Coral reefs are on the front line of climate change, and they are losing ground. Unfortunately, reef inhabitants have to combat more than coral bleaching and elevated temperatures alone: their pristine homes are also getting dirtier. And while we know that poor water quality causes species decline and ecological changes in reefs, the mechanisms at play remain mysterious. Perhaps sediments directly influence the physiology of reef fishes by irritating or damaging their gills and impairing oxygen uptake into the blood. Additionally, indirect effects like the loss of visual range can prevent larvae from finding shelter or prey. Wading into the murk and gloom, Sybille Hess and her colleagues, based at James Cook University and the University of Queensland, Australia, sought a mechanistic link between water quality and physiology. They focused on three species, cinnamon clownfish (Amphiprion melanopus), false orange clownfish (Amphiprion percula) and spiny chromis (Acanthochromis polyacanthus), with different distributions. False orange clownfish strongly prefer clear water, and are rarely found in turbid reefs, while the cinnamon clownfish and spiny chromis members of the family are common in both turbid inshore and clear offshore reefs.
Using a microscope to take a close look at thin sections of the tiny gills, the team investigated whether suspended sediments lead to gill damage and reduced gas exchange. The structures on the gills that absorb oxygen in the blood and remove waste products (lamellae) of cinnamon clownfish and spiny chromis were shorter after sediment exposure, implying less functional gas exchange surface. In contrast, the team found that the distance that gases had to travel across the lamellae was reduced in all three species. This bucks the usual trend – sediment-exposed gills usually take defensive action, like thickening the protective cell layers or increasing mucus secretion, which shield their fragile gill structures at the cost of increasing the distance that gases must diffuse through the gill. However, whether this reduced diffusion distance was a response to compensate for gill damage, or simply due to abrasion of the gill tissue, was unresolved.
The team then moved on to investigate whether the alterations that they had identified in the gill influenced the fish's metabolism, as damaged gills can limit metabolic performance. They found that the cinnamon clownfish was unable to increase its metabolic rate as much as unexposed fish when performing exercise and had a higher baseline metabolic rate while at rest. This leaves them with less energy available for ‘extracurricular activities’, such as reproduction and locomotion, which are not critical to moment-to-moment survival – like having no emergency fund saved up at the bank.
Contrastingly, spiny chromis and false orange clownfish protected their ability to increase their metabolic rate, even at very high sediment concentrations. The authors chalked this up to compensatory responses somewhere in the myriad of physiological steps involved in translating oxygen uptake into metabolic performance, like heart or haemoglobin function. That the false orange clownfish, which is only found in clear water, outperformed the cinnamon clownfish, which tolerates turbid reefs, took the authors by surprise. They suggest that the poor tolerance of the false orange clownfish's sea anemone partner to suspended sediments may limit the distribution of its clownfish lodger.
The frantic pace of human development in scenic coastal locales stirs up the water and challenges the inhabitants of the reef. Given the complex interactions between the direct effects of sediment exposure on fish physiology, the potential responses of other reef inhabitants – such as host anemones – and other stressors such as ocean acidification, the future of coral reefs is as clear as mud.