Despite the fact that Hydra has been studied for more than 200 years, we know surprisingly little about its life history. We show that Hydra vulgaris embryos hatch sporadically over a period ranging from a few days to nine months. We also report, for what seems to be the first time, the presence of Hydra in a vernal pool. Phylogenetic analysis and sexual crossing show that this Hydra is a member of the cosmopolitan Vulgaris clade and is not reproductively isolated from other members of the clade. Our findings lead us to hypothesize that Hydra evolved in an unstable freshwater habitat in which survival required that its life cycle include the use of a bet-hedging reproductive strategy and the formation of an embryo that is desiccation resistant and that can remain dormant for long periods of time.

中文翻译：

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作为九头蛇生活史的组成部分的春季池中的生殖对冲和存在

尽管 Hydra 已经被研究了 200 多年，但我们对其生活史知之甚少。我们表明，Hydra vulgaris 胚胎会在几天到九个月的时间内偶尔孵化。我们还报告，似乎是第一次，在春季水池中出现了九头蛇。系统发育分析和有性杂交表明，这种九头蛇是世界性寻常型进化枝的成员，与该进化枝的其他成员没有生殖隔离。我们的研究结果使我们假设九头蛇是在不稳定的淡水栖息地进化的，在这种栖息地中生存需要其生命周期包括使用对冲繁殖策略和形成抗干燥且可以长时间休眠的胚胎时间。