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A vicious marine predator washes ashore in Harwichport

Ralph MacKenzie

My friend Ralph – as I’ve mentioned before on these broadcasts, has an uncanny eye for unusual things that most of us would overlook. But he outdid himself the other day when he showed me a picture he had taken of something that had washed up on the beach in Harwichport. I had never seen anything like it. It reminded me of the baby creature in Alien, a movie that still haunts my dreams. It was about nine inches long and resembled something of a cross between a lobster and a shrimp, with several other appendages whose function I could not guess at the time.

Ralph told me it was “mantis shrimp,” though it is neither a mantis nor a shrimp. Mantis shrimp apparently broke off from the evolutionary line of true shrimp several hundred million years ago and evolved into one of the most vicious and efficient marine predators, rivaling sharks for the title of ‘killing machines.” Let me describe for you a few of their unique features:

Mantis shrimps are generally divided into two groups, depending on the nature of their hunting arsenal. One group possesses a pair of club-shaped appendages that smash its prey with incredible power; the other group is armed with spear-like structures with barbed tips which they use to skewer their prey.

One reason a mantis shrimp is such an effective predator is that, in both cases, the shrimp shoots out its club or spear with incredible force and speed, said to be in excess of that of a .22 caliber bullet, making it the fastest motion in the natural world. Its clubs are so powerful that it has been known to break the finger of a person that handles it carelessly. It has actually been known to shatter the glass of an aquarium in which it has been placed. One researcher claimed that if a mantis shrimp were the size of a human being, it would be capable of piercing a sheet of steel.

In addition, mantis shrimp have one of the most complex vision systems in nature. Its two compound eyes are mounted on mobile stalks that can move independently. Each eye has more than 10,000 photoreceptor cells, which are divided into three zones, essentially giving each eye trinocular vision and extraordinary depth perception.

But just as nothing in nature is simple, there are other aspects to the mantis shrimp that complicate its image as a super-predator. For one thing, many mantis shrimp species are monogamous, committed to long-term relationships that can last up to twenty years. In these species, the males help to care for the females’ eggs, in some cases by tending part of the egg clutch in a separate nest, in others by hunting food which it provides to the nesting female.

Despite their menacing appearance, mantis shrimp are highly valued in many Asian cultures as a seafood delicacy. In Cantonese cuisine, it is called the “urinating shrimp” due to its habit of squirting a stream of water when picked up.

I asked Ralph if he had ever seen a mantis shrimp before. “No,” he said. “Nantucket Sound is apparently the northern limit of its range. Or was.” Given the increase in ocean temperatures, the mantis shrimp may be one of those marine creatures extending its range north.

Will we someday see signs at the Cape’s beaches, signs similar to those cautioning swimmers about great white sharks, that will warn us of the presence of these diminutive but ferocious crustaceans? Stay tuned.

A nature writer living in Wellfleet, Robert Finch has written about Cape Cod for more than forty years. He is the author of nine books of essays. A Cape Cod Notebook airs weekly on WCAI, the NPR station for Cape Cod, Martha's Vineyard, Nantucket, and the South Coast. In both 2006 and 2013, the series won the New England Edward R. Murrow Award for Best Radio Writing.