Based on a combination of intensive literature review, electronic database searches, re-identification of museum specimens and fieldwork, we hereby provide an updated checklist of the freshwater fishes of continental and insular Costa Rica. This checklist, systematically arranged at the ordinal and familial level, includes nomenclatural revisions, distributional information, and when appropriate, cross-references on the basis of Bussing (1998). According to our results, the native Costa Rican freshwater fish fauna is composed by 250 species, divided into 119 genera, 49 families and 19 orders; increasing in 108 the number of species originally reported by Bussing (1998). By far, the vast majority of these species, according to their supposed tolerance to salinity, are peripheral (63.2%), followed by secondary freshwater fishes (23.6%); only 13.2% are primary freshwater fishes. 24 species in this checklist appear to be endemic to Costa Rica. In addition to the native fauna 8 exotic species are reported.

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A checklist of the marine and estuarine fishes of the inner part of the Gulf of Nicoya, Pacific coast of Costa Rica, Central America, was compiled by examining a museum fish collection, resulting in 72 families and 274 species. Of these species, 127 (46.4%) were marine species and 147 (53.6%) were estuarine-associated species. In terms of their life history and considering the habitat type classification, 188 (almost 70% of the total) were categorized as species inhabiting soft-bottom habitats, reflecting the large estuarine environment and rich fish diversity of the Gulf despite its relatively small area in the tropical Eastern Pacific region. Furthermore, the list contains 13 threatened species of IUCN Red List, which need further research to understand their abundance and their exposure to habitat loss in the Gulf. Further detailed studies on its fish fauna and habitat are needed to better understand and conserve biodiversity within the whole Gulf.

A new species of Chimaera Linnaeus 1758 is described from three specimens collected from off the Pacific coasts of Costa Rica and Peru. Chimaera orientalis n. sp., the first species of the genus described from the eastern Pacific Ocean, is distinguished from its other congeners by a combination of coloration and morphology. Additionally, new records of occurrence for another four species of chimaeroid fishes (Harriotta raleighana (Goode & Bean 1895), Rhinochimaera africana Compagno, Stehmann & Ebert 1990, Hydrolagus colliei Lay & Bennett 1839, and H. macrophthalmus de Buen 1959) previously unknown for the continental shelf of the Pacific coast of Costa Rica and Central America are reported. A key to the eastern Pacific species of the order Chimaeriformes is also presented.

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Astyanax anai, a new species of characid fish, is described from the Sixaola River basin, eastern Costa Rica-western Panama, Central America. The new species can be distinguished from all other congeners by the following combination of characters: premaxillary teeth 4-5 at the inner series and 4-6 at the outer series; maxillary teeth tricuspid, 2-4; predorsal scale series irregular and incomplete, with an unscaled space behind tip of supraoccipital process and 12-14 scales; lateral line scales 34-39; humeral region with a conspicuous black and rounded to horizontally ovate spot and two diffuse brown and vertically elongate bars (the first through the rounded to horizontally ovate spot, the second 2-4 scales behind the first); body depth 36·6-42·3% of standard length (L ); midlateral stripe formed by a series of 10-14 anteriorly-directed dermal herringbone, or chevron-shaped, marks, most apparent in juveniles and in preserved specimens, extending above the lateral line from the black humeral spot or just behind it (from the second vertical bar) to the caudal peduncle; scale rows from lateral line to base of first dorsal-fin ray 8-9; scale rows from lateral line to base of pelvic fin 7-8; pre-anal distance 53·9-61·9% of L ; total anal-fin elements 29-33; caudal spot elongated, rhomboid or rectangular, with its anterior margin surpassing the middle of the caudal peduncle, usually reaching the anal-fin insertion, posteriorly covering 4-7 principal caudal-fin rays and not extending onto the ventral and dorsal margins of the caudal peduncle, covering 3-5 horizontal scale rows. In order to test the phylogenetic relationships of the new taxon in relation to the other North and Central American species of the genus, a new phylogenetic hypothesis based on a reanalysis of the morphological matrix by Schmitter-Soto (2016) is proposed. A key to the lower Central American (southern Nicaragua to eastern Panama) species of Astyanax is also provided.