744 



Fishery Bulletin 97(4), 1999 



and S. elongatus (13.5-15 cm BL) (Munroe et al., 

 1995), and this size difference is observed in larvae 

 also. All stages of development in S. williamsi occur 

 at smaller sizes (flexion between 5.0 and 8.0 mm and 

 transformation >11.9 mm) than in other species. The 

 flexion stage in S. atricaudus occurs between 8.6 and 

 10.8 mm BL (Ahlstrom et al., 1984; Matarese et al., 

 1989), and the transformation stage between 19 and 

 24.2 mm BL (Kramer, 1991 ). The flexion stage in S. 

 elongatus is >6.8 mm and <18.9 mm BL and trans- 

 formation takes place from >18.9 mm to <28.3 mm 

 BL (Charter and Moser, 1996). In Symphurus sp. 

 transformation occurs between 21 and 38 mm BL 

 (Yevseyenko, 1990). 



Usually the size range at metamorphosis that is 

 plesiomorphic for the order is ca. 10 to 25 mm BL 

 because most pleuronectiforms metamorphose in this 

 range (Hensley and Ahlstrom, 1984). 



Metamorphosis in S. williamsi occurs at ca. 11.9 

 mm BL; however, as described by Hensley and 

 Ahlstrom ( 1984), size at metamorphosis is an impor- 

 tant characteristic for lai-val identification, but its 

 use for inferring phylogenetic relations in most in- 

 stances is premature. 



Symphurus williamsi has a larger head at all de- 

 velopmental stages, compared with those of S. 

 atricaudus and S. elongatus. The distance from snout 

 to anus in S. williamsi is relatively longer than it is 

 in S. atricaudus but is similar to that of S. elongatus. 

 In the juvenile stage, this distance is longer in S. 

 williamsi than it is in S. elongatus. Larvae of 

 Symphurus sp. as described by Yevseyenko (1990) 

 have an abdominal cavity extending downward. This 

 projection has the form of a greatly elongated sac, 

 and the lower end terminates in a wormlike process 

 that S. williamsi does not have. The loop of the in- 

 testine in Symphurus sp. is almost one half of the 

 body length. 



The body depth at the preflexion stage is greater 

 in S. williamsi compared with that in S. atricaudus 

 but is similar to that of larvae of S. elongatus. In 

 postflexion and prejuvenile stages, this proportion 

 is smaller in S. williamsi than in both of these other 

 species. 



Finally, body depth at the anus is shallower in S. 

 williamsi than it is in S. elongatus at all stages in S. 

 atricaudus during flexion and juvenile stages. 



Acknowledgments 



This study was funded by the Direccion de Estudios 

 Profesionales - I.P.N, through the following research 

 projects: Planton del Noroeste de Mexico (Clave DEPI 

 86804); Investigaciones Ecologicas del Planton del 



Noroeste de Mexico (DEPI 868043); Caracterizacion 

 de la Zona de Transicion Templado-Tropical del 

 Pacifico Mexicano con base en las Comunidades 

 Plantonicas (DEPI 874264); and Bionomia Plantonica 

 de la Parte Central del Golfo de California (Clave 

 DEPI 903388). We especially thank our colleague 

 Andres Levy who assisted in critically revising the 

 manuscript and translating it into English, Biol. 

 Raymundo Avendaho Ibarra who made the drawings, 

 G. Moser (Southwest Fisheries Science Center) who 

 provided helpful comments that improved the manu- 

 script, and Elhs Glazier, CIBNOR, who edited the 

 English-language text. 



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