About
#mesothermic #fish-like
#species and their
#energy needs to keep their bodies well
#heated.
#Masshomeothermy describes a phenomenon in which animals store
#bodyheat due to their size. The larger an animal is, the more its ratio between skin surface area and body volume shifts towards body volume.
But further specific
#adaptations are necessary to maintain body heat permanently. In rare cases of so called "fish"
#physiological mesothermy evolved.
A veritable giant among
#actinopterygeous fish is the
#oceansunfish #Mola #mola (Tetraodontiformes). Yet it is incapable of mesothermy. It relies on lying near the water's surface for extended periods to utilize solar heat, which provides its body with thermal energy. This fish species' unique adaptation to its environment consists of a combination of size, body shape (well-suited for absorbing solar heat), and an economical, and therefore
#energysaving, mode of activity.
Other actinopterygeous fish, on the other hand, retain the heat energy generated by
#muscleactivity within their bodies through special adaptations of their
#bloodvessels. This includes
#tuna or
#opahs of the genus
#Lampris (Lampriformes), which are also known as ocean sunfish, but may not be mistaken with Mola mola.
That mesothermy in fish-like organisms is the result of independent evolution (
#convergence) can be deduced from the fact that further examples of mesothermic species belong to the cartilaginous fish (
#Chondrichthyes). One needs to be aware in this context that animals referred to as "fish" are a
#paraphyletic group and not a closed reproductive lineage. Mesothermic
#sharks include the
#makosharks Isurus oxyrinchus and Isurus paucus, the
#porbeagle shark Lamna nasus, the
#salmonshark Lamna ditropis, the
#greatwhite shark Carcharodon carcharias, and the
#baskingshark #Cetorhinus #maximus. It can be reconstructed that in the stem species of
#Lamnidae, corresponding features of the circulatory system favoring mesothermy were already existing, putatively even as
#apomorphic character state. The tuna, the opah, the basking shark and the Lampriformes evolved the countercurrent heat exchange system convergently. Convergent evolution, means
#independentevolution, is based on comparable
#selectionpressures resulting from very similar
#environmentalconditions and lifestyles.
The so-called
#Rete #mirabile, responsible for mesothermy, is formed by arteries, and it could therefore arise convergently multiple times because the arrangement of blood vessels in Chondrichthyes and Actinopterygii exhibits fundamental similarities, which channeled the direction of possible adaptation pathways through evolution.
A
#Retemirabile consists of a network of arterial branches that subsequently converge to form a single artery once again. In mesothermic "fishes," warm venous blood does not flow to the gills—where it would cool down—but instead transfers its heat via the Rete mirabile to an adjacent artery, thereby warming the latter's formerly colder blood.
The authors N. L. Payne et al. (2026) have developed a modern methodology to determine the energy consumption of "fish" in relation to their size and body temperature. The researchers concluded that mesothermic species require four times as much energy to maintain their body heat as do
#ectothermic species. Consequently, these species are dependent on cool ambient temperatures; otherwise, they would
#overheat. Thus
#globalwarming may place large mesothermic species at risk of
#extinction.
©
#StefanFWirth, May 2026, Berlin
Reference:
N. L. Payne et al. (2026):
doi.org/10.1126/science.adt2…
#Illustrations:
© Stefan F. Wirth;
#AI-assisted
#artistic illustrations based on my hand-drawn storyboard sketches, manually edited:
1) Large fish: great white shark, tuna, and ocean sunfish (*Mola mola*); of these, only the first two mentioned are mesothermic.
2) tuna in a close-up
3) ocean sunfish Mola mola uses sunlight to warm up its body
4) large "fish" isolated