Molecular phylogenies confirm the presence of two cryptic species in the Mediterranean and reveal the polyphyly of the genera and (Demospongiae: Poecilosclerida).

Background: Sponges are particularly prone to hiding cryptic species as their paradigmatic plasticity often favors species phenotypic convergence as a result of adaptation to similar habitat conditions. is a sponge genus (Family Hymedesmiidae, Order Poecilosclerida) with four formally described species, from which only has been recorded in the Atlanto-Mediterranean basin, on shallow to 80 m deep bottoms. Contrasting biological features between shallow and deep individuals of suggested larger genetic differences than those expected between sponge populations. To assess whether shallow and deep populations indeed belong to different species, we performed a phylogenetic study of across the Mediterranean. We also included other and species from the Red Sea, with the additional aim of clarifying the relationships of the genus .

Methods: was sampled across the Mediterranean, and Adriatic Seas. and were collected from the Red Sea and Pacific. From two to three specimens per species and locality were extracted, amplified for Cytochrome C Oxidase I (COI) (M1-M6 partition), 18S rRNA, and 28S (D3-D5 partition) and sequenced. Sequences were aligned using Clustal W v.1.81. Phylogenetic trees were constructed under neighbor joining (NJ), Bayesian inference (BI), and maximum likelihood (ML) criteria as implemented in Geneious software 9.01. Moreover, spicules of the target species were observed through a Scanning Electron microscope.

Results: The several phylogenetic reconstructions retrieved both and polyphyletic. Strong differences in COI sequences indicated that from the Red Sea might belong in a different genus, closer to than to the Atlanto-Mediterranean spp. Molecular and external morphological differences between and the Atlanto-Mediterranean also suggest that fit in a separate genus. On the other hand, the Atlanto-Mediterranean Crellidae appeared in 18S and 28S phylogenies as a sister group of the Atlanto-Mediterranean . Moreover, what was known up to now as is formed by two cryptic species with contrasting bathymetric distributions. Some small but consistent morphological differences allow species distinction.

Conclusions: A new family (Hemimycalidae) including the genus and the two purported new genera receiving and might be proposed according to our phylogenetic results. However, the inclusion of additional Operational Taxonomic Unit (OTUs) appears convenient before taking definite taxonomical decisions. A new cryptic species ( sp. nov.) is described. Morphologically undifferentiated species with contrasting biological traits, as those here reported, confirm that unidentified cryptic species may confound ecological studies.