THE EFFECTS OF MANIPULATING WATER TEMPERATURE, PHOTOPERIOD, AND EYESTALK ABLATION ON GONAD MATURATION OF THE SWIMMING CRAB, PORTUNUS TRITUBERCULATUS

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 degrees C, and 20 degrees 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 degrees 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 degrees 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 degrees 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.