A biting commentary: Integrating tooth characters with molecular data doubles known species diversity in a lineage of sea slugs that consume "killer algae"

A New Branchipolynoe (Aphroditiformia: Polynoidae) Scale Worm from the Onnuri Deep-sea Hydrothermal Vent Field, Northern Central Indian Ridge

Deep-sea hydrothermal vents are dynamic environments with exotic fauna, including bathymodiolin mussels and scale worm annelids that are often in close association. In this study, we found a new species of Branchipolynoe (Aphroditiformia: Polynoidae) living in the recently discovered mussel Gigantidas vrijenhoeki in deep-sea hydrothermal vents and methane seeps at 2,014-2,023 m depth. Based on the morphology and full mitochondrial genome sequences of specimens of Branchipolynoe from the Onnuri vent field (OVF) on the northern Central Indian Ridge, we describe them as a new species: Branchipolynoe onnuriensis sp. nov. This species resembles B. longqiensis and B. tjiasmantoi, but can be distinguished from these species by the shape of the notopodial acicular lobe and the tips of the subacicular neurochaetae. This identity is well-supported by genetic distance and phylogenetic analyses based on the mitochondrial c oxidase subunit I (COI) gene, with the new species being closest to the Western Pacific species B. tjiasmantoi. Phylogenetic analyses support close relationships between the Indian Ocean and Western Pacific hydrothermal polychaetes. Our data provide a foundation for exploring the evolutionary relationship between scale worms and bathymodiolin mussels.

Ecological speciation by sympatric host shifts in a clade of herbivorous sea slugs, with introgression and localized mitochondrial capture between species

Host shifting in insect-plant systems was historically important to the development of ecological speciation theory, yet surprisingly few studies have examined whether host shifting drives diversification of marine herbivores. When small-bodied consumers feed and also mate on a preferred host, disruptive selection can split a population into host races despite gene flow. Support for host shifts is notably lacking for invertebrates associated with macroalgae, where the scale of dispersal by planktonic larvae often far exceeds the grain of host patchiness, and adults are typically less specialized than terrestrial herbivores. Here, we present a candidate example of ecological speciation in a clade of sea slugs that primarily consume green algae in the genus Caulerpa, including highly invasive species. Ancestral character state reconstructions supported 'sea grapes' (C. racemosa, C. lentillifera) as the ancestral host for a tropical radiation of 12 Elysia spp., with one shift onto alternative Caulerpa spp. in the Indo-Pacific. A Caribbean radiation of three species included symaptric host shifts to Rhipocephalus brevicaulis in the ancestor of E. pratensis Ortea & Espinosa, 1996, and to C. prolifera in E. hamanni Krug, Vendetti & Valdes 2016, plus a niche expansion to a range of Caulerpa spp. in E. subornata Verrill, 1901. All three species are broadly sympatric across the Caribbean but are host-partitioned at a fine grain, and distinct by morphology and at nuclear loci. However, non-recombining mtDNA revealed a history of gene flow between E. pratensis and E. subornata: COI haplotypes from E. subornata were 10.4% divergent from E. pratensis haplotypes from four sites, but closely related to all E. pratensis haplotypes sampled from six Bahamian islands, indicating historical introgression and localized "mitochondrial capture." Disruptive selective likely fueled divergence and adaptation to distinct host environments, indicating ecological speciation may be an under-appreciated driver of diversification for marine herbivores as well as epibionts and other resource specialists.

A molecular phylogeny of Thuridilla Bergh, 1872 sea slugs (Gastropoda, Sacoglossa) reveals a case of flamboyant and cryptic radiation in the marine realm

