Pilargidae is a family of free-living and burrowing marine annelids. A lack of available molecular data for most of these species has precluded a molecular assessment of their phylogenetic relationships and has left uncertain the placement of Antonbruunia, which is hypothesized to be either a member of Pilargidae or its sister clade, the monotypic family Antonbruunidae. In this study, we describe the new species Antonbruunia milenae sp. nov., found at 845 m of depth off the coast of San Diego, California, USA, and we address the phylogeny of these organisms using 15 novel mitogenomes and multiple Sanger-sequenced loci. Our results show that Antonbruunia falls within Pilargidae, making Antonbruunidae a junior synonym of Pilargidae. Glyphohesione was transferred from Pilarginae to Synelminae, the previously unassigned genera Otopsis and Antonbruunia were shown to belong within Synelminae, and Hermundura was assigned to Phyllodocida incertae sedis. Sigambra was found to be non-monophyletic. Four different mitogenome gene orders were found among Pilargidae. Changes between the gene orders and the ancestral state gene order of the family were inferred. Two species have introns within the COI gene. These efforts represent a significant expansion of the available molecular resources for pilargids, as well as the basis for a more stable taxonomy.

Nereidid polychaetes are well known from shallow marine habitats, but their diversity in the deep sea is poorly known. Here we describe an unusual new nereidid species found at methane seeps off the Pacific coast of Costa Rica. Specimens of Pectinereis strickrotti gen. nov., sp. nov. had been observed dating back to 2009 swimming just above the seafloor at ~1,000 m depth but were not successfully captured until 2018. Male epitokes were collected as well as a fragment of an infaunal female found in a pushcore sample. The specimens were all confirmed as the same species based on mitochondrial COI. Phylogenetic analyses, including one based on available whole mitochondrial genomes for nereidids, revealed no close relative, allowing for the placement of the new species in its own genus within the subfamily Nereidinae. This was supported by the unusual non-reproductive and epitokous morphology, including parapodial cirrostyles as pectinate gills, hooked aciculae, elfin-shoe-shaped ventral cirrophores, and elongate, fusiform dorsal ligules emerging sub-medially to enlarged cirrophores. Additionally, the gill-bearing subfamily Dendronereidinae, generally regarded as a junior synonym of Gymnonereidinae, is reviewed and it is here reinstated and as a monogeneric taxon.

The methane seeps on the Pacific margin of Costa Rica support extensive animal diversity and offer insights into deep-sea biogeography. During five expeditions between 2009 and 2019, we conducted intensive faunal sampling via 63 submersible dives to 11 localities at depths of 300–3600 m. Based on these expeditions and published literature, we compiled voucher specimens, images, and 274 newly published DNA sequences to present a taxonomic inventory of macrofaunal and megafaunal diversity with a focus on invertebrates. In total 488 morphospecies were identified, representing the highest number of distinct morphospecies published from a single seep or vent region to date. Of these, 131 are described species, at least 58 are undescribed species, and the remainder include some degree of taxonomic uncertainty, likely representing additional undescribed species. Of the described species, 38 are known only from the Costa Rica seeps and their vicinity. Fifteen range extensions are also reported for species known from Mexico, the Galápagos seamounts, Chile, and the western Pacific; as well as 16 new depth records and three new seep records for species known to occur at vents or organic falls. No single evolutionary narrative explains the patterns of biodiversity at these seeps, as even morphologically indistinguishable species can show different biogeographic affinities, biogeographic ranges, or depth ranges. The value of careful molecular taxonomy and comprehensive specimen-based regional inventories is emphasized for biodiversity research and monitoring.

