. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 20 M. A. SEWELL ET AL YGM N YEAR N+1 N+2 Germ eel! in basis Primary tubules Secondary tubules Fecund tubules Resorbed tubules Figure 2. Schematic diagram to show progressive recruitment of tubules in the ovary of Paraslichapus californium. The gonad morphology is shown for three consecutive years: N, N+l, N+2. In each year the gonad has three cohorts of tubules (YGM = year-round gonad morphology), as well as primary germ cells at the very anterior and the remains of resorhed tubules at the posterior end of the gonad basis. T
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 20 M. A. SEWELL ET AL YGM N YEAR N+1 N+2 Germ eel! in basis Primary tubules Secondary tubules Fecund tubules Resorbed tubules Figure 2. Schematic diagram to show progressive recruitment of tubules in the ovary of Paraslichapus californium. The gonad morphology is shown for three consecutive years: N, N+l, N+2. In each year the gonad has three cohorts of tubules (YGM = year-round gonad morphology), as well as primary germ cells at the very anterior and the remains of resorhed tubules at the posterior end of the gonad basis. Tubule cohorts derived from the same primary germ cells are shown with the same pattern (open, hatched, filled, etc.). The germ cells of year N migrate on the gonad basis (M) to become the primary tubules of year N + 1. Oogomal proliferation (P) in this year results in secondary' tubules in Year N+2. Vitellogenesis (V) in the secondary tubules results in fecund tubules whose gametes are spawned (S) and the empty tubules resorbed in Year N + 3. Consequently, the gametogenic process in P. californicus from primary germ cells to spawn- ing takes 4 Evidence from Gonad Morphology Order Aspidochirotida A priori we might expect that the strongest support for the tubule recruitment model would be found in as- pidochirote holothurians. considering that the model was based on Parastichopus californicus (Family Sticho- podidae). The model can be tested in many aspidochi- rotes because of reproductive studies prompted by their importance as beche-de-mer fisheries (Conand and Byrne, 1993). In holothurians of the family Stichopodidae (30 spe- cies; Smiley, 1994) the gonad is present as two tufts of tubules, one on each side of the dorsal mesentery. Al- though tubule morphology varies considerably in species of this family, none of the other species examined are described as having distinct size classes of tubules on the gonad basis (Stichopus variegatits—Conand, 1993a; S. mollis—S
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