Deviating vertical distribution and increased conspicuousness of parasitized Calanus

Abnormally colored yellow and red Calanus spp. occurred in the Oslofjord (southeast Norway) in late summer. These specimens were infected with an extensive parasitic growth consisting of large branched hyphae-like tubes filled with spores. This parasite has previously been referred to as Ichthyosporidium sp. (now Ichthyophonus, Ichthyosporea), and suspected to be a stage in the life cycle of the fish pathogen Ichthyophonus hoferi. This assumption was not supported by our examination. Infected copepods were virtually confined to the upper meter, while distribution of the uninfected specimens was much deeper. We argue that the change in color and distribution is induced by the parasite, facilitating near-surface, visual predation, dispersal of spores, and, hence, increasing transmission to new hosts.