Munroe: Systematics of western Atlantic Symphurus 
1 15 
Symphurus plagiusa usually has a well-developed 
black spot on the outer surface of the upper lobe of 
the opercle and 5-6 small ctenoid scales on blind sides 
of posterior rays of the dorsal and anal fins (opercu- 
lar spot absent in S. diomedeanus, and scales either 
absent or only 1-2 scales at bases of fin rays in S. 
diomedeanus). Other differences between these spe- 
cies are discussed in the “Comparisons” section of the 
account for S. diomedeanus. In number of caudal-fin 
rays, S. plagiusa agrees with S. trewavasae but differs 
from that species in many features discussed in the 
“Remarks” section of the account for S. trewavasae. 
Symphurus plagiusa differs from S. minor , S. 
parvus , S. kyaropterygium, andS. ommaspilus, other 
Atlantic tonguefishes characterized by 10-caudal-fin 
rays, in lacking a pupillary operculum and membrane 
ostia (present in these others); in having a conspicu- 
ous black opercular spot (no opercular spot in the 
others), in lacking a caudal blotch on the ocular side 
(caudal blotch present in S. minor, S. parvus, and S. 
kyaropterygium) , and S. plagiusa does not have con- 
spicuous spots on the dorsal and anal fins (ocellated 
spot present on dorsal and anal fins in S. ommas- 
pilus). Meristic features of S. plagiusa are much 
lower and completely beyond ranges of those observed 
in S.jenynsi, and S.jenynsi lacks scales on blind sides 
of dorsal- and anal-fin rays characteristic of S. plagiusa. 
Other differences between these taxa and S. plagiusa 
are discussed in the “Comparisons” sections in the re- 
spective accounts for these other species. 
There is nearly complete overlap in most meristic 
features of S. plagiusa and those of the sympatric 
(but not syntopic) western North Atlantic S. 
urospilus. These species differ, however, in number 
of caudal-fin rays (10 in S. plagiusa vs. 11 in S. 
urospilus); S. plagiusa also lacks a spot on the cau- 
dal fin and usually has a conspicuous spot on the 
ocular-side opercle (vs. caudal-fin spot present and 
no spot on outer surface of ocular-side opercle in S. 
urospilus). Additionally, S. plagiusa lacks a well-de- 
veloped pupillary operculum (present in S. urospilus) 
and has small ctenoid scales on blind sides of dorsal- 
and anal-fin rays that are absent in S. urospilus. 
South of Cape Hatteras, North Carolina, and in 
the Gulf of Mexico, S. plagiusa are frequently col- 
lected with S. civitatium . Symphurus civitatium, S. 
plagusia, and S. caribbeanus have similar meristic 
features, body sizes, and somewhat similar pigmen- 
tation patterns to those observed in S. plagiusa. 
Symphurus plagiusa is readily distinguished from 
all three by number of caudal-fin rays (10 in S. 
plagiusa vs. 12 in these others) and S. plagiusa has 
4-8 small, ctenoid scales on blind sides of posterior 
dorsal- and anal-fin rays (vs. scales absent, or only 
1-2 scales at bases of fin rays in these others). Many 
S. plagiusa have a conspicuous black spot on the 
outer surface of the upper ocular-side opercle (these 
others lack a conspicuous black spot on outer sur- 
face of ocular-side opercle, although some S. 
civitatium and S. caribbeanus have a diffuse dark 
brown blotch on the ocular-side opercle resulting from 
pigment of the inner opercular lining showing through 
to outside). In S. plagiusa, inner linings of both opercles 
are heavily pigmented, whereas in S. civitatium, S. 
plagusia, and S. caribbeanus usually only the inner lin- 
ing on the ocular-side opercle is pigmented. 
Symphurus plagiusa also has relatively larger eyes 
(8.3-12.6% HL), compared with those ofS. civitatium 
(7.0-11.0% HL) and S. plagusia (6. 4-9. 5% HL), and 
are more nearly equal in position (eyes noticeably 
subequal in S. civitatium and S. plagusia). In addi- 
tion, in S. plagiusa the posterior margin of the max- 
illa extends posteriorly only as far as the vertical 
through the mideye and often only reaches the ver- 
tical through the anterior margin of the pupil of the 
lower eye, whereas in S. civitatium and S. plagusia 
the posterior margin of the maxilla usually reaches 
a point between verticals through the posterior mar- 
gin of the pupil and the posterior margin of the eye 
(less frequently), or the jaws may even extend slightly 
beyond the vertical through the posterior margin of 
the lower eye in some S. plagusia. Another differ- 
ence between S. plagiusa and S. civitatium is that 
in S. plagiusa, both sexes have uniformly pigmented 
dorsal and anal fins that are generally only lightly 
shaded or moderately dusky. In comparison, male S. 
civitatium have dorsal and anal fins that become 
progressively darker and become nearly completely 
black on their posterior extent (especially pronounced 
in sexually mature males). Symphurus plagiusa also 
has a pronounced longitudinal streak of black pig- 
ment running the length of the gill filaments, 
whereas in S. civitatium the gill filaments are often 
spotted but lack a prominent dark black band. 
Symphurus plagiusa differs further from S. 
caribbeanus in lacking the alternating series of blotches 
and unpigmented areas on the vertical fins character- 
istic of S. caribbeanus (vertical fins in S. plagiusa uni- 
formly pigmented without dark blotches along their 
lengths). Symphurus plagiusa has a fleshy ridge on the 
ocular-side lower jaw that is lacking in S. caribbeanus. 
Among eastern Pacific Symphurus , meristic fea- 
tures and overall body shape of S. plagiusa overlap 
those of S. williamsi. Symphurus plagiusa differs 
from this species in number of caudal-fin rays (10 
vs. 12) and ID pattern (1-4-3 vs. 1-5-3 in S. williamsi). 
Symphurus plagiusa differs further from S. williamsi 
because it lacks a well-developed pupillary operculum 
(present in S. williamsi) and it has a black blotch on 
the ocular-side opercle that is lacking in S. williamsi. 
