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SUMMARY Lake Malawi (LM) cichlids have undergone heterochronic shifts in the expression of their cone opsin genes, the genes responsible for color vision. These shifts have generated species with short-, middle-, and long-wavelength-sensitive cone photoreceptors and visual systems. However, it is unclear when during the evolution of African cichlids these shifts occurred, or whether they could account for similar short- and middle-wavelength-sensitive profiles among unrelated cichlids in Lake Tanganyika (LT). To address these questions, we surveyed opsin expression in developing fry of two African cichlids, Astatotilapia burtoni from LT and Melanochromis auratus from LM. We found that A. burtoni expresses a series of three different single-cone opsins over the course of development, while M. auratus exhibits variation in the expression of only two. Neither A. burtoni nor M. auratus exhibits much variation in the expression of its double-cone opsins. These patterns reveal that A. burtoni exhibits progressive development in the sensitivity of its single-cone photoreceptors, but direct development in the sensitivity of its double-cone photoreceptors. M. auratus exhibits neotenic development in the sensitivity of both photoreceptor sets. Given the intermediate phylogenetic placement of A. burtoni between cichlids from LT and LM, our results suggest that the ancestor of LM's cichlids exhibited a progressive developmental pattern of opsin expression. These results indicate that the heterochronic shifts which produced the short- and middle-wavelength-sensitive profiles of LM's cichlids occurred recently, and suggest that the presence of similar profiles among LT's cichlids are due to parallel heterochronic shifts.

Migration rates among nine populations of the endemic Lake Malawi cichlid Melanochromis auratus were estimated along a 42-km stretch of habitat in the southern end of the lake. Allele frequencies were surveyed at four simple sequence repeat (SSR) loci. The data suggest migration rates among populations are quite low. Exact tests indicate that statistically detectable allele frequency differences exist between many adjacent populations in the study. The F ST value among all populations was estimated to be 0.151 ( P < 0.0002). A biogeographic survey suggests that the highest levels of genetic differentiation exist between populations separated by stretches of deep water. Migration is more common between populations separated by shallower water or with shoreline dispersal routes. Reduced allelic diversity was observed at more recently created habitat patches, suggesting that either bottlenecks are associated with the colonization of new habitat patches or that these shallower sites were all founded by genetically depauperate ancestral populations. The extreme philopatry of M. auratus , coupled with the patchy distribution and transient nature of its preferred habitat, provides opportunities for both selection and genetic drift to produce genetic differentiation among populations. Both processes may be important to the evolution of taxonomic diversity in the East African cichlid species flocks.

The cichlid Melanochromis exasperatus has a natural distribution which is restricted to a single island in Lake Malawi, Africa. Diving observations indicated that it does not occur below 4.5 m depth. Measurements of depth equilibration in a pressure tank showed that males can equilibrate to a depth of 25 m and females to 18 m. Mean maximum sustained rate of depth equilibration was 2 m day−1. M. exasperatus survived a pressure decrease of 40% every 12 h from the maximum equilibration depth to the surface. The fish swim actively during the day but rest on the bottom at night when they are equilibrated to a depth 1 m deeper than ambient. Reduced activity at night is reflected in a drop in oxygen consumption to nearly half of the daytime active value.