Lipid synthesis and reproduction in the polar shrimp Chorismus antarcticus
Chorismus antarcticus (Pfeffer, 1887) is a small benthic hippolytid shrimp which breeds seasonally, producing a batch of large, lipid-rich eggs that are brooded over winter by the female, and hatch the following spring. Ovary size increased linearly during the 2 mo prior to spawning, and lipid accumulated at a rate of 3.7 mg per 10 d in a female shrimp of 2 g fresh weight. Hepatopancreas reserves were insufficient to account for all the material necessary for vitellogenesis, and the hepatopancreas remained constant in size until the last few weeks of ovarian maturation when lipid was used at a rate of 2.2 mg per 10 d. Total lipid synthesis was measured by incubation of isolated whole ovary or hepatopancreas in crustacean saline containing tritiated water. The observed tritium incorporations showed clearly that vitellogenesis in summer was accompanied by a small increase in de novo synthesis, estimated at about 0.197 mg per 10 d for a 2 g shrimp. Thus only a small fraction of the lipid required for eggs is provided by an increase in de novo synthesis. Much of the egg lipid, including all the essential ω3 and ω6 polyenoic fatty acids, thus comes directly from the food. This is a strategy different from planktnic euphausiids and some starfish, where vitellogenesis is fuelled largely by utilisation of previously stored reserves. However, food availability for C. antarcticus at South Georgia in summer is sufficient to provide for both somatic growth and complete ovarian maturation within 6 mo before spawning in late summer. There is thus no need for large lipid stores, or substantial lipid synthesis, and the hepatopancreas acts as a metabolic centre than a concentration of reserves.