Better red than dead? Potential aposematism in a harpacticoid copepod, Metis holothuriae

The conspicuous, red harpacticoid Metis holothuriae grows to a large size (∼600 μm length) and accounts for 29.51% of the numerical meiofaunal abundance within blooms of the toxic, benthic cyanobacterium Lyngbya majuscula. Despite this, the meiobenthic juvenile trumpeter whiting, Sillago maculata, consume M. holothuriae at only 2.16% of numerical meiofaunal biomass within simulated blooms, despite their apparent ease of predation. We compared the predation rates of copepods that had been dyed red (primarily Canuellidae and not known to be toxic) to M. holothuriae by S. maculata, to assess whether avoidance by predators is possibly a response to an aposematic signal conveyed by the colouration of the copepods and reinforced by their potential toxicity from exposure to L. majuscula. M. holothuriae were again strongly avoided, with only 6.25% of M. holothuriae consumed, whereas dyed copepods were consumed with relative alacrity, indicating that predation was not deterred by colouration alone. M. holothuriae copepodites were consumed in preference to adult individuals, supporting the idea that toxin accumulation or other factors relating to maturation might explain avoidance by benthivorous fishes.

► The harpacticoid, Metis holothuriae is highly abundant in Lyngbya majuscula blooms.
► However, M. holothuriae are only a minor component of fish diet.
► Its conspicuous red colouration suggests aposematism.
► M. holothuriae was consumed significantly less than similarly sized, red-dyed copepods.
► Not aposematic, likely toxin accumulation from exposure to L. majuscula.