This common carp had a birth defect that resulted in the absence of a gill flap. Image by Guitardude012 – Own work, CC BY 3.0, via Wikimedia Commons
Here’s something to think about. How do fish optimize gas exchange in their gills to efficiently take up oxygen (favoring large, thin, permeable membranes) while at the same time limiting water and ion movement across the surface? If they reduce surface area, then oxygen transfer will be limited, but ions and water transfer will be optimized. What is a fish to do?
A new review article published in Physiology discussed how some fish have shown evidence of gill remodeling to deal with this conundrum. When the fish have higher access to oxygen, but oxygen demand by the body is low, they grow cell masses over the course of several days in the gills to lower surface area for gas exchange, and hence the area available for water and ions to move as well. With low oxygen availability (i.e. hypoxia) and an increase in cellular demand for oxygen, these cell masses show signs of reduction within as little as ~30 mins, depending on the stimulus, to help quickly promote gas exchange:
Figure 2A showing scanning micrograph images of gills adapted from Gilmour et al., 2018.
For more information about other triggers of gill remodeling, see: Gilmour KM, Perry SF. Conflict and compromise: Using reversible remodeling to manage competing physiological demands at the fish gill. Physiology. 33(6): 412-422, 2018. https://doi.org/10.1152/physiol.00031.2018