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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. 

Hesionidae Grube, 1850 currently comprises over 175 species in 28 genera, placed in several subfamilies. Discoveries in recent years have largely been of deep-sea taxa. Here we describe a further four new hesionid species, mainly from methane ‘cold’ seeps at around 1000–1800 m depths off the Pacific coast of Costa Rica and new record of another species. Several of these taxa also occur at methane seeps in the Guaymas Basis (Mexico) and off the USA west coast (California and Oregon). The phylogenetic relationships within Hesionidae are reassessed via maximum parsimony and maximum likelihood analyses of DNA sequences from nuclear (18S rRNA and 28SrRNA) and mitochondrial (16SrRNA and Cytochrome c oxidase I) loci for the new samples. On the basis of these results, we refer one of the new species to Gyptis Marion & Bobretzky in Marion, 1874, one to Neogyptis Pleijel, Rouse, Sundkvist & Nygren, 2012, and two to Sirsoe Pleijel, 1998. The new species Gyptis robertscrippsi n. sp., Neogyptis jeffruoccoi n. sp., Sirsoe dalailamai n. sp. and Sirsoe munki n. sp. We refer to a collection of individuals from seeps ranging from Oregon to Costa Rica as Amphiduropsis cf. axialensis (Blake & Hilbig, 1990), even though this species was described from hydrothermal vents off Oregon. Neogyptis jeffruoccoi n. sp. was generally found living inside the solemyid clam Acharax johnsoni (Dall, 1891). The position of Hesiolyra bergi Blake, 1985 is resolved on the basis of newly-collected specimens from near the type locality and, as a result, Hesiolyrinae Pleijel, 1998 is synonymized with Gyptini Pleijel, 1998 (and Gyptinae Pleijel, 1998).http://zoobank.org/urn:lsid:zoobank.org:pub:9C0E88EE-34F8-4F25-9EC8-91797618AC86