1 3 Mar Biol (2014) 161:1179–1193 DOI 10.1007/s00227-014-2409-7 ORIGINAL PAPER Acid–base regulation in the Dungeness crab (Metacarcinus magister) Stephanie Hans · Sandra Fehsenfeld · Jason R. Treberg · Dirk Weihrauch Received: 5 July 2013 / Accepted: 12 February 2014 / Published online: 14 March 2014 © Springer-Verlag Berlin Heidelberg 2014 consumption, were reduced in crabs acclimated to elevated pCO 2 , demonstrating that either (amino acid) oxidation is reduced in response to this particular stress, or nitrogenous wastes are excreted in an alternative form. Introduction In many marine organisms, a rise in seawater pCO 2 (envi- ronmental hypercapnia) causes an increase in body fluid pCO 2 in order to maintain an outwardly directed gradient for the passive excretion of metabolic CO 2 (Fabry et al. 2008; Melzner et al. 2009). Hypercapnic environments can therefore lead to a respiratory acidosis, where the extracellular pH decreases if the H + produced from the hydration of excess CO 2 is not buffered in the extracellu- lar fluids (Melzner et al. 2009). The decline in pH could prove detrimental since a stable internal pH is essential for many physiological and biochemical processes (Somero 1986; Riggs 1988; Wheatly and Henry 1992; Fabry et al. 2008). To counteract the internal pH drop active metazo- ans, such as teleost fish and brachyuran crabs, are equipped with mechanisms to excrete acid equivalents and/or create an influx of bases from the surrounding medium (Boron 2004). Green crabs Carcinus maenas and Dungeness crabs Metacarcinus magister exposed to medium- and short-term hypercapnia for example actively accumulate HCO 3 - in the hemolymph to compensate for the resulting acidosis and successfully restore hemolymph pH (Pane and Barry 2007; Appelhans et al. 2012; Fehsenfeld and Weihrauch 2013). In aquatic crabs, acid–base regulation occurs mainly in the gills and uses many of the transporters also involved in osmoregulatory processes (Henry and Wheatly 1992). Na + / K + -ATPase in the gills of the hyper-regulating C. maenas generates the sodium gradient between the cytoplasm of Abstract Homeostatic regulation allows organisms to secure basic physiological processes in a varying envi- ronment. To counteract fluctuations in ambient carbonate system speciation due to elevated seawater pCO 2 (hyper- capnia), many aquatic crustaceans excrete/accumulate acid–base equivalents through their gills; however, not much is known about the role of ammonia in this response. The present study investigated the effects of hypercapnia on acid–base and ammonia regulation in the Dungeness crab, Metacarcinus magister on the whole animal and isolated gill levels. Hemolymph pCO 2 and [HCO 3 - ] increased in M. magister acclimated to elevated pCO 2 (330 Pa), while pH remained stable. Additionally, hemolymph [Na + ], [Ca 2+ ], and [SO 4 2- ] were significantly increased. When challenged with varying pH during gill perfusion, the pH of the arti- ficial hemolymph remained relatively unchanged. Over- all, ammonia production and excretion, as well as oxygen Stephanie Hans and Sandra Fehsenfeld have contributed equally to this work. Communicated by H.-O. Pörtner. S. Hans · S. Fehsenfeld (*) · J. R. Treberg · D. Weihrauch Department of Biological Sciences, University of Manitoba, 190 Dysart Road, Winnipeg, MB R3T2N2, Canada e-mail: Sandra.Fehsenfeld@umanitoba.ca S. Hans e-mail: umhans2@cc.umanitoba.ca J. R. Treberg e-mail: Jason.Treberg@umanitoba.ca D. Weihrauch e-mail: Dirk.Weihrauch@ad.umanitoba.ca J. R. Treberg Department of Human Nutritional Sciences, University of Manitoba, 190 Dysart Road, Winnipeg, MB R3T2N2, Canada