213 I ntroductIon Intertidal beachrock formations are produced by near-shore lithiication and erosion processes that result in shallow, structurally complex formations. Where they occur at beach margins, beachrock zones are ideally suited as nursery grounds for juvenile ishes, many of which later recruit to nearby reefs. Although common to the Bahamas and Bermuda (Purdy 1963), beachrock nursery habitats are rare in the United States, with the Loggerhead Key formation (24°38.08′N 82°55.24′W), in Dry Tortugas National Park (DTNP) being the best studied (Ginsburg 1953; Shinn 2009). The Loggerhead Key formation is a thriving nursery area that helps support DTNP ish biodiversity and reef health. Longley and Hildebrand (1941), for example, reported that as many as 10% of Tortugas ish species utilize the formation as a nursery, and Lindeman et al., (2000) designated the area as essential nursery habitat. Several damselish species are known to utilize the Loggerhead Key nursery (Rummer et al. 2009) including, cocoa damselish (Stegastes variabilis), night sergeants (Abudefduf taurus), sergeant majors (Abudefduf saxitilis), and beaugregory (Stegastes leucostictus). Damselishes are considered to be a keystone group in coral reef habitats (Hixon and Brostoff 1983), and one that plays an important role in shaping the structure, fauna and ecology of patch reefs in DTNP (Wallman et al. 2004; Wilkes et al. 2008). At low tide, Loggerhead Key beachrock areas are reduced to a series of shallow rockpools that experience rapid shifts in temperature. Rummer et al., (2009) reported that temperatures in these pools during June 2001 ranged from 25.9 to 33.2 °C, and Fangue et al., (2001) measured temperatures of nearly 36°C during a June, 2000 rockpool study. Even higher temperatures are likely during the later summer months, for example, Pearse (1934) measured August water temperatures of 42.9 °C in brackish pools on nearby Long and Garden Keys in DTNP. Little is known about how juvenile damselishes cope Caribbean Journal of Science, Vol. 47, No. 2-3, 213-219, 2013 Copyright 2013 College of Arts and Sciences University of Puerto Rico, Mayagüez Thermal tolerance and metabolic responses of two damselish species from a hyperthermic rockpool nursery in Dry Tortugas National Park, USA Theresa F. Dabruzzi 1,3 , Ana G. Jimenez 2 , Justin E. Speaks 1 , Sarah R. Stoler 1 , Melanie A. Sut- ton 1 , Christopher M. Pomory 1 and Wayne A. Bennett 1 1 University of California, Department of Wildlife, Fish, and Conservation Biology, One Shields Avenue, Davis, CA 95616, USA. 2 University of North Carolina at Wilmington, Biology and Marine Biology Department, 601 S. College Rd., Wilmington, NC 28403, USA. 3 Corresponding author-tfdabruzzi@ucdais.edu AbstrAct- Beachrock formations on Loggerhead Key in Dry Tortugas National Park, USA are an important nursery for many juvenile reef ishes. Wide variations in temperature are common in these areas and can markedly inluence ish metabolism and thermal tolerance. We determined routine resting metabolic rate and temperature quotient (Q 10 ) at 24 and 32°C. Thermal tolerance (measured as critical thermal maximum, CTmax) was also measured for sergeant major (Abudefduf saxitilis) and cocoa damselish (Stegastes variabilis) acclimatized to beachrock nursery temperatures. Sergeant majors occupied shallow rockpool margins that experience rapid, extreme temperature changes. These ish exhibited relatively low metabolic rates, reduced Q 10 values (2.40), and relatively high levels of temperature tolerance (CTmax = 40.0°C). Cocoa damselish selected deeper, more thermally stable rockpool areas. These ish had higher metabolic rates, were more sensitive to temperature increase (Q 10 = 2.84), and were less tolerant of high temperatures (CTmax = 38.6°C) than sergeant majors. Metabolic and temperature tolerance adaptations of juvenile sergeant majors and cocoa damselish allow these ishes to exploit a variety of microhabitats found in beachrock areas on Loggerhead Key. Keywords. — Cocoa damselish, Loggerhead Key, Q 10 , sergeant major, tidepool