ズワイガニに関する漁業生物学的研究-IV

Abstract

This paper deals with the duration of the planktonic larval stages of the Tanner Crab, Chionoecetes opilio, estimated from rearing experiments of the larvae at different temperatures. Ovigerous female crabs were collected in moter trawl net which was operated in Wakasa Bay in the Japan Sea on January 11, 1969. The specimens were kept in a tank with approximately 500 liters of sea water at 7-13°C. The larvae hatched on January 27, 1969. As food, rotifers and newly hatched brine shrimp nauplii were given during the first and second stages of the larvae, and grown brine shrimp of the size from 5 to 10mm in total length during the megalopa stage. The rearing of the larvae was attempted at six different grades of temperature, ranging from 4 to 20°C, and the water temperature was regulated by the use of cold water and a heater on one or each side of the containers (Fig. 1). The highest temperature to keep the larvae alive for a long time was about 18°C. It was found that between the temperatures of 4 and 18°C, the warmer the water the shorter becomes the duration for one molting stage. The following relationship was observed between the number of days required for one molting (y) and the mean temperature of the water used for rearing (x); y=300.89/x1.1117 in the first stage zoea, y=367.45/x1.1619 in the second stage zoea. The integral temperatures for an intermolt were found to be as follows; 215-255 day•°C in the first stage zoea, 217-299 day·°C in the second stage zoea and 262-442 day•°C in the megalopa stage (Table 1). In a suspected duration of hatching season for this species of crab, the water temperature throughout the surface to the depth of 150m in the fishing grounds of Wakasa Bay was 11-13°C (Fig. 3). It was estimated from the relations above that the total duration of the planktonic larval stages in this stratum may be 63-66 days. In other words, the duration might be limited to within three months even though the larvae migrate up from or down to a stratum of cold water deeper than 150m when they rise to the surface in the first stage zoea, or when they sink to the sea-bottom in the megalopa stage or first adult type stage. As the carapace width (2.6-2.9mm) and the external appearance in the first molting stage in the specimens of adult form obtained in these experiments are nearly the same as those reported in the previous paper, it is considered that those reported previously were also specimens equivalent to this first molting stage (Fig. 2).