The methane seeps on the Pacific margin of Costa Rica support extensive animal diversity and offer insights into deep-sea biogeography and phylogeography. 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 267 newly published DNA sequences to present a taxonomic inventory of macrofaunal and megafaunal diversity with a focus on invertebrates. We identified 487 morphospecies, of which 132 are described species, at least 57 are undescribed species, and the remainder include some degree of taxonomic uncertainty, likely representing additional undescribed species. This total represents the highest number of distinct morphospecies published from a single seep or vent region to date. Of the described species, 38 are known only from the Costa Rica seeps. We also report 13 range extensions for species known from Mexico, the Galápagos seamounts, Chile, and the western Pacific; 17 new depth records; and three new seep records for species known to occur at vents or organic falls. We note that no single evolutionary narrative explains the patterns of biodiversity at these seeps, as even morphologically indistinguishable species can show different phylogeographic affinities, biogeographic ranges, or depth restrictions. We emphasize the value of careful molecular taxonomy and comprehensive specimen-based regional inventories for biodiversity research and monitoring.

biodiversity, biogeography, Central America, chemosynthetic ecosystem, DNA ‘barcodes’, COI, deep sea, molecular taxonomy