Something Fishy Is Going On
By Dr. Rob Jones, "The Aquarium Vet"
In my last few articles, we discussed osmoregulation in freshwater and marine teleosts. Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids to maintain the homeostasis of that organism's body water.
This month, we will look at elasmobranchs (sharks, rays, and skates). Ninety percent of elasmobranch species live predominantly in a marine environment. The critical difference between marine teleosts and marine elasmobranchs is that the teleosts are hypotonic to seawater, while the elasmobranchs are, in fact, slightly hypertonic. Normally, seawater has a salinity of approximately 35 parts per thousand (ppt). The internal salinity of a teleost is between 9 and 10 ppt, while for an elasmobranch it is 36 to 37 ppt.
Teleosts produce ammonia as an end product of protein metabolism. All elasmobranchs (except for the Potamotrygonidae family) are ureotelic, in that they produce urea as the end product of protein metabolism. Other ureotelic organisms are land (adult) amphibians and mammals. This metabolic process occurs in the liver.
Elasmobranch ion levels (sodium, chloride, and potassium) are only slightly higher than those of teleosts, and so this is not the main reason for the higher osmolality of elasmobranchs. In elasmobranchs, the osmolality is mainly due to high levels of two non-ionic substances:
- tri-methylamine oxide (TMAO).
Plasma urea levels usually contribute about 30 percent of plasma osmolality. Urea is needed for normal cell function in elasmobranchs. Because the blood urea level is so high and the seawater level is lower, there is an outward diffusion gradient across the gills. Various aspects of gill structure and function prevent massive losses of urea across gill membranes.
The kidney plays a major role in maintaining high serum urea levels, by reabsorbing most of the urea from the urine. Less urea is lost via the kidneys than occurs across the gills.
Following death, it is the high urea and TMAO tissue levels that cause elasmobranchs to smell so quickly. This is compounded by the presence of enzymes and bacteria in the tissues that break down the urea to ammonia.
The higher osmolality of elasmobranchs compared to their environment causes a slight water influx to occur from the surrounding seawater. This extra water is then removed via the kidneys through urine production, which occurs at a slightly higher level than in marine teleosts. The excess salt levels caused by the seawater influx are dealt with by the rectal (or salt) gland.
Marine elasmobranchs can drink, but generally do not take in much water. Some elasmobranch species, however, can control their drinking rate to regulate their osmolality, in response to rapidly changing exposures to salinity.
Diagram summarizing marine elasmobranch osmoregulation
It is this very different osmoregulatory control that creates problems when considering a hyposalinity treatment for parasites in a mixed teleost and elasmobranch exhibit. The teleosts are usually fine, but it can create issues for the elasmobranchs, which can retain water and swell up.
For many years, the coelacanth (Latimeria chalumnae) was considered extinct, until a specimen was found in South Africa in 1938. If you want some fascinating reading, there are various books on the rediscovery of the coelacanth. Interestingly, coelacanth osmoregulation is very similar to that of elasmobranchs, with high levels of urea and TMAO.
I look forward to seeing some of you at the AZA Annual Conference in Seattle later this month. Come visit me at Booth 133, and drop by and say g'day. I would love to talk to you about our new penguin and aquatic birds module.
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