Physiological responses of the intertidal starfish Pisaster ochraceus, (Brandt, 1835) to emersion at different temperatures Iain J. McGaw a,b,1 , Alexander M. Clifford b,c , Greg G. Goss b,c a Department of Oceans Sciences, 0 Marine Lab Road, Memorial University, St John's, NL A1C 5S7, Canada b Bamfield Marine Sciences Centre, 100 Pachena Road, Bamfield, BC VOR 1B0, Canada c Department of Biological Sciences, University of Alberta, 11455 Saskatchewan Drive, Edmonton, AB T6G 2E9, Canada abstract article info Article history: Received 24 July 2014 Received in revised form 26 March 2015 Accepted 27 March 2015 Available online xxxx Keywords: Acid–base Emersion Respiration Starfish Temperature The physiological responses of the intertidal starfish Pisaster ochraceus were recorded during 6 h emersion in air at temperatures of 5 °C, 15 °C or 25 °C, followed by a subsequent return to seawater (12–14 °C). After 6 h in 5 °C air the body temperature of the starfish had equilibrated with the medium. In 15 °C and 25 °C air, evaporation across the body surface prevented equilibration, and the body temperature remained several degrees cooler than the air. In air, both oxygen consumption and CO 2 production increased with increasing temperature. The oxygen consumption in air did not change when the body temperature of the starfish was similar to that of seawater (11–13 °C), but appeared to be lower in the 25 °C air treatment. This suggests that the relationship between oxygen consumption and water and air temperature is complex and both the magnitude of temperature change, and the duration of aerial exposure may influence oxygen consumption. The low respiratory quotient suggested that CO 2 excretion was not complete, possibly due to collapse of the tube feet and aboral papulae in air. This leads to a temperature related increase in PCO 2 and a concomitant acidosis in 15 °C, and 25 °C air. In 5 °C air the PCO 2 of the coelomic fluid did not increase significantly and there was no change in pH, possibly due to a temperature related pH increase. There was a slight increase in calcium ion levels in 15 °C and 25 °C air and a drop in 5 °C air; in other species this represents a release of bicarbonate reserves from the exoskeleton, however, no compensation of the acidosis was observed for P. ochraceus. It is likely therefore that this was a passive dissolution of the exoskeleton caused by the acidosis, rather than an active compensatory mechanism. Ammonia and lactate levels were very low or below detection threshold in most specimens which is typical for echinoderms. The body temperature, gas levels and the pH of the coelomic fluid were rapidly regained when the animals were re-immersed in seawater of 12–14 °C. Oxygen consumption remained higher during the recovery period, possibly reflecting the up-regulation of processes associated with cellular repair. The results of this study suggest that the physiological mechanisms reported here make P. ochraceus ideally suited for life in the intertidal zone. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Organisms that inhabit the intertidal zone are usually exposed twice daily to the air, where they are faced with the problem of oxygen con- sumption, carbon dioxide removal and maintaining acid–base balance (deFur, 1988). Many of the organisms that inhabit the intertidal zone possess some mechanism that allows continued oxygen consumption, however there may be challenges associated with the removal of carbon dioxide (deFur, 1988; Newell, 1973). Coupled with changes in the respi- ratory medium, intertidal zone animals often have to deal with sharp changes in temperature. Because of its higher thermal inertia the ocean remains relatively stable over a tidal cycle, however animals can be subject to abrupt changes of up to 20 °C occurring both when exposed to air and upon re-immersion. These rapid changes in temper- ature are a defining factor affecting the physiology of intertidal organ- isms (Helmuth and Hofmann, 2001). Extensive work has been carried out on the physiological responses of intertidal molluscs and crustaceans to emersion (e.g. Burnett, 1988; McGaw et al., 2009; McMahon, 1988; Newell, 1973; Tagliarolol et al., 2012; Widdows et al., 1979), but there is comparatively less information for other invertebrate phyla. For example, despite being represented by several hundred different species, much fewer studies have investigated the physiological responses of intertidal echinoderms to emersion (Burnett et al., 2002; Fly et al., 2012; Murphy and Jones, 1987; Pincebourde et al., 2008, 2009, 2013; Spicer et al., 1988). In general the metabolic rate of asteroids, echinoids, and holothurians tends to be lower compared with other invertebrates because they lack the muscle mass and have a large coelomic fluid volume (Cole and Burggren, 1981; Fly et al., 2012). For echinoderms in water, respiratory Journal of Experimental Marine Biology and Ecology 468 (2015) 83–90 E-mail address: ijmcgaw@mun.ca (I.J. McGaw). 1 Tel.: +1 709 864-3272, fax: +1 709 8643220. http://dx.doi.org/10.1016/j.jembe.2015.03.019 0022-0981/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe