EPIZOIC DIATOMS ON THE EUPHAUSIID NYCTIPHANES-AUSTRALIS - CONSEQUENCES FOR GUT-PIGMENT ANALYSES OF WHOLE KRILL

We report the first record of infestation by the epizoic pennate diatoms (Licmophora sp.) of the euphausiid Nyctiphanes australis (G. O. Sars, 1883) collected from Otago Harbour and coastal waters, New Zealand (45°50′S; 170°37′E) between October and April 1986-1989: at times up to 50 to 70% of the krill examined were infested. Infestation by epizoic diatoms introduces a significant source of error into determinations of egestion rates using gut fluorescence of whole individuals, and prevents measurement of background fluorescence in krill. Diatom infestation can lead to overestimation of ingestion rates by a factor of 2 to 5. We suggest that investigators using the gut fluorescence method to measure grazing rates should determine the occurrence of epizoic diatoms in samples prior to analysis. If diatoms are present, it is necessary to hold krill in the laboratory until diatoms are shed at moult before making in vitro gut-pigment measurements. Alternatively, the stomachs must be dissected for analyses, negating the benefits of rapid sample preparation and quantitative recovery of pigments that are the advantage of the technique. Epizoic diatoms can significantly contribute to variability of apparent in situ ingestion rates in N. australis. © 1990 Springer-Verlag.