Contribution of moulting and eggs to secondary production in Nyctiphanes australis (Crustacea: Euphausiacea)

Abstract

This study was designed to assess the contribution of moulting and eggs to production of the euphausiid Nyctiphanes australis G. O. Sars. For this purpose, live specimens were collected from Storm Bay, south-eastern Tasmania, between August and December 1981, while preserved samples, collected between February 1980 and February 1981, were also examined. The intermoult period of N. australis increased exponentially with increase in body length and weight. Larvae moulted approximately every 2.5 to 3 d and adults every 4 to 5 d at 15°C. A decrease in temperature from 15° to 10°C resulted in the intermoult period almost doubling. The mean weight of exuviae produced represented nearly 6% of the body dry weight of the individual. Continuous maturation of ova was observed, with an individual female capable of releasing a total of ∼1 100 eggs in a lifetime. These are deposited as a series of batches into a pair of external ovisacs every 30 d. The size of the batch of eggs was dependent on the size of the female. Moulting of euphausiids forms a significant contribution of organic matter to detrital food webs. A value of 42.01 mg m^-3 yr^-1 was obtained for the production of exuviae. Egg production was calculated to be between 1.41 and 4.22 mg m^-3 yr^-1. Production and mean annual biomass of N. australis was previously calculated as 78.29 mg m^-3 yr^-1 and 5.39 mg m^-3, respectively. Thus, the total production integrated for the whole of Storm Bay was 125 mg m^-3 yr^-1 or 2 309 tonnes dry wt yr^-1, representing an overall P:B ratio of 23.1.