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Four species of Pterotrachea are currently recognized,
two ofwhich (P. hippocampus Philippi, 1836, and
P. minuta Bonnevie,1920) have very similar
morphologies. These two species havebeen distinguished mainly on the
basis of eye and visceral nucleusshapes; the former with wide,
triangular eyes and a short, broadnucleus, and the latter with
narrower, triangular eyes and ataller, more slender
nucleus. Quantitative and qualitative morphologicaldata were obtained
from specimens of P. hippocampus and P. minuta
collectedduring two oceanographic sampling programs in the North
AtlanticOcean. Comparisons of eye and visceral nucleus shapes
(representedby 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
showedthat the width of the retina increases disproportionately with
bodygrowth (by elongation and medial upturning) beginning at a length
ofabout 21-22 mm. As a result, the overall appearance of the eyeat
this body size changes such that smaller animals (less than21-22 mm)
have eyes corresponding with those of P. minuta, whilethe
eyes of individuals larger than this body length match thoseof
P. hippocampus. Several authors have distinguished femalesof
the two species by the presence (P. hippocampus) or absence
(P.minuta) of cuticular spines anterior to the
eyes. Examinationof female specimens showed that those less than
about 30 mmlacked 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
previousauthors to distinguish the two species were examined and
rejected.Because P. hippocampus was described prior to
P. minuta, itis herein regarded as the senior synonym of
P. minuta. (Received 12 June 1997; accepted 21 May 1999) 相似文献
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