Key to species of fishes from the Coto River basin, Pacific coast, Costa Rica The following key is based on our research and data available in the literature (Bussing 1987, 1998, Angulo et al. 2015 and Robertson & Allen 2015). 1 Body asymmetrical, strongly compressed laterally; both eyes on the same side of head.............................. 2 - Body symmetrical, not strongly compressed laterally; one eye on each side of head................................. 3 2 Pigment and eyes on the left side of fish (PARALICHTHYIDAE) …....................... Citharichthys gilberti (Fig. 8C; Other diagnostic features include: teeth equally developed on both sides of the jaws, in a single series of immovable teeth on each jaw, no canines but front teeth a little enlarged; gill rakers slender, moderately long, lower rakers 12–15; length of the pectoral fin of the eyed side 43–59% of head length) - Pigment and eyes on right side of fish (ACHIRIDAE) ….. Trinectes fonsecensis (Fig. 8B; Other diagnostic features include: interbranchial foramen absent; dorsal fin rays 58–60; scales on eyed side without black hair like filaments) 3 Body covered by bony plates or rings..................................................................... 4 - Body not covered by bony plates or rings, naked or with normal scales.......................................... 7 4 Body robust to elongated, covered by a kind of armor composed by a series of bone segments in the form of plates; mouth ventral, intermediate in size, with teeth, prominent lips with papillae (small projections) and chins, snout non-tubular; eyes in dorsolateral position; pelvic fins present (LORICARIIDAE)................................................... 5 - Body slim and extremely elongated, covered by a kind of armor composed by a series of bone segments in the form of rings; mouth small, without teeth or prominent lips, located at the end of a long and tubular snout; eyes in lateral position; pelvic fins absent (SYNGNATHIDAE) ….......................................................................... 6 5 Ventral portion of the body (abdomen) naked, not covered by no or few bone plates (the species has abdominal plates in other portions of its range); adipose fin present; caudal peduncle not very elongated (postdorsal portion of body shorter or about equal than the predorsal portion) nor dorsally compressed …...... Hypostomus aspidolepi s (Fig. 8 H–I; Other diagnostic features include: general coloration greenish-brown to reddish-brown with black circular blotches on fins and the ventral portion of the body, which tend to be more pronounced at the anterior portion of the body; adipose dorsal fin with strong spines) - Ventral portion of the body (abdomen) covered by bone plates; adipose fin absent; caudal peduncle elongated (postdorsal portion of body larger than the predorsal portion) and dorsally compressed …..................... Rineloricaria uracantha (Fig. 8J; Other diagnostic features include: general coloration yellowish-brown to gray, darker dorsally, with four or five gray or black bars crossing the dorsum, abdomen straw-colored, fins yellowish-brown to grayish with black spots) 6 Dorsal fin rays 37–44; snout short 35–37% of head length; head with a dark lateral stripe …............... Pseudophallus elcapitanensis (Fig. 8M; Other diagnostic features include: general coloration dark brown, usually darker dorsally, with lighter specks along the body) - Dorsal fin rays 30–35; snout long 38–43% of head length; head without marks ….... Pseudophallus starksii (Fig. 8M; Other diagnostic features include: general coloration dark or light brown dorsally with the belly always pale, light yellow to straw) 7 Pectoral and pelvic fins absents; dorsal, anal and caudal fins vestigial (SYNBRANCHIDAE) … Synbranchus marmoratus (Fig. 8L; Other diagnostic features include: body elongated, anguilliform; scales absent; general coloration grayish-brown or yellowish-brown, darker above and paler with dark speckles ventrally) - Pectoral, pelvic, dorsal, anal and caudal fins presents and well developed …...................................... 8 8 Pelvic fins united into an adhesive disc..................................................................… 9 - Pelvic fins separated, not in the form of an adhesive disc...................................................