This study investigated fine-scale population substructuring in an apparently monogamous,
biparental mouth-brooding cichlid. Microsatellite allele frequencies were determined at four
polymorphic loci for nine populations of Eretmodus cyanostictus. We provide empirical
support for the hypothesis that a species employing this breeding strategy should exhibit
high levels of population substructuring. Stretches of sand represent considerable barriers
to dispersal and, in contrast to the rock-dwelling cichlids of Lake Malawi, distance alone,
along a continuous rocky shoreline is sufficient to reduce gene flow significantly. There
was a significant pattern of isolation by distance both along the whole study area and over
the stretch of continuous shoreline suggesting that this species has poor dispersal
capabilities and that juveniles establish territories close to their natal site. Despite
limited dispersal, E. cyanostictus populations are not significantly more inbred than
a more-widely dispersing rock-dwelling cichlid from Lake Malawi. This finding may cast doubt
on the hypothesis that polyandry has evolved as a mechanism for maintaining genetic diversity
in Lake Malawi cichlids. High levels of substructuring may not always promote high levels of
speciation, and other factors such as the intensity of sexual selection, may be more important
in determining the speciation potential of a lineage.