Being a fussy eater is a problem for reef fish who live in rapidly changing environments or on deep reefs. But, scientists discovered, coral prey on deep reefs can support their fussy predator fish—by altering their own diets.

GSO professor Kelton McMahon co-authored the new study published in Functional Ecology along with lead author Dr. Chancey MacDonald and other colleagues from James Cook University (JCU). 

Coral bleaching, large storms and other destructive events are increasing on tropical reefs. This is rapidly degrading these shallow-water coral habitats. 

“Some animals are likely to be pushed close to the borders of their environmental range, where living successfully will be more difficult,” Dr MacDonald said. “On coral reefs this could mean deeper waters for fish. And it’s here you’d expect them to experience a reduced quantity and quality of their preferred foods,” he said.

“However, we found that fish who are fussy about the corals they feed on can continue to thrive in deeper reef waters if their prey engage in a less ‘precious’ approach to what they themselves eat.”

“Habitats such as deep reef systems will likely become important refuges for species persistence as climate change and other disturbances impact shallow reef habitat quality,” said Dr. McMahon. “This study couples classic ecological observational techniques with cutting-edge molecular isotope techniques to explore how the availability of food resources may regulate the quality of refuges in peripheral habitats of coral reef fishes.”

Fish that eat coral, such as the Triangle and Eight-Band butterflyfish, can live on deeper reefs by either feeding more on their usual resource—or by adapting their diets. 

Triangle butterflyfish are fussy eaters, or dietary specialists. The Eight-band butterflyfish are not fussy eaters—dietary generalists. The study compared the diets of the two from shallow to deep depths.

The team found that while overall feeding rates did not change with depth, the Triangle butterflyfish—a dietary specialist—fed more selectively on their preferred corals, which are sparser at greater depths than in shallow waters. In contrast, the dietary flexibility of the Eight-band butterflyfish increased with depth as the relative abundances of different coral types changed. 

Surprisingly, the energy content of the corals that the Triangle butterflyfish preferred to feed on did not decline with depth as expected. However, the pathways through which carbon passed from the corals to the fish did.

“Our results suggest that the quality declines expected for deeper prey corals are buffered by increased plankton intake via their polyps,” co-author Dr. Geoff Jones, also from JCU, said. 

“Climate change and other disturbances have increasing impacts on the habitats and compositions of coral reefs,” said Dr. MacDonald. “Our study shines a light on the importance of the versatile relationship between species as they seek refuge on the edges of their environmental range—even if one species is fussier than another.”