The genus Thuridilla Bergh, 1872 comprises mostly tropical sap-sucking sea slugs species with flamboyantly coloured forms. However, the potential for cryptic or pseudocryptic species masked by convergent or polymorphic colour patterns has not been tested using molecular characters. In this study, we sampled 20 of the 23 recognized worldwide species and performed the most comprehensive molecular phylogenetic analysis of the genus to date using a multi-locus approach combining two mitochondrial (cytochrome c oxidase subunit I, 16S rRNA) and two nuclear (Histone H3, 28S rRNA) genes using maximum likelihood, maximum-parsimony and Bayesian criteria. Three molecular species delimitation methods (ABGD, GMYC, bPTP) and the morphology of radular teeth were additionally used to aid in species delimitation. Our analyses supported 35 species within Thuridilla, of which more than one-third (13) are part of a single radiation here named the Thuridilla gracilis (Risbec, 1928) species-complex. This complex includes T. gracilis, T. splendens (Baba, 1949), T. bayeri (Er. Marcus, 1965), and T. ratna (Er. Marcus, 1965), plus nine additional undescribed species. All 13 species are distinguishable by radular characters, external morphology and their DNA. The detection of this radiation led diversity of Thuridilla to be underestimated by about 25% and provides a new comparative system for studying the role of colour patterns in marine diversification.

Phylogeny and Biogeography of Branchipolynoe (Polynoidae, Phyllodocida, Aciculata, Annelida), with Descriptions of Five New Species from Methane Seeps and Hydrothermal Vents

The four named species of Branchipolynoe all live symbiotically in mytilid mussels (Bathymodiolus) that occur at hydrothermal vents or methane seeps. Analyses using mitochondrial (COI and 16S) and nuclear (ITS) genes, as well as morphology, were conducted on a collection of Branchipolynoe from Pacific Costa Rican methane seeps and West Pacific hydrothermal vents. This revealed five new species of Branchipolynoe, and these are formally described. The new species from Costa Rica live in three species of Bathymodiolus mussels (also new) at depths ranging from 1000 to 1800 m. Branchipolynoe kajsae n. sp. and Branchipolynoe halliseyae n. sp. were found in all three undescribed Bathymodiolus species, while Branchipolynoe eliseae n. sp. was found in Bathymodiolus spp. 1 and 2, and Branchipolynoe meridae n. sp. was found in Bathymodiolus spp. 1 and 3. Hence, Bathymodiolus sp. 1 hosted all four of the new species, while the other two Bathymodiolus hosted three each. Most mussels contained only one specimen of Branchipolynoe; where there was more than one, these were often a female and smaller male of the same species. The newly discovered species from the West Pacific, Branchipolynoe tjiasmantoi n. sp., lives in unidentified Bathymodiolus at depths ranging from 674 to 2657 m from hydrothermal vents in the North Fiji (Fiji) and Lau Basins (Tonga) and also from New Zealand, Vanuatu, and the Manus Basin (Papua New Guinea). The phylogenetic and biogeographical implications of this diversity of Branchipolynoe are discussed.

Phylogenetic systematics of the shelled sea slug genus Oxynoe Rafinesque, 1814 (Heterobranchia : Sacoglossa), with integrative descriptions of seven new species

An integrative approach to investigate the species-level diversity in Oxynoe (Mollusca, Heterobranchia, Sacoglossa) revealed the existence of 11 distinct taxa. Oxynoe viridis (Pease, 1861) and Oxynoe antillarum Mörch, 1863 are redescribed; Oxynoe natalensis E. A. Smith, 1903 and Oxynoe azuropunctata Jensen, 1980 are regarded as valid. Species originally described from empty shells are regarded as nomina dubia. Seven new species are described, four from the tropical Indo-West Pacific: Oxynoe kylei, sp. nov., Oxynoe neridae, sp. nov., Oxynoe jordani, sp. nov. and Oxynoe jacksoni, sp. nov.; and two from the tropical Atlantic: Oxynoe struthioe, sp. nov. and Oxynoe ilani, sp. nov. The name Oxynoe panamensis Pilsbry & Olsson, 1943 has been applied to eastern Pacific specimens, but was introduced based on material collected from the Caribbean; therefore, the new name Oxynoe aliciae, sp. nov. is introduced for eastern Pacific specimens. Species are delineated using molecular and morphological traits, as well as algal host and reproductive biology. Results from morphological comparisons are concordant with molecular and integrative species delimitation analyses, providing robust evidence for species hypotheses. As Oxynoe is one of the few groups specialised to feed on the green algal genus Caulerpa, which includes highly invasive species, clarifying the taxonomy of Oxynoe may inform efforts to predict community response to disruptive algal invasions.