Comparative morphology of spermatozoa in Echiura

The ultrastructure of spermatozoa of two echiuran species, Thalassema thalassemum (Pallas, 1766) and Echiurus echiurus (Pallas, 1767), was studied. Comparison of these two species together with all available data on the spermatozoal fine structure in other Echiura displays major differences in cell-size, number of mitochondria, and acrosome composition. The conical, bell-shaped acrosomal vesicle is oblate in E. echiurus, Urechis caupo Fisher and MacGinitie, 1928 and Ikedosoma gogoshimense (Ikeda, 1904), but is apically elongated to different degrees in all other echiurans studied thus far as well as in T. thalassemum. The filiform spermatozoa of the bonelliids Bonellia viridis Rolando, 1821 and Hamingia artica Danielssen and Koren, 1881, have extremely elongate acrosomes and show several other characteristics of their sperm structure as well as general morphology that are unique within Echiura. In contrast to previous descriptions of other species, we discovered a single c-shaped mitochondrion in E. echiurus and a variable number of mitochondria in T. thalassemum, indicating that the number of mitochondria in Echiura is less phylogenetically informative than previously suggested. The intraspecific variability of the number and shape of mitochondria may be explained by incomplete fusion of mitochondria during spermiogenesis.