NOTE Lauth and Blood; Description of embryonic development of Pleurogrommus monopterygius 



573 



One week after blastopore closure (day 22), the tail 

 region began lifting away from the yolk, and within 

 two days, the length of the embryo was 50% of the 

 circumference of the yolk. As the tail grew longer, it 

 curved further away from its longitudinal axis, causing 

 the embryo to curl around the top quarter of the yolk 

 rather than at the equator. The tail was 75% of its way 

 around the yolk by day 29 and touched the snout by 

 day 36. Lengthening further, the tail extended past the 

 anterior edge of the eye (day 40) and past the posterior 

 edge of the head (day 48); the maximum length was 

 reached on day 64 when it was 1.75 times the circum- 

 ference of the yolk. 



Pigmentation 



Embryo pigmentation first appeared on day 31. Sparse 

 melanophores appeared in the eye surrounding the lens 

 and along the dorsal midline, nape, and gut (Table 1; 

 Fig. 2). The eye became completely pigmented and much 

 darker by day 38. The beginning of the dorsal midline 

 series was observed as scattered melanophores in the 

 area between 20% body length (BL) and 50% BL. By day 

 44 the dorsal midline series extended as a double row 

 anterior to the nape and posterior to 50% BL where it 

 joined to form a single row of irregularly spaced melano- 

 phores that continued to 70% BL. On day 49, the single 

 row of dorsal midline pigment extended to 5-6 myomeres 

 anterior to the caudal peduncle. At hatching, the dorsal 

 midline series was a double row of continuous pigment 

 extending from the nape to three myomeres anterior 

 of the caudal peduncle (Fig. 3). Dorsal gut pigment 

 first appeared halfway along the gut where the pig- 

 ment progressively spread posteriorly toward the anus, 

 and then anteriorly toward the head. On day 42, the 

 dorsal gut pigment extended the full length of the gut, 

 excluding the anus. During subsequent development, a 

 few melanophores migrated ventrally onto the lateral 

 gut wall and others moved close to the newly developed 

 pectoral fin buds. By the last week of development (day 

 71), there was heavy dorsal gut pigment, moderate pig- 

 ment on the lateral gut, and no pigment on the ventral 

 gut or anus (Fig. 3). 



The head remained unpigmented until day 36 when 

 melanophores appeared on the snout, forebrain, mid- 

 brain, and nape (Fig. 2). Initially, head pigment was 

 scattered lightly across the dorsal surface except where 

 it outlined the posterior edge of both lobes of the nape. 

 The nape pigment converged toward the double row of 

 melanophores extending anteriorly from the dorsal mid- 

 line series. During days 37-61, the dorsal midbrain and 

 nape pigment increased, leaving an unpigmented area 

 along the midline (Fig. 2) and several melanophores 

 also became visible on the opercle between the eye and 

 pectoral-fin base. By day 61, opercular pigment con- 

 centrated into one dark spot and a second smaller spot 

 appeared on the opercle below the right lower quadrant 

 of the eye (Fig. 3). Head pigmentation was complete by 

 day 64; stellate melanophores coalesced on the midbrain 

 and nape and heavy pigment covered the nape and mid- 



Table 1 



Description of embryonic development by day for Atka 

 mackerel (Pleurogrammus monopterygius) eggs incuba- 

 ted at 6.2°C. 



Day 



Description 



Single blastomere; yolk contains group of >50 

 oil globules; meroblastic clevage begins 



1 4 blastomeres 



2 32 blastomeres 



3 64-28 blastomeres 



4 Blastodermal cap; individual cells 

 indistinguishable and begin to flatten 



7 Gastrulation, epiboly, and formation of 



embryonic shield begin 

 9 25% epiboly 



11 50% epiboly 



12 75% epiboly 



15 Blastopore closed 



17 Optic vesicles begin to form 



18 Myomeres visible on anterior body behind nape 



19 Kupffer's vesicle visible; optic vesicles formed; 

 myomeres visible on central G0'7c of body 



21 Posterior 50<X of body thick and raised from 

 surface of yolk; eye lenses are formed 



22 Midbrain enlarges to form bump; tail region lifts 

 away from yolk 



23 Midbrain differentiates and forms a bump 

 behind eyes 



24 Embryo arches over 50% of the yolk 



25 Tail curves away from longitudinal axis 

 26-27 Otic capsules formed 



28 Dorsal finfold on posterior 1/3 of body extends 

 anteriorly to 609^ BL 



29 Dorsal finfold extends anteriorly to nape; embryo 

 75'7c of the way around yolk 



31 Pigmentation first appears in eye and along 



dorsal midline, posterior dorsolateral nape, and 

 dorsal gut 



35 Pectoral fin buds formed 



36 Head pigment on snout, midbrain and nape; 

 embryo tail touches snout 



40 Nares visible; embryo tail extends past anterior 

 edge of eye 



41 Nares formed 



42 Lower jaw beginning to form 



48 Embryo tail extends past posterior edge of head 



51 Postanal ventral midline pigment appears 



55 Nape enlarges to form hump 



58 Opercular pigment; lower jaw fully formed 



62 Internal notochord pigment visible 



64 Embryo tail length is 1.75x circumference of yolk 



65 External lateral pigment visible 



71 Pigmentation indistinguishable between 



embyros and hatched larvae 

 74 First hatching 



