No abstract

The monthly variation in reproductive cycle, condition factor (fatness), gonad index (GI), ovarian egg diameter and biochemical composition [RNA, DNA content and their ratio RNA/DNA (R/D)] were compared to determine the spawning season of the purplish Washington clam, Saxidomus purpuratus, collected from the west sea of Taean, Korea. All these factors were correlated to the spawning season from May to October. Monthly variation and concentration of the R/D ratio especially represented indicative information about the processes of gonad development and spawning patterns. The increases in R/D ratio in the ovaries corresponded with increases in ovarian egg diameters and the GI during the spawning season. The peak of R/D and RNA content in females was a good indicator of sexual maturation and the DNA content in males. During winter between November and January, the value of fatness, GI, egg size, RNA and DNA content were lower than in other seasons, indicating depletion of energy reserves and lower metabolic demands because of colder temperatures.

An experiment to induce maturation and spawning of Portunus trituberculatus by manipulating seawater temperature, photoperiod, and eyestalk ablation, and to study the effect these factors have on the variation of nucleic acid content in gonadal tissue, was conducted. Non-incubating adult females were divided among three temperature treatments (natural, 10 • C, and 20 • C) and two photoperiod treatments (15 h and 9 h daylight periods per day, respectively). Within each temperature-photoperiod combination, subjects were further divided into each of two groups, i.e., unilaterally eyestalk ablated and unablated. The gonadosomatic index, spawning rate, and nucleic acid variation were examined for each treatment at 3-week intervals. Holding ablated females at 20 • C and providing 15 h of light photoperiod had the most profound effect on the induction of ovarian maturation and spawning. Gonad maturation also occurred in the unablated, 20 • C, 15 h light group, although to a lower extent. Gonad maturation did not take place for groups exposed to lower temperatures, despite either ablation or photoperiod treatments.RNA to DNA ratios were strongly correlated to the state of gonad maturation, with the highest values obtained for the ablated groups held at temperatures under 20 • C for the 15 h light photoperiod. Under these conditions, increasing DNA concentration, reflecting gonad differentiation (small cell size and a large number of cells per unit of tissue), began three weeks earlier than for the other treatments. These results suggest that P. trituberculatus requires not only a raised temperature but also another stimulus, such as eyestalk ablation or a long photoperiod, to induce off-season maturation and spawning.