The circumstances that favor or disfavor habitat specialization in species are not well understood, despite a vast literature on particular cases. The taxonomic, phylogenetic and geographical distribution of limpet-like gastropods specialized for life on the coiled shells of other gastropods and hermit crabs were surveyed. Predation-related traits were measured to evaluate the refugial status of shells as habitats from predation. There are 41 shallow-water marine shell-specialized limpets, all members of the Lottiidae, Calyptraeidae and Hipponicidae, which account for 3.5% of the global diversity of shallow-water marine limpet-like gastropods. These species are distributed mainly along tropical and some temperate coastlines but are notably few in the highly diverse Indo-West Pacific. The northeast Pacific stands out as the region with the most diverse and most host-specific shell-dwelling limpets. As expected, limpets on shells have shells with little or no external sculpture, all traits associated with low predation risk. Hermit crabs are hosts to specialized limpets living inside their shells, but no limpet specializes on the exteriors of hermit-crab shells. Immobile encrusting and shell-extending epibionts (hydrozoans, anthozoans, sponges and bryozoans) often specialize on hermit-crab hosts. Comparisons between these epibionts and shell-dwelling limpets indicate that specialization for life on shells is greatest when the host takes an active role in attracting or moving organisms to their shells. This highlights the important role of behavior in modifying the direction and extent of natural selection in host–guest associations.

中文翻译：

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栖息地专业化的选择和演变：贝壳上的生命案例

尽管有大量关于特定案例的文献，但人们对有利于或不利于物种栖息地专业化的情况尚不清楚。调查了以其他腹足类动物和寄居蟹的卷壳为生的类帽贝类腹足动物的分类、系统发育和地理分布。测量捕食相关性状以评估贝壳作为捕食栖息地的避难状态。浅水海洋贝类特化帽贝41种，全属Lottiidae、Calyptraeidae和Hipponicidae，占全球浅水海洋类帽贝类腹足动物多样性的3.5%。这些物种主要分布在热带和一些温带海岸线，但在高度多样化的印度-西太平洋地区很少见。东北太平洋是拥有最多样化和最具有寄主特性的贝壳帽贝的地区。正如预期的那样，贝壳上的帽贝具有很少或没有外部雕刻的贝壳，所有特征都与低捕食风险相关。寄居蟹是生活在其壳内的特殊帽贝的宿主，但没有一种帽贝专门研究寄居蟹壳的外部。不动的结壳和延伸壳的外生生物（水螅、珊瑚虫、海绵和苔藓虫）通常专门研究寄居蟹寄主。这些外生生物与壳栖帽贝之间的比较表明，当宿主在将生物吸引或移动到它们的贝壳方面发挥积极作用时，贝壳上的生命的专业化程度最高。