Changes in gill neuroepithelial cells and morphology of threespine stickleback (Gasterosteus aculeatus) to hypoxia and simulated ocean acidification.

Coastal marine environments are characterized by daily, seasonal and long-term changes in both O₂ and CO₂, driven by local biotic and abiotic factors. The neuroepithelial cells (NECs) of fish are thought to be the putative chemoreceptors for sensing oxygen and CO₂, and, thus, NECs play a key role in detecting these environmental changes. However, the role of NECs as chemosensors in marine fish remains largely understudied. In this study, the NECs of marine threespine sticklebacks (Gasterosteus aculeatus) were characterized using immunohistochemistry. We then determined if there were changes in NEC size and density, and in gill morphology in response to either mild (10 kPa) or moderate (6.8 kPa) hypoxia and two levels of elevated CO₂ (1,500 and 3,000 µatm). We found that the NECs of stickleback contained synaptic vesicles and were innervated, and were 50-300% larger and 2 to 4 times more abundant than in other similar sized freshwater fishes. NEC size and density were largely unaffected by exposure to hypoxia, but there was a 50% decrease in interlamellar cell mass (ILCM) in response to mild and moderate hypoxia. NECs increased in size, but not abundance in response to elevated CO₂. Moreover, fish exposed to moderate or elevated CO₂ had 53-78% larger ILCMs compared to control fish. Our results demonstrated that adult marine sticklebacks have NECs that can respond to environmentally relevant pCO₂ and likely hypoxia, which highlights the importance of NECs in marine fishes under the heterogeneity of environmental conditions in coastal areas.