PHYLOGENETIC TRENDS IN SPERM TRANSFER AND STORAGE COMPLEXITY IN DECAPOD CRUSTACEANS

Abstract

Possible phylogenetic trends in male sperm transfer and female sperm storage structures are described. Degree of complexity of genitalia is defined as the relative modification or differentiation from a hypothesized primitive condition. It is proposed that the degree of complexity of male gonopods and female sperm storage organs exhibited by a taxon is a measure of phylogenetic distance from the ancestral state. In a male decapod ancestor, the endopod of pleopod 1 is considered to have been an unmodified natatory ramus with pleopod 2 similar but with an appendix masculina. The female of such a hypothesized ancestor received sperm via an external spermatophoric mass; sperm storage in a thelycum or internal spermatheca had not developed. Primitive dendrobranchiate shrimps are characterized by open thelyca, morphologically elaborate external spermatophores, and open petasmata or semiclosed petasmata without terminal funnels or spouts. In the most advanced groups, females have closed thelyca and true spermathecae in which spermatophoric masses are deposited; males have semiclosed petasmata with terminal spouts. Several variations in genitalia between these primitive and advanced extremes are found in penaeoid species. In the Pleocyemata, a trend of increasing insemination complexity can be constructed from the Stenopodidea-Caridea to Astacidea and culminating in the Brachyura. Sperm transfer and storage in both the Palinura and Anomura stand apart from this major pleocyemate trend. The proposed phylogenetic trends in insemination morphology are compared to various schemes of decapod evolution. Best agreement is found with Burkenroad's (1963) hypothesis on decapod phylogeny; the Natantia of Boas (1880) is not supported by a review and analysis of decapod sperm transfer and storage morphology.