海鞘可能并不漂亮，但这种其貌不扬的海洋生物却引起了科学家的浓厚兴趣！海鞘可能受到许多物种的羡慕嫉妒，它从不必担心“避孕节育”或试管受精等问题，便能够有效地控制好生育繁殖。

这项由澳大利亚昆士兰州大学进行的研究报告发表在8月26日美国权威专业期刊《美国国家科学家学报》上，研究人员揭示海鞘这种海洋生物天然具有控制生殖循环的能力，可依据需要或多或少地进行有节制繁殖。

该项研究的负责人是澳大利亚昆士兰州大学综合生物学院博士生研究员安吉拉－克林，她指出，海鞘能够依据海洋环境中性别比例状况，“适时定制”自己的生殖细胞。比如：当海洋环境中存在着大量的雄性海鞘，它们试图竞争与雌性的卵细胞结合繁殖生育，因此雄性将生产出更大、更具竞争性的精子，便于存活更长的时间。同样地，当雌性海鞘探测到过多的雄性竞争交配结合卵细胞时（过多的精子将杀死一些生物体的卵细胞），雌性海鞘将生产出更小的卵细胞，很精子很难探测到。

克林说，“如果为了避免在高竞争环境中失控，它们必须更具竞争力地繁殖生育，从而提高繁殖高成功率。”据了解，在莫尔顿海湾的实验地点的研究过程中，他们发现事实上海鞘并未移动离开该海域。

在该实验中，大型的海鞘放置在狭窄海域进行深入研究分析，克林说，“我们分别将1只（低密度）或15只（高密度）海鞘放在实验笼中进行1个月的观测分析。这项研究告诉我们关于海鞘性特征的部分秘密。当海鞘精子变得很小时，卵细胞就会发育得更大，因此繁殖生育的数量会很少。”

推荐原始出处：

PNAS Published online before print August 29, 2008, doi: 10.1073/pnas.0806590105

Gamete plasticity in a broadcast spawning marine invertebrate

Angela J. Crean* and Dustin J. Marshall

Sperm competition has classically been thought to maintain anisogamy (large eggs and smaller sperm) because males are thought to maximize their chance of winning fertilizations by trading sperm size for number. More recently it has been recognized that sperm quality (e.g., size, velocity) can also influence sperm competition, although studies have yielded conflicting results. Because sex evolved in the sea, debate has continued over the role of sperm competition and sperm environment in determining both sperm and egg size in externally fertilizing broadcast spawners. Remarkably, however, there have been no direct tests of whether broadcast spawners change the traits of their gametes depending on the likelihood of sperm competition. We manipulated the density (and thus, sperm environment) of a broadcast spawning ascidian (Styela plicata) in the field and then determined whether the phenotype of eggs and sperm changed. We found that sperm from adults kept at high density were larger and more motile than sperm from low-density adults. In vitro fertilizations revealed that sperm from high-density adults also lived longer and induced less polyspermy. Adult density also affected egg traits: eggs from high-density adults were smaller targets for sperm overall but produced larger ovicells than eggs from low-density adults. This suggests that broadcast spawning mothers balance (potentially conflicting) pre- and postzygotic selection pressures on egg size. Overall, our results suggest that sperm competition does not represent a strong force maintaining anisogamy in broadcast spawners. Instead, sperm limitation seems to select for large eggs and smaller, more numerous sperm.