Maria,

I am attaching a few papers that may shed some light on the possible explanation of the red colour ranging from dietary source, sexual dimorphism, protection from ultraviolet radiation or predatory counter-shading. I hope you still have the samples and you can still have a closer look at the copepods, specifically their gut content and sexual organs, since I have seen intensely coloured egg masses in females of some species. I hope that you can follow up with more analysis of the samples, it would be interesting to find out what is the cause of that colouration.

Tomas

During intensive feeding copepods (including Calanus finnmarchicus) accumulate fat as drops of red colour. Red calanus occurs in feeding season.

Thank you, Tomas for the extensive info on possible causes. And Andrey, yes I sampled them during algae blooms, I was just startled by not having seen that a few days before or after in the same species.