“Breath holding” as a thermoregulation strategy in the scalloped hammerhead, a deep-diving tropical ectothermic shark

Fish moving between different thermal environments experience heat exchange via conduction through the body wall and convection from blood flow across the gills. Here we report a strategy of preventing convective heat loss at the gills during excursions into deep cold water by the tropical scalloped hammerhead shark (Sphryna lewini). Adult scalloped hammerhead sharks dive rapidly and repeatedly from warm (~26ºC) surface waters to depths exceeding 800 meters and temperatures as low as 5°C. Biologgers attached to adult sharks show that warm muscle temperatures were maintained throughout the deepest portion of each dive. Substantive cooling only occurred during the latter stages of the ascent phase, and once initiated, was rapid. Heat transfer coefficient modeling indicated that convective heat transfer was suspended, probably by suppressing gill function during deep dives. This previously unobserved strategy has broad similarities to marine mammal “breath hold” diving., Note: (Refer to References section of publication for numbered citations)
Measuring swimming performance, environment, and body temperature
To measure scalloped hammerhead shark swimming performance, depth, ambient water and muscle temperatures we used an instrument package consisting of a tri-axial accelerometer tag and a depth and temperature archival tag housed in a syntactic foam float (2,000 m depth rating) equipped with a timed release mechanism and radio beacon transmitters to facilitate recovery.  The tri-axial accelerometer tag was either a TDR10-XB-340 (56 x 38 x 24mm 69g; Wildlife Computers., Redmond, WA) or a TDR10-Daily Diary-278 (74 x 57 x 36mm, 117g; Wildlife Computers., Redmond, WA). Tri-axial acceleration was sampled at either 16Hz or 32Hz.  Depth, water temperature and body temperature were sampled every 5 or 10 seconds using a MK9 archival tag with an 8cm thermistor stalk (Wildlife Computers, Redmond, WA). Each package also contained a SPOT5 or SPOT6 satellite-linked ..., Thermal coefficient modeling of all scalloped hammerhead shark deep-diving was run using Matlab code \"Script5.m\" with Matlab version 2021b. (included in this dataset). 
Thermal coefficient modeling of the dead scalloped hammerhead sharks was run using Matlab code \"Script3.m\" (included in this dataset) with Matlab version R2020a. 
Telemtry was analyzed using IgroPro 8 (Wavemetrics, Portland, OR).