… 11 9 Upper jaw teeth minute, compressed; gill rakers absent; adhesive disc circular, its width greater than its median length; males with the first dorsal fin spines elongated (filamentous).................. Sicydium salvini (Fig. 6 M–O; Other diagnostic features include: general coloration dark brown to grayish or yellowish brown, darker dorsally and paler ventrally, usually with 6–7 dark bars on the sides and/or a longitudinal dark band; dorsal margin of the dorsal, anal and caudal fins usually red) - Upper jaw teeth conspicuous, conical to caniniform; gill rakers present; adhesive disc oval, its width less than its median length; first dorsal fin spines not elongated................................................................... … 10 10 Snout pointed in lateral view; scales in lateral series 67–73; second dorsal fin short, with 9–12 rays; anal fin short, with 9–11 rays; caudal fin short, about equal to head length, and rounded … Awaous transandeanus (Fig. 6 J–K; Other diagnostic features include: general coloration light brown dorsally, with the sides light yellow to tan; ventral surface white or pale yellow; sides of head with 3 black slanted bars; body with about 7 irregular black bars on the upper side and 7–8 black blotches on the flanks; back with a series of black irregular small spots, in front of and along the first dorsal fin; dorsal fin yellowish) - Snout rounded in lateral view; scales in lateral series 55–65; second dorsal fin long, with 12–15 rays; anal fin short, with 13–15 rays; caudal fin relatively long, larger than head length, and pointed......................................................................... Ctenogobius sagittula (Fig. 6L; Other diagnostic features include: head and body light tan, with 4 dark brown blotches along the middle of the sides plus a dark brown spot at the middle of the caudal fin base; a brown stripe on the cheek and a brown blotch just behind the center of the gill cover; a dark blotch on the side of the nape, above the pectoral fin base; second dorsal and caudal fin with brown spots) 11 Body naked; barbels present........................................................................... 12 - Body covered by scales; barbels absent.................................................................. 15 12 Opercular odontodes present; pectoral and dorsal fin spines absent; dorsal adipose fin absent; caudal fin rounded to truncate (TRICHOMYCTERIDAE)............................... Trichomycterus striatus (Fig. 8K; Other diagnostic features include: general coloration brown, light brown, yellowish or light orange, darker on dorsal surface, and paler ventrally, usually with a continuous black lateral band (of variable thickness), extending from opercle to caudal-fin base; paired, dorsal and anal fins light brown, yellowish (usually darker at base) or translucent; caudal fin brown or yellowish) - Opercular odontodes absent; pectoral and dorsal fin spines present; dorsal adipose fin present; caudal fin forked (HEPTAPTER- IDAE)............................................................................................. 13 13 Occipital process continuous with the osseous plate of the dorsal spine (see Fig. 22a in Bussing 1998, page 143); dorsal fin spine strong and pungent, like the pectoral spine................................ Pimelodella chagresi (Fig. 8D; Other diagnostic features include: general coloration light brown to light gray with a lateral band of intensive black color that extends from the tip of the snout to the edge of the caudal fin and another dark band, a bit thinner, in the dorsal margin, from the nape to the caudal peduncle; maxillary barbels extending to the end of the dorsal fin) - Occipital process no continuous with the osseous plate of the dorsal spine, i.e., not reaching the dorsal plate (see Fig. 22a in Bussing 1998, page 143); dorsal-fin spine weak and usually flexible............................................ 14 14 Maxillary barbels long, usually extending beyond the origin of the dorsal fin; serrations present on both margins of the pectoral spines; space between the dorsal and adipose fins shorter than the length of the base of the dorsal fin................... Rhamdia guatemalensis (Fig. 8 E–F; Other diagnostic features include: general coloration dark gray to grayish-brown, darker dorsally and paler ventrally, some specimens shows a lateral black band of variable thickness) - Maxillary barbels short, not extending beyond the pectoral fins; serrations present only on the inner (posterior) margin of the pectoral spines, sometimes without serrations; space between the dorsal and adipose fins usually larger than the length of the base of the dorsal fin........... Rhamdia laticauda (Fig. 8G; Other diagnostic features include: general coloration gray to grayish-brown or yellowish-brown, darker dorsally and paler ventrally) 15 Dorsal adipose fin present............................................................................. 16 - Dorsal adipose fin absent.............................................................................. 22 16 Anal fin rays 8–10................................................................................... 17 - Anal fin rays more than 20............................................................................ 