12/27/2023 0 Comments Concerning body fluid compartments![]() Since extant ray-finned fishes, such as teleosts, have plasma osmolality much lower than SW, they must cope with osmotic loss of water and with ion entry across the gills, in contrast to the overhydration and hyponatremia that were experienced in FW during earlier evolutionary times. The situation for water regulation is the same in ray-finned fishes that moved into the seas in the Jurassic period of the Mesozoic era. The first tetrapods, amphibians, evolved from lobe-finned fishes (lungfishes) that ventured onto land in the late Devonian period, and in this desiccating habitat, they had to combat dehydration. Early bony fishes in FW may have suffered from excess water entry into the body. The extant hagfishes are an exception to this scheme, as they seem not to have experienced FW as judged by their high plasma ion concentrations, which are equivalent to those in SW (and are characteristic of marine chordates), and the small numbers of glomeruli, which would be insufficient to filter the large amount of water that would enter the body by osmosis from a FW environment ( Colbert et al., 2001). It is thought that primitive armored fishes once entered inland fresh waters (FWs) in the early Devonian period of the Paleozoic era and flourished there from that environment, they next moved onto land or re-entered the sea ( Colbert et al., 2001). Thus, the earliest vertebrates likely had an excellent ability to swim against water currents ( Romer, 1968). Vertebrates appear to have evolved from chordates in the near-shore seawater (SW) or brackish water, where rivers flow into the ocean ( Carroll, 1988). Then an attempt was made to discuss the evolution of the mechanisms from the two perspectives transitions from aquatic to terrestrial habitats and from hydrating (FW) to dehydrating (land and SW) habitats. In the initial part of this short review, interesting differences in the body fluid regulation between mammals and teleosts are introduced, particularly with regard to water acquisition (drinking and intestinal absorption). Like regulation of drinking, we found that the inhibitory mechanisms are dominant for intestinal water absorption in teleosts. Imbibed water becomes body fluid only after absorption by the intestine, and there is a distinct difference in the mechanisms for water absorption between mammals and teleosts. This contrast is also reflected in regulatory hormones dipsogenic hormones such as angiotensin II play pivotal roles in water homeostasis in mammals, whereas antidipsogenic hormones such as atrial natriuretic peptide are essential in teleosts. ![]() Thus, the apparent difference was found not between hydrating and dehydrating habitat, but rather between terrestrial and aquatic habitats. The major route for water acquisition is by oral drinking in terrestrial tetrapods (represented here by mammals) and in SW fishes (represented by teleosts as they are dehydrated in SW), but the regulation is contrasting between the two groups mechanisms inducing thirst have developed in mammals, whereas inhibitory mechanisms are dominant in marine teleosts as observed in FW teleosts. ![]() The evolution of the mechanisms for acquisition of water surely must have accompanied these dramatic environmental changes. As both land and sea are desiccating environments, animals must change their strategies for body fluid regulation from protecting against overhydration in FW to coping with dehydration in seawater (SW) or on land. It is generally accepted that ancient fishes first experienced freshwater (FW), and then variably by lineage moved onto the land or re-entered the seas during evolution.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |