Dowd et al : Metabolic rates of luvenile Caichorhinus plumbeus 



329 



Because sandbar sharks are continuously active, we 

 chose to measure SMR in immobilized and artificially 

 ventilated animals in flow-through respirometers. As a 

 check of this technique, a two-point power-performance 

 curve was constructed by using the logarithms of self- 

 paired SMR and RMR and the mean swimming speed 

 of the animal in the annular respirometer. The average 

 slope for 15 sharks (0.38 ±0.04) at 24-26°C was similar 

 to the slopes of power-performance curves for four other 

 ectothermic shark species (0.2, Scharold et al., 1989; 

 0.34, Scharold and Gruber, 1991; 0.38, Carlson et al., 

 1999; 0.32, Lowe, 2001), and we interpret these data as 

 additional evidence that the technique provides accept- 

 able results. This approach, moreover, avoids the expense 

 and difficulties of developing a swimming tunnel large 

 enough to accommodate juvenile sandbar sharks and may 

 well be generally applicable for generating power perfor- 

 mance curves in other continuously active fish species. 



Elasmobranch metabolic rates and the cost of growth 



The published SMRs of active elasmobranchs are well 

 below those of high-energy demand teleosts (e.g., the 

 endothermic tunas; Korsmeyer et al., 1996), with the 

 exception of the endothermic mako shark Usurus oxy- 

 rinchus) (Graham et al., 1990) (Fig. 5). Brill (1987, 

 1996) proposed that the high SMRs of tunas are an 

 unavoidable consequence of their morphological, bio- 

 chemical, and physiological adaptations for extremely 

 high maximum aerobic metabolic rates, specifically 

 that their large gill surface areas have led to high 

 osmoregulatory costs (but see Brill et al., 2001). Most 

 elasmobranchs, including the sandbar shark, have less 

 than one third the gill surface area of a similar-size 

 tuna (Muir and Hughes, 1969; Emery and Szcepan- 

 ski, 1986). These modest mass-specific gill surface 

 areas and corresponding low rates of oxygen delivery 

 in ectothermic sharks likely dictate slow asymptotic 

 growth rates (Pauly, 1981), in contrast to those of 

 tunas, whose cardiovascular systems are adapted for 

 meeting multiple metabolic demands (including growth) 

 (Bushnell and Brill, 1991; Korsmeyer et al., 1996; Brill 

 and Bushnell, 2001). 



Significant levels of specific dynamic action (SDA, the 

 elevation in metabolic rate in conjunction with protein 

 synthesis after a meal [Brown and Cameron, 1991]) can 

 probably not be met by the cardio-respiratory systems 

 of elasmobranchs, particularly in continuously active 

 species such as the sandbar shark, while they sustain 

 routine activity levels. The oxygen consumption rate fol- 

 lowing a meal can exceed 2-3 times the SMR (DuPreez 

 et al., 1988; Sims and Davies, 1994; Ferry-Graham and 

 Gibb, 2001), whereas the RMR of sandbar sharks was 

 1.6-1.8 times SMR. Assuming that the maximum meta- 

 bolic rate is 1.8 to 2.75 times the SMR (Scharold et al., 

 1989; Lowe, 2001), we determined that sandbar sharks 

 are using between 34% and 100% of their metabolic 

 scope just to sustain routine activity levels. 



Given these limitations, sandbar sharks and other 

 active elasmobranchs probably make tradeoffs among 



metabolic demands at the expense of SDA or growth to 

 remain within their available metabolic scope, or they 

 may adjust their behavior to seek cooler waters during 

 digestion (Matern et al., 2000). Because rapid incorpo- 

 ration of ingested amino acids into body proteins is not 

 possible, many elasmobranchs may have to reduce the 

 rate of digestion or integrate SDA over a longer period 

 (or do both). For example, estimated daily rations for 

 several elasmobranch species average 1-2% of body 

 weight per day (e.g.. Bush and Holland, 2002; Dowd 

 et al., 2006), compared to 4% or more in fast growing 

 teleosts (e.g., Olson and Boggs, 1986). Not surprisingly, 

 sandbar sharks in the Northwest Atlantic mature only 

 after 13-15 years and grow less than 10 cm per year 

 during that time (Sminkey and Musick, 1995). Growth 

 rates for many other large, active elasmobranch spe- 

 cies are also slow (Branstetter, 1990). Future research 

 might well focus on exploring the relationship between 

 SDA, active metabolic rates, and metabolic scopes of 

 slow growing, continuously active sharks. 



Acknowledgments 



We thank Mark Luckenbach, Reade Bonniwell, and the 

 entire staff of the VIMS Eastern Shore Laboratory for 

 their hospitality, advice, and assistance in capturing 

 and maintaining sandbar sharks. This work was sup- 

 ported through the National Shark Research Consor- 

 tmm (NOAA/NMFS grant no. NA17FL2813 to JAM); an 

 Indiana University South Bend Faculty Research Award 

 to PGB; and VIMS GSA Mini-Grant, William & Mary 

 Minor Research Grant, and Oceanside Conservation Co., 

 Inc., awards to WWD. We thank Medtronics for donating 

 the membrane oxygenator used in the RMR experiments. 

 All procedures were approved by the College of William 

 and Mary Research on Animal Subjects Committee. 



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