Congruent geographic variation in saccular otolith shape across multiple species of African cichlids

The otoliths of teleost fishes exhibit a great deal of inter- and
intra-species shape variation. The ecomorphology of the saccular otolith
is often studied by comparing its shape across species and populations
inhabiting a range of environments. However, formal tests are often
lacking to examine how closely variation in otolith shape follows the
genetic drift of a neutral trait. Here, we examine patterns of saccular
otolith shape variation in four species of African cichlid fishes, each
sampled from three field sites. All four species showed the greatest level
of otolith shape variation along two principal component axes, one
pertaining to otolith height and another to the prominence of an anterior
notch. Fish collected from the same site possessed similarities in
saccular otolith shape relative to fish from other sites, and these
‘site-difference’ signatures were consistent across species and observable
in both sexes. Sex-differences in saccular otolith shape differed in
magnitude from site to site. Population differences in saccular otolith
shape did not covary with neutral genetic differentiation between those
populations. Otolith height, in particular, displayed large site
similarities across species, weak correlation with neutral genetic
variation, and strong sex differences, collectively suggesting that
otolith shape represents a selectively non-neutral trait.