These fish would rather walk

Slowpokes of the sea, frogfish and handfish barely manage to swim

spotted handfish

The spotted handfish takes its name from the long, flared shape of its pectoral fins.

CSIRO/Wikimedia Commons (CC BY 3.0)

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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.

Painted frogfishes (one shown) and their relatives prefer to walk along the sea bottom instead of swimming, but they’re not going to trudge up any beaches. If lifted to dry land, frogfishes mostly slump into immobility. Bernard Dupont/Flickr (CC BY-SA 2.0)
The odd anatomy of frogfishes led to taxonomic excess. More than 350 possible Latin names accumulated by the 1980s, but they turned out to represent just 40 species. Recent work turned to genetics, sampling such species as the hairy frogfish, Antennarius striatus. Silke Baron/Flickr (CC BY 2.0)
Biologists suspect that the resemblance of the giant frogfish (Antennarius commerson) to a sponge helps it lull prey into swimming unwisely close. The giant grows to an average 30 centimeters in length — about the size of a football. Bernard Dupont/Flickr (CC BY-SA 2.0)
A female spotted handfish (Brachionichthys hirsutus) guards her fertilized eggs. Once widespread across eastern Tasmania, known populations of the critically endangered species now occupy just one estuary. Mark Green/CSIRO
Also facing extinction, red handfish (Thymichthys politus) are covered with warty skin. They come in two different color patterns, both dominated by reds. Mark Green/CSIRO

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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