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Fishery Bulletin 96(1 ), 1998 
Antitropical species pairs are well known from many 
groups of marine organisms (Hubbs, 1952; Hedgpeth, 
1957), and their origin has been the focus for much 
study and debate (White, 1985, 1986; Briggs, 1987; 
Stepien and Rosenblatt, 1996). Antitropical taxa 
originate by allopatric speciation (Mayr, 1963), 
wherein populations diverge owing to random genetic 
drift or natural selection following genetic isolation 
(or both), and where gene flow has been disrupted 
by some type of barrier to migration or by distance 
(Stepien and Rosenblatt, 1996). Barriers to gene flow 
may arise through vicariant events (White, 1985, 
1986) or dispersal events (Stepien and Rosenblatt, 
1996). In the eastern Pacific, antitropical distribu- 
tions of several species pairs were best explained by 
dispersal and varying temperature tolerances by the 
species than by vicariance (Stepien and Rosenblatt, 
1996). The tropics presented more of a barrier to some 
species than to others, so that genetic isolation var- 
ied among respective members of the different spe- 
cies pairs. Tropical Atlantic waters are an effective 
barrier separating antitropical species pairs of 
Symphurus. Whether these species arose owing to 
restricted dispersal capabilities or through vicariant 
events is unknown. Little is known of temperature 
tolerances and dispersal capabilities of Symphurus, 
especially those of deep-sea species. However, be- 
cause both S. billykrietei (Munroe, unpubl. data) and 
S. ginsburgi (Kurtz and Matsuura, 1994) have plank- 
tonic larval stages, as do other deep-sea Symphurus 
species, climatological events occurring in surface 
waters could directly affect species survival and dis- 
persal at this point in their life histories. 
Symphurus parvus and S. kyaropterygium repre- 
sent a north-south Atlantic species pair with mem- 
bers having discontinuous distributions on opposite 
sides of the Amazon River outflow. The Amazon River 
plume as a barrier to dispersal of tropical Atlantic spe- 
cies was shown to be incomplete for some taxa (Collette 
and Riitzler, 1977). More detailed studies of fish com- 
munities off northeastern Brazil will augment our 
knowledge of fish distributions across this region. 
Symphurus plagusia and S. civitatium are a spe- 
cies pair that occur in the Caribbean Sea and south 
( S . plagusia), and on the inner continental shelf in 
the Gulf of Mexico and off the southeastern United 
States {S. civitatium). The ranges of these species 
are separated by the Florida Current, which appar- 
ently serves as an effective barrier to dispersal. 
Several species pairs with western (listed first) and 
eastern Atlantic components are S. nebulosus: S. 
ligulatus; S. pusillus : <S. nigrescens', and S. rhytisma : 
S. reticulatus. Symphurus nebulosus and S. ligulatus 
occur on the outer continental shelf and upper slope, 
whereas S. pusillus and S. nigrescens inhabit the 
outer continental shelf. Formation of species pairs 
with discontinuous distributions on opposite sides 
of the Atlantic perhaps results from a distance fac- 
tor where once common gene pools have now become 
widely separated by seafloor spreading of the Atlan- 
tic basin with subsequent separation of the respec- 
tive continental shelves. The other east-west species 
pair, S. rhytisma and S. reticulatus, in contrast, are 
insular species, which probably reach island habi- 
tats by dispersal of larval stages. Usually, founder 
populations at remote locations comprise a small 
number of individuals, which if they survive to re- 
produce, subsequently diverge through time owing 
to genetic drift and isolation characteristic of small 
populations at remote island locations (compared 
with species on the continental shelf with large and 
contiguous populations). 
Species pairs with component members distributed 
on either side of the Isthmus of Panama are com- 
mon in a wide variety of taxa (Briggs, 1974; White, 
1985, 1986; Crabtree, 1987). The emergence of the 
Isthmus of Panama some three million years ago 
separated the Atlantic Caribbean fauna from that of 
the eastern Pacific (Briggs, 1974). Hypothesized spe- 
cies pairs of New World tonguefishes with compo- 
nent taxa occurring in western Atlantic (listed first) 
and eastern Pacific areas include S. pelicanus : S. 
gorgonae; S. tessellatus: S. chabanaudi; S. urospilus: 
S. fasciolaris ; and S. marginatus: S. diabolicus. 
Ecological distributional patterns 
In the western Atlantic, Symphurus species inhabit 
a broad diversity of substrates located throughout a 
considerable range of bathymetric environments, 
from shallow estuarine areas only a few centimeters 
in depth (Reichert and Van der Veer, 1991) to demer- 
sal habitats 810 meters deep on the continental slope. 
Nearly equal numbers of species of western Atlantic 
Symphurus are found in shallow-water (<50 m) and 
deepwater ( >50 m) environments. Symphurus 
nebulosus, S. marginatus, and S. billykrietei have 
been collected at depths ranging from 500 to 810 m, 
and are among the deepest dwelling of western At- 
lantic flatfishes. Other deep-dwelling flatfishes in 
this region include Reinhardtius hippoglossoides and 
Hippoglossus hippoglossus, reportedly from depths 
reaching 2,000 m (Nielsen, 1986), Glyptocephalus cyno- 
glossus, collected at about 1,570 m (Scott and Scott, 
1988), and Chascanopsetta lugubris danae, ranging 
from 160 to 460 m (Amaoka and Yamamoto, 1984). 
Previous researchers (Ginsburg, 1951; Topp and 
Hoff, 1972; Menezes and Benvegnu, 1976; Munroe, 
1990, 1991) noted that Atlantic tonguefish species 
generally inhabit rather discrete depth zones. Data 
