Dutch scientists have found that the Lake Victoria cichlid, Haplochromis pyrrhocephalus, a species that nearly went extinct with the upsurge of the introduced Nile perch (Lates niloticus) in the 1980s, is capable of considerable morphological change in twenty years.
Publishing their results in a recent issue of the Biological Journal of the Linnean Society, Frans Witte and co-authors measured fishes collected by bottom trawls in 1977"1981 and 1993"2001. The authors found that the total gill area of the fishes had increased by 64% in the span of two decades. At the same time, head length, eye length, and head volume decreased in size, whereas cheek depth and the depth of the levator posterior muscle (located dorsal to the gills) increased.
Haplochromis pyrrhocephalus, a species that fed on zooplankton, was almost wiped out by the great increase in numbers of introduced Nile perch in Lake Victoria in the 1980s. Intense fishing pressure on the Nile perch by the 1990s alleviated the problem, allowing haplochromine cichlids in the lake (including H. pyrrhocephlaus) to recover (to the point that H. pyrrhocephalus became the most common species in the Gulf of Mwanza in the lake). At the same time, the water clarity and dissolved oxygen levels of the lake had decreased due to increased eutrophication and algal blooms.
The authors conclude that the increase in gill area is a direct response to the hypoxic (low oxygen) conditions created in the lake. At the same time, reductions in eye size and depth of the rostral part of the sternohyoideus muscle, and reallocation of space between the opercular and suspensorial compartments of the head may have permitted accommodation of larger gills in a smaller head. The increase in the depth of the levator posterior muscle is thought to aid the fish in processing larger and tougher prey types (this shift to larger prey is possibly caused by the increased turbidity of the water). The fact that such striking morphological changes in a haplochromine species can arise within as little as two decades after major environmental perturbations is much faster than previously thought, and may have favoured the successful recovery of H. pyrrhocephalus in the perturbed ecosystem of Lake Victoria.
For more information, see the paper: Witte, F, M Welten, M Heemskerk, I van der Stap, L Ham, H Rutjes and J Wanink (2008) Major morphological changes in a Lake Victoria cichlid fish within two decades. Biological Journal of the Linnean Society 94, pp. 41"52.