Fig 1.
(A) Eyestalk pair before cleaning; (B) cleaned left eyestalk, with yellow arrows indicating the cutting axis; and (c) thin section of the whole eyestalk showing the growing edge or the epicuticle (green arrow), the endocuticle (white arrow), and the location that shows clear growth bands (red circle). Scale bar indicates 200 μm.
Fig 2.
Thin sections of known-age Antarctic krill eyestalks.
Transverse thin sections (170 μm) in the eyestalk of known-age individuals of Antarctic krill showing the annual growth bands indicated by dots. Dots are colored green in Figure D for contrast. Green and white arrows indicate the epicuticle and endocuticle layers, respectively. Scale bars indicate 20 μm. A: 37.8 mm total length (TL) and 1 year (y) 4 month (m) old; B: 41.1 mm TL and 3 y 5 m old; C: 43.7 mm TL and 3 y 5 m old; D: 52.2 mm TL and 4 y old.
Table 1.
Information on known-age krill used in age validation.
AAD–Australian Antarctic Division; PNPA–Port of Nagoya Public Aquarium. Estimated absolute age in years (y) and months (m). Plusses (+) and minuses (-) represent the relative difference in the age given the hatch year, and the date of death assuming a January 1 birthdate.
Fig 3.
Thin sections of Antarctic krill eyestalks collected from the wild.
Transverse thin sections (170 μm) in the eyestalk of individuals of Antarctic krill collected from the wild in summer 1992. (A: 33 mm TL; B: 47 mm TL; C: 46 mm TL; D: 48 mm TL) and samples collected in 2015 (E: 29 mm TL; F: 45 mm TL). The images show the annual growth bands indicated by dots. Green and white arrows indicate the epicuticle and endocuticle layers, respectively. Scale bars indicate 20 μm.
Fig 4.
Scatterplot of variability in lengths at age for Antarctic krill.
Krill lengths varied widely with age, beginning at age two. Formalin preserved summer samples are represented by the black symbols while winter samples preserved in glycerol are represented by the grey symbols.
Fig 5.
Bias plot of Antarctic krill band count readings.
Age bias graph for Antarctic krill aged by Readers 1 and 2 that counted the growth bands in thin sections of eyestalk. Each error bar represents the 95% confidence interval about the mean age assigned by Reader 2 to all krill individuals assigned a given age by Reader 1. The values indicate the number of individuals aged at each age group. The solid line represents 1:1 equivalence.