to the disastrous outbreaks. Huge coral heads, hundreds of years old, have been destroyed by the ravages of the starfish. These scientists argue that control measures are necessary, and advocate the removal of Acanthaster whenever seen by divers. (Note: The crown-of-thorns is spiny, and some people have a toxic reaction to its thorns. Do not touch one unprotected.)
Another opinion suggests that the outbreaks are a natural phenomenon, and point to core samples taken on the Great Barrier reef that show periodic accumulations of Acanthaster spines. These scientists say the outbreaks remove dominant coral species, and may be necessary to increase the species diversity of tropical reefs. They note that the reefs have recovered relatively rapidly from the outbreaks, and suggest removal of crown-of-thorns would in the long run be counter-productive.
Brittle stars. Brittle stars are quite similar in appearance to starfish, but have thin, flexible arms. These arms are easily bro ken off, hence the name. While starfish move mainly by the action of the tiny tube feet on the underside of their arms, brittle stars move by movements of the whole arm.
Many brittle stars have spines on their arms which are very sharp and can give the unwary diver a nasty sting. Despite these spiny defences and their unappetizing appearance, brittle stars are preyed upon by several species of fish, and thus tend to remain well-hidden.
On the shallow reef flats one can sometimes find literal "fields" of brittle stars, their bodies flat on the bottom and their arms wriggling in the water, filtering plankton and debris. On deeper areas of the reef, these animals are less bold, and extend just an arm or two from the safety of their crevices.
Serpent stars are brittle stars smooth arms, and often very striking colors. These animals can sometimes be seen with their arms coiled in tight loops around gorgonians.
Basket stars are the most highly developed filter-feeding brittle stars. They only come out to feed at night, when they extend their branched arms to capture planktonic animals drifting past. Basket stars are beautiful creatures to watch, and they are particularly common on Indonesian reefs, where they can grow to over a meter across.
Crinoids. The crinoids or feather stars are survivors of the sea lilies, animals that once were among the most common in the seas. Although there are still some stalked crinoids extant, those seen on Indonesian reefs are unstalked. They perch on the edge of sponges or gorgonians with a set of small clasping legs, and deploy their delicate arms— of which they have 30 or more—to strain plankton from the water, Feather stars can also walk on these long arms, and if dislodged may swim with them in a beautiful but rather inefficient manner.
Many species of shrimps act as cleaners. This is a Leandrites sp. at work on the mouth of a coral grouper, Cephalopholis miniata.
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,
