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Fishery Bulletin 96(1 ), 1998 
unpigmented isthmus and unpigmented inner oper- 
cular linings, whereas those of S. plagiusa are heavily 
pigmented. Further differences between S. diome- 
deanus and S. plagiusa are modally higher counts 
in S. diomedeanus (dorsal-fin rays 86-96 vs. 81-91 
in S. plagiusa ; anal-fin rays 69-80 vs. 66-75; total 
vertebrae 47-50, usually 48-50 vs. 44—49, usually 
45—48, in S. plagiusa). 
Symphurus diomedeanus is similar to the sympa- 
tric S. urospilus in some meristic features and in size, 
and both species also possess a well-developed pu- 
pillary operculum. Besides its different caudal-fin ray 
count, S. diomedeanus differs markedly from S. 
urospilus in possessing a series of spots on posterior 
regions of dorsal and anal fins (only in rare instances 
is there ever a spot on the caudal fin and when 
present, the caudal spot occurs in combination with 
spots on posterior portions of dorsal and anal fins), 
whereas S. urospilus always possesses a single, 
ocellated spot on the caudal fin and lacks pigmented 
spots on dorsal and anal fins. Further differences are 
that S. diomedeanus has 79-96 longitudinal scales 
(vs. 67-82 in S. urospilus), and the ocular surface of 
S. diomedeanus is more or less uniformly pigmented 
(only occasional specimens having faint crossbands), 
whereas most S. urospilus have well-developed 
crossbands. The two species can also be distinguished 
by differences in vertebral counts (47-50, usually 48- 
50, total vertebrae in S. diomedeanus vs. 44-48, usu- 
ally 45-46, in S. urospilus). 
Some meristic features of S. diomedeanus overlap 
those of S. civitatium, S. plagusia, S. tessellatus, S. 
oculellus, and S. caribbeanus, which occur sympat- 
rically, and occasionally syntopically, with S. 
diomedeanus (Munroe, 1991). Symphurus diome- 
deanus differs from these in having 10 caudal-fin rays 
(vs. 12 in the others), a pupillary operculum (lacking 
in these other species), and a series of rounded black 
spots on posterior rays of the dorsal and anal fins. In 
contrast, vertical fins of S. civitatium, S. plagusia, 
and S. tessellatus are more or less uniformly pig- 
mented, without distinctive pigmented blotches or 
spots. Those of S. caribbeanus and S. oculellus have 
an alternating series of somewhat rectangular- 
shaped, pigmented blotches and unpigmented areas 
beginning in the midbody region and continuing pos- 
teriorly (inclusive of the caudal fin in S. oculellus), a 
pattern quite different from that observed in S. 
diomedeanus (dorsal and anal fins with fewer, nearly 
spherical spots only on posterior regions of these fins 
and usually exclusive of caudal fin). Symphurus 
diomedeanus also differs from S. tessellatus in lack- 
ing small scales on blind sides of the posterior dor- 
sal- and anal-fin rays. It can be further distinguished 
from both S. tessellatus and S. oculellus because it 
lacks a black spot on the outer surface of the ocular- 
side opercle (vs. a black spot usually present on outer 
surface of ocular-side opercle in these other species). 
Some meristic and pigmentation features of S. 
diomedeanus overlap, at least partially, those of at 
least 14 other species of western Atlantic tongue- 
fishes. Among these, S. diomedeanus is most similar 
to, and occurs sympatrically with, S. kyaropterygium, 
S. minor, S. parvus, and S. ommaspilus. All species 
possess 10 caudal-fin rays and have a well-developed 
pupillary operculum and unpigmented peritoneum. 
Differences between these species are discussed in the 
“Comparisons” sections in accounts for S. kyarop- 
terygium, S. parvus, S. minor, and S. ommaspilus. 
Symphurus diomedeanus is similar in overall body 
shape, caudal-fin ray count, and has some meristic 
features that overlap partially those of S. trewavasae, 
which co-occurs with S. diomedeanus in the western 
South Atlantic. The two are easily distinguished by 
the presence in <S. diomedeanus of a pupillary oper- 
culum (absent in S. trewavasae), and S. diomedeanus 
has spots on the dorsal and anal fins (usually), whereas 
S. trewavasae does not have spotted fins. Other differ- 
ences between these species are discussed in the “Com- 
parisons” section in the account for S. trewavasae. 
Among eastern Pacific species of Symphurus, S. 
diomedeanus is most similar to S. fasciolaris in that 
both have overlapping meristic features, 10 caudal- 
fin rays, pupillary operculum, unpigmented perito- 
neum, and 1-4-3 ID pattern. Symphurus fasciolaris, 
however, lacks pigmented spots on dorsal and anal 
fins that characterize S. diomedeanus , and S. 
diomedeanus lacks the ocellated spot on the caudal 
fin featured in S. fasciolaris . In addition, none of the 
S. diomedeanus have the combination of crossbands 
and rounded spots on the ocular surface that is found 
on many S. fasciolaris. 
