III. Development of the larvae.

At hatching, the yolk-sac larvae measure between 2.5mm and 3.2mm body length. They show the typical clupeoid characteristics of a slender, sparsely pigmented body, a posteriorly placed anus, and coarsely granular yolk. The head of the young larva is still curved over the yolk, and the mouth is not yet developed. The fin folds are smooth-outlined and erect, with a slight constriction at the anus. Dendritic melanophores develop along the dorsum either side of the fin fold, and as the larva grows they page 7

Figs. 8-9: Final stages of development and hatching of the sprat embryo—8, 80-86 hrs, tail lengthening; 9, 90-98 hrs, hatching. Fig. 10: Sprat yolk-sac larva measuring 4.0mm b.l. Figs 11-13: Sprat larva measuring 4.7mm, 8.5mm, and 12.0mm b.l., respectively.

page 8 migrate down the flanks to a position level with the dorsal side of the abdomen anterior to the pylorus, and along the ventral edge of the intestine posteriorly (Figs. 10 and 11). At about 4.7 - 5.0mm body length the yolk sac is very reduced, the mouth forms terminally, and the caudal and pectoral fin lepidotrichia develop. The slenderness of the body is

page 9

Fig. 14: Post-larval sprat measuring 19.5mm b.l. Fig. 15: Late post-larval sprat of 31.0mm b.l. at beginning of metamorphosis. Fig. 16: Juvenile sprat of 43.0mm b.l. in advanced metamorphosis. Fig. 17: Adult sprat measuring 125.0mm b.l.

page 10 accentuated by a decrease in height of the fin folds relative to the body depth (Fig. 11).

By 7.0 - 9.0mm body length (Fig. 12) the yolk sac has been completely absorbed and the mouth has assumed its functional shape. The dorsal fin is prominent by 8.0mm, and the intestine is tightly convoluted, with a row of superficial melanophores above its dorsal side. There is also a single melanophore just above the origin of the pectoral fin. The notochord turns dorsally and the hypurals develop at about 12mm. The anal fin also appears at this length, and the tail assumes a slightly heterocercal shape (Fig. 13). At 19.0mm pelvic fins arise alongside the pyloric region of the gut (Fig. 14), and the full complement of vertebrae (44-46) are evident in stained specimens. The tail is now homocercal.

Changes in pigmentation and body form take place when the sprat metamorphoses from post-larva to juvenile at body lengths between 30 and 50mm (Figs 15-18). The colour change begins with silver pigment developing on the sides of the head and the opercula, and spreading along the flanks. The whitish-silver colour covers all but the dorsal one-quarter or one-fifth of the flanks before the dark pigment becomes fully developed along the dorsum. As the silver spreads, a multiple row of melanophores extends from the caudal peduncle along the middle of the flanks to the top corner of the opercula; melanophores also appear along the dorsum, in the caudal rays, and around the tip of the snout (Fig. 16). As the lateral melanophores are over-coloured by silver pigment, the dorsal strip

Fig. 18: The changing proportions of the sprat body during metamorphosis 30-50mm b.l.) and subsequent growth. Triangles represent greatest depth of body, dots represent greatest thickness of body (n = 134).

page 11 gradually becomes darker until it reaches the adult bluish-grey. The flanks and belly remain whitish-silver.

During the colour change the body deepens markedly, becomes laterally compressed (Fig. 18), and the ventral edge of the belly becomes noticeably curved and armed with enlarged, pointed scales which give it a serrated appearance (Fig. 17). There is also considerable migration of fins and the anus along the long axis of the body during late post-larval life; the pelvic fins migrate backward over two myotomes (17th-18th), the dorsal fin migrates forward over seven myotomes (25th-18th), and the anus migrates forward over four myotomes (35th-31st).