Dive Dry with Dr. Bill

#216: "Deadly" Zoanthids

Back when my near vision was a bit better, I first noticed small beige to grey anemone-like critters on the rocky reefs of the Casino Point dive park. They were quite abundant if I looked closely. In fact someone with adequate imagination (unlike my rational scientific mind) might think they looked like fields of tiny flowers. At first I just assumed they were a form of sea anemone that I wasn't familiar with.

When I'm confronted with something I don't recognize, I usually jump on the Internet and either e-mail pictures of the critter to experts I know, or do a little independent research on my own. I discovered that these were cnidarians (relatives of the sea anemones, corals and soft corals) known as zoanthids which include about 300 species. Some call them zoanthid corals, some zoanthid anemones... in part because they share certain characteristics of both groups. To make things more confusing, each zoanthid species may assume different morphs or types, often depending on where they are located on a coral or rocky reef.

Later while exploring more of the island on the King Neptune, I found several other types of zoanthids. One is an orange species abundant on Ship Rock, Farnsworth Bank, Little Farnsworth and other cold deep water sites. It has yet to be scientifically described. Others were white, gray or light yellow in color. What did these zoanthids have in common? First, they all grew in colonies that were connected at their bases by either a fleshy stolon (a tube- or root-like structure) or a mat. Most anemones are solitary, although they may grow in large assemblages because they often reproduce asexually by budding. Corals are generally colonial forms.

The individuals in zoanthid colonies are polyp-like, looking very much like anemones. They are soft bodied and lack the hard calcium carbonate skeleton common to reef-building hard or stony corals. Some of the individuals are male and others female, although it is possible they may function as either... but not at the same time. Like corals, they release either sperm or eggs and are synchronized spawners. Of course reproduction wouldn't be successful if only sperm or only eggs were released! It takes two to tango. New polyps can also be reproduced by asexual budding like in many anemones.

Although I haven't stopped to count them all, I'm sure I've seen Carl Sagan's proverbial "billions and billions" of these zoanthids on my dives. Obviously they are very successful in "mating" whichever path they choose. However, reproduction is of no use if the new individuals can't find plenty to "munch" on! Again, obviously they must be successful in this endeavor as well.

Like many anemones and most corals, zoanthids have zoaxanthellae or tiny algae in their tissues. Since the zooxanthellae are algae and photosynthesize, zoanthids are often limited to regions where sufficient light can penetrate, although they generally require less light than corals. These tiny algae use carbon dioxide and waste products (fertilizer!) from the zoanthid to create food for themselves... and for their hosts. Several of our local species are found at depth, and are often white or grey in color, so they may not have zoxanthellae in them.

If this "munching" mechanism were sufficient for all zoanthids, there would be little need for the many tentacles each possess at least for feeding. After all, evolution usually gets rid of unnecessary structures due to the energy an organism must expend to create and maintain them. Scientists know that zoanthids do use their tentacles to capture plankton and organic detritus from the surrounding waters, so apparently they need "supplemental" feeding. This explains why many of them are found at sites like Ship Rock where currents may constantly replenish the smorgasboard table. At least one and possibly two zoanthids I've observed are parasitic on gorgonians or soft corals. They begin growing over the gorgonian's stalks and although they probably don't feed on them, the gorgonian's polyps are killed apparently by suffocation.

Of course the tentacles may also have been retained because they serve a different porpoise... er, purpose. It is well known that many cnidarians, including anemones and corals, have stinging cells known as nematocysts on their tentacles. They are used to defend the polyps against potential predators, although some "munchers" like the shell-less snails known as nudibranchs are able to eat the polyps, stinging cells and all. That's a some spicy meal!!

Some zoanthids are known to incorporate a highly poisonous toxin known as palytoxin in the tips of their stinging tentacles. Apparently this is the most toxic organic substance "known to man" (maybe women are wiser and know of others). There is no known antidote for this poison which affects the nervous system. Of course a diver would have to touch "billions and billions" of zoanthids, or ingest a lot of them, to cause death. In fact palytoxin may prove to be a life saver since medical researchers are investigating its possible role in treating hypertension, heart disease and other human diseases. Choose your poison!

© 2006 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. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles" or "Gentle Giants: Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

The undescribed zoanthid from Ship Rock, open and closed; a very common zoanthid from
the dive park, a parasitic zoanthid that grows over soft corals (gorgonians or sea fans).

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia