# Phylogenetic synthesis of morphological and molecular data reveals insights on the classification of diogenid hermit crabs (Crustacea: Decapoda: Anomura)

## TL;DR

This study combines genetic and physical traits to better understand the evolutionary relationships of diogenid hermit crabs, revealing that traditional classifications may be incorrect.

## Contribution

The study provides the largest phylogeny of diogenid hermit crabs using combined molecular and morphological data.

## Key findings

- Combined molecular and morphological data improved phylogenetic resolution and revealed a paraphyletic Diogenidae.
- The genus Diogenes was found nested within Paguristes, suggesting paraphyly in Paguristes.
- Morphological traits like carapace shield and telson were phylogenetically significant, while gill number was not.

## Abstract

The family Diogenidae Ortmann, 1892 is a diverse and abundance group of hermit crabs, but their systematics and phylogenetic relationships are highly complex and unresolved. Herein, we gathered nucleotide sequence data from two mitochondrial (16S rRNA and COI) and two nuclear (NaK and PEPCK) genes for a total of 2,308 bp in length across 38 species from six extant diogenid genera. Molecular data were combined with 41 morphological characters to estimate the largest phylogeny of diogenid hermit crabs to date with the aim of testing the proposed taxonomic scheme of Diogenidae and addressing intergeneric relationships within this family. Despite conflicts between mitochondrial and nuclear DNA trees, the combined-data tree reflects the contributions of each dataset, and improves tree resolution and support for internal nodes. Contrary to traditional classification, our total evidence revealed a paraphyletic Diogenidae based on internally nested representatives of Coenobitidae Dana, 1851. Within Diogenidae, the studied diogenid hermit crabs were split between two clades with high support, which contradicts recent morphological classification scheme for Diogenidae sensu lato based on fossil records. The genus Diogenes Dana, 1851 was found nested inside Paguristes Dana, 1851, which formed a clade being separated from the remainder, pointing towards paraphyly in Paguristes. In another clade, Dardanus Paulson, 1875 occupied a basal position relative to the other diogenids, while Calcinus Dana, 1851 and Clibanarius Dana, 1852 showed sister relationships and formed a cluster with Ciliopagurus Forest, 1995. Among the morphological characters examined, carapace shield and telson were identified as phylogenetically significant for grouping diogenid genera, while phylogenetic insignificance of gill number was evidenced by its mosaic pattern in diogenid phylogeny. The present study sheds light on the controversial generic phylogeny of Diogenidae and highlights the necessity for thorough taxonomic revisions of this family as well as some genera (e.g., Paguristes) to reconcile current classifications with phylogenetic relationships.