What is It Like to be a Crab? A Complex Network Analysis of Eucaridan Evolution

Abstract

Eucaridan evolution involved a process starting from a body organization characterized by an elongate and cylindrical cephalothorax, a well-developed abdomen composed of swimming appendages, ending in a tail fan formed by flattened uropods and a telson. This process would lead, ultimately, to a body organization characterized by a shortened and depressed cephalothorax, and a reduced and ventrally folded abdomen. This ultimate process is typically known as carcinization, and is commonly defined as the process of becoming a crab. In this work, the evolution of the superorder Eucarida was studied using complex networks. A new definition of crab and carcinization are given based on the results obtained. A crab is a topological structural closure that determines the formation of a triadic central core. The evolution of the crab implied the formation of a triadic structure with high closeness centrality, formed by the cephalon, the fused thoracomere 1–4 and the carapace, which represented a highly stable hierarchical core deeply buried or enclosed in the topological structure of the network, responsible for the generation of a highly integrated and robust topology. Under this new definition, the representative of the infraorder Anomura used in this work, which is commonly considered as a crab, is not. This network seemed to be characterized by the presence of a quasi-dyadic structure, formed by the cephalon and the carapace, which was not sufficient for generating the topological closure.