AHLSTROM ET AL.: PLEURONECTIFORMES 



659 



subfamilies recognized (see introduction), the Pleuronectinae is 

 the largest with 26 genera, representing % of the genera in the 

 family. Three contributions that summarize egg and larval in- 

 formation for pleuronectine flatfishes from the eastern North 

 Atlantic and Mediterranean are Ehrenbaum (1905-1909), Pa- 

 doa (1956k). and Nichols (1971). Bigelow and Welsh (1925). 

 Bigelow and Schroeder (1953), Martin and Drewry (1978), and 

 Fahay (1983) give information on eggs and larvae of western 

 Atlantic pleuronectine flatfishes. The most comprehensive work 

 dealing with early life history stages of flatfishes from the western 

 North Pacific is Pertseva-Ostroumova (1961). 



Yolk-sac larvae of pleuronectine flatfishes can be as small as 

 1.7 mm (Hypsopseila giittulata) or as large as 10-16 mm (Rein- 

 hardtius hippoglossoides) and size at hatching is a primary char- 

 acter for identifying yolk-sac larvae (Table 1 76). The pigment 

 pattern can be quite distinctive, as for example in the genus 

 Pleuronichthys. but in many pleuronectines the body pigment 

 migrates during the yolk-sac stage, and is variable from speci- 

 men to specimen of the same species. The yolk sac itself can 

 lack pigment (as in Parophrys vetulus. Hippoglosstis stenolepis 

 or Eopsetta jordani), can be moderately pigmented (as in Lyop- 

 setta exilis, Lepidopsetta bilineata or Psettichthys melanostictus) 

 or can be heavily pigmented (as in Pleuronichthys decurrens or 

 I'erasper variegaliis). Similarly the finfold can lack pigment or 

 be variously pigmented and useful in identification. 



Early preflexion pleuronectine larvae are slender; the head is 

 of moderate size; snout-anus length can be as much as 50% NL 

 (as in four species oi Pleuronichthys larvae, Sumida et al., 1979) 

 but usually is shorter (i.e., 35-45% NL). The gut is initially 

 straight but develops a coil soon after the completion of yolk 

 absorption. Greatest body depth after the gut becomes looped 

 is either at the anus or slightly anterior to it. Body shape of 

 preflexion larvae is quite similar from species to species. There 

 are few distinctive characters unique to the larval period of 

 pleuronectine flatfishes. Only a few kinds of pleuronectine larvae 

 develop head spination. Preopercular spines form in larvae of 

 Athercslhcs. Glyptocephalus, Tanakiiis and E opsei! a; oiic spines 

 develop on larvae of Microstomus (at least on 2 species), Hyp- 

 sopsetta, and Pleuronichthys ( 1 species); Athercsthes has a spi- 

 nous supraocular crest. Head spination develops during the pre- 

 flexion stage, but usually is best developed on flexion or early 

 postflexion larvae. 



The caudal fin begins forming either slightly before or together 

 with the dorsal and anal fins. The first caudal supporting bones 

 to form as cartilage are the hypurals. Usually several caudal rays 

 (2 + 2 or 3 + 3) are formed before flexion begins. In late flexion 

 and early postflexion larvae, the end of the notochord can project 

 beyond the hypural plates. The complete complement of caudal 

 rays is usually laid down during the flexion period. 



The dorsal and anal fins form in the finfold at some distance 

 from the main part of the body. The intervening space becomes 

 filled with the pterygiophores that support the dorsal and anal 

 fin rays, causing an increase in body depth. In both dorsal and 

 anal fins the rays begin forming at the anterior ends of the fins 

 and the differentiation proceeds posteriad. The first few rays in 

 both fins are reduced in size and the terminal ray is often minute. 



Pelvic fin buds usually form during the flexion stage but pelvic 

 rays usually are not developed until the postflexion stage. As in 

 all flatfishes, formation of pectoral fin rays is delayed to the end 

 of the transformation stage. 



The vertebral processes ossify before the centra. In the caudal 

 group of vertebrae, ossification of haemal and neural processes 

 proceeds posteriad. Ossification of abdominal neural processes 

 can follow several patterns, but usually proceeds anteriad. The 

 last neural and haemal processes to ossify are the truncate spines 

 of the 2 or 3 vertebrae anterior to the urostyle. Centra ossify 

 initially at the bases of neural and haemal processes and ossi- 

 fication proceeds peripherally until a complete ring is formed. 

 On first formation only the middle portion of a vertebral cen- 

 trum is ossified, hence the space between adjacent centra may 

 be as wide as the ossified portions of the centra. The ural centra 

 are the first to ossify in some pleuronectines or they can ossify 

 at the same time as other centra. The last centra to form are 

 those of the 2 (or 3) vertebrae anterior to the urostyle. 



All pleuronectine larvae that have been described have body 

 pigment. The pigment pattern changes with growth, often mark- 

 edly. Also, there is often considerable variation in pigmentation 

 of larvae of similar sizes of the same species. Notwithstanding, 

 body and finfold pigment constitutes a primary character for 

 identification of flatfish larvae during the preflexion stage. 



To show the variety of pigment patterns found on preflexion 

 stage pleuronectine larvae, preflexion larvae of 1 7 species from 

 the North Pacific are illustrated (Figs. 349-351). Heavily pig- 

 mented larvae are in the genera Pleuronichthys. Hypsopsetta, 

 and I'erasper (Fig. 349). The posterior portion of the tail is 

 unpigmented or pigment is confined to marginal spots along the 

 notochord. The unpigmented tail area is more extensive in some 

 species than in others. Finfold pigment is very useful in iden- 

 tifying these larvae to species taken in conjunction with larval 

 size and extent of tail pigment. 



In the other 14 kinds of larvae representing as many genera, 

 tail pigment appears in a number of patterns. The larvae illus- 

 trated in Figs. 350 and 35 1 are arranged in the order of increasing 

 complexity. In the simplest pattern pigment is concentrated 

 along the ventral midline with only moderate dorsal or lateral 

 pigment, as in Hippoglossus stenolepis or Reinhardtius hippo- 

 glossoides. Although Parophrys vetulus and Lyopsetta exilis have 

 more ventral margin pigment than dorsal, it is almost contin- 

 uous on both margins. Platichthys stellatus has more diffused 

 pigment over the tail portion of the body, but it is not in a 

 pattern. The most unusual pigment is found in Atherestes. There 

 are two conspicuous dorsal patches as opposed to almost no 

 ventral pigment. Pigment on Eopsetta jordani is limited to a 

 mid-tail band and a terminal notochord patch. A more common 

 pattern is encountered in Isopsetta, which has two pigment bands 

 across the tail together with the terminal notochord pigment. A 

 basically similar pattern is found in Lepidopsetta bilineata. Pset- 

 tichthys is unusual in having alternating dorsal and ventral 

 blotches. Hippoglossoides elassodon has three tail pigment areas 

 (i.e., opposing dorsal and ventral pigment patches) together with 

 terminal notochord pigment. This is also the basic pattern in 

 Microstomus. Emhassichthys increases opposing tail patches to 



Fig. 349. Larvae and transforming specimens of Pleuronectidae. (A) Pleuronichthys coenosus. 3.7 mm, from Sumida et al., 1979; (B) P. 

 coenosus. 8.9 mm. ibid; (C) Hypsopseila giitndala. 2.6 mm, ibid; (D) //. giittulata. 6.6 mm, ibid; (E) I'erasper variegaliis. 5.6 mm, from Pertseva- 

 Ostroumova. 1961 after Uchida, 1933; (¥) \'. variegatus. 12.4 mm. ibid. 



