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

寄居蟹类有1000多种，是十足类甲壳类中最成功的类群之一。这两个群体最明显的特征是软壳企鹅，在大多数物种中，它隐藏在蜗牛壳中。在系统发育上，帝王蟹和陆生底栖动物也属于寄居蟹。同样有趣的是第一条行走的腿，它们被形成为蜥脚类动物，与身体长度相比可以达到惊人的尺寸。有右侧突出的寄居蟹(Paguridae，石蟹总科)和左侧突出的(Digenidae，Coenobitidae)爪(Chela)，但有些种类的爪子大小没有差异(Pylochelidae)。这些爪子正在执行各种任务(即生物角色)，例如食物操纵和消费。它们还可以用来封闭有人居住的蜗牛壳，或者用作武器来对抗潜在的捕食者。根据分类单元的不同，贝壳的闭合可以由右足、左足或两者兼而有之。对于食物消费，爪子的整体形状显然没有固定和可移动手指之间的刀刃那么重要。在功能上，锯齿的牙齿特征明显类似于不同类型的钳子。拟议的项目旨在了解寄居蟹中贝壳类动物的形态、功能和生物学作用的进化。为此，我们采用了三种不同的方法来应对不同水平的进化转型。首先，基于最近的系统发育假说，我们将研究每一科的各种物种，并研究进化转化。将使用μCT、数字3D重建和几何形态测量等新技术来确定爪子的整体形状。其次，为了了解导致这种差异的进化转变的基础，我们研究了两个丰富的寄居蟹物种的爪子的种内表型变异：普通寄居蟹(Pagurus Bernhardus)和小型寄居蟹(Digenes Pugilator)。这两个物种都以使用它们的优势爪子作为它们栖息的蜗牛壳的闭合而闻名。除了种群间爪子形状的差异外，还将研究爪子形状与各自蜗牛壳入口之间的相关性。第三种方法是基于这样一种现象，即刚刚蜕皮的寄居蟹可以根据蜗牛壳的入口处调整爪子的形状。在饲养实验中，新鲜蜕皮的普通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可变性分开。后者对于理解可变性作为进化转变的起点很重要。

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.