Fine structure of the gametes and spermiogenesis in Spiophanes uschakowi (Annelida: Spionidae) from the Sea of Japan, with comments on fertilization biology in broadcast-spawning spionids

Micron : the International Research and Review Journal for Microscopy
Vasily I RadashevskyYana N Alexandrova

Abstract

Spiophanes uschakowi is a common polychaete living in tubes in sandy sediments in shallow waters of the Sea of Japan. Females and males release their gametes into the water where fertilization and holopelagic, planktotrophic larval development occur. In females, oogenesis is intraovarian: vitellogenesis occurs when the oocytes grow in paired ovaries attached to genital blood vessels in fertile segments. The developed oocytes are accumulated in the coelomic cavity prior to spawning. The newly released oocytes are lentiform, 185-200 μm in diameter, with honey-combed envelopes 5-7 μm thick. Each oocyte has 41-49 cortical alveoli regularly arranged in a peripheral circle, a nucleus 80-83 μm in diameter, and a single nucleolus about 30 μm in diameter. In males, spermatogonia proliferate in testes and the rest of spermatogenesis occurs in the coelomic cavity. During spermiogenesis, the acrosomal vesicle migrates from the posterior to the anterior part of the spermatid. The spermatozoa are ect-aquasperm with a plate-like acrosome 0.58 ± 0.06 μm thick and 2.14 ± 0.13 μm in diameter, barrel-shaped nucleus 2.23 ± 0.13 μm long and 3.18 ± 0.13 μm in diameter, short midpiece 0.93 ± 0.09 μm long with five spherical mitochondria, two centriol...Continue Reading

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