Histological changes of the digestive system and its associated glands, and structures of the jaw were studied in summer flounder Paralichthys dentatus from hatching (day 0) until day 44. Specimens for this study were hatched from artificially spawned broodstock and maintained in the laboratory (20 1 C). During the first 3 days after hatching, the formation of the oral jaw apparatus, lengthening of the digestive tube, yolk resorption, and mucosae differentiation are the most conspicuous elements of development. The larval digestive system is morphologically ready to process external food at the time of mouth opening (3-4 days after hatching). Epithelial cells of the anteromedian and the posterior intestine show evidence of lipid and protein absorption, respectively, after first feeding. The most noticeable events occurring during the next month of independent life are an increase in mucosal folding, cellular differentiation in the luminal epithelia, gut segmentation and looping, and liver growth. Gastric glands and pyloric caeca appear by day 31 and complete the morphological digestive features characteristic of the juvenile stage. 1995 The Fisheries Society of the British Isles

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Hypo-osmoregulatory ability in juvenile Atlantic salmon, Sulrno salur L., was improved by cortisol treatment. Implantation of a vegetable shortening pellet containing cortisol (50 mg kg-') resulted in elevated plasma cortisol titres. Maximum cortisol levels (160-170 ng ml-') were observed at days 6 and 12 after the implantation and dropped significantly by day 55. Cortisolimplanted fish in fresh water developed a twofold increase in gill Na+/K+-ATPase activity at days 6 and 12, and a threefold increase by day 55. Intestinal mucosa Na+/K+-ATPase activity was not affected by cortisol. Cortisol-implanted fish exposed to 28 ppt sea water for 48 h tended to show an improved ability to regulate their plasma osmolarity and reduce their ionic load. The osmoregulatory ability attained at days 12 and 55 was further evaluated by exposing fish to 37 ppt sea water for 96 h. While all the control fish died relatively early in these tests, cortisol-implanted fish showed a clear reduction in their mortality rate. These results indicate that cortisol can induce biochemical and organismal changes during winter months that typify preadaptive events normally occurring in the spring.

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ABSTRACT. Survival, growth, and starvation times were studied in summer flounder Paraljchthys dentatus larvae hatched in the laboratory a t 12.5 and 21°C. The observations spanned the time interval from hatching throughout the period of feeding on rotifers. Survival and growth in length and weight were strongly dependent on water temperature and delay of the initial feeding. At either temperature, the percentage of summer flounder larvae surviving beyond the rotifer phase increased if food was made available at the time of mouth opening. At 12 5"C, hatching started 85 h after fert~lizatlon. All feeding delays resulted in 2 separate periods of mortality, which caused low final survival. The point of no return ranged from 11 to 12 d after hatching. Larvae fed at mouth opening showed a maximum survival of 40%. No significant growth in length or weight was evidenced by any group at 16 d after mouth opening At 21°C, hatching started 60 h after fertilization. Larvae fed at mouth opening showed 90% survival and significant growth in length and weight in 10 d. A delay of 48 h in initial feeding led to a final survival of 36%, but also resulted in significant growth. Time to the point of no return was 6 to 7 d from hatching. These results illustrate the interdependence of temperature and food availability and their effects on survival and growth of summer flounder larvae. These observations provide crucial information for the development of a culture system for this species and demonstrate the strong influence of the temperature-food relationship on larval survival and growth, suggesting that this relationship is a determinant of recruitment in certain areas of the ocean.