Key to the Species of Ctenogobius
This key is from Pezold 1984

1a. Second dorsal and anal fins usually with 12 and 13 elements, respectively; shoulder patch present or absent ...................................................................... 4

1b. Second dorsal usually not having 12 and 13 elements respectively; shoulder patch present .................................................................................................................2

2a. Second dorsal and anal fins with 13 and 14 elements respectively; scales in lateral series 49-66, usually more than 55; nape scaled to midopercle or preopercle; pectoral fins very short, not reaching to anus 17% SL ....................... Ctenogobius sagittula

2b. Second dorsal and anal fins with 11 and 12 elements respectively; scales in lateral series not more than about 45; pectoral fins reaching anus or anal fin origin ......... 3

3a. Head and sometimes trunk with scattered ocelli formed by dark rings around light centers; lateral scales 31-45, usually near 40; nape commonly with scales to rear margin of opercle; third spine of first dorsal elongate; no well developed V pattern on trunk....................................................................................... Ctenogobius smaragdus

3b. No ocelli on head or trunk; internal scales 29-35 (mean = 31); nape usually with few or no scales, not to rear margin of opercle; third spine of first dorsal not produced; dorsal extensions of midlateral blotches producing V pattern on trunk ..................................................................................................Ctenogobius boleosoma

4a. Darkly pigmented along preopercle margin of cheek ............................................5

4b. Preopercular margin, if pigmented, not more intense than other head pigmentation, and not as distinctly defined .....................................................................................6

5a. Preopecle margin of cheek with thin edging; nape generally with 10-12 scales before D1 origin, usually reaching midopercle in Bermuda to preopercle elsewhere; females with very dark vergules on anal fin; first dorsal spines not produced in males ........................................................................................... Ctenogobius stigmaturus

5b. Preopercle margin with broad edging, widest and most intense at angle of preopercle; nape with few or no scales; vergules not present on anal fins of females; 3rd spine of first dorsal often greatly produced in males ...... Ctenogobius fasciatus

6a. Eyes greatly reduced, not filling sockets ........................... Ctenogobius thoropsis

6b. Eyes normal, not reduced ..................................................................................... 7

7a. Cheek with three dark broad vertical bars; sides of trunk with 7-10 unpigmented vertical stripes; laterally projecting sometimes nearly horizontal tusk-like canine in middle of lower jaw on each side ......................................... Ctenogobius stigmaticus

7b. Cheek not as in 7a; sides of trunk without vertical stripes; canine present midlaterally in lower jaw of some species, but not projecting out laterally or nearly horizontal ................................................................................................................ 8

8a. Broad strip of dark pigment crossing lower cheek from lower preopercle angle to just above the corner of the jaw; males often with elongate 3rd spine in first dorsal and large recurved canine midlaterally in lower jaw .............. Ctenogobius pseudofasiatus

8b. Broad strip not present as described; males with or without elongate spine and midlateral large canine in lower jaw ...................................................................... 9

9a. Cheek pigmentation dominated by distinct suborbital bar following a vertical from the corner of the jaw to the lower rim of the orbit, in some populations of C. saepepallens reaching only a third of the distance to the jaw ............................... 10

9b. Cheek pigmentation not dominated by suborbital bar described, instead horizontal bar across midcheek from upper preopercle canal to corner of jaw or streak on snout from eye to midlateral upper jaw may be stronger ................................................. 11

10a. Cheek often with numerous dark striations following sensory papillae; scale pockets in midlateral trunk region darkly outlined forming lattice-like or rows of diamonds especially towards anterior end; sides peppered with dark dots above and paralleling faint midlateral blotch series (the series of five blotches typical of the genus); pelvic fins dusky in males; anal fin in females dusky; 3rd dorsal spine moderately elongate in both sexes ...................................... Ctenogobius lepturus

10b. Sides of trunk not as above, instead midlateral blotches frequently dash-like with dots in between, giving a dot-dash effect; cheek without other dark markings; pelvic fins of males with margins; anal fins of females clear or only pigmented on rear half; 3rd dorsal spine elongate only in males ........................... Ctenogobius saepepallens

11a. Horizontal cheek bar, dark, connected to angular blotch at corner of the jaw to form a dark prominent hatchet shaped mark; males with a large black spot around the end of the fifth dorsal spine and a dark margin on a dusky pelvic disk ................................................................................................. Ctenogobius manglicola

11b. Cheek pigmentation not as above; Ctenogobius manglicola dorsal may have dark spots, but largest is located along first spine; small but not black spot at tip of fifth dorsal seen in some, especially females ................................................................. 12

12a. Caudal fin very elongate in both sexes (42-53% SL in males, 39-50% in females); jaws large usually to posterior margin of orbit in both sexes (13-16% SL in males, 13-14% in females); dark well defined shoulder patch present, most prominent marking of trunk, V pattern of midlateral blotches with dorsal extensions frequently formed in adults .................................................................................... Ctenogobius phenacus

12b. Caudal fin moderately produced (to 44% SL in males, to 39% in females); jaw large in adult males of C. shufeldti and C. sp. A, but not reaching posterior margin of orbit in females (only to 13% SL in C. shufeldti, to 12% SL in C. sp A); shoulder patch frequently present in C. sp A, but rarely as dark as midlateral blotches; V pattern not formed, only single dorsal arms may be present .................................................... 13

13a. Pelvic fins in adult males dusky, in females with bilateral streaks paralleling innermost ray running posteriorly from base of fin; adult males usually with 3rd spine of first dorsal elongate ............................................................................................14

13b. Pelvic fins in adult males with the bilateral streaks typical of females in other species described above; pelvic fins of female clear; adult males without elongate spine in first dorsal ................................................................ Ctenogobius shufeldti

14a. Teeth of outermost row of upper jaw greatly enlarged in males; recurved canines in midlateral lower jaw of males much larger than innermost rows; pigmentation generally very light, with exception of spots associated with dorsal spines in some specimens ............................................................................... Ctenogobius claytonii

14b. Teeth of outer row in males only slightly enlarged; small recurved canine present in males midlaterally on lower jaw; darkly pigmented, with horizontal cheek bar and streak from eye to midlateral jaw on snout prominent; faint shoulder patch usually present ....................................................................................... Ctenogobius sp A.