4 LINES OF SPERMATID DEVELOPMENT AND DIMORPHIC SPERMATOZOA IN THE SEA-URCHIN ANTHOCIDARIS-CRASSISPINA (ECHINODERMATA, ECHINOIDA)

The process of sperm development in the sea urchin Anthocidaris crassi spina was studied by light and electron microscopy. Similar to other e chinoids studied, a single flagellum, striated rootlet and nuage-like materials were present in spermatogonia of A. crassispina. Spermatocyt es near the diplotene stage showed intracellular localization of the a xoneme which appeared to be a retracted flagellum prior to cell divisi on. Fibrous filaments were associated with a proximal centriole in spe rmatocytes and spermatids and might be involved in movement of the pro ximal centriole. An acrosomal vesicle was developed and a residual bod y was formed in spermatids. The special development patterns in A. cra ssispina attributed to the presence of two patterns of tail developmen t and two patterns of mitochondrial development during spermiogenesis. These four lines of spermiogenesis resulted in the formation of four morphological types of sperm cell, i.e. sperms with: (1) a symmetrical midpiece and posterior tail, (2) an asymmetrical midpiece and posteri or tail, (3) a symmetrical midpiece and bent tail and (4) an asymmetri cal midpiece and bent tail. Sperm cells with bent tails (type 3+4) wer e probably still at the late spermatid stage because results of scanni ng electron microscopy demonstrated gradual detachment and eventual st raightening of the bent tail, and their percentage occurrence in the s perm population decreased significantly (P<0.05) towards the spawning season of A. crassispina. Spermatozoa with a symmetrical midpiece were dominant (averaging 70% occurrence in the sperm population) over thos e with an asymmetrical midpiece. The dimorphic spermatozoa in A. crass ispina (types 1, 2) are both considered to be euspermatozoa as their m orphology is typical for Echinoida.