The parasitic copepod, Sarcotretes scopeli Jungersen, 1911 castrate its mesopelagic fish host Benthosema glaciale

Abstract The mesopelagic fish Benthosema glaciale [Actinopterygii: Myctophiformes: Myctophidae], abundant in open ocean and deep fjords, is targeted for future exploitation despite existing knowledge gaps on population vital rates. Among these gaps, the vulnerability to pathogens is poorly understood. Parasites rely on the metabolic energy of their host and infections might compromise host physiology and behaviour, which in turn, may affect growth and survival. Parasitic infections by the copepod Sarcotretes scopeli [Crustacea: Copepoda: Pennellidae] have frequently been recorded from the North Atlantic, including west Norwegian fjords. In this paper we present data on how S. scopeli infection affect reproductive investment, energetic status, and growth of its host, the mesopelagic fish B. glaciale . Our results indicate that (i) S. scopeli reduce reproductive investment (measured as gonadosomatic index GSI) of both female and male B. glaciale ; (ii) that the energy storage (hepatosomatic index HSI) appeared unaffected in female B. glaciale , while infected male hosts have higher HSI than uninfected, and (iii) that infected fish are in poorer condition than uninfected. These findings may indicate that S. scopeli could have a significant regulatory impact on B. glaciale populations.