Beetle fights bass in mouthwash duel

A whirligig beetle seeps white goo when pestered, and that slow ooze—instead of a big squirt—gives the beetle a chance in a life-or-death contest inside a fish’s mouth, say Cornell University researchers.

Bass appears to struggle to rinse a bad taste off a tainted mealworm marked with charcoal. It alternately sloshes darkened water from its mouth (above) and gills (below). Eisner and Aneshansley/Cornell

Eisner and Aneshansley/Cornell

Close-ups of a beetle’s rear end show white repellant and its release. Eisner and Aneshansley/Cornell

When a largemouth black bass gets a mouthful of beetle, the fish doesn’t swallow immediately, report Thomas Eisner and Daniel J. Aneshansley. Instead, the bass starts flushing water through its mouth, spits out the beetle for a few seconds, and then snaps it up and sloshes it around in more water, as if trying to rinse off a vile taste.

If the whirligig beetle Dineutes hornii makes its precious slime supply last until the fish gives up rinsing, the beetle swims away free. If the slime runs out, however, the fish wins dinner.

The researchers analyze the goo-versus-swish struggle in an article scheduled for the Oct. 10 Proceedings of the National Academy of Sciences.

Accounts of oral-flushing fish have popped up sporadically, but Eisner and Aneshansley propose that the strategy may be more common than reported. Likewise, they suggest that the beetle’s slow doling out of repellant could be a common defense in small water creatures.

Whirligig beetles skitter in swirls along water surfaces worldwide. Compared with long-legged water striders, whirligigs have stubby bodies and short legs.

Two glands at the beetle’s rear secrete goo, which Eisner describes as looking like yogurt. The glop gets its punch from gyrinidal, a terpene hydrocarbon like such powerful scents as camphor and menthol.

Working in a laboratory with wild-caught bass, the researchers checked the repellant’s power. They inserted whirligigs randomly in a series of mealworms, and the bass ate all 197 mealworms but only 3 of 96 beetles offered.

When researchers smeared mealworms with beetle glands, the fish went into a frenzy of mouth flushing before swallowing.

The hungrier the fish, the longer it worked to rinse tainted mealworms. A bass midway through feeding typically devoted 1.3 minutes to a washing. That’s not quite as long as a beetle typically sustained its discharge—1.5 minutes.

“Each is attuned to the law of averages,” Eisner concludes.

Scientists haven’t much explored the possibility of a bad-taste defense by water insects, comments J. David Allan of the University of Michigan in Ann Arbor. “The demonstration of this role in whirligig beetles is exciting,” he says. More familiar strategies among aquatic insects include hiding during the day or when a fishy scent drifts by, but defense by bad taste is well known among terrestrial insects, he adds.

The argument for the beetle’s dribbling chemical defense “makes perfectly good sense,” comments Todd Crowl of Utah State University in Logan. During the past decade, he says, scientists interested in the chemistry of danger underwater have revealed “a whole lot of elaborate chemical signaling going back and forth,” he says.

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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