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

Abstract

Predicting biotic resistance to highly invasive strains of "killer algae" (Caulerpa spp.) requires understanding the diversity and feeding preferences of native consumers, including sea slugs in family Oxynoidae. Past studies reported low algal host specificity for Oxynoe (6 spp.) and Lobiger (4 spp.), but these taxonomically challenging slugs may represent species complexes of unrecognized specialists that prefer different Caulerpa spp. Here, we assess global diversity of these genera by integrating gene sequences with morphological data from microscopic teeth and internal shells, the only hard parts in these soft-bodied invertebrates. Four delimitation methods applied to datasets comprising mtDNA and/or nuclear alleles yielded up to 16 species hypotheses for samples comprising five nominal taxa, including five highly divergent species in Lobiger and five in Oxynoe. Depending on the analysis, a further four to six species were recovered in the O. antillarum-viridis complex, a clade in which mitochondrial divergence was low and nuclear alleles were shared among lineages. Bayesian species delimitation using only morphological data supported most candidate species, however, and integrative analyses combining morphological and genetic data fully supported all complex members. Collectively, our findings double the recognized biodiversity in Oxynoidae, and illustrate the value of including data from traits that mediate fast-evolving ecological interactions during species delimitation. Preference for Caulerpa spp. and radular tooth characteristics covaried among newly delimited species, highlighting an unappreciated degree of host specialization and coevolution in these taxa that may help predict their role in containing outbreaks of invasive algae.

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

“…There is no consensus on a minimum COI distance required to delimitate species, and this varies markedly across taxa [65]. However, substantially larger minimum interspecific distances over maximum intraspecific distances often support the recognition of new species [66,67].…”

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

“…Partial sequences of cytochrome c oxidase subunit I (COI), 16S rRNA (16S), Histone H3 (H3), and 28S rRNA (28S) were amplified by polymerase chain reaction (PCR) using the universal primers: LCO1490 and HCO2198 (Folmer et al., 1994) and custom versions of these primers (see Berriman et al., 2018) for the COI gene (658 bp); 16S ar‐5′ and 16S br‐3′ (Palumbi, 1996) for the 16S gene (435 bp); H3F and H3R (Colgan et al., 2000) for the H3 gene (328 bp). For the 28S gene (1384 bp) three fragments were amplified using the following pairs of primers: 28SC1 and 28SD2R (Vonneman et al., 2005), 28SF2 and 28SR3 (Morgan et al., 2002), and 28SF3 and 28SR1 (Morgan et al., 2003).…”

A New Branchipolynoe (Aphroditiformia: Polynoidae) Scale worm from Deep-Sea Hydrothermal Vents in The Onnuri Vent Field on The Northern Central Indian Ridge

“…There is no consensus on the minimum COI distance required to classify a species, and each taxon has a different minimum COI distance [37]. Therefore, a minimum interspecies distance greater than the maximum interspecies distance is often applied to identify new species [38].…”