Evolutionary biology: evidence for sympatric speciation?

Sympatric speciation is difficult to demonstrate in nature and remains a hotly debated issue. Barluenga et al. present a case of putative sympatric speciation for two cichlid species in the Nicaraguan crater lake Apoyo, but they overlook or reinterpret some key published information on the system.

The schistosome intermediate host, Bulinus nyassanus, is a 'preferred' food for the cichlid fish, Trematocranus placodon, at Cape Maclear, Lake Malawi.

Over the last decade, transmission of the schistosome parasites of humans has increased in parts of Lake Malawi, possibly because over-fishing has led to a decline in the numbers of molluscivorous fish. The stomach contents of 51 wild-caught Trematocranus placodon from the Cape Maclear area of Lake Malawi have now been investigated and compared with the field abundance of snail species at two sites, close to where the fish were caught. The snails found in the fish stomachs were identified to genus or species on the basis of their shell fragments and/or opercula.

Mercury in fish from two Nicaraguan lakes: a recommendation for increased monitoring of fish for international commerce.

We measured total mercury concentrations in water and fish of Lake Managua and Lake Apoyo. Water mercury concentrations were 10-fold higher in Lake Managua than in Lake Apoyo, although differences in mercury concentration in the most common native fish were not significant. One-fourth of the commercially fished tilapia in Lake Managua exceeded maximum recommended mercury levels for consumption among pregnant women and other at-risk groups, although bioavailability to fishes was lower than in previously studied sites in Brazil and Western Maryland.

Evolution of NADH dehydrogenase subunit 2 in east African cichlid fish.

The complete sequence (1047 bp) of the mitochondrially encoded ND2 gene was obtained from 31 species of cichlid fishes to investigate the evolutionary history of the species flocks of the East African lakes. The observed pattern of nucleotide substitution in this sequence is typical of mitochondrial genes, showing a high transition bias and rapid mutational saturation, especially at codon positions where base frequencies are unequal. The base composition of the third position of codons is heterogeneous among species, suggesting frequent shifts in the pattern of substitution.

Similar morphologies of cichlid fish in Lakes Tanganyika and Malawi are due to convergence.

The species flocks of cichlid fishes in the lakes of East Africa are the most spectacular example of adaptive radiation among living vertebrates. Similar highly derived morphologies are found among species in different lakes. These similarities have been variously interpreted either as evidence for migration of ancestral species between the lakes, or of striking convergence of morphology.