Figure 3. The reproductive stages of Diadema setosum from the Gulf of Aqaba.

Histological photomicrographs of ovaries (a,c,e,g) and testes (b,d,f,h). Cross-sections through acini representing reproductive stages I–IV. Stage I (spent): Gonads are largely devoid of contents showing ova-free lumen in females (a), and spermatozoan-free lumen in males (b) and may contain unspawned ova and spermatozoa undergoing lysis. A thin layer of NPs is present along the ascinal walls in both sexes and may form a pale meshwork across the ascinus. Strongly basophilic previtellogenetic oocytes or primary spermatocytes, staining dark purple with Hematoxylin and eosin, may be present along the ascinal wall. Stage II (recovering): NPs proliferate throughout the gonads from the ascinal wall to the centre, gradually filling the lumen of ovaries (c) and testis (d). Limited groups of primary spermatocytes and clusters of previtellogenetic oocytes start appearing in the testicular and ovarian germinal epithelia, respectively, and may occasionally project centrally. Stage III (growing): With the onset of vitellogenesis oocytes grow in size and become decreasingly basophilic. Both early and late vitellogenetic oocytes may be present along the ovarian wall and gradually migrate to the ovarian lumen as they mature (indicated by arrow) (e). All stages of germ cells are evident in the male germinal epithelium and continuously increase in number as new spermatogonia develop basally while spermatocytes migrate to the testicular lumen, where they accumulate as mature spermatozoa, forming visible columns of darkly stained cells (f). NP deplete and progressively occupy less space in both males and females. Stage IV (mature): By the end of this stage the NP layer in both ovaries and testes is largely exhausted. Ovaries are packed with mature ova, while oocytes at different maturation stages may still be evident in the germinal epithelium (g). The testicular lumen is densely packed with spermatozoa. Occasionally some ova and spermatozoa may be evident in the coelom (h). Scale bars represent 100 μm. Ge germinal epithelium; C coelom; Po previtellogenetic oocyte; EVo early vitellogenetic oocyte; LVo late vitellogenetic oocyte; NP nutritive phagocytes; Ov ova; L lumen; Sc spermatocytes; Sz spermatozoa; Ps primary spermatocytes.