Molecular phylogenies confirm the presence of two crypticHemimycalespecies in the Mediterranean and reveal the polyphyly of the generaCrellaandHemimycale(Demospongiae: Poecilosclerida)

Abstract

BackgroundSponges 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.Hemimycaleis a sponge genus (Family Hymedesmiidae, Order Poecilosclerida) with four formally described species, from which onlyHemimycale columellahas been recorded in the Atlanto-Mediterranean basin, on shallow to 80 m deep bottoms. Contrasting biological features between shallow and deep individuals ofHemimycale columellasuggested 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 ofHemimycale columellaacross the Mediterranean. We also included otherHemimycaleandCrellaspecies from the Red Sea, with the additional aim of clarifying the relationships of the genusHemimycale.MethodsHemimycale columellawas sampled across the Mediterranean, and Adriatic Seas.Hemimycale arabicaandCrella cyathophorawere 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.ResultsThe several phylogenetic reconstructions retrieved bothCrellaandHemimycalepolyphyletic. Strong differences in COI sequences indicated thatC. cyathophorafrom the Red Sea might belong in a different genus, closer toHemimycale arabicathan to the Atlanto-MediterraneanCrellaspp. Molecular and external morphological differences betweenHemimycale arabicaand the Atlanto-MediterraneanHemimycalealso suggest thatHemimycale arabicafit 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-MediterraneanHemimycale. Moreover, what was known up to now asHemimycale columella,is formed by two cryptic species with contrasting bathymetric distributions. Some small but consistent morphological differences allow species distinction.ConclusionsA new family (Hemimycalidae) including the genusHemimycaleand the two purported new genera receivingC. cyathophoraandHemimycale arabicamight 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 (Hemimycale mediterraneasp. nov.) is described. Morphologically undifferentiated species with contrasting biological traits, as those here reported, confirm that unidentified cryptic species may confound ecological studies.