18 17 Body rhomboidal, higher in the middle portion of the body (approximately at the origin of the dorsal fin), slightly compressed laterally; teeth absent; eye diameter greater than the length of the mouth; longitudinal series with 36–37 scales (CURIMATI- DAE).. Cyphocharax magdalenae (Fig. 2 L–M; Other diagnostic features include: general coloration silver to silvery-golden, without marks on body; fins transparent, yellowish or brownish) - Body elongate, fusiform, height relatively uniform along the body, robust, not compressed laterally; eye small, eye diameter less than the length of the mouth; teeth present; longitudinal series with 26–27 scales (LEBIASINIDAE)................ Lebiasina boruca (Fig. 3 A–B; Other diagnostic features include: general coloration silver to gray above and on the flanks and white below, with three longitudinal red stripes on each side formed by a red blotch on each scale of the three lateral rows; fins transparent, yellowish, reddish or brownish) 18 Upper jaw projected anteriorly (i.e., its anterior margin clearly surpasses the anterior margin of the lower jaw in the vertical plane); upper jaw with three or four rows of teeth (in ventral view); adults easily exceeding 15 cm in total length (BRYCONI- DAE). Brycon behreae (Fig. 2 C–E; Other diagnostic features include: general coloration greenish-silvery to pinkish-silvery, greenish-gray dorsally, whitish-gray dorsally; fins transparent to reddish or yellowish-gray) - Upper jaw not very projected anteriorly (i.e., its anterior margin about at the same level as the anterior margin of the lower jaw in the vertical plane); upper jaw with one or two rows of teeth (in ventral view), in Roeboides (see below) it is also possible to observe two pairs of external teeth in the premaxilla; adults not exceed 15 cm in total length (CHARACIDAE)......... 19 19 Body strongly compressed; nape concave; two pairs of external teeth on premaxillary; anal fin very long, with 42–45 rays.. Roeboides bussingi (Fig. 2 I–K; Other diagnostic features include: general coloration yellowish-silvery; fins transparent to yellowish-orange or red) - Body not strongly compressed; nape convex or straight; no external teeth on premaxillary; anal fin short, with 20–29 rays..................................................................................................... 20 20 Premaxillary teeth in two rows; humeral spot present, always shaped like the letter P; caudal fin usually without yellow spots; maximum body size about 130 mm SL................................................... Astyanax orstedii (Fig. 2F; Other diagnostic features include: general coloration grayish to withish-silvery, darker dorsally, usually greenish-brown or gray, and paler ventrally; fins transparent to yellowish-orange or red) - Premaxillary teeth in a single row; humeral spot absent or, if present, not shaped like a letter P; caudal fin with two yellow spots, above and below the dark caudal fin blotch; maximum body size about 50 mm SL................................ 21 21 Maxillary long, its length 52.6–58.5% of eye diameter; upper margin of the eye usually without pigment or brownish; anal fin rays 20–23... Odontostilbe dialeptura (Fig. 2G; Other diagnostic features include: general coloration yellowish-silvery; fins transparent to yellowish-orange) - Maxillary short, its length 28.6–43.5% of eye diameter; upper margin of the eye usually reddish; anal fin rays 23–27....... Pseudocheirodon terrabae (Fig. 2H; Other diagnostic features include: general coloration whitish to yellowish-silvery; fins transparent to yellowish-orange or red) 22 One dorsal fin...................................................................................... 23 - Two dorsal fins..................................................................................... 48 23 Fins without spines; caudal fin rounded.................................................................. 24 - One or more fins with prominent spines; caudal fin slightly rounded or truncated to forked......................... 36 24 Maxillary bone long, extending beyond the posterior margin of the eye in the vertical plane; anal fin origin located posterior to the posterior insertion of the dorsal fin in the vertical plane, closer to the caudal fin origin than to the dorsal fin origin (ERY- THRINIDAE)........... Hoplias microlepis (Fig. 2 N–O; Other diagnostic features include: mouth large, with a few long, sharp teeth; pelvic fins placed well back on the abdomen; general coloration brown to yellowish-brown, darker dorsally, paler ventrally, with darker blotches on the back and light brown to white blotches on the lower part of the flanks and the abdomen; all fins dark brown with some clear areas alternating with brown blotches) - Maxillary bone long, usually not reaching the anterior margin of the eye in the vertical plane; anal fin origin usually located anterior to the dorsal fin origin, not very close of the caudal fin origin.......................................... 25 25 Eyes with free borders; dorsal fin origin at or very close to the middle of the body; males with anal fin modified into a copulatory organ called gonopodium; third anal fin ray not branched (POECILIIDAE).................................. 