Variations in photoprotective compounds in surface-dwelling copepods in subtropical open waters

Abstract

We measured the concentrations of photoprotective compounds, carotenoids and mycosporine-like amino acids (MAAs), in surface-dwelling copepods in relation to their vertical distribution patterns along a zonal transect in the subtropical North Pacific to better understand their adaptation to strong ultraviolet (UV) radiation. All the domi-nant copepod groups analysed had detectable levels of both photoprotective compounds, while the concentrations dif-fered among taxonomic groups. Farranula spp. had high carotenoid and MAA content (mean carotenoids: 0.0656; mean MAAs: 0.0230 µg mg dry wt−1) and the accumulation of MAAs correlated positively with in-situ UV penetra-tion, explaining their constant occurrence in the surface layer throughout the transect. Oncaea spp. and Acrocalanus spp. had higher levels of carotenoids than MAAs and showed variable vertical distribution patterns regardless of UV penetration, suggesting that carotenoids are used only temporally to mitigate the effect of UV radiation. In contrast, Clausocalanus spp. and Calocalanus spp. had the lowest concentrations of both photoprotective compounds (mean carotenoids: 0.0148 and 0.0209; mean MAAs: 0.0015 and 0.0029 µg mg dry wt−1). The mean weighted depth of Clau-socalanus spp. showed a significant negative relationship with UV penetration, suggesting that they mitigated the harm-ful effects of UV radiation through behavioural responses. Calocalanus spp. may have adapted to subtropical surface waters with relatively low photoprotective compound concentrations. Our study demonstrated that surface-dwelling subtropical copepods exhibit taxon-specific patterns in photoprotective compound concentrations that are adaptations to intense UV radiation, which is crucial for understanding the high species diversity of copepod communities in subtropical open waters.