Immunophysiology of Atlantic sturgeon, Acipenser oxyrinchus oxyrinchus (Mitchill), and the relationship to parasitic copepod, Dichelesthium oblongum (Abilgaard) infection M S Sokolowski 1 , B A Allam 1 , K J Dunton 1 , M A Clark 2 , E B Kurtz 2 and M D Fast 1,3 1 School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA 2 Mount Sinai High School, Mount Sinai, NY, USA 3 Hoplite Research Group, Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, PEI, Canada Abstract The copepod parasite, Dichelesthium oblongum, is known to infect the Atlantic sturgeon, Acipenser oxyrinchus oxyrinchus, within the area near New York city, USA, known as the NY Bight. The gross pathology associated with the juvenile and adult copepod stages along with the parasiteÕs link in causing changes in sturgeon osmoregulatory capa- bilities has led us to investigate the host immuno- physiology in relation to this host–parasite system. All the host variables, which included gill Na + -K + - ATPase activity, serum alkaline phosphatase (AP) and white blood cell differential counts, were affected in a non-linear manner by the copepod parasite. The parasites increased the host gill Na + - K + -ATPase activity and serum AP along with the percentage granulocytes while decreasing the per- centage lymphocytes. A new method, developed to sample and preserve white blood cells in the field for future flow cytometry analysis, proved adequate. The effects of fish size, location and time of sam- pling were accounted for by the use of generalized linear models, and their effects on the host variables are discussed. Keywords: Atlantic sturgeon, copepod parasite, flow cytometry, gill Na + -K + -ATPase, leucocyte, serum alkaline phosphatase. Introduction The Atlantic sturgeon, Acipenser oxyrinchus oxyrin- chus (Mitchill), is a diadromous species with a long life span (>50 years), of which the majority is spent in salt water. Juveniles of this species are known to leave the river system within the first few years of life, and genetics suggest they may return to their natal rivers (Waldman, Hart & Wirgin 1996; Grunwald et al. 2008), following maturation (12– 14 years (Van Eenennaam & Doroshov 1998)), to spawn at intervals, which may be >3 years (Boreman 1997). There is currently an information gap regarding the movements and habitat usage for juvenile Atlantic sturgeon upon leaving their natal river, but what is known suggests significant migrations across varying osmotic environments. Osmotic competence is therefore extremely impor- tant for adeptly negotiating nearshore environments during spring and autumn months and because the energetic demands required for undergoing long migrations are significant. Stressors these juveniles encounter within and around these habitats may have severe repercussions, as elevated levels of the stress hormone cortisol have been shown to disrupt the osmoregulatory capacity and reduce the immu- nocompetence of other fish species (Pickering & Duston 1983; Pickering & Pottinger 1989; Bonga 1997). For Atlantic sturgeon, there is little known about their immunophysiology within hypersaline environments. Marine-phase Atlantic sturgeon migrations are thought to be limited to narrow corridors of waters Journal of Fish Diseases 2012 doi:10.1111/j.1365-2761.2012.01390.x Correspondence M D Fast, Novartis Chair in Fish Health, Department of Pathology and Microbiology, Atlantic Veterinary College–UPEI, 550 University Avenue, Charlottetown, PEI, C1A4P3, Canada (e-mail: mfast@upei.ca) 1 Ó 2012 Blackwell Publishing Ltd