Hawaiian Reef Fauna — The Invertebrates

The fringing reefs of Maui support a rich and varied community of organisms and are a microcosm of the planet's coral reef ecosystem. Comprising the core of the reef are the corals themselves. About 50 species of shallow-water coral inhabit the waters surrounding the Hawaiian Island chain. Most important are the massive, reef-building hermatypic species but nearly all of them are contributors to the reefs as a whole. The most common reef-building coral in Hawaiian waters is the Lobe Coral (Porites lobata). Found in shallow waters along semi-protected stretches of the coastline, Lobe Coral forms massive colonies in more protected waters, but tends to encrust the substrate in higher energy environments. The second most common reef-building species is the Finger Coral (Porites compressa). Preferring calmer, more tranquil waters, Finger Coral is usually found in shallow, well-protected bays or in deeper waters, at depths greater than 30 feet. When flows of molten lava pour into the sea, the cooled basaltic rock presents a new surface for sessile marine invertebrates to attach to. One of the first colonizing organisms to take advantage of this situation is the important branching coral known as the Cauliflower Coral (Pocillopora meandrina). This species thrives along exposed coastlines in waters of moderate to high energy.

A number of other important coral species combine to form the reef structures surrounding the Hawaiian Islands. Abundant in Honolua Bay, Maui, the distinctive Blue Rice Coral (Montipora flabellata) forms vivid blue encrustations on the reef surface. A related species of rice coral, Montipora capitata, can form both massive and encrusting colonies. Hawaii's largest species of branching coral, the Antler Coral (Pocillopora eydouxi), occurs at depths greater than 20 feet where it forms impressive, tubercle-covered branches. In shallower waters, another branching species, the Lace Coral (Pocillopora damicornis) can be found in abundance.

The Mound Coral (Porites lutea) is another massive reef-builder that can be found in shallow waters along protected stretches of coastline. Common along the west coast of Maui, the Plate-and-Knob Coral (Porites monticulosa) forms encrusting patches on the reef surface, but may also produce massive structures comparable to other reef-building species. The coral group that includes the beautiful and distinctive "brain corals" of the family Faviidae is not common in Hawaiian waters but a few species are important contributors to the reef systems of the islands. These include the Ocellated Coral (Cyphastrea acellina), the Bewick Coral (Leptastrea bewickensis), and the Transverse Coral (Leptastrea transversa).

Any invertebrate that attaches to the substrate and secretes a CaCO₃ shell or hard part is a potential contributor to the framework structure of a reef. In Hawaiian waters, a multitude of organisms qualify as reef contributors, but a few deserve special mention. First and foremost in importance are the coralline algae. These carbonate-secreting algae are significant reef-builders in their own right! Indeed, a large part of the reef framework in Hawaii is the product of a species of red coralline algae known as Porolithon gardineri. This important reef-building protist prefers heavy wave action and strong currents where it forms beautiful pink, red, and purple mounds and encrustations.

Some organisms are more subtle and localized in their reef-building activities. Tube-shell snails construct hollow, cylindrical, tube-like structures out of calcium carbonate which they then cement to coral reefs or lava rock. In Hawaiian waters, the most common of these tube-building snails is the Variable Worm Snail (Serpulorbis variabilis).

Other organisms are more cosmopolitan in their distribution, covering or encrusting virtually everything that floats or is submerged in shallow seawater. The tiny invertebrates known as bryozoans are just such a creature. Sometimes forming hard crusts on the substrate, sometimes upright and branching, bryozoan colonies occur in waters of all depths. Commonly called "lace corals", bryozoans are actually more closely related to brachiopods. But their colonies do look like corals. Because the encrusting species resemble mats of delicate moss, divers and biologists who study them sometimes refer to them as "moss animals" and "sea mats". About 200 species of bryozoans occur in Hawaiian waters.

A host of invertebrates and reef fish make their homes in the coral reef systems of Hawaii. Indeed, the reef communities of these islands are extremely rich and varied and include some of the most beautiful and bizarre life forms in the marine world. The cnidarian, molluscan, arthropodan, and chordate phylums are all well-represented in these Hawaiian reef communities.

Soft corals are not common in Hawaiian waters. Only a few species of gorgonians occur here, and of these all but one are found in very deep waters. Known as octocorals or "horny corals", the gorgonians as a whole do best in warm, tropical waters. This group includes the sea fans, sea whips, and the precious red coral known as Corallium. In Hawaiian waters, many of the octocorals sport vivid hues of blue, pink, or red, and add a splash of color to the reef. Two of the most beautiful octocoral species are the Blue Octocoral (Sarcothelia edmondsoni) and the gorgeous, multicolored gorgonian known as Acabaria bicolor.

Black corals and wire (or whip) corals also occur in the waters surrounding the Hawaiian Island chain. Known as antipatharians, this group of corals does not require direct sunlight to live. Consequently, they are found in deeper waters than the more conventional types of coral. Black coral, which was once used extensively in jewelry, is now rare and its collection is regulated. In Hawaiian waters, the Common Wire Coral (Cirrhipathes anguina) is one of the more frequently encountered antipatharians.

Over 1300 species of mollusks inhabit the seas surrounding the Hawaiian Island chain. Some 250 of these are found nowhere else in the world. This impressive population of mollusks includes chitons, snails, clams, and oysters but also includes the most advanced invertebrates in the animal kingdom, the squid, cuttlefish, and octopi.

Chitons are the most primitive mollusks and are one of the few creatures able to tolerate the turbulent, high energy wave action along the reef front. Chitons mostly graze on algae growing on the surface of the reef, but some species can actually break down the coral rock itself. Hawaiian chitons are small and few in number. The largest species, the Green Chiton (Acanthochiton viridis), attains a maximum length of only 1½ inches.

Marine snails are quite abundant in Hawaiian waters. Nearly 600 species of gastropod have been identified so far, with many more yet to be discovered. Nearly every conceivable form or type of shell can be found in the gastropod populations of Hawaii. Simple, cone-shaped shells are characteristic of the limpet family who, like the chitons, prefer the wave-battered surge zones of exposed coastlines or reef fronts. Edible species like the Black-Foot Limpet (Cellana exarata) are called "opihi" by the native Hawaiians.