670 



ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



the head, gut and gas bladder. In some species, large blotches 

 in the finfold distal to the dorsal and ventral midline blotches 

 give the larvae a barred appearance. In later stages the midline 

 blotches become more numerous and some species develop a 

 series along the horizontal septum. Early larvae of Symphurus 

 have small melanophores along the ventral midline, and in some 

 species, also along the dorsal midline. Most species have a single 

 bar posteriad on the tail and at least one, 5. athcauda, has large 

 blotches at the finfold margins. The head (particularly ventrally), 

 gut, gas bladder and horizontal septum become pigmented and 

 later-stage larvae have pigment patterns similar to Cynoglossus 

 species. 



Metamorphic stages 



Pleuronectiforms undergo a remarkable metamorphosis dur- 

 ing which one of the eyes, the left in dextral and the right in 

 sinistral species, migrates around or through the head to a po- 

 sition dorsal to the non-migrating eye. Metamorphosis occurs 

 over a wide size range among flatfishes, from about 5 mm in 

 achirine soles (Houde et al.. 1970) to greater than 120 mm in 

 some bothines (Amaoka, 1971). Capture of specimens of the 

 enormous flatfish larva observed by Barham ( 1 966) from a div- 

 ing saucer may double the maximum size for flatfish larvae. 

 Most flatfishes metamorphose within the range of 10-25 mm 

 (see preceding sections and Tables 173-178); the size interval 

 over which the process occurs is smaller in species which meta- 

 morphose at a small size. 



Metamorphosing specimens are relatively rare in plankton 

 collections because 1) the process is transitory, 2) avoidance is 

 increased at larger sizes, and 3) metamorphosing individuals 

 may change habitat. Existing information indicates a variety of 

 mechanisms of eye migration among flatfishes. In groups where 

 the dorsal fin origin in larvae is at the posterior margin of the 

 eye or more rearward (psettodids, citharids, scophthalmids, most 

 paralichthyids, pleuronectids), a depression forms in the inter- 

 ocular region and the eye migrates over the dorsal midline an- 

 terior to the fin origin. Subsequently the dorsal fin extends for- 

 ward to its adult position (except in psettodids). In larvae of 



bothids and the paralichthyid genera Cyclopsetta. Syaciiun and 

 Cithanchlhys (some species), the dorsal fin is attached to the 

 skull anterior to the eye and, during metamorphosis, the eye 

 migrates through a slit which forms between the fin base and 

 the skull. In some metamorphosing soleids the dorsal fin projects 

 forward above the snout and the eye migrates through the space 

 between this protuberance and the skull; subsequently the fin 

 projection fuses to the skull (Houde et al., 1970; Palomera and 

 Rubies, 1977; Minami, 1981b). Seshappa and Bhimachar( 1955) 

 described the process of eye migration in a captive specimen of 

 Cynoglossus macrostomus. Just before eye migration a fleshy 

 hook-shaped protuberance grew forward from the region of the 

 head anterior to the dorsal fin origin. The right eye migrated 

 through the space between the protuberance and the skull, after 

 which the fleshy appendage fused to the dorsal region of the 

 skull. The entire process took place over a 5-hour period during 

 the night. A similar structure appears on advanced larvae of an 

 unidentified cynoglossid illustrated by John (1951b) and this 

 mechanism of eye migration may be widespread among cyno- 

 glossids. 



During eye migration in flatfishes a number of other meta- 

 morphic events occur: 1) larval spines are lost, 2) elongate rays 

 assume their j uvenile proportions, 3) gut protrusions are brought 

 into the body cavity and internal organs are rearranged, 4) gas 

 bladder, if present, is lost, 5) pectoral fins develop rays, except 

 in cynoglossids, some soleids, some bothids and Mancopsetta, 

 where (one or both) fins are lost altogether during this period, 

 6) larval pigment patterns are replaced by juvenile patterns, 7) 

 ossification of the vertebral column and other bony structures 

 is completed, 8) intermuscular bones appear in bothids. and 9) 

 scales form. 



(H.G.M., B.Y.S.) National Marine Fisheries Service, 

 Southwest Fisheries Center, P.O. Box 271, La Jolla, 

 California 92038; (K.A.) Faculty of Fisheries, 

 Hokkaido University, Hakodate, Japan; (D.A.H.) De- 

 partment OF Marine Sciences, University of Puerto 

 Rico, Mayaguez, Puerto Rico. 



Pleuronectiformes: Relationships 

 D. A. Hensley and E. H. Ahlstrom 



BASICS of the current working model for evolution of pleu- 

 ronectiforms were proposed by Regan (1910, 1929) and 

 Norman (1934). In his monograph, Norman treated the floun- 

 ders (Psettodidae, Bothidae, Pleuronectidae), and though he did 

 not publish a revision of the remaining pleuronectiforms, his 

 key and classification of the soleoids were published posthu- 

 mously (1966). Norman's model and classification with the 

 modifications of Hubbs (1945), Amaoka (1969), Futch (1977), 

 and Hensley (1977) represent the most recent, detailed hypoth- 

 esis for pleuronectiform evolution. We will refer to this as the 

 Regan-Norman model (Fig. 358) and classification (preceding 



article, this volume) and consider it the working hypothesis to 

 be reexamined using adult, larval, and egg characters. 



Formation of the Regan-Norman model involved an eclectic 

 approach, i.e., a combination of phyletic and phenetic methods. 

 Although some of the groups currently recognized appear to be 

 based on synapomorphies, many are clearly based on symple- 

 siomorphies and were recognized as such by the authors. This 

 search for horizontal relationships among pleuronectiforms us- 

 ing eclectic methods, with one exception, has been the only 

 approach used in this group. The exception is the recent work 

 of Lauder and Liem (1983) in which a cladogram for flatfishes 



