The helmet conch, Casis cornuta. Indonesians call this kirn a kepala kambing, the "goat's head shell."

      Some of the segmented worms have evolved unusual reproductive strategies, perhaps the most famous being that of the palola worms, (Palola siciliensis). Called nyale in parts of Indonesia, these worms spend their lives in coral crevices, but one night a year, their tail parts metamorphose into a sexual form, containing either eggs or sperm.

      These sexual forms, called epitokes, break off and swarm to the surface. The timing of the event is set by the moon, and in parts of Indonesia, most notably western Sumba and southern Lombok, the appearance of the epitokes is an important event in the ritual or cultural calender. It is also a great culinary event, as the rich-tasting epitokes are highly prized for eating.

      Molluscs

      Molluscs are one of the largest and most familiar groups of invertebrate animals, and thousands of species live in Indonesian waters. The phylum is organized into either five or eight classes, the main ones found on Indonesian reefs being: Gastropoda (univalves—single shells), including snails, cowries and conches, as well as the shellless sea slugs; Pelecypoda (bivalves—two-part shell), including clams, oysters and mussels; and the Cephalopoda, including octopi, squid and cuttlefish. Despite their differences, animals in these three groups all possess a soft, fleshy body (mollusc means "soft") and most— octopi and nudibranchs are exceptions—have the ability to produce a calcareous shell.

      Nudibranchs and snails

      Gastropods are abundant on Indonesian reefs, but they are easily overlooked because most are small, many are nocturnal, and some are very well camouflaged. Nevertheless the diver who develops the habit of carefully scrutinizing the reef surface will soon find many of these delightful creatures.

      Nudibranchs. Nudibranchs, the "naked gilled" sea slugs, are the most interesting to the diver. Like common garden slugs, they are snail-like animals that have lost their shells. Nudibranchs are often strikingly colored.

      In some species the coloration is clearly cryptic, allowing them to blend in with their chosen prey. Nudibranchs are carnivorous, and most are very prey specific, feeding for example only on particular types of soft coral or sponges. Since these prey animals are often very colorful, so are the nudibranchs.

      In other species, it seems certain that coloration serves as a warning to would-be predators that the animals are foul-tasting or poisonous. Nudibranchs are known to produce some very concentrated toxins. Some even have nematocysts, which they obtain from their cnidarian prey and concentrate in the outer layers of their own skin.

      Most nudibranchs are small, although a few mainly nocturnal forms grow to 20 centimeters or more in length. One of the largest and certainly the most spectacular nudibranch found in Indonesia is the Spanish dancer, Hexabranchus sanguineus, a beautiful, crimson-colored animal. This nudibranch only wanders out at night, and if it is found and gently picked up, it will begin its "dance." The wild undulations of its body and surrounding skirt are thought to serve as part of a warning display. Like many nudibranchs, Spanish dancers lay their eggs in huge numbers, in spiral ribbons that can look like flowers.

      Tridacna gigas is the largest of the seven species of giant clams. It can be distinguished by its size and the pebbly texture of its mantle. Scientists believe these to be among the longest-lived animals, some surviving as long as 200 years. A speciman as large as the one pictured here is probably well over 50 years old.

      Rock shells. The rock shells or murex (Murex spp.) feed heavily on small bivalve molluscs such as oysters. This is not an easy task, because the bivalves clamp their shells shut when attacked. A murex shell overcomes this resistance by chipping away at the edge of the oyster with its sharp radula or mouthpart, and then pokes its proboscis into this opening to feed on the fleshy tissues within. Another species, with similar tastes in prey, is the drill (Thais). This small gastropod literally drills a hole through the oyster's shell. The large number of bivalve shells with neat holes drilled in them that are washed up on Indonesian beaches testifies to the efficiency of this feeding method.

      Tritons. The triton shell (Charonia tritonis) is famous as a predator of the troublesome crown-of-thorns starfish, which has devastated Australian reefs. This large shell (to more than 30 cm.) is a popular souvenir, and over-harvesting has been blamed for population explosions of the crown-of-thorns. The helmet conch (Casis cornuta) is another large, predatory gastropod found in Indonesia. Because of collectors, both the triton and helmet conch are endangered in parts of Indonesia.

      Cone shells. Cone shells (Conus spp.) are even more rapacious predators. Their radulas are modified as barbs, with which they stab their victims. They then immobilize their prey by injecting a neurotoxic poison. Most cone shells eat worms, although a few are piscivorous. The poison of some of the fish-eating Conus species is powerful enough to kill a human, so treat them with respect.

      A predatory gastropod prying open a bivalve.

      Cowries. Cowries (Cypraea spp.) are common, small (most just a few cm.) gastropods with a smooth shell that is completely covered by the animal's fleshy mantle. Both the shells and mantles can be beautifully marked, often with very different patterns. The cowries are omnivorous, feeding on algae as well as a variety of sedentary animals such as soft corals.

      Clownfish and their Sea Anemone Hosts

      There is perhaps no sight more charming than a pair of bright clownfishes nestled in one of the colorful giant reef anemones. Although known to possess powerful stinging cells, the anemones clearly don't harm the clownfishes, which look downright snug tucked into the soft tentacles of their host.

      The colorful magnificent anemone, one of the largest clown-fish anemones.

      The relationship between the fish and the actinian is commensal; the anemonefishes clearly benefit, receiving protection for themselves and their offspring. They even pluck at the tentacles and oral disk of the anemone, eating the organic material that has collected there.

      The benefit to the anemone is less clear. The constant prodding, cleaning and stimulation provided by the fishes certainly seems enjoyable, but this maybe just to us. Anemonefishes are never found without anemones; anemones, however, are sometimes found without the fish.

      A Delicate Operation

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