Disco clams may flash chemical-weapons warning

Puzzling streaky light show might tell predators not to bite

ICK, ICK  A clam flashing reflected light off its lips may be warning predators that attempts to bite will not end well.

Courtesy of L. Dougherty

WEST PALM BEACH, FLA. — A disco clam’s flashing light show might warn predators of — with no comment on the 1970s — bad taste.

In lab tests, usually voracious mantis shrimp backed off or showed suspiciously little interest in clams as if the flashy flesh was distasteful or toxic, Lindsey Dougherty of the University of California, Berkeley reported January 4 at the annual meeting of the Society for Integrative and Comparative Biology. So the antipredator idea is a strong hypothesis among several in the ongoing puzzle of the clam’s flashing, Dougherty said.

Ctenoides ales clams put on a steady show not known in any other mollusk. At least once a second, they uncurl the soft lips along the edge of their bodies and reveal embedded silica spheres that reflect a quick streak of ambient light (SN Online: 6/25/14). Then the lip curls closed again, hiding the light.

When Dougherty and her colleagues identified the silica reflectors last year, the next question became: What benefits could a clam possibly get from flashy lips? In a lab test, Dougherty staged a clam-versus-predator encounter. The predator, a peacock mantis shrimp with enough smash power in its club to break mollusk shells, approached and grappled briefly with the clam. Then the mantis shrimp backed off for intense cleaning of its mouthparts. It did not attack again, even when the clam opened up, exposing vulnerable — and one would think, tasty — tissue. (The crab apparently was so successful in conveying its inedibility to the shrimp that the shrimp in confusion later tried to mate with it.)

Some mollusks defend themselves by releasing toxic, or just disgusting, chemicals, including floods of acid. Disco clam tissue carries an unusual abundance of sulfur, Dougherty has found, and she plans to explore whether the clams release sulfuric acid, especially around the fringed tentacles that stick out past the clams’ lips.

That idea seemed plausible when Dougherty offered tidbits of clam tentacles to mantis shrimp in the lab. The shrimp wanted nothing to do with them. Nuggets of muscle from deeper in the clam’s body, however, were a hit as a shrimp snack.

The predator-warning hypothesis is worth following up on, said Melissa Bowlin of the University of Michigan–Dearborn, who heard Dougherty’s talk at the meeting. That’s exactly what Dougherty plans to do.

But Dougherty said she isn’t yet ruling out other hypotheses, such as flashing as a lure for the tiny plankton that clams feed on. Microprey should be able to see the flashes, she and her colleagues hypothesize. She has prepared mock clams, some of them flashing and some not, that she will take to disco clam country in Indonesia to see if light shows enhance the capture of food.

Another possible function of the flash, as a clam-to-clam cue for attracting mates, seems less likely, Dougherty said. Lab tests she and Berkeley colleague Alex Niebergall ran found no evidence that flashing attracts other clams.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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