Scientists from the University of Basel have discovered the evolutionary origin of egg spots in haplochromine cichlids.
Bernd Egger and coauthors report in a recent issue of the online journal PLoS ONE that female haplochromine cichlids exhibit a pre-existing sensory bias for egg-shaped structures and egg coloration (red, yellow and orange), which may have been enough to trigger the evolution of egg spots in the anal fins of the male fish.
Egg spots or egg dummies are round or oval-shaped markings found on the anal fins in males of approximately 1500 haplochromine cichlid species. The markings consist of a conspicuous yellow, orange, or reddish inner circle and a transparent outer ring.
Given that these markings resemble real fish eggs, it has been proposed that they mimic freshly laid eggs in order to attract females and to maximise fertilisation success.
The authors test the hypothesis that the exploitation of a pre-existing bias in female haplochromine cichlids has triggered the evolution of this male trait.
Using the Egyptian mouthbrooder (Pseudocrenilabrus multicolor) as their experimental organism, the authors conducted a series of mate choice experiments to test if the females showed a bias for egg-shaped spots and egg colour. The Egyptian mouthbrooder is a basal (i.e. ‘primitive’) haplochromine whose males do not possess egg spots.
In the experiments, female fish were given a choice of two male fish presented in the form of computer-animated images. The time spent by the female fish in front of each image as well as the time spent in following the image were measured and used as a proxy for the mating preference of the female fish.
The authors first gave female mouthbrooders a choice between a male mouthbrooder and a male Burton’s mouthbrooder, Astatotilapia burtoni (a species with egg spots), as a benchmark to test whether the females were able to distinguish heterospecifics from conspecifics. After testing twelve females, they found that focal female cichlids spent significantly more time and interacted significantly more often with the animation showing the conspecific male.
The authors next tested the female cichlids’ preference for male conspecifics with and without the red fringe in the anal fin (the red fringe was digitally removed in the images presented to the female fish). After testing 15 females, they found that the females did not show any preference for the presence of the red fringe in the anal fin of male fish.
Finally, they tested the female cichlids’ preference for male conspecifics with and without egg spots (the egg spots were digitally added to the images presented to the female fish) and found that focal females spent significantly more time in front of and following the image of a male with the artificial egg-spot.
The authors also tested the preference of female haplochromines for egg-spot colour in conspecific males. Testing was carried out in outdoor ponds located on the shore of Lake Tanganyika.
Four haplochromine species with egg spots (Astatotilapia burtoni, Petrochromis polyodon, Tropheus duboisi and T. moorii) and 14 species without egg spots (Altolamprologus calvus, A. compressiceps, Chalinochromis brichardi, Cyphotilapia frontosa, Cyprichromis leptosoma, Julidochromis dickfeldi, J. regani, Neolamprologus sexfasciatus, Ophthalmotilapia nasuta and Xenotilapia papilio) were used. The authors presented these 18 species with a transparent foil placed at the bottom of the pond bearing five conspicuous colour dots (yellow, orange, red, green, and blue) arranged in a pentagonal shape.
The fish were allowed to approach the foil and the number of times they pecked at the dots during a five-minute observation period was counted. The experiment was repeated in the laboratory with three species (Egyptian mouthbrooder, Burton's mouthbrooder and Julidochromis ornatus).
In both sets of experiments, the authors found a marked preference for egg-spot like colours (yellow, orange and red).
Taken together, the experiments suggest that sensory exploitation of a pre-existing bias was responsible for the evolution of anal fin egg-spots in haplochromine cichlids.
For more information, see the paper: Egger B, Y Klaefiger, A Theis and W Salzburger (2011) A sensory bias has triggered the evolution of egg-spots in cichlid fishes. PLoS ONE 6, e25601.
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