Graduate School of Human and Environmental Studies, Kyoto University, Sakyo, Kyoto, Japan.
PLoS One. 2018 Aug 22;13(8):e0197719. doi: 10.1371/journal.pone.0197719. eCollection 2018.
Flattening of coiled shells has occurred in several gastropod lineages, while the evolutionary process of shell flattening is little known. The subfamily Fossarininae of the top shell family (Trochidae) is unique, because it includes four genera at various stages of shell flattening. Broderipia and Roya, have zygomorphic shells that has lost coiling, while the sister genera, Fossarina and Synaptocochlea, have respectively turbiniform and auriform shells. Therefore, comparisons of biology, habitats and detailed morphology among these four genera may help us to detect selection pressure driving shell flattening and loss of coiling. Although Broderipia has recently been identified as living symbiotically in the pits of sea urchins, the habitats and biology of the other three Fossarininae species, especially Roya are poorly known. After an extensive search on rocky shores of the Japanese Archipelago, we found live Roya eximia snails on intertidal/subtidal rock surfaces exposed to strong waves. Roya snails crept on the bare rock surface to graze periphyton at low tide, and fled into vacant barnacle shells at high tide. Comparison of the morphology of soft bodies in Fossarininae revealed that the columellar muscle of flattened species has been drastically elongated and arranged in posterior semi-outer edge of the flattened shell as observed in true limpets. The flattering and loss of coiling of the shell in Roya caused acquisition of a zygomorphic flat body, retraction of coiled visceral mass, and expansion of the foot sole. All of these changes improved tolerance against strong waves and the ability to cling to rock surfaces, and thus enabled a lifestyle utilizing both wave-swept rock surfaces and the inside of vacant barnacle shells.
卷曲壳的压扁在几个腹足纲动物谱系中发生过，而壳压扁的进化过程却鲜为人知。宝螺科（Trochidae）的骨螺亚科（Fossarininae）很独特，因为它包括四个处于壳压扁不同阶段的属。Broderipia 和 Roya 具有失去旋卷的左右对称壳，而姊妹属 Fossarina 和 Synaptocochlea 则分别具有涡轮形和耳形壳。因此，对这四个属的生物学、栖息地和详细形态进行比较，可能有助于我们发现驱动壳压扁和失去旋卷的选择压力。尽管 Broderipia 最近被鉴定为与海胆的坑中共生，但其他三个 Fossarininae 物种，特别是 Roya 的栖息地和生物学仍知之甚少。在对日本列岛的岩石海岸进行广泛搜索后，我们在潮间带/亚潮带暴露于强浪的岩石表面上发现了活体 Roya eximia 蜗牛。Roya 蜗牛在低潮时会在裸露的岩石表面上爬行以啃食附生藻类，而在高潮时则会逃入空的藤壶壳中。对 Fossarininae 软体动物形态的比较表明，压扁物种的柱肌已大大伸长，并排列在压扁壳的后半外缘，就像真正的石鳖一样。壳的压扁和旋卷的丧失使 Roya 获得了左右对称的扁平身体，卷曲的内脏团缩回，足垫扩张。所有这些变化都提高了对强浪的耐受性和紧贴岩石表面的能力，从而使它们能够利用被浪冲击的岩石表面和空藤壶壳的内部这两种生境。
