Detection of growth zones in the eyestalk of the Antarctic krill Euphausia superba (Dana, 1852) (Euphausiacea)

No reliable measures of age currently exist in the Antarctic krill, Euphausia superba (Dana, 1852). The eyestalks from 51 individuals were dissected, cut in longitudinal sections and studied for identifying growth zones. The krill was collected at the South Orkney Islands during January and February 2015, and varied between 30 and 53 mm in total body length. Up to six growth zones were identified, each zone consisting of one light and one dark section. The width of the longitudinal sections increased with increasing body length, although there were differences between sexes. Females tended to have narrower growth zones from the third zone and onwards compared with males. Data show that male subadult stages (MIIA1, MIIA2 and MIIA3) had 2.2 ± 0.8 (average ± SD) zones and adult male stages had 3.8 ± 0.8 zones. The female juvenile stage (FIIB) had 1.7 ± 0.5 zones and adult females (FIIIA-E) had 3.7 ± 1.0 zones. There were positive relationships between the number of zones and the maturity stage, and between the number of zones and body length. Further knowledge about molting process in the Antarctic krill and a verification of the ageing procedure from krill with a known age is needed before the number of growth zones can be definitely established as an indicator of age. The detection of growth zones in the Antarctic krill will be an important contribution to the understanding of the biology of the species if the zones actually represent annual growth.