The evolution of the swimming crab morphotype – an extraordinary case of convergence?

“游蟹”一词通常是指在系统发育上已确立的真蟹分类群（短尾目）中的一种特定形态表现（形态型）。游泳蟹与其他短肢目动物的不同之处在于，它们表现出一系列形态功能特征，这些特征使它们能够利用它们的第五感觉足类（P5）来执行一种独特的游泳行为模式。游蟹（连同一些形态相似的非游蟹类群，如众所周知的绿蟹（Strandkrabbe） Carcinus meaenas）已被归入葡萄牙蟹总科。游泳蟹是一种单系群体的假设最近受到了挑战，似乎它们特定的形态适应可能有多个起源，可能已经进化了多次。该项目探讨的主要问题是：(1)不同葡萄牙纲动物的游泳腿（以及轴向骨骼的功能相关结构）是同源的，还是第5类知觉足类动物的游泳能力是独立进化的？(2)哪些特征组合使第五感觉足类具有游泳腿的功能？(3)导致游泳蟹进化的先决条件（在体型、轴向骨骼、表足类结构等方面）是在异肢类短肢纲中存在的，而在足肢类和胸肢类（短肢纲中另外两个主要分类群）中没有？为了回答这些问题，我们将利用μCT和数字三维重建和可视化技术对各种短爪蟹的运动器官结构进行研究。由此获得的详细描述将用于对字符和字符状态进行概念化，并且将在当前系统发育假设的基础上研究字符状态的转换。运动能力将通过建模（Maya 2018）在体外研究，通过虚拟重建所有P5肢体关节的最大活动范围（ROM），并在体内通过可以研究活体标本的物种的运动形态（vrom）的视频重建。这两种方法都将揭示P5个体特征在游泳行为方面的意义。

The term "swimming crab" usually refers to a specific morphological manifestation (morphotype) within the phylogenetically well-established taxon true crabs (Brachyura). Swimming crabs differ from all other members of Brachyura in displaying a range of morpho-functional features that together enable them to perform a unique mode of swimming behavior using their 5th pereiopods (P5). Swimming crabs (together with some morphologically similar non-swimming taxa, such as the well-known green crab (Strandkrabbe) Carcinus meaenas) have been assigned to the superfamily Portunoidea. The assumption that swimming crabs are a monophyletic group has recently been challenged, and it seems that their specific morphological adaptations could have multiple origins and possibly have evolved multiple times. The main questions explored in the proposed project are: (1) Are the swimming legs (and functionally related structures of the axial skeleton) of different Portunoidea homologous or did the swimming capacity of the 5th pereiopods evolve independently? (2) Which combination of features enables the 5th pereiopods to act as swimming legs? (3) What prerequisites (in terms of body shape, axial skeleton, structure of the pereiopods) which led to the evolution of swimming crabs were present in heterotrematan brachyurans and not in podotrematan and thoracotrematan crabs (the two other major taxa within Brachyura)? To answer these questions, the structure of the locomotor apparatus of various brachyuran crabs will be studied using μCT and digital 3D reconstruction and visualization. The detailed descriptions thus obtained will be used to conceptualize characters and character states, and the transformation of character states will be studied on the basis of current phylogenetic hypotheses. Locomotory capability will be studied ex vivo using modeling (Maya 2018) by virtually reconstructing the maximum range of mobility (ROM) of all P5 limb joints together, and in vivo via video reconstruction of moving morphology (VROMM) in species where living specimens can be studied. Both methods will shed light on the significance of individual features of the P5 in terms of swimming behavior.