To learn more or opt-out, read our Cookie Policy. Twitter can be a wasteland of heart-wrenching breaking news, trolls, and poorly placed promoted content — but then there are the metal cats and the random factoids, like this viral Twitter thread about Greenland sharks that threw the Verge newsroom into a frenzy today. So I called some scientists to fact-check this thread. And I even found one that had eaten that fermented dish. Urea is just a by-product of protein breakdown that we humans filter out in pee.
Sharks — all ocean sharks, and rays, and skates — retain urea so that their bodies are at the same salt concentration as the salt water outside. This is called osmotic balance, and without it, sharks would lose or gain water to the ocean and die.
But urea is toxic — it breaks down proteins in tissue. So to balance the urea and protect its tissue, sharks also have high concentrations of a compound called trimethylamine oxide, or TMAO. Bushnell ate some Greenland shark meat on a research expedition, after soaking it in milk and frying it up with a lot of soy sauce. Greenland sharks are also the longest-living vertebrates known on Earth. In fact, these sharks can be years old. Think about that: there are sharks on this planet that have been swimming in the Arctic sea since well before the French Revolution.
Why do they live so long? So it could just take longer for those DNA glitches that cause cancer to occur. Although they grow slowly, they can become massive: up to 24 feet long and over 2, pounds heavy. Take action. Animals Encyclopedia. Endangered Animals. Plants Encyclopedia. Endangered Plants. Breaking News. Contact Us. They fuse to form a collecting tube. This unit forms the countercurrent system of the nephron of elasmobranch.
Two different urea transporters were identified in the ventral and in the dorsal zone of the little skate, Raja. Figure 3. Schematic drawing of an elasmobranch nephron as described by Lacey and Reale [37]. The authors showed that in the dorsal zone urea transport was not saturable, was sensitive to the non competitive inhibitors phloretin and HgCl 2 and was significantly reduced in the presence of an urea analogue.
By these results they suggested the presence of one or more facilitated urea transporters possibly coded by skUT skate urea transporter which expression had been already evidenced in both the dorsal and ventral section [41]. It was suggested that the presence of homologues in two species that have diverged in the elasmobranch evolution [43] speaks in favour of a conservation of this isoform.
However in the Atlantic stingray another phloretin transporter, strUT-2, was also identified [44]. It has an high degree of sequence identity with shUT but has an unique extension of the hydrophilic COOH-terminal region of about 51 aminoacids.
In addition strUT-2 has a protein kinase regulatory site lacking in the shark urea transporter. The authors proposed that protein kinase may alter the function of the stingray transporter thus regulating urea reabsorption during the movement of the animal between habitats of different salinities. Indeed Atlantic stingray is an elasmobranch that, unlike the spiny dogfish, can move to habitats covering a large range of salinities.
Most elasmobranches live in seawater and are stenohaline while the stingrays of the genus Potamotrygon are the only elasmobranch permanently adapted to freshwater [45,46]. However a number of species are marginally euryhaline since they can enter in the brackish environment of river mouths [47] and a small number can move from seawater to freshwater, generally on a seasonal basis [13,46,48,49].
Since the kidney is the primary organ responsible of urea retention and euryhaline elasmobranchs remain ureotelic in environment with a low salinity [30,34,46,] the renal regulation of urea retention in habitat with different salinities is crucial for the osmotic homeostasis of these species.
Indeed an increase in urea excretion relative to urea absorption in a diluted environment was described in the euryhaline skates [30,31]. Morgan et al. Since kidney osmolality and urea concentration decreased while water content increased, the authors suggested that the downregulation of skUT plays an important role in lowering tissue urea levels in response to the osmolality of the external medium.
A recent study of Yamaguchi et al. It was suggested that the transfer of this fish to a low salinity did not change the expression of UTs, as observed in R.
It was found that in the control T. On the return to full seawater the apical distribution recovered its initial level Figure 4. These observations led the authors to conclude that UT distribution is regulated by its production and degrada-.
Figure 4. Redistribution of UT transporters filled circles in a cell lining the collecting duct of T. In fish transferred to a diluted medium they significantly decrease but the most pronounced change is in the apical membrane.
On the return to full seawater the distribution recovers its initial level see text for the references. It was suggested that vasotocin can act as possible regulator in T. This hypothesis was based on two observations:. However it is also possible that plasma and tissue levels of urea change as a consequence of a change of hepatic urea production.
