HOUDE: NORTHERN SENNET 



all larvae until about 13.5 mm SL. Remnants 

 of the larval finfold persisted until 12.5 mm SL 

 (Figure 3B). 



Pigmentation 



Except where specifically mentioned, referen- 

 ces to pigment are to melanophores. Xantho- 

 phores were common on larvae, and both silver 

 iridophores and blue chromatophores were pre- 

 sent. (See Fujii, 1969, for chromatophore ter- 

 minology.) Some variations in melanophore 

 patterns were present among S. borealis larvae 

 of similar size but the following description gives 

 the typical sequence of development. 



Melanophores and xanthophores were present 

 on newly hatched larvae but the latter faded 

 after preservation. Small melanophores were 

 distributed in a dorso-lateral and ventro-lateral 

 row on each side of the larva (Figure IB) . The 

 two rows converged just above the yolk sac and 

 ran anterior as a single row to the posterior 

 cephalic region, joining a series of melanophores 

 located over the hindbrain. Other melanophores 

 were present on the anterior half of the oil glob- 

 ule and near the posterior of the yolk sac. 



One to two days after hatching (about 3.8 mm 

 SL) , melanophores became larger and more nu- 

 merous (Figure 2A) . Those located in the later- 

 al rows and on the cephalic region became stel- 

 late. They were more numerous on the yolk 

 sac and two small melanophores appeared near 

 the developing mouth. A series of two to six 

 small, contracted melanophores were noted near 

 the tip of the notochord. The eye became pig- 

 mented at 2 days after hatching. 



As larvae developed, both melanophores and 

 xanthophores became more numerous. Xantho- 

 phores were distributed over much of the body 

 and consisted of elongate yellow cells forming 

 a loose network on the body. In life, larvae ap- 

 peared green because of the presence of both 

 yellow and black pigments. Blue iridophores on 

 the hindgut and some iridescent pigment in the 

 eyes also were present. 



By 7 days after hatching (about 5.3 mm SL, 

 Figure 2C) stellate melanophores had appeared 

 over the brain, on the tip of the upper jaw, lower 

 jaw, angle of the jaw, ventral margin of the 



opercular region and over the foregut. Each 

 dorso-lateral row of stellate melanophores was 

 well developed while the ventro-lateral rows 

 were condensing into a single ventro-medial row, 

 posterior to the anus. A mid-lateral series of 

 melanophores was developing posterior to the 

 anus. 



At 9 days after hatching (about 7.4 mm SL) 

 pigmentation became more intense (Figure 2D) . 

 About 5 melanophores were present in the de- 

 veloping caudal fin. The fleshy tip of the lower 

 jaw began to become darkly pigmented on some 

 specimens at this stage. The extent and inten- 

 sity of pigmentation continued to increase on 

 older larvae (Figures 3A and 3B). No changes 

 in pattern were observed, except for develop- 

 ment of a line of pigment that bisected the eye, 

 and a migration of melanophores from the tip 

 of the developing urostyle to the ventral margin 

 of the hypural plate. 



When specimens were 20 to 22 days old (about 

 12.5 to 14.5 mm SL), the juvenile pattern of 

 dorsal and lateral blocks of pigment began to ap- 

 pear (Figure 3C). Stellate melanophores also 

 developed in the second dorsal and anal fins of 

 individuals of this size. Longer specimens from 

 plankton collections had the typical juvenile pig- 

 ment pattern (Figure 3C) (cf. de Sylva, 1963; 

 Figure 4). 



GROWTH AND MORTALITY 



Larvae of S. borealis grew most rapidly dur- 

 ing the first 14 days after hatching, but more 

 slowly during the next 7 days (Figure 4). 

 Larvae were 2.6 mm SL at hatching, averaged 

 5.5 mm SL at 7 days after hatching, 11 mm at 

 14 days, and about 13.5 mm at 21 days. The 

 decrease in growth rate of the sennets during 

 the third week may have been caused by the 

 scarcity of fish larvae in their diet. Average 

 growth rate during the rearing experiment was 

 about 0.5 mm per day. 



A total of 78 sennet eggs were incubated but 

 only about 50 ';r hatched. From this small num- 

 ber of hatched larvae, 9 survived until 14 days 

 after hatching, when they had developed most of 

 the characters of juvenile sennets. Mortality 

 in the first 14 days included 11 larvae preserved 



193 



