The common banded coral shrimp, Stenopus hispidus, has well-developed front claws, and is sometimes called the boxer shrimp. Various species of Periclimenes also serve as cleaners.
Cleaner shrimp often set up a "station," that fish visit repeatedly. It is quite a sight to watch a tiny shrimp crawl into the mouth and gills of a grouper or large angelfish. If approached slowly enough cleaner shrimps will climb onto a diver's outstretched hand, to see if it too needs cleaning, or even into his mouth.
Pistol shrimp. These animals (Alpheus and Synalpheus) have well-developed pincers, one much larger than the other. By some means that is not well understood, the pistol or snapping shrimp is able to create an audible clicking sound with its large claw.
Some of the blind or near-blind pistol shrimp have developed interesting relationships with small gobies. In lagoons and on sandy patches around the reef you can see these small fish sitting up on their fins outside a small burrow. Next to the fish will be one or more pistol shrimps. The shrimps rely on the gobies, with which they keep in contact by their long antennae, to warn them of the approach of any danger. The gobies benefit from this relationship by having a safe burrow dug for them.
Crabs
Many species of crabs live on Indonesian reefs, but they are not always easy to find. Crabs would soon be eaten by strong-jawed fish such as wrasses if they ventured out boldly by day. Many species are therefore only seen at night when they come out under the cover of darkness to feed. If you look closely at a well-protected coral thicket, however, you will likely see a few small crabs safely wedged in.
Hermit crabs. These familiar, and comical creatures use the discarded shells of gastropods as portable refuges. Some of these small animals are very colorful, particularly Aniculus and the demon hermit crabs, Trizopagurus. A few species of hermit crabs go one stage further, carrying small sea anemones on their shells as additional discouragement to potential predators.
The large terrestrial coconut or robber crab (Birgus latro), a delicacy in the Moluccas and other parts of Indonesia, is actually a hermit crab that abandons its shell when it reaches adulthood. Small land hermit crabs (Coenobita) are common along the high tide line on some Indonesian beaches.
Decorator crabs. These are types of spider crabs that protect themselves by sticking live sponges, gorgonians or other material onto their fuzzy or spiny backs as camouflage. Small decorator crabs may be spotted at any time on sea fans or black coral trees. But look out for the large nocturnal species that carry massive chunks of soft coral or sponge on their backs, held on with their last pair of legs.
The small and colorful boxer crab, Lybia tesselata, grasps a pair of tiny sea anemones in its claws which it then uses for both defense, and to collect food.
Porcelain crabs. The porcelain crabs (Neopetrolisthes), so-named for their smooth, colorful shells, are sometimes called "half-crabs," for they are structurally similar to prawns and lobsters. They are commensals on the giant anemones where, protected from predators, they strain plankton from the water with mouthparts that have been modified for filter-feeding.
Echinoderms
Everyone is familiar with the common starfish or sea star. But starfishes are only one of five groups that together form the Echinodermata, "hedgehog-skinned" animals. The others are the sea urchins, the brittle stars, the feather stars and the sea cucumbers. Most echinoderms have a skeleton of spiny plates— most developed in the sea urchins, and least developed in the sea cucumbers—and five-sided symmetry.
Starfish. 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.