Ultrastructure of spermatozoa and spermiogenesis inSpirula spirula (L.): systematic importance and comparison with other cephalopods

Abstract

Spermatozoa and spermiogenesis in the deep-water cephalopodSpirula sprirula (L.) are examined using transmission electron microscopy. Mature spermatozoa (taken from spermatophores) are elongate cells 115–120 μm long, composed of a conical acrosomal vesicle, cylindrical nucleus (6.8–7 μm long), flagellum and a loose mitochondrial sleeve — the latter concealing the proximal 6–8 μm of the flagellum. The acrosomal vesicle is 2.8 μm long with fibro-granular contents and an electron-lucent apical zone. Subacrosomal material, organized as closely packed granules, fills a basal invagination of the acrosomal vesicle. In early spermatids the flagellum is derived from a triplet substructure centriole positioned close to the developing nuclear invagination. As flagellum formation proceeds, the acrosomal vesicle (produced evidently through Golgi secretion) attaches to the condensing nucleus. Spermatids are connected by cytoplasmic bridges throughout their development, and exhibit a perinuclear sheath of microtubules from the onset of the fibrous stage of nuclear condensation (mid-, late spermatids). In mid-spermatids, mitochondria collect posterior to the nucleus and subsequently are packed into a cylindrical extension of the plasma membrane to form the periflagellar mitochondrial sleeve. These features of spermiogenesis and mature spermatozoa ofSpirula clearly associate the Spirulidae with the Sepiida, Teuthida and Sepiolida — particularly with the latter order. However, pending results of a thorough review of coleoid sperm morphology, the Spirulidae are here included in their own order — Spirulida (of Reitner & Engeser, 1982) — rather than in either the Sepiida or Sepiolida.