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Genyochromis mento - A Wolf In Sheep's Clothing?
|By Ad Konings, 1998.|
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(This article was originally published in Cichlid News Magazine, Jan-98 pp. 18-21, It is reproduced here with the permission of author Ad Konings and Aquatic promotions).
An OB female Pseudotropheus elegans (left), a sand dwelling species from Nkhata bay, and its mimicking Genyochromis mento (right) from Chadagha, north of Nkahata bay. Photos by Ad Konings.
One of the most widespread cichlids of Lake Malawi is the well-known fin-biter and scale-eater, Genyochromis mento. Although its formal description - by the late Ethelwynn Trewavas in 1935 - is relatively recent compared to other common mbuna, it can easily be recognized by collectors in the field by its comparatively prominent lower jaw ... a fortunate occurrence because Genyochromis mento will not hesitate to attack other fishes when placed together in a drum for transport and shipping!
Genyochromis mento is an active and skillful predator that does not appear to have a particular preference for prey species; instead it attacks all types of cichlids in a manner which at first glance seems to be random. Typically Genyochromis mento takes up residence in a biotope and just "hangs out," usually positioning itself near the substrate and allowing other fishes to pass by overhead. When a potential victim comes within striking distance, Genyochromis mento makes a lightning-fast strike and either snaps off a mouthful of finnage or (less frequently) grabs a mouthful of scales from the flank or peduncle of the victim. After the attack, the predator returns to its former position as though nothing has happened. Often the speed of the attack is such that the victim appears not to have a clue as to what has hit him! However, within a relatively short time, resident fish seem to learn to avoid an area where such attacks occur. Many mbuna species have a home area within which an individual spends much of its time, even displaying a regular pattern of movement through the area. Such patterns favor the feeding strategy of Genyochromis mento in that by simply shifting his position by a few feet, the predator moves away from the previous attack zone (now avoided by the biotope's residents) into an area that fish are moving freely through. Not surprisingly, conspicuously colored, territorial male mbuna are attacked at a greater frequency - as evidenced by the condition of their finnage - than reclusive brooding females or non-breeding individuals, which tend to form schools that hover or move randomly above the substrate.
The feeding strategy just described is most often seen in adult females and sub adults of both sexes of Genyochromis mento. Large, adult males often employ a different technique. I have often witnessed such individuals respond to territorial disputes and fights between two males of other cichlid species by "sneak attacks" upon the dueling males. Indeed, large male Genyochromis mento are often observed cruising at high speeds (sometimes too quickly for a diver to follow) through habitats alert to the occurrence of "feuding" neighbors; whenever a fight is happened upon, it quickly launches an attack on the distracted participants. While the combatants are engaged, they are fair game for a male Genyochromis mento, who attacks both, biting chunks from their erect fins. Amazingly, it appears as though the fighting males can't figure out that a third "interloper" has entered the fray. Rather, each acts as though the other is the source of the heightened molestation and responds by displaying even more vigorously (which in turn provides even easier targets for the sneaky Genyochromis mento!).
On other occasions I have observed male mbuna displaying to females in an attempt to lead them to spawning sites, interactions that again attract the ubiquitous fin-biter. As such displays usually include erect, brightly-marked fins, courting males are again a prime target for Genyochromis mento. Indeed when one swims through most rocky habitats in the lake, you will observe that almost all territorial males have notched fins, many of which are testimony to the successful foraging strategy of Genyochromis mento.
Victims are usually mbuna, but larger non-mbuna are also attacked, especially species that hang out in rocky areas within which Genyochromis mento can hide. On one occasion I witnessed an individual Genyochromis mento repeatedly attacking two male Oreochromis lidole which were each three times the length and at least fifty times the biomass of their attacker. The male Genyochromis mento went for the scales on the caudal peduncles of the tilapias which it was unable to dislodge because the scales were larger than the predator's head! Fin-biting may have evolved from the aggressive behavior known as tail-chasing. Species of the genus Melanochromis are particularly inclined to this behavior, which involves two males (less frequently two females) simultaneously attempting to bite each other's flank and at the same time avoid being bitten by the other. The result is a brisk circling by the two opponents with little if any injury to either. Somewhere in Genyochromis mento's evolution, tail-chasing behavior may have escalated into feeding from an opponent's caudal peduncle.
The peduncle is also the preferred site of attack of other scale-eating species, such as Corematodus taeniatus and C. shiranus. It is, of course, also the site farthest away from the mouth of the victim, i.e., from possible retaliation.
