This study presents details of the ultrastructure of the spermatozeugmata and spermatozoa of the South American catfish, Trachelyopterus lucenai, an inseminating species. Spermatozoa are tightly packaged into unencapsulated bundles, spermatozeugmata, with as many as 130 cells comprising a single packet. Within an individual spermatozeugma, cells are aligned side by side such that the anterior region of the bundle is comprised of nuclei, the middle region midpieces, and the posterior region flagella. Integrity of the packet appears to be maintained by specialized junctions between adjacent cells along their entire lengths. Each spermatozoon is characterized by an elongate nucleus containing condensed chromatin and a fossa at the posterior end. The elongate midpiece contains multiple mitochondria, abundant glycogen deposits, and accessory microtubules that run along the length of the midpiece. The ultrastructural modifications of the spermatozoa are discussed as possible adaptations associated with the reproductive mode of insemination.

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