CHARACTERIZATION OF SPERMATOZOON ULTRASTRUCTURE IN TETRAGONOCEPHALUM SP. (CESTODA: LECANICEPHALIDEA: TETRAGONOCEPHALIDAE) FROM THE WHIPRAY, UROGYMNUS ASPERRIMUS 1 (DASYATIDAE: UROGYMNINAE).

The Lecanicephalidea represents one of 11 currently recognized elasmobranch-hosted tapeworm lineages. It is quite speciose, with nearly 100 validly recognized species to date. Yet spermatozoon features have been fully characterized for only a single species; spermatozoon characters for a second species were previously included in a data matrix used to infer phylogenetic relationships among cestode orders, but data are limited and no images were provided. Specimens of Tetragonocephalum sp. were collected from the whipray, Urogymnus asperrimus 1, from the Solomon Sea off the Solomon Islands. The mature spermatozoa of Tetragonocephalum sp. are distinctly different from the other lecanicephalidean species for which spermatozoon ultrastructure has been thoroughly investigated. Tetragonocephalum sp. spermatozoa represent the Type IV (sensu Levron, 2010) morphology possessing a single axoneme, crested bodies, cortical microtubules running parallel to the axoneme, and nucleus, which is helical to the axoneme. Although spermatozoa for both lecanicephalideans examined to date are of the Type IV spermatozoon morphology, they differ in the number of crested bodies and cortical microtubules, the relative size of the spermatozoon-specifically as it relates to the nuclear region-and the presence or absence of the anterior spiral structure. Significant spermatozoon character variation is present across these 2 representatives of 2 lecanicephalidean families, indicating that additional studies of representatives of the other families in the order are needed to better understand the extent of character variation present in the Lecanicephalidea. Overall, spermatozoon ultrastructure is understudied in elasmobranch-hosted tapeworm lineages compared to other vertebrate-hosted tapeworm orders at both the species and family levels.