Spatial distribution and morphological characters of the eggs and larvae of the slender mola Ranzania laevis from the tropical waters of the western Pacific Ocean

摘要

An ichthyoplankton survey was carried out during October 1978 on board R/V Xiang yanghong 5 in the area 14o00'-17o00'S、171o00'-174o00'E of the western Pacific Ocean. In the investigated area, 9 sampling stations were set up. The abundance of Slender mola Ranzania laevis , eggs and larvae were collected by vertically towing a net (mouth diameter 80 cm, 270 cm in length, mesh size 0.50 mm) from 200 m depth to the sea surface at a speed of 1 m/sec at each station. A total of 27 eggs and 11 larvae of R . laevis were captured respectively . Fish eggs were mainly distributed in the area 15o28.4'-16o17.3'S,172o43.8'-173o29.0'E where the temperature and salinity were 25.72-26.22℃ and 35.10-35.32, respectively. Fish larvae were mainly distributed in the area 15o28.4'-16o17.3'S,172o43.8'-173o29.0'E and 14o29.0'S,171o48.4'E, the temperature and salinity were 25.72-27.02℃ and 34.80-35.50, respectively. Abundances of fish eggs and larvae were 1-14 ind./100m³ and 1-3 ind./100m³, respectively. The distribution areas of fish eggs and larvae were located in the oceanic front. The R . laevis eggs were spherical in shape, 1.40-1.60 mm (1.50±0.054 mm, n = 27) in diameter, and they contained 20 to 30 yellowish oil droplets of 0.06-0.16 mm (0.10±0.064 mm, n = 27) in diameter. The eggs were buoyant, with a transparent, unsculptured egg chorion，and an unsegmented yolk. As the embryonic development processes reached to tail-bud stage, many melanophores had distributed on the dorsal surfaces of head, trunk and yolk sac. 20 pairs clearly distinguishable myotomes and the dermal sac were clearly seen. Before hatching, the dermal sac encased the entire embryo, except tail and pectoral fins. The dermal sac was densely covered with minute vesicles whose function was unknown. Newly hatched larvae possessed a dermal sac, which enclosed the head and trunk. The dermal sac was entirely collapsed at snout to anus length (SAL, the distance from the tip of the snout through the pectoral base to a vertical from the anus) 1.20 mm (1.80 mm TL and 1.68 mm BT). The body spines begun to decrease in size at SAL 3.20 mm (4.00 mm BT). Concurrently, the caudal fin fold degenerated and disappeared completely, the notochord atrophy was nearly complete, and the spines on the ventral midline became connected by extensions small spines, which develop into a ventral keel. The morphological characters, included egg shape and size, egg chorion structures, number of oil droplet, distribution of melanophores, and shape and number of spines etc., were regarded as the basis to distinguish eggs and larvae between R . laevis and other Tetradontiform fishes [ Acta Zoologica Sinica 51(6)：1034–1043，2005].