Ontogeny and metamorphosis in the Dover sole (Microstomus pacificus) : a description of the early life history stages, with comments on the sister-group relationship between Microstomus and Embassichthys Public Deposited

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  • The early life history stages and metamorphosis in Dover sole (Microstomus pacificus) are described from 201 larvae and juveniles. Unlike other pleuronectids, with the possible exception of the deep-sea sole (Embassichthvs bathvbius), initial migration of the left eye is not associated with metamorphosis or transition from plankton to benthos. The left eye exhibits a stasis during its migration, so that premetamorphic larvae are optically asymmetrical while planktonic. Seven characters are used to quantify the progress of metamorphosis: completed eye migration, dorsal fin position, dentition, pectoral fin development, condition of posterior process of coracoid, pigmentation, and elongation of the intestine into a secondary body cavity. Other characters associated with metamorphosis are reduction in body depth associated with reductions in lengths of neural and hemal spines and associated dorsal- and anal- fin pterygiophores, decreasing interorbital distance, increasing right eye diameter and right premaxilla length, and development of body scales. Metamorphosis appears to require 9- 11 or 12 months to complete. The planktonic period of premetamorphic and metamorphic larvae is about 18-24 months. Metamorphic larvae appear to move inshore into shallower water as metamorphosis progresses. Settlement occurs from about December to April, primarily from January to March. One of the earliest phylogenetic hypotheses about the intergeneric relationships of the pleuronectid genera Microstomus, Embassichthys, Glyotocephalus, and Tanakius was by Norman (1934), who considered Microstomus plus Embassichthvs and Glyotocephalus plus Tanakius to be sister groups. An alternative hypothesis by Richardson (1981) suggested that Embassichthvs larvae resembled larvae of Glyptoceohalus and Tanakius based on a "leptocephaluslike" body shape, and that Microstomus was the primitive sister group of these three genera because of its dorsoventrally deepened body shape. In order to evaluate Richardson's hypothesis of sister group relationships based on body shape, I examined morphological characters associated with eye migration and metamorphosis, and changes in body shape during ontogeny from post-flexion larvae through benthic juveniles. Richardson's-character of a moderate "leptocephalus-like" body shape is an artifact of the paucity and small size range of larvae available for her examination. The body shape of larger Embassichthvs larvae is also dorsoventrally deepened and the larvae appear to undergo changes in body shape during metamorphosis similar to Microstomus. Three synapomorphies that support Norman's hypothesis of an Embassichthys plus Microstomus sister group relationship are dissociation of initial eye migration with metamorphosis, a stasis during eye migration in premetamorphic larvae, and a retention of a larval-like appearance to the eyes of newly settled specimens.
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