Extremely common, Genyochromis mento is found in all habitats except the open water column and at all localities around the lake. The species occurs as many variants, which are not necessarily geographically-based. Besides olive-green, yellowish-grey, black, and brown forms, there are also bright yellow, orange, and OB (=orange-blotched) morphs. The most common pattern is medium brown with darker bars, not unlike females of many other mbuna species.
Genyochromis mento seems to adopt a dark coloration when hunting in rocky habitats so as to blend in with the substrate. It is also found in sandy habitats where it seems to mimic the silvery coloration of many other species found there. I believe that to some extent each individual Genyochromis mento can alter its coloration to match its background. The pattern expressed is dependant upon the type of habitat it is in, which suggests that coloration has a camouflaging effect. But the situation in Genyochromis mento goes further than this. In order to enhance its abilities to "blend in with the crowd" it can also mimic the coloration of some species of the local community, usually members of the Pseudotropheus elongates complex, but also those of the genus Labeotropheus. I don't think that these color variants are subject to change within a given individual; instead offspring of Genyochromis mento with aberrant, genetically-based color patterns (different from their parents but similar to that of a local Pseudotropheus Elongates population) have better chances of approaching victims, and thus of feeding successfully (and ultimately reproducing successfully and increasing in frequency within the population).
At several locales around the lake I have found Genyochromis mento mimicking local Pseudotropheus elongatus species. For example, at Chinyankwazi Island, home of the "Dinghani" (=Pseudotropheus flavus), many Genyochromis mento have a yellow cast to the body. Potential victims can be deceived by the similarity, allowing the Genyochromis mento the split second timing it requires for a successful approach and attack. At the same site, there is also a solid-black elongatus (Pseudotropheus ater); as you may have guessed, there are also solid-black individuals of Genyochromis mento. Similarly, at Mbenji Island, you can find a yellowish Genyochromis mento that mimics Pseudotropheus sp. "lime Nkhomo" which can be abundant in some areas around the island. I don't believe that such examples of "mimicry" are meant to provide Genyochromis mento with a better chance of feeding on the model species; instead, they allow the mimics to deceive other species into perceiving Genyochromis mento as a harmless herbivorous co-habitant of the area, rather than the fin-biting predator it really is.
Pseudotropheus ater, a member of the Pseudotropheus elongatus complex around Chinyamwezi island to the left, the mimicking Genyochromis mento to the right, notice the teeth! Photos by Ad Konings.
The situation gets even wilder. At some locations you find very conspicuously colored Genyochromis mento that would seem to refute any ideas about camouflage. At Mpanga Rocks I have found several bright yellow individuals of Genyochromis mento and at many places OB-morphs of Genyochromis mento occur. How can these brightly colored individuals exist in populations wherein other members mimic more cryptically marked species?
An OB female Metriaclima zebra from Nkhata bay, and its mimicking Genyochromis mento. Photos by Ad Konings.
Closer examination of each individual situation reveals an explanation. For example, near Masinje OB individuals of Metriaclima zebra have relatively large black blotches on an orange ground color. The rare OB morph of Genyochromis mento found at Masinje also has large black blotches. At Nkhata Bay the black markings of female OB zebras are smaller; the occasional OB Genyochromis mento you observe there also has smaller blotches. At Minos Reef in Mozambique I have seen a blotched Genyochromis mento with a bright orange body color matching in every detail that of the local OB morph of Pseudotropheus estherae. And the yellow (non-blotched) morph of Genyochromis mento found at Mpanga Rocks resembles the local yellow morph of Labeotropheus trewavasae. So again we see that these conspicuous Genyochromis mento forms can survive by the grace of the presence of similar forms of other harmless species.
An OB female Pseudotropheus estherae from Minos Reef, Mozambique, and its mimicking Genyochromis mento. Photos by Ad Konings.
I've described a similar situation earlier for the scale-eater Plecodus straeleni and its model Cyphotilapia frontosa in Lake Tanganyika (Konings, 1992). It seems though that Genyochromis mento not only mimics several different species, but also "tracks" their geographical variants! I just hope that nobody describes the variants of Genyochromis mento as different species, because that would surely nullify such a remarkable example of adaptation.
© Copyright 1998 Ad Konings, all rights reserved
Konings, Ad. (July 03, 1999). "Genyochromis mento - A Wolf In Sheep's Clothing?". The Cichlid Room Companion. Retrieved on May 23, 2013, from: http://www.cichlidae.com/article.php?id=118.