Pressure tolerance of tadpole larvae of the Atlantic ascidian Polyandrocarpa zorritensis: potential for deep-sea invasion

Abstract How deep-sea fauna evolved is a question still being investigated. One of the most accepted theories is that shallow water organisms migrated to deeper waters and gave origin to the deep-sea communities. However, many organisms are prevented from performing long vertical migrations by the increasing hydrostatic pressure. Tadpole larvae of the ascidian Polyandrocarpa zorritensis were submitted to pressure treatments of 1, 50, 100 and 200 atm. Survival, settlement and metamorphosis rates were verified after 24 hour incubation in a pressure chamber. The majority of larvae settled (84%, 62%, 83% and 77% respectively) and successfully underwent metamorphosis (93%, 59%, 85% and 60%) in all pressure treatments. Larval mortality was of less than 15% in all treatments, except for the 50 atm treatment, which presented 38% mortality. Nearly 100% of the surviving larvae underwent metamorphosis in the treatments of 1, 50 and 100 atm. However, 1/3 of the individuals were still in their larval stages in the 200 atm treatment and presented delayed development. These data suggest that ascidian larvae can withstand the hydrostatic pressure levels found in the deep-sea. It is therefore feasible that the current abyssal ascidian species may have colonized the deep-sea through vertical migration and in only a few generations.

摘要 深海动物群的演化机制仍是学界持续探索的课题。目前最广为接受的理论认为，浅海生物通过迁徙进入深海环境，进而演化形成深海群落。然而，不断升高的静水压力会阻碍多数生物开展长距离垂直迁徙。本研究对被囊动物（ascidian）Polyandrocarpa zorritensis的蝌蚪状幼虫施加1、50、100及200标准大气压（atm）的压力处理，将其置于压力舱中孵育24小时后，检测幼虫的存活率、附着率与变态率。结果显示，各压力处理组的幼虫附着率分别为84%、62%、83%与77%，且多数幼虫成功完成变态发育，对应成功率分别为93%、59%、85%与60%。除50标准大气压组的幼虫死亡率达38%外，其余各组死亡率均低于15%。在1、50及100标准大气压处理组中，存活幼虫的变态率近乎100%；但在200标准大气压组中，有三分之一的个体仍处于幼虫阶段，发育出现延迟。上述数据表明，被囊动物幼虫可耐受深海环境中的静水压力水平。由此推测，现生深渊被囊类物种可能仅需数代即可通过垂直迁徙完成深海定居。