The ultrastructure of spermatozoa of the Australian freshwater crocodile, Crocodylus johnstoni Krefft, IB73 (Crocodylidae, Reptilia)
B.G.M. JAMIESON, D.M. SCHELTINGA and A.D. TUCKER
Department of Zoology, University of Queensland, St. Lucia, Qld, Australia
SUMMARY Mature spermatozoa of Crocodylus johnstoni are filiform with a curved head, consisting of acrosome complex (length 1.6 µm) and nucleus-(length 10.7 pm); midpiece (length 4.8 pm); and elongate tail (length 49-61 µm). The acrosome vesicle and underlying subacrosomal material form a conical sheath around the anterior tapered nuclear rostrum. One to three endonuclear canals, each containing putative perforatorial material, extend posteriorly from the tip of the nuclear rostrum for approximately 5 pm, nearly half the length of the nucleus. The midpiece is composed of 11 oblique rings of varying regularity, each usually of six mitochondria, surrounding the elongate distal centriole. The mitochondria are subspheroidal to slightly elongate and possess few septate to (more externally) concentric cristae; a central dense body reported for Caiman crocodilus is questionably present. Nine peripheral dense fibres, closely associated with the triplets of the distal centriole, are lost within the anterior principal piece; those at doublets I and 8 are the longest and are separated from their doublets in the principal piece, as in other reptiles. A well defined annulus separates the midpiece from the principal piece. The principal piece consists of a 9+2 axoneme, investing fibrous sheath and plasma membrane and is followed bv the endpiece which lacks the fibrous sheath. The spermatozoon of C. johnstoni is apomorphic relative to Chelonia and Sphenodon in reduction of concentric mitochondrial cristae but is less similar to that of ratites than is that of Caiman crocodilus, differing from ratites in the longer, multiplied perforatoria. The compact dense sheath around the central singlets of the distal centriole is a possible crocodilian autapomorphy.