These fish would rather walk

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Frogfishes were mistaken for true frogs in the 18th century, which isn’t as silly as it sounds. These are strange fishes. And the first genetic study of their evolutionary relationships raises 21st century puzzlements over how to classify them. 

“I tell people they’re fish that can’t swim,” says Rachel Arnold of the University of Washington in Seattle. That’s only a slight exaggeration: “They’re kind of wagging their tails, trying to push themselves through the water, but they’re just not going very fast,” she says. 

The fish can get an assist by gulping water and jetting it out of the small opening behind their gills. “It sounds like it might be fast — ‘jet propulsion,’ right? — but it’s still remarkably inefficient,” Arnold says. 

Instead of swimming, the several dozen fish species in the family Antennariidae usually walk. Moving one pectoral fin forward at a time, they trudge along the sea bottom in warm waters worldwide. And some other under­water pedestrians, including the family of handfishes, may belong among frogfishes, Arnold’s work suggests. The 14 squat, lumpy handfishes grow fin tips so splayed that the fish look as if they’re walking on humanlike hands.

None of these fishes let being slowpokes of the sea interfere with voracious eating. They fish the way people do, with patience and lures. The fishes’ dorsal fins’ front spines have evolved flamboyant deceptions: tendrils that wiggle like worms, a pom-pom on a stick, a lump with stripes and an eyelike spot. Any creature lured in has 10 milli­seconds or less to appreciate the extreme suction created when these fishes’ huge mouths suddenly gape.

Arnold’s work hints at a genetic — and ovarian — divide between two big evolutionary branches of the expanded company of frogfishes. Females of one group grow sheetlike organs that roll into a pair of tubes — “double-scroll ovaries,” they’re called. Some can extrude long egg-containing gelatinous strips. After fertilization, the strip dissolves and embryos waft away.

Females of the other group, including handfishes, grow lump-style ovaries, and fertilized eggs get days of parental tending. In a few of these species, the parent picks up the glop of eggs and wears it as a stick-on body patch. Another species cradles the mass in its tail. Which of course makes swimming attempts even clumsier.