Eye Morphometrics of Cave and Spring Populations of Gammarus minus (Amphipoda: Gammaridae)

Abstract

Number of ommatidia, total eye area, and head length were measured for the amphipod Gammarus minus Say, 1818, from four karst drainages in southern West Virginia. Data were analyzed using the allometric equation y = axb. For spring populations, eye area showed positive allometry for all populations (b = 1.55), while numbers of ommatidia showed a variety of patterns, ranging from negative allometry to positive allometry. Compared to spring populations from the same basin, cave populations showed reduced eye area and numbers of ommatidia (both small and large individuals). There were no consistent differences in allometric coefficients (b) between cave and spring populations. Cave populations also differed among themselves, both in eye area and numbers of ommatidia. In one population (Organ Cave), the numbers of ommatidia showed a statistically significant decrease with increasing size of the animal. Some populations showed unimodal and other populations showed bimodal distributions of numbers of ommatidia.