SPECIES DISCRIMINATION AMONG PELAGIC HETEROPODS: RESOLUTION OF THE PTEROTRACHEA HIPPOCAMPUS--P. MINUTA PROBLEM

Abstract

Four species of Pterotrachea are currently recognized, two of which (P. hippocampus Philippi, 1836, and P. minuta Bonnevie, 1920) have very similar morphologies. These two species have been distinguished mainly on the basis of eye and visceral nucleus shapes; the former with wide, triangular eyes and a short, broad nucleus, and the latter with narrower, triangular eyes and a taller, more slender nucleus. Quantitative and qualitative morphological data were obtained from specimens of P. hippocampus and P. minuta collected during two oceanographic sampling programs in the North Atlantic Ocean. Comparisons of eye and visceral nucleus shapes (represented by their length to width and length to retinal width ratios, respectively), plotted against body length showed linear decreases, with no justification for the recognition of two separate species. Examination of eye shape across a wide range of body sizes showed that the width of the retina increases disproportionately with body growth (by elongation and medial upturning) beginning at a length of about 21‐22 mm. As a result, the overall appearance of the eye at this body size changes such that smaller animals (less than 21‐22 mm) have eyes corresponding with those of P. minuta, while the eyes of individuals larger than this body length match those of P. hippocampus. Several authors have distinguished females of the two species by the presence (P. hippocampus) or absence (P. minuta) of cuticular spines anterior to the eyes. Examination of female specimens showed that those less than about 30 mm lacked these spines, while those above this size possessed them. Thus, cuticular spines represent a secondary female sexual characteristic. Other morphological features that have been used by previous authors to distinguish the two species were examined and rejected. Because P. hippocampus was described prior to P. minuta, it is herein regarded as the senior synonym of P. minuta.