A decrease of urea production during the acclimation to fresh water was suggested for the little skate R. Tam et al. The ability to modify urea production is different in the various elasmobranch species and it is probably related to their ability to survive in habitats with different salinities. The stenohaline fresh-water rays Potamotrygon spp, that permanently dwell in fresh water, being confined in the Amazonian river basin away from ocean for million years [60,61] are ammoniotelic and are not able to produce urea neither to counteract an experimental salinity stress [45,62,63] nor as strategy to detoxify ammonia.
In was observed that Potamotrygon motoro is not able to upregulated urea synthesis via the ornithine-urea cycle when exposed to environmental ammonia [64]. It is known that the high urea concentration found in marine elasmobranch fish, similar only to that found in mammalian kidney [65], has a destabilizing effect on many macromolecules.
In order to explain the mechanisms responsible of the chemical denaturation of urea two mechanisms have been proposed: the indirect and the direct mechanism. According to the indirect mechanism urea denatures protein by destroying water structure that in turn weakens the hydrophobic interaction responsible of the globular structure of the protein [66,67]. According to the direct mechanism urea unfolds protein through a direct interaction mediated by either electrostatic interaction or van der Waals attraction [].
The loss of structure influences enzyme kinetic properties [71,72], alters the melting point transition temperature of protein [73,74], inhibits functions as ligand binding [75]. To counteract the destabilizing effect of urea, elasmobranchs accumulate methtylamine organic osmolytes [14, 71,], since they are stabilizer of protein structures and activator of many functional properties of proteins [71,72].
Trimethyl amine oxide TMAO and betaine are the most effective [15] and the predominant methylamines found in the muscle of elasmobranches [79,80].
The level of TMAO, which primary role is that of an osmolyte, varies in the euryhaline species adapted to different salinities [76,81]. However in seawater elasmobranches TMAO is the major component while in the transition from euryhaline to freshwater elasmobranches betaine becomes the major methylamine [82]. A concentration ratio of urea to these counteracting compounds is optimal for preserving proper protein function [15,71,72].
In addition urea could increase the fluidity of membranes by a destructive effect on the hydrophobic core of the membrane or by indirect effects related to changes of integral membrane proteins. Barton et al. Regarding the synthesis of the predominant methylamines it was suggested that betaine synthetic pathways are probably present in most or in all elasmobranches, like in the other vertebrates, since they play an important role in the catabolism of choline introduced by the food [83].
Measurable levels of these enzymes were found in various elasmobranch species [82]. This enzyme is sporadically present in elasmobranches [82,] and when present its activity is high in the liver that should be the major site of TMAO synthesis [84,86,87].
It was suggested that the ability to maintain TMAO plasma concentration in species lacking of TMAox without an exogenous supply of TMAO for a long period depends on a release from muscle [85] as well as on a very low loss from the animal [81,87,88]. This can be due to a sustained reabsorption of the filtered TMAO [30,89]. In addition, in order to maintain elevated methylamine levels to counteract the destabilizing effect of urea in species lacking of TMAox, an activation of the enzyme involved in betaine synthesis during a period of exogenous supply of TMAO can occur [82].
In summary, studies hitherto performed suggest that, in order to maintain a large outwardly direct concentration gradient, both gills and rectal gland of elasmobranchs have an unusual permeability allowing the movement of water but not of urea.
However in the gills an homologue of a renal urea transporter was also detected, suggesting a back-transport of the osmolyte in the basolateral membrane that contributes to avoid urea loss from the body. Various facilitated urea transporters have been identified in the elasmobranch renal tubules allowing the reabsorption of urea freely filtered by the glomerulus.
The adaptation to different salinities involves also a variation of TMAO concentrations, the second osmolyte in elasmobranchs, that plays also the important role to counteract not only the destabilizing effect of urea on the structure and hence on the function of many proteins but also the urea effect on membrane fluidity.
However a multidisciplinary approach ranging from organism to molecular techniques is still necessary to complete the picture of the strategies adopted by different elasmobranch species for living with high urea concentrations. This approach will be necessary also to better understand how the urea levels are modulated to allow euryhaline species to live in habitats with different salinities. Pang, P. American Zoologist, 17, Shuttleworth, T.
In: Shuttleworth, T. In: Evans D. Barton, K. Hazon, N. Comparative Biochemistry and Physiology B, , In: Carrier, J.
Hammerschlag, N.
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