Intraspecific variation in the pollution indicator polychaetePolydora ligni(Spionidae)

Abstract

The extent of intraspecific variation in the pollution indicator Polydora ligni Webster was assessed on evidence from morphology, reproductive biology, physiological response, and population genetics. Five populations from Florida were compared morphologically and genetically; three of them were also analysed for differences in reproduction and physiological response. Morphological differences were observed between populations with respect to the setae of the fifth setiger and the presence of the nuchal antenna, both important taxonomic characteristics. Significant differences were observed in gametic distribution in mature individuals, egg size, and spermatophore morphology. Interpopulational crosses suggested the possibility of incipient reproductive isolation between some of the populations. Physiological responses, expressed as survival and growth rate, were compared between populations using a central composite factorial design experiment consisting of three levels each of temperature, salinity, and dissolved oxygen. Experimental results were analysed using response surface methodology and revealed highly significant differences in both five-day survival and growth rates between populations. Gene frequencies were determined by use of horizontal starch gel electrophoresis. Standard genetic distance, based upon ten loci, was not significantly different between most populations. However, one population varied from all others at a level corresponding to sibling species. For all populations pooled, 90 % of all loci were polymorphic. The evidence indicates that this cosmopolitan pollution indicator has undergone considerable divergence throughout much of its range and may require separation into. discrete species.