Dive Dry with Dr. Bill

#225: Flatworms: Fencing Fools

This past weekend saw a gathering of SoCal SCUBA divers from here in Avalon for the monthly Wrinkles Dive. These divers gather each month to dive a site in mainland Los Angeles County, Catalina, Orange County or San Diego on a rotating basis. It is a great way to join a group to meet other divers and find buddies from our region, and to socialize with some really great folks. One of my goals during this event was to join Ken (Mo2vation) and Claudette (HBDiveGirl) to search for and film nudibranchs, the often very beautiful and colorful shell-less snails loved by underwater imagers. Claudette has a great eye for finding them, and Ken takes great pictures of them. They saw six different species on their early morning dive, but when I joined them in the afternoon we spotted only one. I guess I'm a bit of a jinx. However, I did find one nudibranch crawling around in the lobster tank at the El Galleon restaurant!

New divers and aquarium enthusiasts are known to confuse the subject of today's column with the nudibranchs. Heck, even a seasoned veteran like myself has made this mistake on occasion. Certainly today's featured creature can exhibit nearly equal beauty, at least in the tropics where bright colors and unusual patterns are required to identify others of your own species to mate with. Or is that Hollywood? I'm speaking of the flatworms, a group including some 30,000 species world-wide. Of course this includes some pretty ugly ones that live in the terrestrial realm. Only about 4,000 species in this group are found in our oceans.

For a group with so many species, I am surprised that I rarely encounter them on my dives. When I do, they are often hiding beneath rocks or in rock crevices. Their paper thin bodies make it easy to move in such restricted places. These soft-bodied animals lack protective spines and other defense mechanisms and often must rely on the protection afforded by their cryptic habitats. When they venture out into the open, their soft bodies make them easy prey for some hungry fish or invertebrate. If they are munched, they can't mate and keep their species going.

These relatively simple invertebrates possess some sensory structures to detect food, or avoid being munched by an approaching predator. They have a pair of palps at the anterior (head) end of their body which sense the environment through touch. They lack advanced eyes, but have light-sensitive nerves that allow them to detect changes in illumination... such as the shadow of a predator approaching from above! This sense also drives them to escape areas of bright sunlight where they might be more easily detected and filmed... or eaten! Occasionally I see them "rushing" (a worm? rush?) for cover when I overturn rocks and expose them to sunlight.

Although many of their land-based relatives may be parasitic, flatworms in the ocean tend to be carnivores or scavengers. Many are cannibalistic and may make a meal out of their neighbors. Not quite my idea of warm hospitality. They may roam freely in search of food, especially under the cover of night. Flatworms crawl along the bottom, sometimes using a trail of mucus to smooth the way, and may even swim with undulating movements of their bodies.

Another method for gaining nutrition in some species is the presence of algae within the worm's tissues. The internal algae use waste products from the worm, and light energy from the sun to create food through photosynthesis. Of course those species living under rocks would hardly benefit from this. I can't imagine the algae in flatworms that are active only at night are able to photosynthesize by starlight or moonlight, even on the night of a full moon.

The sex life of flatworms poses an interesting dilemma. Individuals are hermaphroditic, containing both male and female organs. However they will rarely fertilize themselves, and greatly prefer mating with another individual... definitely something I have in common with these simple critters. However, if the species is cannibalistic, "getting together" could involve either of the two primary functions of any animal, "munching" or "mating." I guess if a flatworm is horny enough, it just has to let the dice roll and take its chances.

Scientists in Sydney, Australia, recently discovered an interesting mating behavior in a species of flatworm from "down under" known as an oyster leech. Reports in the popular press refer to this behavior as "penis fencing." When two of these flatworms encounter one another and "get the urge," they try to stab the other with their male genitalia. The first one to penetrate the other, inserts "his" sperm into "his" mate, and then goes on to duel with other members of "his" species. The "loser" in this fencing match, hereafter referred to as "her," lays "her" fertilized eggs and broods them until they hatch. Although fencing was a popular sport at Harvard, I now have yet another reason why I'm glad I never took it up myself!

© 2007 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!

Dark upper surface and lighter lower surface of a flatworm;
elongated flatworm crawling on rock, translucent flatworm on scallop shell (can you see it?).

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