Endothermy makes fishes faster but does not expand their thermal niche

Abstract

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High resolution acoustic telemetry reveals swim speeds and inferred field metabolic rates in juvenile white sharks (Carcharodon carcharias)

“…This in turn, along with behavioral adaptations to maximize swim efficiency in a three dimensional environment [ 6 , 46 ], is argued to underpin the success of hunting strategies employed by larger white sharks [ 4 ]. Our estimate of cruising speed (0.6 m s -1 , 0.3 U TL s -1 ) is markedly lower than that recorded by both Carey et al (1982 [0.89 m s -1 , 0.51 TL s -1 ], shark size 457 cm TL, 943 kg) [ 33 ], more than half that of Semmens et al (2013) [2.25 m s -1 , 0.62 TL s -1 , shark sizes 280–450 cm TL, 195–839 kg] [ 29 ], and ~ 2/3 the mean swim speed reported by Watanabe et al (2019) [0.94 m s -1 , shark sizes 290–420 cm TL, 218–721 kg] In fact, our cruising speed estimates are more in line with those reported from biologging data by Harding et al, (2021) [ 7 ]. Our mean estimated fRMR was approximately half that reported by Semmens et al (723 mg O 2 kg −1 h −1 ) [ 29 ].…”

“…Previous studies in adults and sub-adults of the species have suggested that white sharks have developed behavioral strategies that help offset the energetic costs of regional endothermy [ 6 ], including elevated average swim speeds (cruising speeds) [ 3 ] in comparison with species that do not exhibit regional endothermy, that provide a competitive advantage in resource exploitation. This hypothesis was supported by the results of a recent study that used a synthesis of bio-logging data from both ectotherm and endotherm fish, and concluded that the convergent evolution of endothermy in a range of fishes more likely driven by competitive advantages gained in ecological interactions and resource exploitation, rather than by thermal niche expansion [ 7 ]. Juvenile white sharks (JWS) exploit a very different habitat niche to their adult conspecifics [ 5 , 8 – 14 ], and rely on a mostly piscivorous based diet [ 15 – 19 ].…”

“…However, when standardized against body length (in TL s -1 ) cruising speeds in JWS included in the study were observed to follow a negative relationship with shark body size, whereby smaller white sharks, with smaller bodies, exhibited higher swim speeds than those of larger conspecifics. Harding et al, (2021) [ 7 ] found modal swimming speeds of sub-adult and adult white sharks (320–430 cm TL; n = 5) to range between 0.8–0.98 m s -1 . Standardized to body length, these equate to modal speeds of 0.21–0.25 m s -1 , with the smaller sharks exhibiting higher modal swimming speeds than larger sharks.…”

Behavioural thermoregulation linked to foraging in blue sharks

“…They are an ectothermic species without the parallel arrangement of arteries and veins that function as counter-current heat exchangers in muscles (Carey and Gibson 1987). In general, ectothermic fishes are thought to have narrower thermal niches than regionally endothermic fishes with well-developed heat exchangers (e.g., tunas, lamnid sharks) (Block et al 1993;Weng et al 2005), although a recent study challenged this view (Harding et al 2021). Blue sharks represent an interesting example as they dive to great depths [occasionally > 1000 m, with the maximum record of 1706 m (Queiroz et al 2017)] and experience extensive water temperature ranges (up to 20 °C) (Stevens et al 2010;Campana et al 2011;Queiroz et al 2012) despite their ectothermic physiology.…”

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“…During the prespawning period, capelin form relatively large and compact schools that are heavily preyed upon by humpback whales (Megaptera novaeangliae) as well as a number of other cetacean species including finback whales (Balaenoptera physalus), minke whales (Balaenoptera acutorostrata), whitebeaked dolphins (Lagenorhynchus albirostris), and short-beaked saddleback dolphins (Delphinus delphis) (Whitehead and Glass, 1985). The white shark's ability to maintain a warmer-thanambient body temperature likely enables expansion of its thermal niche and access to habitats largely limited to other top predators, accepting that regional endothermy is more associated with higher swim speeds rather than broader thermal niches (Harding et al, 2021). It is likely the white sharks are drawn to the abundant food supply of the SES during this period and at times may actively hunt or scavenge on cetaceans in the subzero waters just off the slope of the shelf, though we cannot rule out the possibility of white sharks foraging on the spawning capelin while on the shoal.…”