Dive Dry with Dr. Bill

052: Beach Bunnies (Sea Hares)

Last weekend, after 41 years, I experienced my first serious "emergency" while SCUBA diving. I finished my dives and walked up to the house only to discover my wetsuit zipper was stuck. My housemate and neighbors were not home, and I couldn't get the zipper down by myself. I stopped people in the street but no one else could do it either. After trying for an hour, I admitted defeat and walked down to the fire station. I said to the crew "for years you've been waiting to rescue me at the Dive Park, here's your chance!" After much laughter I was free. Thanks guys!

I had been diving in surge searching for subjects to videotape. I didn't have to wait long... within the first two minutes I located a group of five sea hares in a mating orgy. Sea hares are molluscs related to snails and, like the nudibranchs, they lack an external shell. There are actually two larger species in our waters, the brown and the black. Brown sea hares are smaller, up to about 16" in length. Black sea hares may reach 30" and weigh up to 30 pounds. The brown species has smooth mottled skin that may be tan, red, olive-green or brown in color. To some degree skin color depends on what they eat. Its relative has rough black skin.

The name "sea hare" comes from two structures on the head known as rhinophores which look somewhat like a rabbit's ears (come on, use your imagination). The dorsal (back) surface is dominated by two large flaps which are actually extensions of the animal's foot. These flaps are believed to create currents which help the animal breathe.

Brown sea hares are known from Humboldt Bay, California, to the Gulf of California. Usually found in rocky areas down to about 80 feet, they may also occur on sand or mud. Young ones can be found in the shallows under rocks and in tidepools and are usually reddish because they feed on red algae. They only live about one year.

The group I observed were brown sea hares. This species is a strict vegetarian, feeding on seaweeds, kelp and eelgrass. Salad every night... how boring! At least they could sneak an occasional shrimp or lobster. Their eyesight is very limited and they sense their food chemically. Their digestive systems are very complex and have several stomachs (like cows) lined with grinding teeth allowing them to extract sufficient nutrition.

It is believed that sea hare flesh (as opposed to rabbits) does not taste good and its digestive system may be toxic, so it has few predators. When disturbed, sea hares release a deep purple ink that confuses potential predators. This ink is created from chemicals in the red seaweeds the animal feeds on. I've had this happen occasionally when handling the animals to get a better shot. It can really cloud one's view!

Like many snails, sea hares are hermaphrodites and have both male and female sex organs. However, they are not able to fertilize their own eggs so they require a mate. When mating they may function as female, or male, or both at once. The more the merrier with this species. The group I observed had four individuals actively mating with one another... the fifth one had fallen from the rock and rested nearly motionless on the bottom a few feet away. It probably needed it since actual mating may last several hours to several days (without Viagra)!

Two scientists estimated that a large brown sea hare may produce half a billion eggs each year. More conservative biologists suggest the number is only several to tens of millions. The eggs are laid within long, yellow spaghetti-like strands that form masses attached to the rock and look like Mama Mia's Day Old Spaghetti. One mass of eggs may measure 1/3rd of a mile when stretched out.

The eggs hatch in 10-12 days and the larvae begin a month-long planktonic life fraught with dangers. Most are eaten at this stage by plankton feeders. If this mass slaughter did not occur, the world would soon be overrun with sea hares underwater (and rabbits on land)! If only that would happen with the terrestrial "beach bunnies!"

© 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: Head-on view of rhinophores and oral hood; wings or flaps on dorsal surface;
three sea hares participating in mating orgy; spaghetti-like egg mass on rock

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