Dive Dry with Dr. Bill

019: Cup Corals

As our local waters cool and the kelp recovers from warmer summer temperatures, my thoughts stray to my diving experiences in the warm waters and coral reefs of South East Asia, Australia and the South Pacific Islands. Kelp forests are native to cool, temperate waters and reef-building corals are found in warm, sub-tropical and tropical waters. Looking at global distribution maps of the two groups, it is quite clear they do not overlap because of this.

However, Catalina and southern California waters are home to a few varieties of coral that do not form massive reef structures like their tropical relatives. These are the solitary brown and orange cup corals, and the colonial cup coral. In fact these species may be found as far north as British Columbia south to Baja California. Like their tropical relatives, each of these species forms a hard cup of calcium carbonate to protect the delicate, anemone-like body and tentacles. Although small, these corals can be quite beautiful when viewed close-up. I've never seen it myself, but the pigment in the orange cup coral is apparently fluorescent and appears blue under "black light" (UV).

The solitary species form cups of 1-1.5 inches in diameter scattered along the rocky surfaces. Their orange or almost clear tentacles extend from the protection of these stony cups to feed. Each tentacle bears many stinging cells called nematocysts used to capture their prey. The nematocysts have barb-like projectiles attached by threads which explode on contact, entering the prey and holding it tight so the tentacles can pass it to the central mouth.

In corals the two sexes are separate. Sperm is cast into the water and fertilizes the eggs which are inside the female's stomach cavity (you think you have digestive problems!). The small larval forms are released mainly in the spring and summer, and travel only short distances before settling on rocks and other hard surfaces to attach and develop into adults.

The colonial cup coral occurs in groups and is orange to pink in color. These colonies may be half a foot in width and up to 3" high. Obviously not enough to constitute a coral reef. They are generally not found in the open like their solitary relatives, preferring caves and the protected sides of rocks. There are a few colonies within the Dive Park but divers must search carefully to find them.

Another difference between our local cup corals and the reef-builders is they contain none of the symbiotic algae that make coral reefs appear green. In the tropical reef corals these tiny one-celled algae provide food and oxygen for the corals which in turn give them a habitat to live in, nutrients and carbon dioxide. It is the loss of these algae that is the cause of coral bleaching. However, our cup corals can absorb the organic matter shed by giant and other kelps, and use it for food.

Several years ago I received e-mail from a scientist in the Middle East who is a recognized expert on the cup corals. He had read my research paper on marine invertebrates found on drifting kelp holdfasts. The scientist rightfully questioned how a cup coral could be found on kelp since they only attach to hard surfaces like rock. I explained that drifting holdfasts often contain rocks attached to them, and the cup corals I found were on these rocks. Both of us recognized that, because cup coral larvae only travel short distances, their presence on kelp holdfasts drifting in the open ocean might be a good mechanism to disperse them from the mainland coast to more isolated areas like Catalina and other islands.

As you can see from the pictures, our local coral really can be quite beautiful. However, my thoughts still turn to warmer waters come December. This winter I'll be diving and lecturing on a cruise ship in the Sea of Cortez based out of La Paz. With any luck I may still get a chance after that to return to Thailand or Tahiti, or try Brazil or Venezuela, to enjoy the warm waters... I mean the beautiful reef-building corals there!

© 2003 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Image caption: Calcium carbonate cip of solitary cup coral; colonial cup corals;
tentacles pf solitary cup coral showing nematocysts; tentacles and mouth of solitary cup coral

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia