强壮水螅的特征及其与寡水螅的种间差别问题

一种广泛分布于黑龙江省的大型具柄水螅被暂定为强壮水螅Hydra robusta(IT(?) 1947)。这一中国品系与IT(?)(1947)在日本报告的新种主要特征相同,包括具有精巢乳头。只是精巢乳头不稳定。在实验室条件下第一次有性生殖时每个雄体都发生精巢乳头,但第二次有性生殖时,同一群的后代可全部失去精巢乳头。这是介于强壮水螅与寡水螅Hydra oligactis Pallas(1766)之间的中间性状。从而导致作者做出结论,认为过去其他学者所主张的,以精巢乳头的有无做为鉴别特征来区分强壮水螅和寡水螅是不可靠的。因此,本文又检验了此种水螅的体细胞染色体,证明2n=30,其中第二对染色体上有明显近中位置的次缢痕,最小染色体的长度为最大染色体长度的二分之一以上。这些可做为此种水蝗的鉴别特征。对Niiyama(1944)和Datta(1970)在寡水螅上所做的染色体研究也做了比较及讨论。

THE CHARACTERISTICS OF HYDRA ROBUSTA AND ITS DIFFERENCE FROM H. OLIGACTIS

A large and stalked fresh-water polyp widely distributed over Heilongjiang Province has been tentatively identified as Hydra robusta (ITO 1947). The Chinese strain closely resembles the Japanese strain in, main characteristics described as a new species by ITO in 1947, including the presence of the testis nipple. But the testis nipple in Chinese strain is not a stable feature. In laboratory condition, it has been found that the testes nipples occur in every male hydra during first sexual reproduction and the nipples can disappear completely from the testes of the offspring of the same population during next sexual reproduction. This is an intermediate feature between Hydra robusta and H..oligactis. So the authors are led to the conclusion that the testis nipple as an identifying characteristic to distinguish Hydra robusta from H. oligactis, advocated by some other previous authors, might be unreliable. So the somatic chromosomes in the present species were additionally examined. It has been proved that the diploid number is 2n=30 and each chromosome of the second pair has a clear secondary constriction, and that the length of the smallest chromosome is longer than half of the largest one. These may be regarded as identified characteristics of this species of hydra. A comparison with the studies on chromosomes of H. oligactis by Niiyama (1944) and Datta (1970) and a discussion on the difference between the two species have also been given.