西太平洋热带水域长体翻车鱼卵和仔稚鱼的空间分布及其形态特征

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

1978年10月利用向阳红5号调查船对西太平洋14°00′-17°00′S、171°00′-174°00′E热带水域进行鱼类浮游生物调查。调查期间共采集到长体翻车鱼卵27粒和仔稚鱼11尾。长体翻车鱼卵集中分布于调查海域东南部的15°28.4′-16°17.3′S、172°43.8′-173°29.0′E海区,而仔稚鱼主要分布于调查海域东南部的15°28.4′-16°17.3′S、172°43.8′-173°29.0′E和西北部的14°29.0′S、171°48.4′E海域。鱼卵和仔稚鱼的密度分别为1-14粒/100m3和1-3尾/100m3。出现鱼卵和仔稚鱼分布海域的水温和盐度分别为25.72-26.22℃、35.10-35.32和25.72-27.02℃、34.80-35.50。鱼卵和仔稚鱼均出现在海洋锋面处。长体翻车鱼卵为圆球型、浮性、卵膜光滑、透明,卵子直径1.40-1.60mm(1.50±0.054mm,n=27),多油球,20-30个,其直径为0.06-0.16mm(0.10±0.064mm,n=27),卵黄均匀无龟裂。当胚胎发育到尾芽期,可数肌节20对,胚体背部和油球表面出现黑色素细胞分布,皮质体壳已初步形成。孵化前,皮质体壳的表面布满了浓密、细微的小泡,唯尾部和胸鳍外,整个胚体被体壳包住。仔鱼的肛前长为1.20mm(全长1.80mm,体长1.68mm)时,体壳已退化脱尽,肛前长为3.20mm(体长4.00mm)时,鱼体上的部分棘开始退化、变小,尾鳍完全退化、消失,尾锥体萎缩成瘤状,腹部的棘与棘之间出现许多锯齿状小刺。卵子的形状和大小、卵膜的结构、油球的数量、黑色素的分布以及体棘的形状和数量等是区别于其它形目鱼类的卵子和仔稚鱼的依据。

An ichthyoplankton survcy was carried out during October 1978 on board R/V Xiangyanghong 5 in the area 14°00′ - 17°00′S, 171°00′ - 174°00′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/ see 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 15°28.4′- 16°17.3′S, 172°43.8′- 173°29.0′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 15°28.4′- 16°17.3′S, 172° 43.8′- 173°29.0′E and 14°29.0′S, 171°48.4′E, the temperature and salinity were 25.72-27.02℃ and 34.80- 35.50, respectively. Abundance of fish eggs and larvae was 1 - 14 ind./100 m^3 and 1-3 ind./100 m^3, 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, unsculpturcd 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].