Dive Dry with Dr. Bill

#168: Mantis Shrimp

I had been out of the water for nearly a month when Thanksgiving arrived, and was looking forward to a wonderful weekend of diving. Little did I realize how nice it would be! Friday and Saturday were spent diving on the King Neptune with some old dive friends from ScubaBoard and a group from San Francisco that was to quickly become new dive friends. One of the things I really enjoy about diving is the socializing that goes on between divers during their surface intervals. This group initially caught my attention at the dive shop for some reason. These folks not only shared my passion for diving, they were intelligent and very nice as well. And, of course, to a gilled semi-aquatic male mammal like myself, nothing could be finer that a lady diver covered head to toe in neoprene! Thanks Valerie, Mina and Mark for a great weekend of diving and for future dives and friendship!

Although none of the dives on the first two days were extraordinary, each one had at least one highlight that made it worthwhile. For example, while diving the reef at Yellowtail Point near the Empire Landing Quarry, I floated over the upper reef and then slowly angled down towards its base at about 80 feet. There I discovered a type of fan worm, the first I remember seeing in Catalina waters. It looked very similar to ones I've seen in Mexican waters, but it will take a bit more research before I write about it. I slowly angled down towards a maximum depth of 100 ft. and there encountered the subject of today's column, a mantis shrimp poking its head out its burrow.

Now mantis shrimps are common in Willow Cove located next to Toyon Bay. A long-term study of them was conducted there by UCSB marine biologist Dr. Jack Engle. Noted underwater film producer Howard Hall filmed the mantis shrimp there a few years ago, watching as an electric or torpedo ray dropped down to rest on the camera control, triggered the camera on and "burned" through $3,000 worth of IMAX film in no time. I rarely see them because their burrows are in the sandy bottom, and I focus my research on the kelp forests growing off of rock reefs. This dive gave me an opportunity to film this one for a minute or so until it "freaked out" and dropped down into its burrow. Perhaps it thought I was a black sea bass looking for an easy meal (which I've been known to do on occasion since my cooking is so bad).

Although mantis shrimps are more common in tropical and subtropical waters, we have at least four species here in southern Califonia waters. The largest one (up to about 10") is known from the Gulf of California north to Pt. Conception. These lobster-like crustaceans (known as stomatopods) are vicious predators. They get their name from specialized hinged feeding "arms" that look like a praying mantis' feeding "arms." Different mantis shrimp species use these appendages in different ways to capture prey. Some "spear" their food, while others smash their prey with heavy hammer- or club-like structures. It is said that the force of our local species equals that of a 22 caliber bullet! Using specialized high speed camcorders, researchers at UC Berkeley timed the strike of one mantis shrimp at 75 ft/sec, enough to strike fear even in the heart of Muhammad Ali! The strike is so energetic it even vaporizes some of the water between the mantis and its prey, releasing a quick burst of light. If these guys were the size of big lobsters, they'd be even more dangerous to divers than they are.

Mantis shrimp are more active at night, waiting in their burrows for food to pass by or coming out of their holes under the cover of darkness to feed. Prey includes smaller crustaceans, clams, snails, urchins, brittlestars, worms and even fish. When one was accidentally introduced into an exhibit at the Monterey Bay Aquarium, it devastated other marine invertebrates in its quest for food.

Mantis shrimp are often overlooked by the casual diver because their habitat is the sandy bottom, which appears barren to those who do not look carefully. Often the only thing a clumsy diver will see is their hole since the mantis will have jumped down its burrow at their approach. These protective holes are lined with mucus originating in the saliva which keeps the walls from crumbling. Burrows run just below the surface for 3-10 feet. Some mantis shrimp use a bright yellow fluorescent signal to scare predators or in territorial displays for other mantis shrimp.

The two stalked eyes often seen sticking out of these holes are perhaps the most sophisticated in the animal kingdom. Each individual compound eye is capable of 3-D vision and can detect light that is polarized three different ways. Their eyes contain 12 different color receptors (compared to the human eye's three) and they may be able to detect 10X as many colors as we can, not to mention four colors of ultraviolet light which our eyes cannot detect. I can barely read the DMV chart with my glasses on! No wonder they see me coming long before I detect them.

Divers have reported hearing "groaning" sounds coming from the burrows. It is believed they are made when mantis shrimps try to attract a mate. Interesting, since in human mating the groaning often occurs after the mate has been located. Oops, better watch out for the censors (or is it sensors?). After mating, the female lays a cluster of eggs in the burrow which it guards until they hatch. The larvae are also active carnivores in the plankton. In some the early larval stages are spent within the burrow, and the later stages in the plankton where they may drift for a month and a half before settling. Some day I'm going to settle on the bottom at Willow Cove and get some really good footage of these interesting critters!

© 2005 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. Buy my "Munching and Mating in the Macrocystis" DVD so you can take Dr. Bill home with you... we'll both be glad you did!

Image of a mantis shrimp (courtesy of Rick Coleman), mantis shrimp peeking out of its hole, being a little
bolder showing the stalked eyes, and the way divers usually "see" mantis shrimp... just the burrow opening.

This document maintained by Dr. Bill Bushing.
Material and images © 2005 Star Thrower Educational Multimedia