Evidence for poecilogony in Pygospio elegans (Polychaeta:Spionidae)

Abstract

The spionid polychaete Pygospio elegans displays more than one developmental mode. Larvae may develop directly, ingesting nurse eggs while brooded in capsules within the parental tube, or they may hatch early to feed in the plankton before settling. Asexual reproduction by architomic fragmentation also occurs. Geographically separated populations of P. elegans often display different life histories. Such a variable life history within a single species may be interpreted either as evidence of sibling speciation or of reproductive flexibility (poecilogony). Four populations from the English Channel were found to demonstrate differing life histories and were examined for morphological and genetic variability to determine whether P. elegans is in fact a cryptic species complex. Significant but minor inter-population polymorphisms were found in the distribution of branchiae and the extent of spoonlike hooded hooks. These externally polymorphic characters did not vary with relation to life history, and variation fell within the reported range for this species. Cellulose acetate electrophoresis was used to examine 10 allozyme loci, 5 of which were polymorphic. Overall, observed heterozygosity (H o = 0.161) was lower than that expected under Hardy-Weinberg equilibrium (H e = 0.228). Significant heterozygote deficiencies, detected at the Est' and Xdh * loci in all populations (except Xdh * at Ryde Sand, Isle of Wight, UK), are discussed. F-statistics were used to examine patterns of genetic structuring among both separate and pooled populations. F ST values at all polymorphic loci indicated a significant level of genetic differentiation between populations, most probably related to isolation by geographic distance. No direct relationship between life history and genetic structure could be detected. Overall genetic identity among the 4 populations was high (I=0.977 to 0.992). Overall, populations displaying larval brooding did not appear to be reproductively isolated from populations displaying a fully planktonic larval mode. Present data support the hypothesis that P. elegans is poecilogonous.