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University of Basel

23 September 2015

Wolf in Sheep’s Clothing: How Scale-Eating Cichlid Fish Trick Their Prey

Scale-eating cichlid fish from Lake Tanganyika (Plecodus straeleni)
Scale-eating cichlid fish from Lake Tanganyika (Plecodus straeleni) feed exclusively on the scales of other fish species.

Displaying false information in order to deceive is one of the most ingenious survival strategies in the natural world. Zoologists at the University of Basel have now revealed a particular example of this kind of trickery in an African cichlid that is specially adapted to feeding on the scales of other fish. As reported by researchers in the scientific journal Biology Letters, the scale-eater imitates non-predatory fish to allow it to approach its prey without being noticed.

The lakes of East Africa are home to a rich diversity of species, including hundreds of cichlid species. This family of fish are known for their rapid adaptation into different ecological niches, and for the recurrent origin of novel species. Most cichlids have developed a particular way of feeding, such as scraping up algae, cracking open snail shells or sucking up small animals. One particularly unusual feeding strategy is that employed by so-called scale-eaters, which, as the name suggests, feed exclusively on the scales of other fish.

These scale-eating cichlids, found primarily in Lake Tanganyika, exhibit particular adaptations to their specific diet; for example, hook-like teeth and a mouth that opens at the side. These features enable them to quickly rip out the scales of unsuspecting prey fish in their attacks. Some species, like the blue and white striped scale-eater Plecodus straeleni, also exhibit color patterns that look deceptively similar to those of other cichlids.

Wolf in sheep’s clothing feeding on goats

Until now it was assumed that scale-eaters imitated their prey so that – like a wolf in sheep’s clothing – they could slip into a shoal unnoticed. However, the Basel zoologists have now revealed that the fish in fact uses another strategy: “It appears that scale-eaters feed on a wide range of fish and almost never attack the ones they resemble,” explains Nicolas Boileau, the study’s lead author.

To get to the bottom of this sophisticated tactic, the researchers at Basel University adopted a novel approach. Their study is not based observations of the lightning-quick attacks in the wild, since such attacks are extremely difficult to observe directly; instead, their approach is based on molecular examinations in the lab. At the Zoological Institute of the University of Basel they examined the stomach contents of the blue and white predator and analyzed the genetic fingerprints of the ingested scales. In this way it was possible to unequivocally assign the individual scales to a particular species of prey.

The results reveal the complexity of this so-called “aggressive mimicry”: the scale-eaters are actually imitating several blue and white striped species at once, in order to trick an entire natural community. The leader of the study, Prof. Walter Salzburger, summarizes the findings thus: “The scale-eater pursues the strategy of a wolf that dresses up as a sheep only to then go for goats and cows.”

Original source

N. Boileau, F. Cortesi, B. Egger, M. Muschick, A. Indermaur, A. Theis, H. H. Büscher, W. Salzburger (2015)
A complex mode of aggressive mimicry in a scale-eating cichlid fish.
Biology Letters 20150521, doi: 10.1098/rsbl.2015.0521


Further information

Prof. Walter Salzburger, University of Basel, Department of Environmental Sciences, Zoology, tel. +41 61 267 03 03, email: walter.salzburger@unibas.ch

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