Markle et al.: Metamorphosis of Microstomus pacificus 



287 



assumed for this study. Considering the change in 

 weight of three orders of magnitude between 10 and 

 SOmmSL, the weighing error was considered accept- 

 able for this study. 



A smaller subset of 201 specimens was examined to 

 describe metamorphosis in greater detail, and all were 

 deposited in the Oregon State University Fish Collec- 

 tion (OS). These specimens were either cleared and 

 differentially stained with alizarin red S and alcian 

 blue (Potthoff 1984), radiographed, or both. This sub- 

 set included only postflexion Stage- 1, most Stage-2, 

 and representatives of Stages 3-5 larvae. In addition 

 to routine measurements listed above, we measured 

 right eye diameter, interorbital width, right upper jaw 

 length, length of gastrointestinal tract as measured 



from anus to most posterior part of intestinal loop, 

 length of first caudal neural spine, length of dorsal fin 

 pterygiophore anterior to first caudal neural spine, 

 length of dorsal fin pterygiophore posterior to first 

 caudal neural spine, length of first hemal spine, length 

 of anal fin pterygiophore anterior to first caudal hemal 

 spine, and length of anal fin pterygiophore posterior 

 to caudal hemal spine. Counts of vertebrae and rays 

 of dorsal, anal, caudal, pectoral, and pelvic fins also 

 were made. 



A staging system describing Dover sole ontogeny 

 was developed following the suggestions of Youson 

 (1988). Our terminology deviates from Balon (1979, 

 1984) and Youson (1988) in our use of five numbered 

 stages for early development, rather than numbered 

 stages for metamorphosis only. Dover sole have a pro- 

 tracted metamorphosis, and our stages can be related, 

 generally, to flatfish metamorphosis. We suggest term- 

 inology for each stage that incorporates traditional 

 concepts of larval and juvenile periods as well as the 

 metamorphic phase of the larval period. Metamorphosis 

 occurs in Stages 2-4. 



We were especially concerned with describing the 

 beginning of metamorphosis, the initiation event, and 

 the completion of metamorphosis, the climax event 

 (Youson 1988). The initiation event was described 

 based on six characters that reach the adult state dur- 

 ing the plankton-to-benthos transition (see Results 

 below). Another character, body scale formation, could 

 be documented only in cleared and stained specimens 

 and was concordant with completion of the six ini- 

 tiation-event characters. Development of the intestinal 

 loop in the secondary body cavity, quantified by SINT, 

 is the last character to change in Dover sole meta- 

 morphosis. The climax event was described based on 

 the rate of change of the ratio of natural logarithms 

 of two measurements (SINT and SL). Both initiation 

 and climax events are further corroborated by body 

 shape changes. 



We use the concept of competency, as developed in 

 the marine invertebrate developmental literature, as 

 part of our definition of stages. The term regrettably 

 has become a synonym for metamorphosis, as in the 

 phrase "competent to metamorphose" (Pechenik 1986). 

 Doyle (1975), using the term "delay" stage, noted that 

 the onset of competency included both developmental 

 criteria (strict metamorphosis as used herein) and a 

 behavioral criterion, the ability to settle. In some in- 

 vertebrates, attachment to a substrate is a prerequisite 

 to metamorphosis; thus settlement must occur prior to 

 metamorphosis. In fishes there is not necessarily a 

 connection between metamorphosis (Youson 1988) and 

 competence. However, Cowen (1991) applied the terms 

 to fish and kept the marine invertebrate connection 

 intact. Competency has been defined more narrowly 



