Tropheus differences all in the head

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Differences in the head shapes of Tropheus morphs are likely to be adaptations to female mouthbrooding and population-specific foraging strategies, according to the results of a new study.

The Tropheus genus currently contains six species - moorii, brichardi, annectens, polli, duboisi and kasabae - which exist in at least 120 different colour morphs.

While Tropheus are broadly similar in overall appearance, and their morphology has been described as "relatively static", they do differ in shape between species, populations and the sexes.

Scientists from the University of Graz in Austria used geometric techniques to measure the shapes of six populations of Tropheus moorii and a population of Tropheus polli to determine the extent of the differences.

The results, which have just been published in the journal Frontiers in Zoology, show that mean shapes differed significantly between species, populations and sexes.

Populations and species differed mainly in the position of the mouth and the ventral portion of the head, while the sexes differed in head shape.

Females, which mouthbrood small clutches of large eggs, have a larger head and a much larger buccal area, which allows them to hold the eggs more comfortably. Different Tropheus populations differed mainly in the cranial region.

The study says that the differences found are most likely to be due to adaptations which evolved to allow mouthbrooding and for differences in feeding modes between populations.

For more information see the paper: Herler J, Kerschbaumer M, Mitteroecker P, Postl L, Sturmbauer C. (2010) - Sexual dimorphism and population divergence in the Lake Tanganyika cichlid fish genus Tropheus. Front Zool. 2010 Jan 22;7:4.