672 



ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



or right eye is dorsal with about equal frequency (state referred 

 to here as truly dimorphic). Exceptions to this are species of 

 paralichthyids (sinistral) and pleuronectines (dextral) where the 

 right or left optic nerve, respectively, is always dorsal, even in 

 reversed individuals, i.e., the optic chiasma is monomorphic. 

 The Soleidae and Cynoglossidae, however, retain a truly di- 

 morphic optic chiasma. Subsequent work by Regan (1910) and 

 Hubbs (1945) showed that in the indiscriminately dextral or 

 sinistral Psettodes the optic chiasma is also truly dimorphic. In 

 addition, Hubbs presented evidence of a third state, at least in 

 Citharoides (sinistral), where the nerve of the migrating eye is 

 dorsal even in reversed individuals. He thus interpreted the 

 Citharidae as having a basically dimorphic optic chiasma and 

 predicted the same for scophthalmids, although apparently no 

 one has examined a reversed scopthalmid to test this prediction. 

 A truly dimorphic optic chiasma as found in Psettodes and the 

 soleoids has been interpreted as plesiomorphic for pleuronec- 

 tiforms. The type of optic chiasma found in Citharoides and 

 predicted for scophthalmids (i.e., nerve of the migrating eye 

 always dorsal) was interpreted as an intermediate state between 

 the truly dimorphic and the monomorphic chiasmata as found 

 in pleuronectoids. We agree with this interpretation of polarity. 

 However, some plesiomorphic states have been used to define 

 groups: Psettodidae, truly dimorphic; Citharidae, basically di- 

 morphic; Scophthalmidae, predicted to be basically dimorphic; 

 and Soleoidei, truly dimorphic. 



Major problems exist with the use of the optic chiasma for 

 phylogenetic inference. One of these concerns the feasibility of 

 actually determining which state exists in a group. Demonstrat- 

 ing the occurrence of truly dimorphic chiasmata is relatively 

 simple. All that is needed is to show that either optic nerve is 

 dorsal regardless of which eye has migrated; reversed individuals 

 are not necessary. To demonstrate occurrence of the basically 

 dimorphic state, reversals are needed and the nerve of the mi- 

 grating eye must always be dorsal. Likewise, reversed individ- 

 uals must be examined to show a monomorphic chiasma. Here 

 the nerve to the right eye must be dorsal in all individuals 

 (including reversals) of normally sinistral species and the nerve 

 of the left eye must be dorsal in all individuals of normally 

 dextral species. When one actually examines the data for this 

 character (see Hubbs, 1945), states have been determined for 

 very few pleuronectiform groups. The occurrence of the basically 

 dimorphic state in the Citharidae was demonstrated in only one 

 species. Of greater significance, however, is the fact that a mono- 

 morphic state has been shown for very few pleuronectoid species. 

 Within the pleuronectoids it has been widely assumed that all 

 paralichthyids, bothids, and pleuronectids have monomorphic 

 optic chiasmata, and that because of this they are monophyletic 

 and not closely related to the soleoids (truly dimorphic). It is 

 worthy of note here that a monomorphic optic chiasma has 

 never been demonstrated for four pleuronectid subfamilies 

 (Poecilopsettinae, Rhombosoleinae, Samarinae, Paralichthod- 

 inae), the Bothidae, or the paralichthyid genus Thysanopsetta. 



Ocular asymmetry— T\\K character (sinistral, dextral, indis- 

 criminate) is obviously interrelated with the optic chiasma in 

 certain groups, i.e., those with basically dimorphic and mono- 

 morphic chiasmata. The evolution of ocular asymmetry and its 

 relationship to the optic chiasma is not well understood, al- 

 though there is one major hypothesis (Norman, 1934; Hubbs, 

 1945) which states that primitively, pleuronectiforms were in- 

 discriminate in ocular asymmetry and the optic chiasma was 



truly dimorphic. Soleoids became discriminate (soleids dextral 

 and cynoglossids sinistral), but retained a truly dimorphic chias- 

 ma. Psettodids remained indiscriminate and truly dimorphic. 

