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

1. 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 excu...