Cooper and Chapleau: Monophyly and intrarelationships of the family Pleuronectidae 
689 
al., 1991). The adoption of this reclassification was a 
recognition of the first and only study that attempted 
to define intrarelationships in the Pleuronectidae 
( sensu Norman, 1934). This new classification has 
not been widely accepted (Wheeler, 1992; Rass, 1996). 
It is argued here that because the Pleuronectidae 
used by Sakamoto (1984a) was determined to be poly- 
phyletic (Chapleau, 1993) and given the phenetic 
nature of the analysis, it is unlikely that the nomen- 
clatorial revisions summarized in that work repre- 
sent natural groups. 
The dubious nature of this most recent reclassifi- 
cation (i.e. Sakamoto, 1984a), the uninformative na- 
ture of the previous classification (i.e. Norman, 1934), 
and the commercial importance of this group require 
that a more comprehensive examination of pleuro- 
nectid intrarelationships be based on natural groups. 
A cladistic analysis based on structural variation 
within Pleuronectidae, in contrast with characters 
observed in closely related outgroups will establish 
an hypothesis of genealogical descent (Wiley, 1981). 
Outgroup hypothesis for 
Pleuronectidae 
Relationships within the order reveal that only the 
Pleuronectidae ( sensu Chapleau and Keast, 1988) 
have a caudal skeletal complex, synapomorphic with 
taxa belonging to the Paralichthyidae, Scoph- 
thalmidae, Brachypleura, and Bothidae (Chapleau, 
1993). These taxa have been identified as the bothoid 
lineage within Pleuronectiformes (Hensley and 
Ahlstrom, 1984). This lineage is supported in one of 
18 equally parsimonious trees observed in a cladis- 
tic analysis for the order (Chapleau, 1993). The other 
taxa formerly in Pleuronectidae (Samaridae, 
Rhombosoleidae, and Poecilopsettidae) were placed 
in a clade that included the soles Achiridae, Soleidae, 
and Cynoglossidae (Chapleau, 1993). Paralichthodes 
algoensis was not included in Chapleau’s ( 1993) cla- 
distic revision but has recently been determined to 
be the sister lineage to these other taxa formerly in 
Pleuronectidae (Cooper and Chapleau, 1998). Con- 
sequently, the most likely outgroup for Pleuro- 
nectidae would be represented by species within the 
bothoid lineage. 
There is a wide range of morphological types within 
the bothoid lineage for which the intrarelationships 
are not resolved (Chapleau, 1993). In the absence of 
synapomorphies to determine the sister relationship 
of Pleuronectidae with other bothoid taxa, a compari- 
son of jaw structure can be used to determine the 
most likely candidates. It is assumed that the 
outgroup for the Pleuronectidae should have large 
symmetrical jaws and pointed teeth. Within 
Pleuronectiformes, the evolutionary trend for jaw 
structure and feeding strategy may be considered 
unidirectional. Symmetry of jaw and dentition found 
in piscivorous flatfishes, like Psettodidae and 
Citharidae (de Groot, 1971), is considered to be the 
plesiomorphic condition. The osteological characters 
observed in these two taxa are most similar to the 
generalized acanthopterygian structure (Yazdani, 
1969). Taxa with symmetrical jaw structure are hy- 
pothesized to have given rise to groups with more 
specialized dentition types and jaw asymmetry, as 
observed in the Achiridae, Soleidae, and Cyno- 
glossidae (Yazdani, 1969; Chapleau, 1993), but the 
reverse situation is not indicated in any study of re- 
lationships. 
Within subgroups of Pleuronectiformes, the same 
evolutionary trend is assumed to occur. Left-eyed 
flounders within the bothoid lineage have large, 
nearly symmetrical jaws for piscivory, or a more spe- 
cialized, asymmetrical jaw structure that accommo- 
dates capture of benthic prey (Yazdani, 1969; de Groot, 
1971). Likewise, the Pleuronectidae contains both 
piscivores with nearly symmetrical jaws, Hippoglossus 
stenolepis and Reinhardtius hippoglossoides, as well 
as more specialized predators with asymmetrical jaws, 
such as Glyptocephalus stelleri (de Groot, 1971). As- 
suming that evolutionary trends in pleuronectid jaw 
structure are consistent with trends observed in the 
order, the ancestral pleuronectid would have near 
symmetrically developed jaws. The bothoid family, 
Paralichthyidae, appears to be one of the most 
plesiomorphic groups of left-eyed flounders and is 
chosen as the outgroup for the Pleuronectidae. In 
addition, Psettodes and Lepidoblepharon are also 
chosen as secondary outgroups. These assumptions 
are only valid if the Pleuronectidae is monophyletic. 
If Pleuronectidae is not monophyletic, then multiple 
outgroup taxa with either symmetrical or asymmetri- 
cal jaw structures may account for the variation ob- 
served within the Pleuronectidae. 
Materials and methods 
Fifty-three of 59 pleuronectid species were examined. 
Five outgroup taxa, chosen from the families Psetto- 
didae (Psettodes sp.), Citharidae (Lepidoblepharon 
ophthalmolepis), and Paralichthyidae ( Citharichthys 
arenaceus , Paralichthys lethostigmus , and P. 
squamilentus ) were also examined. The following 
cleared and stained specimens were dissected and 
examined for osteological characters. Nomenclature 
follows the conclusions of this analysis with the pre- 
vious classification of Sakamoto ( 1984a) indicated in 
