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 their large white antennae to attract the attention of passing fish.
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.
