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The ontogeny of the digestive system of cobia Rachycentron canadum from hatching to 22 days post-hatch (dph) (20Á1 mm standard length) was examined with light microscopy. The activities of selected pancreatic enzymes were also determined during this period in order to optimize current rearing methods for this species. At hatching (3Á6 mm), the digestive tract consisted of a relatively undifferentiated, straight tube positioned dorsally to the yolk sac. The major morphological changes in the digestive tract primarily occurred over the first 1-4 dph (3Á6-4Á4 mm). During this time, larvae began exogenous feeding (3 dph) and the digestive tract differentiated into five histologically distinct regions: buccopharynx, oesophagus, stomach anlage, anterior intestine and posterior intestine. Yolk reserves were exhausted by 5 dph (4Á5 mm) and the oil globule began rapidly decreasing in size disappearing entirely by 9-10 dph (6Á3-6Á8 mm). Gastric glands differentiated at this time, and by 12 dph (8Á1 mm) surface mucous cells of the stomach anlage stained positive for neutral mucosubstances. By 16 dph (11Á6 mm), the blind sac (fundic region) of the stomach formed as did the pyloric caecae which initially appeared as a single protrusion of the anterior intestine just ventral to the pyloric sphincter. Generally, enzyme activities (U larva À1 ) for amylase (0Á0-1Á8), chymotrypsin (0Á0-7902Á4), trypsin (0Á2-16Á6) and lipase (9Á3-1319Á0) were measurable at or soon after hatching and increased steadily from c. 8-22 dph (5Á7-20Á1 mm). The results of this study are discussed in terms of current and future weaning practices of this species.

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Fatty acid composition of eggs affects development, growth and ecological performance of fish embryos and larvae, with potential consequences for recruitment success. Essential fatty acids in eggs derive from the maternal diet, and the time between ingestion and deposition in eggs is ecologically important but unknown. We examined the dynamics of diet -egg transfer of arachidonic acid (ARA) in the batch-spawning fish, red drum (Sciaenops ocellatus), by measuring ARA concentrations in eggs after a single diet shift and during a period of irregular variations in diet. ARA concentrations in eggs changed within 2-16 days of a diet shift. The rate of change was proportional to the magnitude of the shift, with no evidence of equilibration. These results are not consistent with eggs being assembled entirely from accumulated body stores. The immediate source of ARA in eggs appears to be the recent diet. We propose that batch spawning produces rapid diet -egg transfer of ARA because it removes large amounts of fatty acids from the body and prevents equilibration. The immediacy of the diet -egg connection suggests that spawning migration combined with short-interval batch spawning may have evolved to take advantage of nutrients critical for offspring survival that are available at the spawning site.