PhD Studentship: Extent and Drivers for Cryptic Benthivory in ‘Pelagic’ Ocean Sunfish

Natasha Phillips

Queen’s University Belfast

Supervisor(s): Jon Houghton and Chris Harrod

The ocean sunfish (Mola mola) is often described as an inactive, obligate predator of gelatinous zooplankton, but new research suggests these long held beliefs need significant revision. A recent study has revealed evidence of cryptic benthivory in juveniles and this project aims to examine the extent of and ecological drivers for benthivory in sunfish ontogenetically. Specifically, we hypothesize that ocean sunfish have a counter-intuitive life history tactic where small sunfish target high energy, benthic prey becoming more pelagic as they grow and shifting to an under-utilised niche as specialist foragers of gelatinous prey.

By combining stable isotope analysis (SIA) of diet with electronic bio-logging of behaviour, the energetic basis, timing and extent of this strategy can be quantified. In collaboration with Monterey Bay Aquarium, USA, techniques will be developed on captive sunfish before being deployed in the field. Tri-axial accelerometers fitted to captive sunfish will provide proxies of energy expenditure for discrete behaviours, and these data can then interpret the accelerometer readings of wild sunfish. The captive sunfish will also enable development of non-lethal sampling techniques for SIA to estimate isotopic fractionation and turnover through diet switch experiments.

Field work will be in Camogli, Italy where there is a local abundance of ocean sunfish and ongoing collaborations with the fishery and Marine Protected Area authorities. Using wild specimens, a description of ocean sunfish age and growth will help determine whether benthivory is restricted ontogenetically. The trophic ecology of wild sunfish will be analysed using gut content analysis and SIA (δ13C, δ15N) to characterise diet. These data will then be compared between populations using ocean sunfish samples collected by colleagues globally.

Overall this project aims to provide an understanding of the cryptic ecology of a large, pelagic consumer that may play a complex role in ecosystem function, alongside policy and conservation recommendations recognising the complexities associated with mixed benthic-pelagic foraging strategies.


Twitter: @sunfishresearch