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

寄居蟹（寄居蟹总科）是十足甲壳类动物中最成功的类群之一，有1000多种。这一群体最显著的特征是软壳pleon，在大多数物种中，它隐藏在蜗牛壳中。在系统发育上，帝王蟹和陆生隐蟹也属于寄居蟹。同样有趣的是最早的会走路的腿，它们形成了足跖动物，相对于身体长度可以达到惊人的大小。寄居蟹有突出的右爪（细爪蟹科）和突出的左爪（螯），但有些种类在爪大小上没有差异（细爪蟹科）。这些爪子正在执行各种任务（即生物角色），例如操纵食物和消费。它们也可以作为居住的蜗牛壳的封闭物，或者用作对抗潜在捕食者的武器。根据不同的分类群，可以由右足、左足或双足来保证壳的闭合。对于食物的消耗来说，爪子的整体形状显然不如固定手指和活动手指之间的刀刃那么重要。切削刃的齿列特征就其功能而言显然类似于不同类型的钳子。该项目旨在了解寄居蟹中足跖动物的形态、功能和生物学作用的演变。为此，采用了三种不同的方法来处理不同层次的进化转换。首先，我们将根据最近的系统发育假说，对每一个类蛙科的各种物种进行研究，并研究它们的进化转变。将利用μCT、数字三维重建、几何形态测量等新技术确定爪的整体形状。此外，还计划了基于切削刃齿列的功能分类。其次，为了理解导致这种差异的进化转变的基础，我们将研究两种丰富的寄居蟹物种：普通寄居蟹（Pagurus bernhardus）和小型寄居蟹（Diogenes pugilator）的种内表型变异。这两个物种都以使用它们的优势爪子来关闭它们栖息的蜗牛壳而闻名。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。第三种方法是基于一种现象，即刚蜕皮的寄居蟹可以调整它们的爪子形状以适应蜗牛壳的入口。在保持实验中，将刚蜕皮的普通寄居蟹和小寄居蟹与3d打印的蜗牛壳进行对比，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。后者对于理解可变性作为进化转换的起点是很重要的。

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.