Abstract:
Adult females andmales of Pleuromamma abdominalis were examined from 26 plankton samples taken at stations in the eastern North Pacific Ocean along two latitudinal gradients, 150 degrees W between 21.55 degrees N and 14.00 degrees S, and 119 degrees W between 17.09 degrees N and 20.00 degrees S. The species was found at all stations except three between 17.9 degrees N and 12.6 degrees N along 119 degrees W. Variation within a sample was observed in the ventral attenuations [= spiniform outgrowths] of the first segment of antenna 1 of females, and specimens referable to P. abdominalis forma edentata could not be distinguished unequivocally from other females. Males with a strongly asymmetrical urosome and re-curved lateral seta on the left caudal ramus, referable to P. abdominalis forma abdominalis, were found, as were males with a quasi-symmetrical urosome and simple lateral seta on the left caudal ramus, referable to P. abdominalis forma abyssalis. Specimens of the latter form were not restricted to deep water but were present in samples taken as shallow as 200 m. Males with a slightly asymmetrical urosome, not as pronounced as that of P. abdominalis forma abdominalis, and with a simple lateral seta on the left caudal ramus, also were found. The degree of asymmetry among this third group of males was variable. There was no discernable pattern in depth or geographical distribution of either female or male morphs. Based on gradations of morphological states traditionally used to diagnose the different forms of P. abdominalis, the species appears to be variable, but the nominal forms should not be considered subspecies. Morphological variation among females and males of P. abdominalis should continue to be registered; in situations where groups of P. abdominalis can be clearly categorized, they should be denoted simply as morphs.