Xyloplax is a genus of three species of sea stars previously found only on sunken wood in the deep ocean. Their circular and petaloid bodies, which lend them their common name “sea daisy”, and their presumed exclusive diet of wood make them an unusual and rare element of deep-sea ecosystems. We describe here the fourth species of Xyloplax from the eastern Pacific Ocean, Xyloplax princealberti n. sp., which ranges from offshore Canada to the Gulf of California (Mexico) and Costa Rica. Though sampled geographically close to another described species of Xyloplax from the northeastern Pacific, X. janetae, this new species is unique morphologically and according to available DNA data. The short abactinal spines are the most obvious feature that distinguishes X. princealberti n. sp. from other Xyloplax. The minimum distance for mitochondrial cytochrome c oxidase subunit I from Xyloplax princealberti n. sp. to the only other available Xyloplax, X. janetae, was 13.5%. We also describe Ridgeia vestimentiferan tubeworm bushes from active hydrothermal vents as a new Xyloplax habitat, the first record of a non-wood substrate, and a new reproductive strategy, simultaneous hermaphroditism, for this genus. We generated the first mitochondrial genome for a member of Xyloplax and analyzed it with other available asteroid data using nucleotide-coding or amino acid (for protein-coding genes) plus nucleotide coding (for rRNA genes). The nucleotide-coding results place Xylopax as part of the clade Velatida, consistent with a previous phylogenomic analysis that included Xyloplax princealberti n. sp. (as Xyloplax sp.), though the placement of Velatida within Asteroidea differed. The amino acid plus nucleotide coding recovered Velatida to be a grade with X. princealberti n. sp. as sister group to all other Asteroidea.

Within Polynoidae, a diverse aphroditiform family, the subfamily Macellicephalinae comprises anchialine cave-dwelling and deep-sea scaleworms. In this study, Lepidonotopodinae is synonymized with Macellicephalinae, and the tribe Lepidonotopodini is applied to a well-supported clade inhabiting deep-sea chemosynthetic-based ecosystems. Newly sequenced “genome skimming” data for 30 deep-sea polynoids and the comparatively shallow living Eulagisca gigantea is used to bioinformatically assemble their mitogenomes. When analyzed with existing scaleworm mitogenomes, deep-sea scaleworms exhibit increased gene order rearrangement events compared to shallow-water relatives. Additionally, comparative analyses of shallow-water vs. deep-sea polynoid substitution rates in mitochondrial protein-coding genes show an overall relaxed purifying selection and a positive selection of several amino acid sites in deep-sea species, indicating that polynoid mitogenomes have undergone selective pressure to evolve metabolic adaptations suited to deep-sea environments. Furthermore, the inclusion of skimming data for already known Lepidonotopodini species allowed for an increased coverage of DNA data and a representation of the taxa necessary to create a more robust phylogeny using 18 genes, as opposed to the six genes previously used. The phylogenetic results support the erection of Cladopolynoe gen. nov., Mamiwata gen. nov., Photinopolynoe gen. nov., Stratigos gen. nov., and Themis gen. nov., and emended diagnoses for Branchinotogluma, Branchipolynoe, Lepidonotopodium, and Levensteiniella.

Specimens of a new species of torquaratorid acorn worm (Hemichordata, Enteropneusta) were video recorded and subsequently collected at abyssal depths in the eastern North Pacific at sites ranging from Oregon to northern Mexico. These worms are described here as Yoda demiankoopi n. sp. by molecular and morphological methods. The new species differs from its only described congener, Yoda purpurata, in three ways. First, the lips are extremely wide and indented by a deep ciliary groove for ingesting substrate and conveying it to the mouth. Second, a connective tissue bulge of unknown significance runs mid‐dorsally along the hepatic and posthepatic regions of the intestine. Third, the posthepatic intestine is strikingly sinuous and packed with gut contents presumably undergoing digestion for extended periods between infrequent defecations. Yoda demiankoopi n. sp. is hermaphroditic, a character so far known only for the genus Yoda in the entire class Enteropneusta. The gonads of each adult worm comprise hundreds of ovaries (each containing a single oocyte) and hundreds of testes located just beneath the dorsal epidermis of the anterior trunk and associated genital wings. In any given animal, at any given time, gametes of only one sex or the other become fully mature. Thus, the worm is a sequential hermaphrodite, alternately spawning purely as a female or purely as a male.