Life if tough for larval fishes; the moment they enter a pelagic ecosystem they are faced with the struggle between development and the hydrodynamic field. As fish transition from feeding off their yolk sac to autonomous feeding, there is a dramatic increase in their mortality rate known as the critical period (Hjort, 1914). Larval fish exist live at low Reynolds numbers, a term used to characterize the viscosity of a fluid where at low Reynolds numbers the ocean feels like a vat of molasses, incredibly sticky. In this world where viscous forces dominate, the ability to successfully feed is critical but successfully feeding at low Reynolds numbers is hard and many larvae end up starving (China and Holzman, 2014).
In this paper the authors search and identify a genetic mechanism that may control the starvation response in larval fishes. Scanning three potential candidate genes, agrp1, npy, and hsp70, researchers highlight genetic pathways and expression levels relating to starvation response. Through in-situ experiments researchers identified agpr1 as the best and most consistent gene associated with starvation in S. aurata and likely other fishes. In fact, they found that expression of this gene increases significantly in response to acute starvation, conversely researches observed decreased expression in well fed larvae. But what drives the expression of this key gene? Is it food availability, age, or size?
Other studies have found consistently high mortality rates even in the presence of high food availability (Yavno and Holzman, 2017). So, to further test the mechanisms of starvation, researchers used dynamic scaling experiments to address the question, how much does size really matter? For these experiments, water is manipulated to feel “stickier” to represent how larvae experience their hydrodynamic environments. In theory, larval fishes should throw all their resources at growing long, not complex, to escape this sticky flow. By manipulating relative viscosity, the authors demonstrated the key importance of viscosity on the signaling of agrp1. When in sticky situations, despite age, agpr1 signaling is enhanced, representing an increase in starvation. Supporting the hypothesis that the starvation response is directly affected by viscosity and fish should grow longer and faster during this critical period. In terms of starvation, development, and hydrodynamics, size truly matters.
By Karly Cohen, PhD student at the University of Washington.
Blog post drawn from an article published by Integrative and Organismal Biology: A journal of the Society of Integrative and Comparative Biology: The expression of agrp1, a hypothalamic appetite stimulating neuropeptide, reveals hydrodynamic-induced starvation in a larval fish
China, V. and Holzman, R. (2014). Hydrodynamic starvation in first-feeding larval fishes. Proc. Natl. Acad. Sci. U. S. A. 111, 8083–8.
Hjort, J. (1914). Fluctuations in the great fisheries of northern Europe. Rapp. Pa-V. Reun. Cons. Perm. Int. Explor. Mer 19, 1–228.
Yavno, S. and Holzman, R. (2017). Do viscous forces affect survival of marine fish larvae? Revisiting the ‘safe harbour’ hypothesis. Rev. Fish Biol. Fish.