Sperm Ultrastructure of the Spider Crab

18 This work describes the morphology of the sperm cell of Maja brachydactyla, with emphasis on 19 localizing actin and tubulin. The spermatozoon of M. brachydactyla is similar in appearance and 20 organization to other brachyuran spermatozoa. The spermatozoon is a globular cell composed of 21 a central acrosome, which is surrounded by a thin layer of cytoplasm, and a cup-shaped nucleus 22 with four radiating lateral arms. The acrosome is a subspheroidal vesicle composed of three 23 concentric zones surrounded by a capsule. The acrosome is apically covered by an operculum. 24 The perforatorium penetrates the center of the acrosome and has granular material partially 25 composed of actin. The cytoplasm contains one centriole in the subacrosomal region. A 26 cytoplasmic ring encircles the acrosome in the subapical region of the cell and contains the 27 structures-organelles complex (SO-complex), which is composed of a membrane system, 28 mitochondria with few cristae, and microtubules. In the nucleus, slightly condensed chromatin 29 extends along the lateral arms, in which no microtubules have been observed. Chromatin fibers 30 aggregate in certain areas and are often associated to the SO-complex. During the acrosomal 31 reaction, the acrosome could provide support for the penetration of the sperm nucleus, the SO32 complex could serve as an anchor point for chromatin, and the lateral arms could play an 33 important role triggering the acrosomal reaction, while slightly decondensed chromatin may be 34 necessary for the deformation of the nucleus. 35