Morphological responses of macrobenthic polychaetes to low oxygen on the Oman continental slope, NW Arabian Sea

Morphological adaptation to low dissolved oxygen consisting of enlarged res piratory surface area is described in polychaete species belonging to the f amily Spionidae from the Oman margin where the oxygen minimum zone impinges on the continental slope. Similar adaptation is suggested for species in t he family Cossuridae. Such morphological adaptation apparently has not been previously recorded among polychaetes living in hypoxic conditions. The re sponse consists of enlargement in size and branching of the branchiae relat ive to similar species living in normal levels of dissolved oxygen. Specime ns were examined in benthic samples from different depths along a transect through the oxygen minimum zone. There was a highly significant trend shown to increasing respiratory area relative to body size in two undescribed sp ionid species with decreasing depth and oxygen within the OMZ, Yet the size and number of branchiae are often used as taxonomic characters. These with in-species differences in size and number of branchiae may be a direct resp onse by the phenotype to intensity of hypoxia. The alternative explanations are that they either reflect a pattern of differential post-settlement sel ection among a highly variable genotype, or represent early genetic differe ntiation among depth-isolated sub-populations. (C) 1999 Elsevier Science Lt d. All rights reserved.