Department of Biological Sciences, Auburn University, 101 Rouse Life Sciences, Auburn, Alabama 36849, USA.
Nat Commun. 2013;4:2738. doi: 10.1038/ncomms3738.
Acorn worms, or enteropneusts, are vermiform hemichordates that occupy an important position in deuterostome phylogeny. Allied to pterobranch hemichordates, small colonial tube dwellers, modern enteropneusts were thought to be tubeless. However, understanding of hemichordate diversity is poor, as evidenced by absence of reports from some oceanic regions and recent descriptions of large epibenthic deep-water enteropneusts, Torquaratoridae. Here we show, based on expeditions to Antarctica, that some acorn worms produce conspicuous tubes that persist for days. Interestingly, recent fossil descriptions show a Middle Cambrian acorn worm lived in tubes, leading to speculation that these fossils may have been pterobranch forbearers. Our discovery provides the alternative interpretation that these fossils are similar to modern-day torquaratorids and that some behaviours have been conserved for over 500 million years. Moreover, the frequency of Antarctic enteropneusts observed attests to our limited knowledge of Antarctic marine ecosystems, and strengthens hypotheses relating more northern deep-sea fauna to Antarctic shelf fauna.
橡果虫，或肠纽虫，是一种蠕虫状的半索动物，在后口动物系统发育中占有重要地位。与翼腕幼虫（小的群体居住在管状栖息地上的海鞘）亲缘关系密切，现代肠纽虫被认为是无管的。然而，由于某些海洋区域没有报告，以及最近对大型深海表栖肠纽虫（Torquaratoridae）的描述，对半索动物多样性的理解很差。在这里，我们基于对南极洲的考察表明，一些橡果虫会产生明显的、持续数天的管子。有趣的是，最近的化石描述表明，一种中寒武纪的橡果虫生活在管状结构中，这导致了这样一种推测，即这些化石可能是翼腕幼虫的祖先。我们的发现提供了另一种解释，即这些化石与现代的旋尾虫科相似，并且一些行为已经被保存了超过 5 亿年。此外，观察到的南极肠纽虫的频率证明了我们对南极海洋生态系统的了解有限，并加强了与更北深海动物群有关的假说与南极大陆架动物群的关系。
