Evolutionary morphology of hermit crab claws (Decapoda, Paguroidea) – what determines shape?

寄居蟹（寄居蟹总科），数量超过1000种，代表了十足类甲壳动物中最成功的群体之一。这群人最显著的特征是软壳的pleon，它隐藏在大多数物种的蜗牛壳中。在系统发育上，帝王蟹和陆生的寄居蟹也属于寄居蟹。同样有趣的是第一条行走的腿，它们是由螯足动物形成的，相对于身体的长度，它们可以达到惊人的尺寸。有寄居蟹与一个突出的权利（Paguridae，Lithodoidea）分别突出左（Diogenidae，Coenobitidae）爪（螯），但有些物种显示没有差异的爪大小（Pylochelidae）。这些爪子执行各种任务（即生物学角色），例如食物处理和消费。它们也可以作为栖息蜗牛壳的封闭物，或者用作对抗潜在捕食者的武器。根据分类单元，壳的闭合可以通过右、左或两个螯足动物来确保。对于食物消耗，爪的整体形状显然不如固定和可移动手指之间的切割边缘相关。建议的项目旨在了解寄居蟹中螯足类的形态、功能和生物学角色的演变。为此目的，追求三种不同的方法来处理不同层次的进化转型。首先，根据最近的系统发育假说，我们将对每一个狸科的各种物种进行检查，并研究进化转变。将使用新技术（如μCT、数字3D重建和几何形态测量学）确定爪的整体形状。此外，功能分类的基础上齿的切削刃planned.Second，为了理解的基础上的进化转型导致这样的差距，种内表型变异的爪应检查两个丰富的寄居蟹物种：常见的寄居蟹（Pagurus bernhardus）和小寄居蟹（Diogenes pugilator）。这两个物种都以使用它们占优势的爪子作为它们栖息的蜗牛壳的闭合物而闻名。除了种群之间爪形的差异外，还将研究爪形与各自蜗牛壳入口之间的相关性。第三种方法是基于这种现象，即刚蜕皮的寄居蟹可以调整它们的爪形以适应蜗牛壳的入口。在饲养实验中，新鲜蜕皮的普通寄居蟹和小型寄居蟹将面对3D打印的蜗牛壳，以确定爪形的表型可塑性程度，并将表型可塑性与遗传变异性分开。后者对于理解变异性作为进化转化的起点是重要的。

Hermit crabs (Paguroidea), numbering over 1000 species, represent one of the most successful groups within decapod crustaceans. The groups’ most conspicuous feature is the soft-shelled pleon, which is hidden within a snail shell in most of the species. Phylogenetically, king crabs and the terrestrial coenobitids, also belong to the hermit crabs. Not less interesting are the first walking legs, which are formed as chelipeds and can reach astonishing sizes in relation to body length. There are hermit crabs with a prominent right (Paguridae, Lithodoidea) respectively a prominent left (Diogenidae, Coenobitidae) claw (chela) however some species display no discrepancy in claw size (Pylochelidae). Various tasks (i.e. biological roles) are being carried out by these claws, e.g. food manipulation and consumption. They also serve as closure for inhabited snail shells or are used as weapons to fight potential predators. Depending on the taxon, closure of the shell can be ensured by the right, the left or both chelipeds. For food consumption, the claws overall shape is apparently less relevant than the cutting edge between the immovable and movable finger. Characteristics of the dentition of the cutting edges apparently resemble different types of pliers regarding their function.The proposed project aims for understanding the evolution of the chelipeds’ form, function and biological role in hermit crabs. For this purpose, three different approaches tackling different levels of evolutionary transformations are pursued. First, a variety of species of every paguroid family shall be examined and evolutionary transformations studied, based on recent phylogenetic hypotheses. The overall claw shape will be determined using new techniques such as μCT, digital 3D reconstruction and geometric morphometrics. Furthermore, a functional classification based on the dentition of the cutting edges is planned.Second, for understanding the basis of evolutionary transformations leading to such a disparity, the intraspecific phenotypic variability of the claw shall be examined in two abundant hermit crab species: the common hermit crab (Pagurus bernhardus) and the small hermit crab (Diogenes pugilator). Both species are well known for using their dominant claws as closure for the snail shells they inhabit. In addition to differences in claw shape between populations, the correlation between claw shape and the entrance of the respective snail shells will be studied. The third approach is based on the phenomenon, that freshly molted hermit crabs can adjust their claw shape to the entrance of the snail shells. In keeping experiments, freshly molted common and small hermit crabs will be confronted with 3D-printed snail shells, to determine the degree of phenotypic plasticity in claw shape and to separate phenotypic plasticity from genetic variability. The latter one is important for understanding variability as starting point in evolutionary transformations.