Free-swimming swordfish, Xiphias gladius, alter the rate of whole body heat transfer: morphological and physiological specializations for thermoregulation

Abstract

Abstract Swordfish (Xiphias gladius) are large, highly-migratory pelagic, fishes that make diel, vertical excursions from the warm, surface layer (e.g. 18–24 °C) to the cold waters (∼8 °C) below the thermocline (300–600 m). They possess anatomical traits [e.g. medial red muscle (RM) position and an associated vascular rete] that could enable metabolic heat-retention and result in RM temperature elevation above ambient, or RM endothermy. We herein provide: (i) expanded anatomical descriptions of the RM-associated vasculature (i.e. central rete and lateral blood vessels), (ii) new measurements of in vivo temperature, and (iii) heat transfer models to assess the capacities for RM endothermy and physiological thermoregulation during vertical movements. Despite the presence of a medial RM and two associated blood-flow pathways (one of which forms a rete), swordfish exhibited a limited capacity for RM endothermy, with muscle temperatures approaching ambient during prolonged periods above or below the thermocline. Our heat transfer models suggest, however, that swordfish may control rates of heat loss or gain during vertical movements, possibly by altering the route of blood flow supplying the RM. Such physiological thermoregulation likely contributes to the ability of swordfish to capitalize on food resources below the thermocline, which are out of range for most other active, pelagic fishes.