Dive Dry with Dr. Bill

#487: Toxic Clouds in the Sea

When I was just a youngster in junior high school, my father and I built a laboratory bench in my parent's basement to use for my chemistry experiments. I had just "graduated" from playing with the HO train set in the adjacent room, and I think we may have used some of the wood from that layout to build my "top secret" lab. I spent hours down there experimenting with various chemicals... and one year won the outstanding award at the Illinois state science fair for breaking down various proteins into their component amino acids. Fortunately I never duplicated my later "success" as a chem lab assistant in high school when I created a cloud of toxic chlorine gas that drove my classmates and teacher out of the room.

Reminiscing about chemistry and toxic substances brings to mind an interesting creature occasionally seen in the dive park that is a creative chemist as well. I'm referring to the California sea hare (Aplysia californica). These beautiful reddish-brown snails with dark lines and blotches look like rabbits due to the sensory structures referred to as tentacles on their heads, and they munch away on vegetation just as a rabbit would do.

Their vegan diet consists primarily of seaweeds including juicy red algae. Their eyes are very simple, responding mainly to the intensity of light in the near ultraviolet with poor vision in the red end of the spectrum which is interesting since red algae are a primary component of their diet. They are able to sense their food chemically using receptors located on the two pairs of tentacles. It is said that people can smell my cooking a mile away... and usually run in the opposite direction. These snails also have taste receptors located on the forward pair of tentacles and the hood surrounding the mouth. The alga or seaweed is captured using the many tiny "teeth" on a structure known as the radula, and pieces are torn off and consumed. Food is initially held in a portion of the esophagus known as the crop, then passed to the gizzard where it is ground up and mixed with digestive enzymes.

Sea hares are gastropods (snails to you common folk). Yes, I know... where is the shell? Actually the sea hare has a small flattened shell, but it is buried in a flap of tissue. So how do they defend themselves against predators anxious to masticate their plump, soft, fleshy bodies? Here is where the story of the sea hare gets really interesting (at least for this chemistry buff). They use their food not simply to maintain the body and grow, but apply their knowledge of chemistry to convert phycoerythrin or phycoerythrobilin, photosynthetic pigments found in red algae, into a poison known as aplysioviolin (or APV for short)! This poison is incorporated into the reddish-purple "ink" that sea hares release when they are disturbed by a predator, or just an annoying garibaldi.

You may be familiar with the ink produced by squid when they are threatened. Italians like to cook their squid in this ink to create calimares en su tinto. Squid ink is apparently not toxic. It simply blinds or distracts the predator so the squid can (hopefully) escape. Don't try this with Aplysia! The sea hare ink isn't quite so benign. It actually interferes with the predator's senses and prevents them from clearly perceiving the snail. Apparently the ink also includes other defenses. A chemical in it causes lobster to initiate grooming and eating behavior. I assume it's like humans... most won't stuff their face while they're brushing their hair or shaving in front of a mirror!

So these chemical defenses seem to work pretty well for the sea hares. The only times I can remember seeing them being munched, usually by crabs, were when they were already dead. However green sea anemones are known to capture them with their "sticky" tentacles and feed on them while still alive. Apparently they stop feeding just before they get to the ink gland and thereby avoid the toxic defenses... a bit too late for the poison to be of any use to the snail.

Can you believe it... an entire Dr. Bill column on munching... without a single mention of that other "M" word animals engage in. I'll have to make up for that in future columns!

© 2012 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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California sea hare in the open on sandy bottom and feeding amongst the red algae;
ink being released by a sea hare and four garibaldi in a cloud of ink.

This document maintained by Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia