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The abundance and distribution of corals in Hawaiian reefs varies according to their distance and depth from shore. The primary cause for this zonation is the effect of wave exposure, although gradients in salinity, sedimentation, and temperature can also be important. There are four main zones in Hawaiian reefs: reef flat (0-2m); reef bench (2-10m); reef slope (10-30m); and the rubble zone (30-40m). Reef flat or boulder zone (0-2m) Close to shore in shallow water, this area of the reef is characterised by an intermittent low cover of cauliflower coral (Pocillopora meandrina) intermixed with algal-covered boulders. In this habitat high wave energy, and run-off of fresh water and sediments from the land, prevents much reef development but is it a good habitat for many species of algae, small invertebrates and shore-fishes. Reef bench zone (2-10m) Immediately seaward, from 2-5 m depths begins the reef bench zone - a wave-swept area dominated by a higher cover of coral. In this habitat the highest diversity of corals is found including lobe coral (Porites lobata), cauliflower coral (Pocillopora meandrina), rice corals (Montipora spp.), and many other common coral species. As the wave energy decreases further down the zone, the massive and encrusting coral forms begin to make way for more branching and columnar forms. The area is a good habitat for a wide variety of seaweeds, other invertebrates, and shore fishes. Christmas tree worms (Spirobranchus gigantea) are commonly found burrowed into lobe coral while feather duster worms (Sabellastarte sp.) burrow between rocks and rubble. Collector urchins (Tripneustes gratilla) are common out in the open grazing on encrusting and filamentous seaweeds which cover the rocks. Laying around on sand patches you can find black sea cucumbers (Holothuria atra). Also found here under rocks and in caves are slipper lobsters (Paribaccus antarcticus) and spiny lobsters (Panulirus marginatus). Fishes are abundant and diverse in this area and are dominated primarily by parrotfish and surgeonfishes, such as the ubiquitous yellow tangs (Zebrasoma flavescens). These fishes are primarily herbivores and graze the rocks and dead coral clean of seaweeds. Some parrotfish also eat live coral and can generate a large amount of sediment which contributes to beach development and reef growth. Other common near-shore fishes are moray eels, several types of wrasses, triggerfish, puffers and butterflyfishes. Reef slope zone (10-30m) Further seaward the reef drops off steeply to the reef slope zone, with a pattern of spurs, or buttresses, radiating out from the reef down to a depth of around 20m with deep channels between them. These alternating spurs and channels may be several meters wide and act to drain debris and sediment off the reef and into deeper water. In Hawaii this area is usually dominated by a very high cover of finger coral (Porites compressa) to a depth of 20-30 m. Here wave forces are minimal and conditions for reef growth are optimal. Hawaiian reefs protected from waves are almost always dominated by finger coral, whose thin vertical branches quickly overgrow other coral species, suggesting that it is the dominant coral competitor on its reefs. Butterflyfishes (Chaetodon spp.) are very common in this area as well as hawkfish (Cirrhitus spp. and Paracirrhites spp.), snappers (Lutjanus spp.), and damselfish such as Hawaiian sergeants (Abudefduf abdominalis), the Hawaiian dascyllus (Dascyllus albisella), which is commonly found in cauliflower coral heads, and a diverse mix of wrasses. Goatfish (Parupeneus spp. and Mulloidichthys spp.) are also frequently seen. Rubble zone (30-40m) Below 20-30 m is the rubble zone which is characterized by accumulated broken coral fragments intermixed with a small amount of live lobe coral and sand. Common invertebrates in this area include black sea cucumbers and many types of burrowing worms. Hawaiian dascyllus may occur here on isolated coral heads along with an occasional triggerfish. And on sand flats in areas with moderate currents garden eels (Gorgasia hawaiiensis) can be seen extending out of their sand burrows. |
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This page was last updated 26 March 2006