Dorvilleidae is a diverse group of annelids, well known from deep sea chemosynthetic environments. Of the dorvilleids, Ophryotrocha is the most speciose genus, but also contains several cryptic species and has been shown to be paraphyletic in many studies. Twenty-one new species of Ophryotrocha are recognized in this study from deep sea methane seep and whale fall environments in the northeast Pacific Ocean, six of which are being formally described. Eight previously known-to-science species were also recovered. The Ophryotrocha specimens were studied using morphology and phylogenetic analyses of DNA sequences from mitochondrial (cytochrome c oxidase subunit I, 16S rRNA, and cytochrome b) and nuclear (18S rRNA and histone 3) genes. The current Ophryotrocha phylogeny is examined and the issue of its paraphyly is addressed, but cannot be resolved in this study.

Caymanostellidae is a group of rarely collected and morphologically unique sea stars that have been exclusively discovered on wood falls in the deep sea. There are currently 3 genera and 7 species within the family, with a wide distribution across the Atlantic, Pacific and Indian Oceans and a depth range between ~414 and 6780 m. Two new species, Caymanostella scrippscognaticausa n. sp. and Caymanostella n. sp., are here identified and described from specimens collected from wood falls in multiple localities across the Pacific Margin of Costa Rica. These records represent the first evidence of caymanostellids occurring on wood falls found at methane seeps and expand the known geographical distribution of caymanostellids. This study also includes the first descriptions of early-stage juvenile caymanostellid sea stars with several collected from experimentally deployed wood, which shows that traits previously considered useful for diagnosis might represent intra-specific ontogenetic variability, with important consequences for caymanostellid taxonomy.

The deep sea, once considered homogeneous and devoid of life, has undergone a transformative shift in understanding since the 19th-century HMS Challenger expedition, with advancements in technology contributing to our growing knowledge of this vast and biodiverse habitat, which comprises 96% of habitable space on Earth and remains largely unexplored. Within this ecosystem, annelids, particularly those inhabiting chemosynthetic environments, play a significant ecological role. Despite the recognized diversity within the order Phyllodocida, which encompasses families such as Siboglinidae, Dorvilleidae, Polynoidae, Serpulidae, and Phyllodocidae, elucidating their intricate relationships has proven challenging due to the scarcity of comprehensive data. Notably, the family Lacydoniidae, found within the order Phyllodocida, stands out. The genus Lacydonia exhibits an extensive bathymetric distribution, occupying a range of deep-sea habitats. However, our understanding of the phylogenetic relationships within Lacydonia as well as aspects such as diet, life cycle, and internal anatomy, remains limited. In this study, we present an updated phylogeny of the genus Lacydonia and describe seven novel species based on meticulous morphological examinations and comprehensive molecular sequence analyses. Specimens were collected from methane seeps in the Eastern Pacific Ocean and Caribbean Sea, resulting from recent research expeditions. By elucidating the taxonomy and evolutionary relationships within Lacydonia, our research provides valuable insights into the remarkable diversity and complex evolutionary history of this enigmatic annelid genus, underscoring the significance of deep-sea exploration for unraveling hidden biodiversity and shedding light on the intricate ecological dynamics of these unique ecosystems.

Nereididae is one of the most diverse families of polychaetes, containing 700 species and 45 genera. Nereidids are commonly found in shallow-water marine biomes, but also occur in deep-sea environments, freshwater habitats, and estuaries. Several species of nereidids have been described from the deep sea; however, only a few species belonging to the genera Nereis have been collected from chemosynthetic-based habitats and two species of Neanthes from a whalefall. These are Nereis sandersi and Nereis piscesae, described from the eastern Pacific vents and Neanthes shinkai from an Atlantic whalefall. New nereidid samples were collected from Costa Rica (seeps), Gulf of California (seeps and vents), California (seeps and whalefalls), Oregon (seeps), and from the North Fiji and Lau Basins (vents), representing three new species. Also, representatives of N. sandersi and N. piscesae from their type localities were obtained as well as specimens of the type species of Nereis (N. pelagica) and Neanthes (N. vaali). Mitogenomes were generated for all these terminals and analysed with existing mitogenome data for Nereididae allowing for the clear delineation of Nereis and Neanthes. It appears there has been at least two origins of chemosynthetic-associated Nereididae.

Osedax is a genus of siboglinid annelids that live on and consume bones in the ocean. Aided by symbiotic bacteria Osedax dissolve the bone matrix for habitat and consume the nutrients inside. Currently there are 27 described and 10 undescribed species of Osedax from 16 localities globally. Using molecular and morphological data we described four new species bringing the total number of Osedax species to 31 and localities to 19. Two species are the first records of Osedax from New Zealand where extensive species diversity is suspected. Two species are from the Gulf of Mexico, one of which is the first species named from a reptile fall. We also expanded the ranges of five described species to Oregon, San Diego, and Costa Rica and conducted population structure analysis on nine species using the COI gene. We found shared haplotypes and evidence of genetic connectivity across broad ranges such as between California and Japan and along the Pacific coast of North and Central America.

