Shell morphometry of the deep sea protobranch bivalve Ledella pustulosa inthe Rockall Trough, north-east Atlantic

The deep sea protobranch bivalve species Ledella pustulosa recently has bee n described as comprised of four subspecies, differentiated on subtle and q ualitative differences in shell morphology. Two Of these, L. p. pustulosa a nd L. p. marshalli, possess overlapping geographic distributions, L. p. pus tulosa occurring on the continental slope, and L. p. marshalli occurring on the continental rise and at upper abyssal depths. Principal components ana lysis was applied to six samples of Ledella pustulosa from the Rockall Trou gh in order to explore patterns of variation in shell shape and to test whe ther the variability follows a depth-related dine which might be under envi ronmental control. Two of these samples had previously been identified as L . p. pustulosa and L. p. marshalli, respectively. The distributions of shap es at every depth overlapped broadly with the distributions from other dept hs and none of the depths had uniformly distinct shells. However, the mean shapes of the six samples divided into two clusters: a shallow cluster char acterized by a lower dorsal profile (smaller 'shoulders') and a deep cluste r with larger 'shoulders'.: No bathymetric dine in shape was found within t hese clusters. A discriminant function based on training samples of L. p. p ustulosa and L. p. marshalli correctly identified 84% of the shells in the training samples and was applicable over a wide range in shell size. It sho wed that the subspecies differed primarily in the dorsal profile. The discr iminant function was applied to 598 shells from depths of 1632-2900 m in th e Rockall Trough in order to test the hypothesis of depth-related segregati on of the subspecies. Although individual variability was high, the results indicated a gradual transition from L. p. pustulosa dominance at similar t o 2000 m to L. p. marshalli dominance at 3000 m depth, but not marked segre gation. A sample from 1632 m depth with large proportions of both subspecie s disrupted the bathymetric trend and suggested a latitudinal gradient in d istribution. Possible environmental factors, including bottom currents, act ing on a common genotype to cause the observed pattern of differentiation, are also discussed.