CHEMICAL EMBRYOLOGY OF THE CRUSTACEA

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Abstract
Summary: 1. The dry weight of a neonate crustacean is sometimes lower than the initial dry weight of the egg, owing to utilization of respiratory substrates, as in Daphnia, where the dry weight decreases by 16–25 %. In Artemia any loss is compensated by an uptake of salts so that there is no change in dry weight during the course of embryonic development. The embryos of Ligia receive nutrients secreted into the maternal brood pouch, and the dry weight of the neonate is greater than the initial dry weight of the egg.2. Early embryos of Simocephalus, Artemia and Balanus consume less oxygen per unit dry weight than older embryos. The increase in oxygen consumption from early to late embryos is approximately six to eightfold.3. Oxygen consumption decreases when the rate of embryonic development of Artemia is reduced by increasing osmotic pressures in the external medium.4. Glycogen is formed rather than utilized in the embryos of Simocephalus and Artemia. In the latter the main carbohydrate reserve is trehalose, which is transformed to glycogen and glycerol. The relative formation of these two substances is influenced by external osmotic pressures. At high external osmotic pressures more glycerol is formed, and this aids osmotic rupture of the tough outer shell of the egg.5. Trehalose seems to be important in relation to the dormant state of Artemia embryos. The embryos which develop rapidly in the maternal brood pouch contain much less trehalose than those which become encysted and dormant.6. The proportion of the initial lipid content of a crustacean egg that is utilized during development is very variable. In Homarus 60% disappears by time the neonate emerges, in Ligia 32%, and in Artemia the decrease is very slight, or there may even be a slight increase.7. Haemoglobin in the eggs of Cladocera does not appear to act as a respiratory pigment, but does serve to accelerate development in poorly aerated water. It is suggested that the main function is to act as a supply of stable protein.8. The haemochromogen found in the gut of late embryos of Daphnia appears to be formed in part at least from haemoglobin, which diminishes in concentration as development proceeds.9. Breakdown of haemoglobin during embryonic development of Daphnia is not accompanied by formation of bile pigments.10. Biliverdin is synthesized in the embryonic eyes of Polyphemus but not elsewhere in the embryo.11. The carotenoproteins in the eggs of many different crustaceans break down towards the end of embryonic development, liberating free carotenoid. It is suggested that these proteins are stabilized and held in reserve by linkage to a carotenoid until a particular stage of embryonic development has been reached.12. Metabolic pathways in the formation of astaxanthin from β‐carotene may involve monoketo and diketo carotenes, or by an alternative route monohydroxy and dihydroxy carotenes.

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