Fin-Eating Fish

Some freshwater fish in sub-Saharan Africa have a rather nasty habit where they bite off and eat the fins of other fish. This behaviour called pterygophagy is rare and the fish have actually evolved highly specialized jaws for fin-eating. As juveniles, some of them eat smaller fish as a whole, but by the time they become adults, they switch to biting of fin pieces.

This behaviour reminds me of the scale-eating cichlid fishes in Lake Tanganyika who became text-book examples for frequency-dependent natural selection. This species (Perissodus microlepis) occurs in two distinct morphological forms. One of them has mouth parts twisted to the left, enabling the fish to eat scales off its victim’s right flank. In contrast, the other morph, whose mouth is twisted to the right, eats scales off its victim’s left flank. The relative abundance of the two forms is regulated by frequency-dependent selection.

A new study on pterygophagous fish reveals some insights into prey selection:

To further investigate pterygophagy in distichodontids and shed some light on evolutionary and ecological aspects of this highly unusual trophic strategy, DNA barcoding was used to identify prey species from fin fragments found in the stomachs of Phago, Eugnathichthys, and Ichthyborus specimens. Information on prey identity was then used to determine whether pterygophagous distichodontids are opportunistic generalists or strict specialists with regard to prey selection, and to test Roberts’s (1990) hypothesis that aggressive mimicry is used as a strategy for successful pterygophagy in distichodontid fishes.

Previous work has suggested these fish target specific prey while employing aggressive mimicry to blend in with their victims. But this study shows these fish (at least the three genera studied) will target just about any species — including their own.

This study demonstrates how DNA barcoding can be used to shed light on evolutionary and ecological aspects of highly specialized ectoparasitic fin-eating behaviors by enabling the identification of prey species from small pieces of fins found in fish stomachs.

I couldn't agree more and I wonder if someone has some gut content of Perissodus microlepis for a similar look at the scale eaters. As far as I can remember colleagues didsome morphological work on their stomach content but there was no DNA Barcoding used so far. I also wonder if there is any connection between the two unusual feeding habits or at least similarities in the way both evolved.