Spermatozoa and sperm aggregates in the vestimentiferan Lamellibrachia luymesi compared with those of Riftia pachyptila (Polychaeta: Siboglinidae: Vestimentifera).

The spermatozoa and the sperm bundles of the vestimentiferans Riftia pachyptila and Lamellibrachia luymesi (Annelida: Siboglinidae) were studied using several microscopical techniques (transmission and scanning electron microscopy, and confocal microscopy) and compared with some other annelid sperm. The spermatozoa and sperm bundles of both species show a similar structure, but they differ in the dimensions of the components of individual cells and in the number of spermatozoa forming each sperm bundle. The spermatozoa of R. pachyptila and L. luymesi are filiform cells composed, in sequence, by an acrosome in the form of a thread-like helical vesicle, an elongated coiled nucleus surrounded by two helical mitochondria, and a long flagellum. In the spermatozoa of both species, the apical portion of the nucleus is completely devoid of chromatin and is delimited by a thickened nuclear envelope with a fibrillar appearance. Both species have sperm bundles that resemble buds, having a calyx-like portion formed by the helical heads, and a stalk-like portion formed by the tightly packed flagella. A parsimony analysis based on spermatozoal characters showed monophyly of the Siboglinidae and the Vestimentifera. We propose a new set of autapomorphies characterizing vestimentiferan spermatozoa. Our analysis suggests that spermatozoal characters are useful to the understanding of the phylogeny of the group.