 Citharids and presumably scophthalmids became discriminate 

 (scophthalmids and citharines sinistral and brachypleurines 

 dextral) but retained some ontogenetic plasticity in regard to 

 the optic chiasma, since reversed individuals still have the nerve 

 of the migrating eye dorsal (basically dimorphic). The remaining 

 pleuronectoids became discriminate (Paralichthyidae and Both- 

 idae sinistral and Plueronectidae dextral) and evolved a mono- 

 morphic chiasma. The only exceptions with regard to ocular 

 asymmetry are certain indiscriminate paralichthyids and pleu- 

 ronectines. However, most of these indiscriminate pleuronec- 

 toids have been shown to have a monomorphic optic chiasma 

 (a possible exception is Tephnnectes). It would thus appear that 

 indiscriminate ocular asymmetry in pleuronectoids developed 

 secondarily from discriminate ancestors (Hubbs and Hubbs, 

 1945). 



Making phylogenetic interpretations from two states of ocular 

 asymmetry is difficult or impossible without corroborative evi- 

 dence. Thus, a statement to the effect that two or more dextral 

 (or sinistral) pleuronectoid groups are most closely related to 

 each other because they are dextral (or sinistral) without addi- 

 tional evidence of synapomorphies is circular, and may lead to 

 the recognition of polyphyletic groups. This reasoning was the 

 basis for the proposed close relationship in the Regan-Norman 

 model between the Pleuronectinae and the remaining pleuro- 

 nectid subfamilies (Poecilopsettinae, Rhombosoleinae, Samar- 

 inae, Paralichthodinae) and for treating the genera Mancopsetta 

 and Thysanopsetta as members of the Bothidae and Paralich- 

 thyidae, respectively. 



Ribs and intermuscular bones. — In pleuronectiforms that pos- 

 sess ribs, these appear to be homologous with the pleural and 

 epipleural ribs of other teleosts, and the presence of these bones 

 should be considered plesiomorphic for the order. Two groups 

 lack both series of ribs, the Achirinae and apparently the Cyn- 

 oglossidae. Chabanaud (1940) reports epipleural ribs in some 

 cynoglossids but mentions no genera or species. We have not 

 seen them in cleared-and-stained Symphurus species or in ra- 

 diographs of several Cynoglossus species. Although it is still 

 commonly believed that all soleoids lack both series of ribs (e.g.. 

 Nelson, 1976; Lauder and Liem, 1983), Chabanaud (1940, 1941) 

 found short epipleural ribs in Solea, Microchirus, and Aesopta, 

 and we have seen them in Aseraggodes. 



Chabanaud (1940, 1950, 1969) found additional rib-like 

 bones ("metaxymyostes") in certain pleuronectiforms. Some of 

 his statements about these were in error, and it is now clear he 

 was referring to Bothus podas and Samaris cristalus (Hensley, 

 1977). Amaoka (1969) found these ("intermuscular") bones in 

 all species of his Bothidae and presented very detailed descrip- 

 tions of their morphology. One of his primary justifications for 

 elevating Norman's ( 1934) Bothinae to the family level was the 

 presence of these bones in the group and their absence in Nor- 

 man's Paralichthyinae. Norman considered Engyophrys. Tri- 

 chopsetta, Monolene. Taeniopsetta, and Perissias to be paralich- 

 thyines. All of these genera have intermuscular bones (Amaoka, 

 1969; Futch, 1977; Hensley, 1977; pers. observ.) and are con- 

 sidered here to be bothids. 



Bothid intermuscular bones are in five series. Amaoka ( 1 969) 

 called these series epimerals, epicentrals, hypomerals, and 

 myorhabdoi (two series). He interpreted three of these (epi- 



