Skeletal stiffening in an amphibious fish out of water is a response to increased body weight

Abstract

Terrestrial animals must support their bodies against gravity, while aquatic animals are effectively weightless because of buoyant support from water. Given this evolutionary history of minimal gravitational loading of fishes in water, it has been hypothesized that weight-responsive musculoskeletal systems evolved during the tetrapod invasion of land and are thus absent in fishes. Amphibious fishes, however, experience increased effective weight when out of water - are these fishes responsive to gravitational loading? Contrary to the tetrapod-origin hypothesis, we found that terrestrial acclimation reversibly increased gill arch stiffness (∼60% increase) in the amphibious fish when loaded normally by gravity, but not under simulated microgravity. Quantitative proteomics analysis revealed that this change in mechanical properties occurred via increased abundance of proteins responsible for bone mineralization in other fishes as well as in tetrapods. Type X collagen, associated with endochondral bone growth, increased in abundance almost ninefold after terrestrial acclimation. Collagen isoforms known to promote extracellular matrix cross-linking and cause tissue stiffening, such as types IX and XII collagen, also increased in abundance. Finally, more densely packed collagen fibrils in both gill arches and filaments were observed microscopically in terrestrially acclimated fish. Our results demonstrate that the mechanical properties of the fish musculoskeletal system can be fine-tuned in response to changes in effective body weight using biochemical pathways similar to those in mammals, suggesting that weight sensing is an ancestral vertebrate trait rather than a tetrapod innovation.

Gill remodelling during terrestrial acclimation in the amphibious fish Polypterus senegalus

“…The gill reduction strategy used by P. senegalus in this study contrasts strongly with our earlier work in K. marmoratus, which increase collagen deposition, mineralization, and stiffness of the gill arches after terrestrial acclimation (Turko et al, 2017). Thus, there does not appear to be a generalizable skeletal response to life out of water that is shared by independently evolved groups of amphibious fishes.…”

“…Lower metabolic rates in fishes versus mammals would presumably slow the process further. In K. marmoratus , for example, stiffening of gill arches occurred within 7 d of terrestrial acclimation but was not reversible after 14 d back in water; stiffness eventually returned to control values after 16 weeks (Turko et al, ). Degradation of bone tissue may explain the lower mineralized volume we found in the first and fourth arches of terrestrially acclimated P. senegalus , but degradation does not typically cause a change in overall bone size (Burr & Allen, ).…”

“…Mechanical properties of gill filaments and arches were measured using a custom tensile testing apparatus described previously (Fudge, Gardner, Forsyth, Riekel, & Gosline, 2003;Turko et al, 2017). Briefly, gill tissue was mounted between a moveable micrometer and the tip of a glass microbeam with cyanoacrylate gel.…”

Evolutionary and cardio‐respiratory physiology of air‐breathing and amphibious fishes

“…Gill remodelling in two amphibious species, K. marmoratus and bichir, Polypterus senegalus , involved an increase in the height of the interlamellar mass, which was associated with a reduced gill surface area in air‐acclimated fish, possibly to fortify delicate lamellar structures or to reduce water loss. As well, air exposure stiffened the gill skeleton of K. marmoratus with increased expression of proteins responsible for bone mineralization and more densely packed collagen fibrils in gill arches and filaments . Although gill remodelling in the air is reversible, it takes 1 week for the interlamellar mass to regress and up to 12 weeks for the gill stiffness to return to control aquatic levels .…”

“…As well, air exposure stiffened the gill skeleton of K. marmoratus with increased expression of proteins responsible for bone mineralization and more densely packed collagen fibrils in gill arches and filaments. 33 Although gill remodelling in the air is reversible, it takes 1 week for the interlamellar mass to regress 36 and up to 12 weeks for the gill stiffness to return to control aquatic levels. 38 Therefore, upon returning to water, there are lag time effects that immediately impact aquatic respiration 40 imply that there are trade-offs to gill remodelling.…”