Direct ageing of Antarctic krill (Euphausia superba) – potential utility of eyestalk sections for age determination

We outline a method for the direct determination of age in Antarctic krill (Euphausia superba). Recent studies (Kilada et al. 2012), have shown that annular growth bands are present in the endocuticle layer of the eyestalks and gastric mill in decapod crustaceans. Here we report on a preliminary proof of concept study using several known age krill grown under controlled conditions at the Australian Antarctic Division and examined using the methods outlined in (Kilada et al. 2012). Two analyses are performed. First, longitudinal sections of the eyestalk from known age krill (2+ and 3+ years old) were examined and were determined to show distinct bands within the endocuticle consistent with their known age. Second, six krill were aged (blind to the reader) from samples collected during summer and winter (2013 to 2014) using this technique to determine whether annuli could be resolved in wild samples across seasons.  Marks consistent with annuli observed in laboratory raised krill were observed. Length at age was plotted against a theoretical Von Bertalanffy growth curve (L­­­_inf=65 (mm); T­­₀ =-0.1; K=0.45) and fall on the line for two and three year old krill. If validated, the technique should allow the development of age-based assessments of krill necessary for management. Further because the annular bands formed within the endocuticle are not eroded in krill preserved in formalin, it may be possible to re-analyse historical patterns of size at age, using archived samples, and to compare size at age among regions of interest, opening up many research questions that have eluded the scientific community.