With or without

Natural History, Sept, 2002 by Stephan Reebs

The eel catfish (Channallabes apus), which lives in the rainforests of central Africa, spends its time slithering under the mud. This fish can detect worms that lie just beneath the surface of very moist soil and it comes up to eat them. C. opus lacks pectoral and pelvic fins--an adaptation suited to its burrowing habits. Dominique Adriaens, of Ghent University in Belgium, was therefore surprised to find that a reproductive pair he was keeping in the laboratory produced two larvae with fully developed pectoral fins. When Adriaens and his colleagues examined adult eel catfish specimens from various museum collections, they found that about two-thirds also had pectoral fins, and a few even had both pectoral and pelvic fins. Interestingly, the bodies of individuals with both types of fins were shaped less like eels and more like the typical fish. Apparently, fin loss and body elongation in the eel catfish go together as adaptations for burrowing.

To demonstrate that limbs can be lost in the course of evolution, biology teachers often arrange illustrations of related species in a series to show gradual reduction in limb size or numbers. Adriaens's study provides a rare example of variation in limb numbers within a single species. ("Intraspecific Variation in Limblessness in Vertebrates: A Unique Example of Microevolution," Biological Journal of the Linnean Society 75, 2002)

Stephan Reebs is a professor of biology at the Universite de Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).