Stomatopoda, otherwise known as mantis shrimp, are a diverse group of marine crustaceans with some notable features. They have enlarged second maxillipeds encompassing the raptorial claw that distinguishes them into two groups: smashers and spearers. Previous studies have focused on morphology or a few gene trees, but only recently have there been whole mitochondrial genome-based phylogenies of mantis shrimp. However, there is only a small set of mantis shrimp taxa with their mitogenomes sequenced. Nine new mitochondrial genomes were generated from genome skimming with a conserved gene order and a combined phylogenetic analysis with available data of the mitochondrial 13 protein coding genes, 12S, 16S, and nuclear 18S. Species from three out of the seven superfamilies were used in this study. Two different rooting options were used: 1) Euphausia pacifica (krill) and 2) Hemisquilla californiensis based on the current hypothesis that Hemisquilla is the sister group to the rest of Stomatopoda. The H. californiensis outgroup datasets had the same tree topology as the E. pacifica outgroup datasets with slight variation at low supported nodes. Squilloidea was found to be highly supported as monophyletic while Gonodactyloidea was non-monophyletic. The position of H. californiensis was found inside its superfamily, Gonodactyloidea, and grouped in a low supported clade containing Odontodactylus havanensis and Lysiosquillina maculata for the E. pacifica datasets. An ancestral state reconstruction was performed on the raptorial claw form to determine the evolution of one of its most characteristic features. The results exhibited spearers as the ancestral state with smashing evolving after.

Dorvilleidae is a well-represented group of annelids that are commonly associated with chemosynthetic habitats. Currently, there are 11 described Parougia species, of which two are found at vents or seeps: Parougia wolfi and Parougia oregonensis. Eight new Parougia species are recognized and described in this study from collections in the Pacific Ocean, all from >586 m depth at whale-falls, hydrothermal vents, and/or methane seeps. The specimens were studied using morphology and phylogenetic analyses of DNA sequences from mitochondrial (cytochrome c oxidase subunit I, 16S rRNA, and cytochrome b) and nuclear genes (18S rRNA and Histone 3). We found six sympatric Parougia spp., from Hydrate Ridge, Oregon and four Parougia spp. capable of inhabiting different types of chemosynthetic habitats. Two new species had a most remarkable distributional range and depth, from Oregon to Costa Rica seeps (>5500 km; 587-1583m). The dorvilleid genus Ophryotrocha has previously been highlighted as diversifying in the deep-sea environment. Our results emphasize the hitherto unknown diversity of other taxa such as Parougia at these chemosynthetic systems.

Galapagomystides is an exclusively deep sea group of phyllodocid annelids, originally erected for Galapgomystides aristata from hydrothermal vents of the Galapagos Rift. In this study, numerous specimens of Phyllodocidae collected from hydrothermal vents and methane seeps from the Pacific Ocean, including specimens that are likely to represent Galapgomystides aristata, were studied using morphology (light microscopy and scanning electron microscopy) and DNA sequence data. Phylogenetic analysis of the newly generated molecular data (cytochrome c oxidase subunit I, 16S rRNA, 18S rRNA, and 28S rRNA) combined with an already available extensive dataset for Phyllodocidae resulted in a monophyletic Galapagomystides. Two new species from hydrothermal vents in the West Pacific, G. bobpearsoni n. sp., and G. kathyae n. sp., as well as one new species from a cold seep in the East Pacific, G. patricki n. sp. were inferred from the phylogenetic results and morphology. These new species are formally described, and a previously known vent species, Protomystides verenae, is redescribed and transferred to Galapagomystides. Galapagomystides verenae was found to occur in both vents and seeps in the eastern Pacific, from Oregon to Costa Rica. The diagnosis of Galapagomystides is amended and the biogeography and habitat evolution of the five species of Galapagomystides is discussed. This study reinforces the conception that species within the genus Galapagomystides likely feed on the blood of tube worms, resulting in the colloquial name applied here, “vampire worms”.

There are currently four named species of Branchipolynoe, all found living symbiotically in mytilid mussels in the genus Bathymodiolus, which all occur at hydrothermal vents or methane seeps. Here, phylogenetic analyses using mitochondrial (CO1 and 16S) and nuclear (ITS) genes, as well as morphological analysis were conducted on a collection of Branchipolynoe from Pacific Costa Rican methane seeps. This revealed four new species of Branchipolynoe and these are formally described here. These newly discovered species live in three different species of Bathymodiolus mussels (also new) at depths ranging from 1000 m to 1800 m. Branchipolynoe kajsae sp. nov. and Branchipolynoe halliseyae sp. nov. are found in hosts Bathymodiolus billschneideri, B. earlougheri, and B. nancyschneiderae. Branchipolynoe meridae sp. nov. is found in hosts Bathymodiolus earlougheri and B. billschneideri. Branchipolynoe eliseae sp. nov. is found in hosts Bathymodiolus billschneideri and B. nancyschneiderae. The phylogenetic and biogeographical implications of this high sympatric diversity of Branchipolynoe are discussed.