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

术语“梭子蟹”通常是指系统发育成熟的真蟹（Brachyura）分类群中的特定形态表现（形态型）。梭子蟹与短尾目其他成员的不同之处在于，它们显示出一系列的形态功能特征，这些特征共同使它们能够使用它们的第五腹足类（P5）进行独特的游泳行为模式。梭子蟹（连同一些形态相似的非游泳分类群，如著名的绿色蟹（Strandkrabbe）Carcinus meaenas）被分配到梭子蟹总科。梭子蟹是一个单系类群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，并可能进化了多次。本研究的主要问题是：（1）不同梭子蟹总科的游泳腿（以及中轴骨骼的功能相关结构）是同源的，还是第五腹足类的游泳能力是独立进化的？(2)哪种特征组合使第五肢类动物能够充当游泳腿？(3)导致梭子蟹进化的先决条件是什么（在体形、中轴骨骼、腹足类结构方面）存在于异孔动物短尾纲中，而不存在于足孔动物和胸孔动物（短尾纲中的另外两个主要类群）中？为了回答这些问题，将使用μCT和数字3D重建和可视化来研究各种短尾蟹的运动器官的结构。由此获得的详细描述将被用来概念化字符和字符状态，字符状态的转换将在当前的系统发育假说的基础上进行研究。将使用建模（Maya 2018）通过虚拟重建所有P5肢体关节的最大活动范围（ROM）离体研究运动能力，并通过在可研究活体标本的物种中进行运动形态视频重建（VROMM）进行体内研究。这两种方法将阐明P5个体特征在游泳行为方面的重要性。

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.