フィロゾマ幼生に関する海洋生物学的研究
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The studies on the phyllosoma larvae of Palinulid and Scyllarid lobster, crustacean decapod, have been made from various points of view, but their larval development, migration in the sea, and the way how to return for recruitment to their habitat from off-sea, are still left to further research. The purpose of the present paper is to study the phyllosoma larvae, reared or collected, in connection with oceanographic conditions that surrounds them. 1. On the rearing of phyllosoma larva. (1) Phyllosoma of genus Panulirus. As to Panulirus phyllosoma Panulius japonicas, the Japanese spiny lobster, was mainly used. In author's experiments, 12 times moulted larva (13th instar), 6.4 mm in length, was obtained, though the attempt to rear up the Puerulus larva was not successful. The suitable water temperature for phyllosoma is 22°-29°C. For their food were used the nauplii of brine-shrimps. The growth of cultured phyllosoma was nearly constant till 5th instar, and after that it became dull gradually. From these results, it may reasonably be assumed that the growth from 6th till 13th instar is abnormal in some degree. Each ecdyses of Phyllosoma occurs at almost regular intervals, which are apt to widen gradually, and the interval is short at first (7-8 days) and later it becomes gradually prolonged (10-15days). These intervals shows ome fluctuations when their food is changed in quality or in quantity. Usually the intervals become shorter when more or better food is given. (2) Phyllosomas excepting genus Panulirus. The rearing up of phyllosoma of Ibacus ciliates to 4th instar, that of Parribacus antarcticus to 3rd instar and that of Scyllarus bicuspidatus to 9th instar were brought forth successfully. The condition of breeding is nearly the same with Panulirus phyllosoma. 2. On the distribution of phyllosoma in the Indian Ocean. In 1963-64, Kagoshima-maru, attached to the Kagoshima University, engaged in the International Indian Ocean Expedition, carrying on the plankton investigations at the areas of 7°N~26°S and 78°E~98°E. Phyllsoma larvae collected were 340 individuals in all ; that is, 201 of genus Panulirus and 139 of other phyllosomas. Almost all the phyllosomas were collected by surface horizontal towing with 160cm larval net at night. The densities of phyllosoma at the surface were calculated from the time required in towing and from the numbers collected at each station.Average density of phyllosoma per km^2 at the areas of 6°30'N~5°N, 5°N~0°, 0°~5°S, 5°S~10°S, 10°S~15°S, 15°S~20°S, 20°S~25°S were 1700, 3200, 1600, 1200, 100, 24, 0, respectively. The fact that phyllosomas were found in many off-sea stations would mean that waters of almost all areas were previously mixed with coastal waters to some degree. The distribution of phyllosoma disappeared at 15°~17°S, and from this, it may be supposed that there is no active mixture between Indian equatorial waters and Indian central waters. A remarkable change of plankton biomass showing the existence of two different watermasses over there was also observed. It is sometimes not by hydrographic elements such as temperature and chrolinity but by plankton that the oceanographic conditions are made clear. Phyllosoma larva may be useful as an indicator of these oceanographic conditions to confirm the existence of watermasses or currents which are not to be indicated completely by ordinary hydrographic elements. To ascertain how many phyllosomas come back to the coast again is very difficult but important. The average density of the late stage phyllosoma in the Indian Ocean (more than 20mm in length ; which are going to be changed into Puerulus larva in a month or so) was about 1,700 per km^2 at the north of equator. Provided that all the Puerulus larvae that are distributed within 100km from the coast can go back to the coast again, the number of larvae becomes 170,000per 1km coast length. Although on their way to the coast, a considerable decrease in their number can not be avoided, still the returning of enough survived number of the larvae might be expected.
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