the plankton stream to become part of the swimming nekton (fish, jellyfish) or the crawling or fixed benthos, or bottom dwellers (sea urchins, gorgonians).
Sapphire damsels, Pomacentrus pavo, take shelter in the sponge Cribrochalina. Halmahera, Maluku.
Sponges
Indonesian reef sponges vary in size from tiny to huge, from the small patches of color provided by encrusting sponges (family Clionidae) to the meter-high barrel sponges (Xestospongia). All sponges are members of the phylum Porifera, "the hole-bearers," and their porous, "spongy" nature is crucial to their mode of feeding. Sponges are the archetypal filter-feeders, straining plankters from the water through myriad microscopic pores.
A cross-section of a sponge shows a very sophisticated system for moving water. Small intake pores lead to an internal system of tiny canals and chambers lined with cells bearing whip-like processes. Beating constantly, these cells create a current through the sponge that moves its own volume of water every 4-20 seconds. Even a relatively small sponge can circulate as much as 5,000 liters a day. The chimney or barrel shape of many larger sponges helps increase surface area and the water flow through the animal.
Sponges are notoriously difficult to identify. Colors vary and even the shape or size of a sponge does not necessarily mark its species; sometimes shape is just a response to local conditions. Scientists would call this new form an "ecomorph."
Scientists rely on detailed examination of the internal "skeleton" to identify sponges. Sponges are made of a proteinaceous secretion called spongin. This fibrous net forms the useful part of the bath sponges (Spongia and Hippospongia) harvested in the Mediterranean and Caribbean. Many sponges also contain spicules of silica or calcium carbonate, or both, bound together with spongin.
There are an estimated 830 species of sponges in Indonesia. The giant barrel sponges are most impressive to divers, but the smaller tube sponges and vase sponges also create colorful and aesthetically pleasing forms.
Marine dinoflagellate plankters, top to bottom: Gymnodinium, Gonyaulax, Peridinium, Ceratium.
Like many invertebrates, sponges can grow to a remarkable age. Experiments with commercial farming of bath sponges in the Caribbean have led researchers to estimate that larger specimens are at least 50 years old, and maybe much older.
Reef sponges create an environment that is exploited by a variety of other creatures. Small crabs and shrimps and even fish hide in the tubes and cavities. Crinoids perch on upright sponges to filter plankton from the current. And sea cucumbers and other detritus feeders graze on the organic material that collects on the sponge's surfaces.
Corals and their relatives
Corals, soft corals, sea anemones, gorgonians, hydroids, jellyfish and the other members of the phylum Cnidaria (formerly Coelenterata) cause a great deal of confusion for the diver trying to identify the teeming mass of branched and tentacled life he sees attached to the reef. Taxonomists identify these animals by their stinging cells, nematocysts, and simple coelenteron, from the Greek koilos, "hollow," and enteron, "gut." All have the form of a polyp at some stage in their lives. Other than these shared characteristics, the form of these animals varies widely.
Aristotle considered them an intermediate form between plants and animals, and they were first placed by taxonomists in a group called Zoophyta, "animal-plants." Only in 1723 were corals properly identified as animals, and Jean Andre Peyssonel, the naturalist who proposed this to the French Academy of Sciences, was laughed at and quit science in disgrace.
Phylum Cnidaria is usually divided into four classes: Hydrozoa, hydroids and fire corals; Anthozoa, corals and anemones; Cubozoa, box jellies; Scyphozoa, jellyfish. Anthozoa, in turn, is split into three sub-classes: Alcyonaria (Octocorallia), containing the soft corals and gorgonians; and Zoantharia (Hexacorallia), containing the stony corals and anemones; and Ceriantipatharia, including the black corals and cerianthids, or tube anemones.
The Stony Corals
The stony or hard corals are the reef-builders. They are in the order Scleractinia, and are sometimes called scleractinian or "true" corals. The skeletons these animals secrete range in shape from the massive, smooth boulders of Pontes and stout-branched Pocillipora that take a pounding at the reef edge to the finely foliated needle coral Seriatopora histrix.
These corals are colonies, comprised of thousands of individual coral animals, or polyps. Each polyp, upon close examination, will be seen to have much the same shape as a sea anemone, with tentacles ringing a central mouth. What makes the stony coral polyp distinctive, and so ecologically important, is that it deposits calcium carbonate around its lower part, forming a skeletal cup. The skeleton is essentially formed of repeated casts of the tiny polyp.
The presence of whip coral gorgonians often indicates very clean water and plenty of plankton. This is Ctenocella (formerly Ellisellaj. Bunaken group, Sulawesi.
Most reef-building corals are nocturnal. During the day, the polyps are retracted, drawn down into the skeletal cup. At night, these corals are transformed from dead-looking lumps of rock into miniature forests thick with polyps, which expand to feast on the abundant night plankton. Tiny plankters are snared by the polyp's tentacles, which are armed with stinging nematocysts. Although they feed on plankton, the vast majority of the nutrition of reef-building corals is provided by the symbiotic zooxanthellae in their tissues. (See sidebar, "Zooxanthellae and Corals," page 25.)
Corals, like other reef animals, also spawn at night, releasing pink clouds of sperm and eggs. To increase the chances of fertilization, corals of the same species tend to coordinate the release of their eggs and sperm. Many reef animals spawn around the time of the full moon, when tidal currents are strongest, to ensure wide dispersal of the larvae. On the Great Barrier Reef of Australia the majority of corals spawn 4-5 days after the November full moon. Some corals in Indonesia spawn at this time too, but the full pattern of coral spawning has not yet been determined here.
According to travel brochure cliches, corals are supposed to be "kaleidoscopic" with color. Divers, of course, know that at least for the reef-building corals, this is not at all the case. Most shallow water corals are a dull brown color, a consequence of the pigments in their zooxanthellae. Still, some are blessed with subtle pastel tints. In particular, the growing tips of Acropora can be colored with a pinkish or purplish pigment, a group of amino acids called S-320 which serves as an ultraviolet filter to protect the still-young polyps.
A detail of the eponymous vesicles of the bubble, or grape coral, Plerogyra sinuosa. These sacs, called acrorhagi, possess stinging nematocysts. During the day they are inflated with water and protect the polyp tentacles. At night they shrivel, and the polyps are extended to snare plankton. The acrorhagi also discourage other corals from over-growing Plerogyra, blocking its sunlight and supply of plankton.
The shape of stony corals, rather than their color, is their most salient characteristic. The form the coral will take is strongly influenced by wave action and currents, and even the same species may take different forms under different conditions. A specimen of the distinctive pal-mate Caribbean elkhorn coral (Acropora palmata) placed in a research ecosystem at the Smithsonian Institute in Washington, D.C. sent up new growth in the bushy form of A prolifera. This, it would seem, further
