Ocean sunfish rewarm at the surface after deep excursions to forage for siphonophores

Abstract

Summary1. Ocean sunfish (Mola mola) were believed to be inactive jellyfish feeders because they are often observed lying motionless at the sea surface. Recent tracking studies revealed that they are actually deep divers, but there has been no evidence of foraging in deep water. Furthermore, the surfacing behaviour of ocean sunfish was thought to be related to behavioural thermoregulation, but there was no record of sunfish body temperature. 2. Evidence of ocean sunfish feeding in deep water was obtained using a combination of an animal-borne accelerometer and camera with a light source. Siphonophores were the most abundant prey items captured by ocean sunfish and were typically located at a depth of 50-200 m where the water temperature was <12°C. Ocean sunfish were diurnally active, made frequently deep excursions and foraged mainly at 100-200 m depths during the day. 3. Ocean sunfish body temperatures were measured under natural conditions. The body temperatures decreased during deep excursions and recovered during subsequent surfacing periods. Heat-budget models indicated that the whole-body heat-transfer coefficient between sunfish and the surrounding water during warming was 3-7 times greater than that during cooling. These results suggest that the main function of surfacing is the recovery of body temperature, and the fish might be able to increase heat gain from the warm surface water by physiological regulation. 4. The thermal environment of ocean sunfish foraging depths was lower than their thermal preference (c. 16-17°C). The behavioural and physiological thermoregulation enables the fish to increase foraging time in deep, cold water. 5. Feeding rate during deep excursions was not related to duration or depth of the deep excursions. Cycles of deep foraging and surface warming were explained by a foraging strategy, to maximize foraging time with maintaining body temperature by vertical temperature environment.

Heat transfer in fish: are short excursions between habitats a thermoregulatory behaviour to exploit resources in an unfavourable thermal environment?

“…The k-coefficient was traditionally presented as an empirically derived lumped parameter that includes conditions both within and outside the animal (Stevens and Fry, 1974;Fechhelm and Neill, 1982;Brill et al, 1994) and is now broadly applied in fish ecology (e.g. Azumaya and Ishida, 2005;Nakamura et al, 2015). Although this model generally fits accurately with recorded body temperature data, the precise factors explaining our results (e.g.…”

“…Holland et al, 1992;Brill et al, 1994;Azumaya and Ishida, 2005;Nakamura et al, 2015) for modelling the body cavity temperature (T b ) relative to the ambient water temperature (T a ):…”

“…The k-coefficient, sometimes defined as the whole-body heat-transfer coefficient (e.g. Holland et al, 1992;Azumaya and Ishida, 2005;Nakamura et al, 2015), is widely used in fish ecology to understand how body temperature responds to water temperature change. Fish mass is an important variable explaining the range of the k-coefficient (Stevens and Fry, 1974;Fechhelm and Neill, 1982).…”

“…Tanaka et al, 2000;Azumaya and Ishida, 2005;Sims et al, 2006). Recently, Nakamura et al (2015) found that ocean sunfish, Mola mola, exhibit vertical migration of hundreds of metres to rewarm at the surface and forage for siphonophores in deeper water, below their thermal preference.…”

Convergent Foraging Tactics of Marine Predators with Different Feeding Strategies across Heterogeneous Ocean Environments

“…Linking movement patterns to habitat use remains, however, a challenging task. Detailed records of prey abundance and distribution and accurate indices of feeding are difficult to obtain for the majority of species and although visual assessment of prey capture is possible for some species (Seminoff et al, 2006;Elliott et al, 2008), in most cases, indirect parameters have been used as a proxy (e.g., gastric or visceral temperature changes, mouth/beak opening or head/jaw movement, accelerometer signatures; Sepulveda et al, 2004;Gleiss et al, 2011aGleiss et al, , 2013Nakamura et al, 2011Nakamura et al, , 2015Carroll et al, 2014;Nakamura and Sato, 2014). For efficient foraging by predators, patterns of habitat use are assumed to reflect the distribution, density and quality of prey resources (Stephens and Krebs, 1986;Austin et al, 2006;Carroll et al, 2017).…”