THE new holder of the world record for smallest vertebrate is the Sumatran fish Paedocypris progenetica, a member of the carp family. And it also has a big secret, or at least the male of the species does.
It was a mature female that clinched the title at 7.9 millimetres long, but it’s the male’s anatomy that is arousing most interest. “The amazing thing is that it has this unusual pelvic fin,” says Ralf Britz of the Natural History Museum in London, a member of the team that found the fish (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2005.3419).
“The fish has unique features that probably help it grip females during mating”
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Projecting downward from about midway along the underside of the male’s body, the fin has unique anatomical features which probably help it grip females during mating. Just in front of the fin is a large group of muscles that form a kind of “gripper”, while just ahead of that is a protruding pad or “knob” of hardened skin.
“We think that most likely, the female is clasped by the muscles and held between the pelvic fin and the knob,” says Britz. Alternatively, the males, which are between 8 and 9.6 millimetres long, might use the fin to manipulate eggs in the swamps in which the fish live. “No other fish has these features,” Britz says.
P. progenetica, which feeds mainly on plankton, is also unusual in that most of the roof of its skull is missing and the brain is exposed.
Until now, the smallest known vertebrate was an 8.8-millimetre-long species of Indo-Pacific goby.
