Spermatozoa of the deep-sea cephalopod Vampyroteuthis infernalis Chun: ultrastructure and possible phylogenetic significance

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Abstract
Sperm ultrastructure in the rare deep-sea cephalopod Vampyroteuthis infernails is described, based on formalin-fixed material held in the Australian Museum (Sydney). The species is the sole member of the coleoidean order Vampyromorpha, which represents a level of organization intermediate between that of the Sepioidea-Teuthoidea and the Octopoda. Spermatozoa of Vampyroteuthis , the simplest observed in any cephalopod, exhibit the following features: (1) a spheroidal acrosome lacking any complex substructure; (2) a short (8.5 µm) fusiform nucleus with a deep (2.2-2.5 µm ) basal invagination (containing an extensive plug of dense material); (3) two triplet centrioles arranged parallel to the sperm longitudinal axis; (4) a short (1 µm) midpiece composed of a triangular cluster of mitochondria surrounding the centrioles; and (5) a tail (length 130-135 µm) that is continuous with one of the centrioles (here considered as a 'distal' centriole). An annulus and membranous skirt are absent, though the coarse fibres do fuse into a ring at the tail-midpiece junction). These cells show some resemblance to sperm or spermatids of sepioids and teuthoids (spheroidal acrosome, short nucleus) but are also remarkably similar to mid- spermatids of Octopus (with the exception of the uncondensed nucleus in Octopus spermatids). Sperm morphology supports the current assignment of Vampyroteuthis to a separate coleoidean order - Vampyromorpha - and also suggests that a close link exists between the Vampyromorpha and Octopoda.