26 - Eyes with or without free borders; dorsal fin origin at the posterior portion of the body, close to the caudal fin origin; males with normal anal fin, i.e. not modified into a gonopodium; third anal fin ray (counting rudiments) branched................ 33 26 Anal fin with a black blotch........................................................................... 27 - Anal fin clear or dusky, without a black blotch............................................................. 28 27 Predorsal distance 45.45–52.63% of standard length; dorsal fin rays 10 o 11....................................... Brachyrhaphis roseni (Fig. 5 C–D; Other diagnostic features include: dorsal fin base short, usually shorter or of the same length than head length; general coloration yellowish-brown to reddish-brown, darker dorsally and paler ventrally, with 12 dark bars on the sides; anal fin usually yellowish; margin of the caudal fin usually orange with a darkened submarginal band in males) - Predorsal distance 50.00–62.50% of SL; dorsal fin rays 12–16............................. Brachyrhaphis terrabensis (Fig. 5F; Other diagnostic features include: dorsal fin base long, longer than head length; general coloration yellowish-brown to reddish-brown, darker dorsally and paler ventrally, with diffuse dark vertical bars, more conspicuous in adults, and a tenuous dark band along the body; anal fin usually yellowish-orange to reddish; margin of the caudal fin usually usually bluish-white) 28 Pelvic fins of mature males modified, with the tip of first ray with a soft swelling and the second ray thickened; gonopodium short, rarely reaching the posterior insertion of the dorsal fin in the vertical plane; dorsal fin origin of females usually above the origin of the anal fin in the vertical plane; dorsal fin usually with black spotting or a blotch proximally; cross-hatch pattern faint; caudal fin profusely spotted its base, often on entire fin; rows of yellow or black spots along sides...... Poecilia gillii (Fig. 5 G–H; Other diagnostic features include: general coloration yellowish-silver to greenish-silver, with blue highlights, the dorsum usually greenish-brown and the ventral portion withish-silver) - Pelvic fins not modified in males; gonopodium relatively long, reaching or extending beyond the posterior insertion of the dorsal fin; dorsal fin origin of females slightly behind the anal fin origin in the vertical plane; dorsal fin usually without black spotting or a blotch proximally (present in Priapichthys panamensis; see below); cross-hatch pattern on body usually evident.................................................................................................. 29 29 Dorsal and caudal fins with small brown blotches at the proximal half of the fin......................... Priapichthys panamensis (Fig. 5N; Other diagnostic features include: predorsal profile slightly convex; dorsal fin inserted behind the anal fin base; general coloration yellowish-brown, with 3–7 thin dark bands on the sides; fins usually transparent or yellowish) - Dorsal and caudal fins without small brown blotches........................................................ 30 30 Two or three oval black spots present on flanks..... Poeciliopsis paucimaculata (Fig. 5J; Other diagnostic features include: predorsal profile convex; gill rakers 19–20; dorsal scales with dark brown posterior borders) - No black oval spots on flanks.......................................................................... 31 31 Sides of body without marks.. Poeciliopsis retropinna (Fig. 5L; Other diagnostic features include: predorsal profile slightly convex; gill rakers 27–34; general coloration grayish-brown; fins usually transparent) - Sides of body with 6–9 narrow bars..................................................................... 32 32 Total gill rakers 29–32; pectoral fin longer, usually, Published as part of Angulo, Arturo, Gil-León, Jorge San, Oviedo-Soto, Alejandro, Abarca- Odio, Juan I. & Umaña-Villalobos, Gerardo, 2020, The fishes from the Coto River basin, Pacific coast, Costa Rica, pp. 1-28 in Zootaxa 4751 (1) on pages 18-24, DOI: 10.11646/zootaxa.4751.1.1, http://zenodo.org/record/3974692, {"references":["Bussing, W. A. (1987) Peces de las aguas continentales de Costa Rica / Freshwater fishes of Costa Rica. Editorial Universidad de Costa Rica, San Jose, 271 pp.","Bussing WA (1998) Peces de las aguas continentales de Costa Rica / Freshwater fishes of Costa Rica. Editorial Universidad de Costa Rica, San Jose, 468 pp.","Angulo, A., Molina-Arias, A., Murase, A., Miyazaki, Y., Bussing, W. A. & Lopez, M. I. (2015) Fishes from the Tusubres River basin, Pacific coast, Costa Rica: checklist, identification key and photographic album. Check List, 11 (3), 1 - 15. https: // doi. org / 10.15560 / 11.3.1666","Robertson, D. R. & Allen, G. R. (2015) Shorefishes of the Tropical Eastern Pacific. Online Information System. Available from: http: // biogeodb. stri. si. edu / sftep / taxon. php (accessed 22 October 2019)"]}