Sciences, Museum Victoria, Melbourne, Victoria 3001, Australia.
Proc Biol Sci. 2010 Oct 7;277(1696):2967-71. doi: 10.1098/rspb.2010.0155. Epub 2010 May 19.
Argonauts (Cephalopoda: Argonautidae) are a group of rarely encountered open-ocean pelagic octopuses with benthic ancestry. Female argonauts inhabit a brittle 'paper nautilus' shell, the role of which has puzzled naturalists for millennia. The primary role attributed to the shell has been as a receptacle for egg deposition and brooding. Our observations of wild argonauts have revealed that the thin calcareous shell also functions as a hydrostatic structure, employed by the female argonaut to precisely control buoyancy at varying depths. Female argonauts use the shell to 'gulp' a measured volume of air at the sea surface, seal off the captured gas using flanged arms and forcefully dive to a depth where the compressed gas buoyancy counteracts body weight. This process allows the female argonaut to attain neutral buoyancy at depth and potentially adjust buoyancy to counter the increased (and significant) weight of eggs during reproductive periods. Evolution of this air-capture strategy enables this negatively buoyant octopus to survive free of the sea floor. This major shift in life mode from benthic to pelagic shows strong evolutionary parallels with the origins of all cephalopods, which attained gas-mediated buoyancy via the closed-chambered shells of the true nautiluses and their relatives.
箭蛸（头足纲：箭蛸科）是一种罕见的远洋浮游章鱼，具有底栖祖先。雌性箭蛸栖息在一个易碎的“纸鹦鹉螺”壳内，这个壳的作用让自然学家们困惑了几千年。这个壳的主要作用被认为是用于产卵和育雏的容器。我们对野生箭蛸的观察揭示了薄的钙质壳也起到了水静压结构的作用，雌性箭蛸利用这个结构来精确地控制在不同深度的浮力。雌性箭蛸在海面用壳“吞”入一定量的空气，用带边的臂密封捕获的气体，并用力潜水到压缩气体的浮力与体重相抵消的深度。这个过程使雌性箭蛸能够在深处达到中性浮力，并在繁殖期间可能调整浮力以抵消卵的增加（且显著）重量。这种空气捕获策略的进化使这种负浮力的章鱼能够在没有海底的情况下生存。这种从底栖到浮游的生活模式的重大转变，与所有头足类动物的起源有着强烈的进化相似性，它们通过真鹦鹉螺及其亲属的封闭腔室壳获得了气体介导的浮力。
