Moulting and body shrinkage in the Antarctic krill, Euphausia superba

Brief description

Euphausia superba were collected from the Antarctic Ocean during the cruise by RV Kaiyo-Maru in January 1980, and were transported to the Australian Institute of Marine Science laboratories at Townsville in February 1980.\n\nSpecimens of Euphausia superba were maintained in a cold room at -0.5°C (0 to -1.0°C) under continuous subdued light (< 0.6 W/m²) for a period of 5 months. Seawater was collected from the adjacent Coral Sea and the salinity adjusted to 34.0 ppt with distilled water. Single specimens were isolated into 1-4 litre glass beakers, depending on size. Approximately 80% of seawater in the beakers was changed once a week, when new food was prepared. One of three feeds was provided to each group of 15 specimens of various sizes: a mixture of laboratory cultures of microalgae (Dunaliella tertiolecta and Phaeodactylum tricornutum); an artificial pet fish food (Tetra Marin, W. Germany); and filtered seawater (HA Millipore filter, 0.45µm pore size) without food. When a specimen died, it was replaced with another of similar size from stock specimens, which were maintained on mixed foods (mixed microalgae and Tetra Marin). Care was taken to ensure that food was always present during this experiment.\n\nSpecimens were checked daily for moults, which were then preserved with a few drops of buffered formalin (40%) in seawater. The exopodite length of the moult uropod was later measured. Changes in body wet weight or length were then tracked by applying allometric equations derived from specimens sacrificed at the end of experiments. Preserved moults were later rinsed in distilled water to remove formalin and salts, and were dried in a desiccator over silica gel at room temperature to obtain dry weights. Fresh moults, collected from stock specimens were used for analysis of carbon and nitrogen using a elemental analyser (Perkin Elmer, model 240) with acetanilide as standard.\n\nIn another experiment, conducted over a period of 211 days, using the same temperature, light and seawater conditions described above, 15 Euphausia superba of various sizes were maintained individually in sea water filtered through a HA Millipore filter, 0.45µm pore size (starved). Two control groups of 15 individuals each were fed, ad libitum, an artificial pet fish food (Tetra Marin) and frozen copepods (Calanus finmarchicus) or a mixture of microalgae (Dunaliella tertiolecta and Phaeodactylum tricornutum). During the experiment, handling of specimens was minimised to avoid possible damage. Changes in body wet weight were tracked as described above.\n\nAt the end of the experiment, physiological rates (oxygen uptake, ammonia excretion, inorganic phosphate excretion, dissolved organic nitrogen (DON) excretion, dissolved organic phosphate (DOP) excretion) were measured by a water bottle method, where individual specimens were placed into 250 to 1000 ml glass bottles filled with the filtered sea water for 24 h in the dark at -0.5°C. Dissolved oxygen, ammonia, and inorganic phosphate were measured by Winkler titration, phenolhypochlorite, and molybdate methods, respectively. DON and DOP were determined by a UV irradiation method.\n\nThe individuals were weighed (wet weight) and freeze-dried for the determination of dry weight and analyses of elemental composition (C, N, P). The analyses of C and N were made with an elemental analyzer (Perkin Ebner, Model 240) using acetanilide as standard. Determination of P was made by the molybdate method after digestion of samples in 50% (v/v) H2SO4 for 1 hour at ~ 100°C followed by neutralization with KOH.

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