Date Published: May 1, 2019
Publisher: Public Library of Science
Author(s): Marta Pola, Sofía Paz-Sedano, Armando Macali, Dan Minchin, Agnese Marchini, Fabio Vitale, Cataldo Licchelli, Fabio Crocetta, Geerat J. Vermeij.
The precise number of Okenia taxa inhabiting the Mediterranean Sea, as well as their general taxonomy, varies according to different specialists. So far, eight valid species have been reported from the area: Okenia aspersa (Alder & Hancock, 1845), Okenia cupella (Vogel & Schultz, 1970), Okenia elegans (Leuckart, 1828), Okenia hispanica Valdés & Ortea, 1995, Okenia impexa Er. Marcus, 1957, Okenia leachii (Alder & Hancock, 1854), Okenia mediterranea (Ihering, 1886), and Okenia zoobotryon (Smallwood, 1910). Of these, only three (O. elegans, O. hispanica, and O. mediterranea) have their type localities in the Mediterranean Sea, whereas the others were described from different biogeographic areas and later included in the Mediterranean biota. We carried out a review on Mediterranean Okenia species through an integrative approach, based on a wide literature search and a morphological and molecular analysis of available type material and samples collected recently. The present study confirmed the presence of O. aspersa, O. elegans, O. hispanica, and O. mediterranea in the Mediterranean Sea, although leaving remaining questions about some of those taxa. The distribution of O. cupella, O. impexa, and O. zoobotryon is limited to the western Atlantic, and of O. leachii to the eastern Atlantic. All specimens previously identified as O. cupella, O. impexa, and O. zoobotryon by different authors in the Mediterranean Sea were repeatedly misidentified. Thus, we describe Okenia problematica sp. nov. and Okenia longiductis sp. nov., from the “Mediterranean” Okenia cupella/impexa and O. zoobotryon. We also consider here Okenia pusilla Sordi, 1974 a nomen dubium and include a redescription of the holotype of O. cupella. A molecular phylogeny, including all the sequenced Okenia species, was performed in order to evaluate the evolutionary relationships of the newly described species with the other congeneric taxa.
The genus Okenia Menke, 1830 (Gastropoda: Nudibranchia: Goniodorididae) includes around 50 valid species worldwide and is composed by small to medium-sized sea slugs, whose distribution spans from cold, temperate, and tropical waters of the Pacific Ocean to the north and south Atlantic Ocean, including the Mediterranean Sea, and bathymetric range goes from the intertidal to the 160 meters depths of Okenia vancouverensis (O´Donoghue, 1921) [1–6]. Little is known about the phylogenetic relationships between Okenia and related genera. Gosliner  synonymized Hopkinsia MacFarland, 1905, Hopkinsiella Baba, 1938, and Sakishimaia Hamatani, 2001 with Okenia based on morphological characters, while Pola et al.  and Paz-Sedano et al.  confirmed the monophyly of the genus Okenia and the synonymy of Hopkinsia and Hopkinsiella proposed by Gosliner  based on preliminary molecular data. An even more intricate situation concerns Okenia alpha taxonomy, since many new species of Okenia have been recently described [2–5, 7–13], some of these lack complete morphological descriptions [5, 12–13], and many new species still require description . The validity of species identification has varied historically according to the views of different authors [2, 15–19], with cryptic or pseudocryptic species still being discovered at the beginning of the twenty-first century . This makes the known distribution of previously described Okenia species uncertain, thereby creating confusion of true geographic ranges [5, 13, 18]. As a result, the overall knowledge of Okenia is incomplete, and reviews focusing on selected biogeographic areas and determining the true identity and spread of Okenia taxa worldwide need to be undertaken through both genetic and anatomical studies.
The taxonomy and biogeography of Okenia species in the Mediterranean Sea has been mainly based on external morphology, with the sole exception of very few works which studied the internal anatomy of selected specimens [17, 18, 57], and the majority of papers dealing with local biota relied only on external resemblances for identification (Table 1 and references therein). However, despite the presence of diagnostic characters even in external morphology (e.g. general colour and shape of papillae), identifications of Okenia taxa have proved to be a challenging task worldwide, resulting in multiple misidentifications that have confused the geographical distribution of species within this genus. This indeed asked for the necessity of an in-depth morphological and molecular review of Mediterranean Okenia species.