particularly M. platyphylla, which can be found growing with massive scleractinian corals at the pounding edge of the reef.
Jellyfish
These familiar animals (class Scyphozoa) are characterized by a dominant medusa stage. Like all cnidarians they form a polyp for part of their lives, but for the jellyfish, this is just temporary. Occasionally, large jellyfish can be seen while diving in Indonesia, particularly in areas of rich plankton. These can be quite beautiful to observe. More bothersome are the cubomedusae, or sea-wasps, tiny jellyfish that can have an irritating sting. Members of the genus Chironex have even been responsible for human fatalities in Australia. Because they tend to inhabit the surface layer during the day, they are more of a bother for snorkelers. The lights of night divers, however, can often attract an unwelcome swarm of these creatures.
Worms
Although the word conjures up a dull, and faintly repulsive animal to many people, the worms found on the reefs of Indonesia show a diversity of form and color that often astounds the observer.
There are many different sorts of worms, but most likely to be seen by divers on Indonesian reefs are those in the following phyla: the flatworms (phylum Platyhelminthes); the ribbon-worms (phylum Nemertina); the tongueworms (phylum Echiura); and the segmented worms (phylum Annelida).
Flatworms. Flatworms often have the strikingly beautiful colors divers associate with nudibranchs (which are molluscs). The species seen on Indonesian reefs rarely grow longer than 10 centimeters, and feed on sessile animals such as tunicates and sponges. Flatworms move by gliding over the bottom, or by muscular undulations. This latter waving action is characteristic, and mimicked by the juveniles of several species of fish. This mimicry and the bright colors suggest the presence of a noxious chemical to deter predators.
Ribbonworms. These animals are longer than flatworms, and not as showy. Many are white, with dark stripes or bands. They tend to live under rocks and corals or in the sand, and are most likely to be seen by divers at night. Some can grow to astonishing sizes, as much as several meters. They feed on molluscs and other worms.
Tongueworms. The tongue-worm Bonellia can be seen on reef slopes. However, it hides its sac-like body in a crevice, with only a forked proboscis protruding, is easily overlooked. These animals have an unusual sex life. All the fully formed Bonellia are females; if a larva settles into an area where there are no worms, it becomes a female. If there are already Bonellia established in the area, the larva passes into the body of an adult, becoming a dwarf male, which lives like a parasite on the female "host."
Two do rid nudibranchs, Notodoris citrina (top) and Nem-broth a sp. Nudibranchs tend to be very prey-specific. Nembrotha, as is shown here, feeds only on hydroids. Notodoris was photographed in Halma-hera, Maluku; Nembrotha in Flores.
Segmented worms. The segmented worms are the most abundant and diverse of all the groups of reef worms. Divers are familiar with the feathery feeding parts of the tiny Christmas tree worms (Spirobranchus) which extend from lumps of living coral. The body of the worm is hidden in a tube within a Pontes coral head. The similar, but larger fanworm, or feather duster worm (Protula, Sabellastarte), secretes a tube of flexible parchment to protect its soft body. From its tube, it periodically extends a crown of colorful "feathers" to collect plankton. These worms make good subjects for macrophotography, but any sudden movement will cause them to withdraw their crowns.
The triton shell, Charonia tritonis. This gastropod preys on crown-of-thorns starfish.
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
