New research on eggspots in Astatotilapia burtoni

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Astatotilapia burtoni was the species used by Wickler in 1962 in formulating his theory regarding the purpose of eggspots. Now scientists have studied the way these spots develop as the young fish grows from fry to adulthood.

Young burtoni were grown on for 14 weeks, both in groups and singly, and regularly measured, weighed, and photographed. 

The photographs were subsequently used to study eggspot development.  The main points of study were the timing of egg-spot development relative to age, size, and weight, and the effect of different rearing conditions on egg-spot ontogeny.

Egg-spot formation starts with a yellowish coloration covering the fin, which then forms blotches that finally become surrounded by an outer transparent circle. 

Four stages of development were recognised. The first was a transparent anal fin with some yellow pigment, particularly on the first three rays.

The second stage involved the appearance of orange pigment, and in the third stage this orange colour aggregated into irregular spots.

Finally, at the fourth stage, fully-developed egg-spots with a transparent surrounding ring became apparent. Interestingly, in all cases there were initially three egg-spots, although this increased later. All males had fully-developed eggspots by a length of 2.5cm/1", though some developed them at a slightly smaller size than others. Females developed egg-spot-like markings much later and these were less conspicuous.

The study suggests that in A. burtoni growth rate and the appearance of eggspots do not depend on age, but instead on the conditions in which the fishes are reared. 

The authors postulate that food intake and/or light conditions may be responsible for the timing of eggspot development.  

The orange colour of the spots comes from carotenoids, which cannot be synthesised by fishes and have to be acquired from food, but it has also previously been shown that light intensity influences egg-spot size in Lake Victoria haplochromine cichlids. 

The authors conclude by suggesting further experiments to clarify this question, and additional similar studies of other haplochromine species.

For further information see:  Heule, C. & W. Salzburger (2011) The ontogenetic development of egg-spots in the haplochromine cichlid fish Astatotilapia burtoni. Journal of Fish Biology (2011) 78, 1588–1593.  doi:10.1111/j.1095-8649.2011.02944.x

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