Humans think they have much to fear from predators like great white sharks and even the much smaller and usually harmless moray eel. Informed folks like the readers of my columns realize that sharks kill very few people each year and we have much more to fear from coconuts or horses or ants. Morays with their poor eyesight may occasionally bite a diver, but it is usually when the human carelessly sticks their hand in a hole to crab a lobster. We have far more to fear from single-celled Plasmodium falciparum or Trypanosoma brucei, viruses, bacteria, ticks and other parasites.
Biologically speaking (what other language do I know?) parasites are organisms which benefit at the expense of another species. Those of you with dogs or cats are probably very familiar with ecto-parasites like fleas and ticks. These tend to be external and of sufficient size to be seen by the human eye. You are also familiar with certain endoparasites like worms that live within the body. However, the definition of parasite has been extended over the years to include microparasites such as the pathogens I mentioned above.
A "wise" parasite usually does not kill its host. Why kill (or even "bite") the hand that feeds? Far better to keep your source of sustenance alive because if you don't, you may die as well! According to Wikipedia, the word entered the English language in 1539 based on a Latinized Greek phrase that translated as "one who eats at the table of another." Darn, I'd like to do that more often since my cooking is barely edible. By siphoning off food or even body tissues, parasites can certainly affect the health of their host. In some cases the parasite may even cause the host to become incapable of reproduction... the ultimate insult since continuing the species is critical to all living things.
In 1913 (when the City of Avalon was first incorporated!) the German Reuter coined another word, parasitoid. This described an extreme case of parasitism wherein the invader actually kills the host. Bad ju ju. You'd better have another viable option when you go to such an extreme!
Today's column is going to focus on one such organism, the zoanthid Parazoanthus lucificum. Zoanthids are members of the phylum Cnidaria which includes the hydroids, sea anemones, coral, gorgonians, jellyfish and their relatives. Zoanthids are said to be intermediate between anemones and coral, exhibiting some characteristics of both groups. We have several species of zoanthids here in Catalina waters including the orange one named for "Dr. Bob," marine biologist Dr. Robert Given who was well known here in Avalon. Other species are also found on tropical coral reefs and even in the deep ocean.
Years ago while investigating the wreck of the Suejac in the Casino Point Dive Park, I noted what I thought was a golden gorgonian or soft coral that was developing much larger than normal polyps on it. Over the past 20 years or so, I've studied this organism, and soon learned that the giant polyps were actually those of its zoanthid relative Parazoanthus lucificum. I realized that the zoanthid was growing over the gorgonian and slowly killing it. Once the gorgonian had expired, the zoanthid utilized its protein colony structure as a base to attach to.
The Parazoanthus murdered the entire gorgonian colony comprised of hundreds of tiny polyps so it could grow on top of it. As far as I know, it does not ingest the living tissue of the gorgonian, but simply smothers it and prevents it from feeding. Although the frequent host of this parasitoid is the golden gorgonian, it has also attacked the related brown gorgonian.
In researching this column, I discovered that the tentacles of the Parazoanthus polyps apparently glow when they are disturbed. The species name lucifum can be translated as luminescent. I must admit I've never tried that on any of my night dives. Although this species was not scientifically named and described until 1960, experiments to test the mechanisms for its luminescence were conducted almost a decade earlier. It was determined that the zoanthid does not utilize the common luciferin-luciferase bioluminescence mechanism favored by the fireflies of my Chicago youth and other glowing critters like the sea pansy in our waters or bioluminescent planktonic dinoflagellates in our salt water toilet bowls! Yep, when on Catalina, try flushing your toilet in the dark and you may see what I mean.
© 2015 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!
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Healthy golden gorgonian and one being overwhelmed by parasitic zoanthid; closer images of the
zoanthid Parazoanthus after it has completely overwhelmed the gorgonian.
This document maintained by
Dr. Bill Bushing.
Material and images © 2015 Star Thrower Educational Multimedia