- Why do sharks, rays, and chimaeras possess specialized organs such as the salt gland, kidney, and gills to maintain their inner ionic composition?
Chondrichthyans must adapt to a range of environments. Therefore, the chemicals and salt found within their bodies and tissues differs from the chemicals and salt in the surrounding salt and freshwater environments. These specialized organs process and excrete chemicals so their bodies remain at the constant chemical balance needed. In the sea, they excrete sodium and chloride ions that diffuse into their bodies because the concentration of these ions in the ocean exceeds that within their bodies. These organs allow the water to enter and pass through the body at a rate needed for the animal to maintain this inner balance between the dissolved solutes and water. This is known as osmoregulation. The gills, for example allow water to pass through the body when its body fluids contain a higher osmolarity than the surrounding environment, the kidneys function will excrete. The relationship of the solutes in tissues can differ, but in any case, these animals must maintain their inner ionic composition to function and live in their environments.
(Klimley, Pg. 84-85)
- How do they maintain the proper functioning of their enzymatic reactions to counteract the denaturing effect of urea on intracellular proteins?
Urea exists as a destabilizer of proteins. Urea’s function is to help the balance of salt concentration in the body to match the salt concentration in the outside environment. It exists in high concentrations in the muscles of cartilaginous fishes. The high concentrations of urea must be counteracted to protect the tissues because urea alone can be toxic. Trimethylamine oxide, or TMAO is a protein stabilizer that exists to counteract the high levels of urea. The ratio of urea to TMAO is 2:1, however, Greenland sharks (Somniosus microcephalus) have a much higher level of urea and TMAO than most other sharks (Praebel, K. 2019). Perhaps the reasoning is to protect the shark at depths through its extremely long lifetime. Therefore, the meat of a Greenland shark is toxic, and if consumed, it must be treated to reduce the toxins first.
- Only a few sharks and rays inhabit freshwaters. Why do you think this is so? Perhaps this has less to do with osmoregulation than with the limited value of electroreception in this nonconductive medium.
The salt in salt water is comprised of very large amounts of sodium and chloride ions. Much larger amounts than that of fresh water, thus salt water is more conductive than fresh water. These sodium and chloride ions allow electrons to pass through materials that vary slightly in how they shed electrons. This is ultimately the electrical force given off. Cartilaginous fishes are special in that they rely heavily on electrical forces emitted by potential prey or threats, or other materials and to detect these electrical forces they rely on their specialized sensory organs the ampullae of Lorenzini. The lateral line is connected from the ampullae down a shark, helping sense vibrations. These develop a highly effective system for sharks. Some sharks have been naturally engineered with higher concentrations and placements of their ampullae of Lorenzini, for instance, the family Sphyrnidae, Hammerhead sharks.
According to S.P. Collin (2010): “The effects of habitat salinity also have influence on the structure of the ampullary organs in teleosts, which are reduced to ‘microampullae’ in freshwater plotosid catfish, that is, Plotosus tandanus. These microampullae consist of short canals (50 μm in length) and contain low numbers of receptor cells (10–15) and appear different to the ampullary organs described for marine Plotosus anguillaris, which are characteristically defined as resembling the ampullae of Lorenzini in elasmobranchs. In contrast, the canals of the ampullary organs in P. anguillaris measure in centimeters and the ampullae include hundreds of receptor cells. Similarly, elasmobranchs such as the bull shark, C. leucas, which migrate between saltwater and freshwater rivers also possess differences in receptor types and therefore the ability to detect bioelectric fields”.
According to Wueriger, Squire, Kajiura, Tibbetts, Hart and Collin (2012): “Freshwater sawfish, Pristis microdon, and two species of shovelnose rays, Glaucostegus typus and Aptychotrema rostrata were tested for their reactions towards weak artificial electric dipole fields. The comparison of sawfishes and shovelnose rays’ sheds light on the evolution and function of the elongated rostrum (‘saw’) of sawfish, as both groups evolved from a shovelnose ray-like ancestor”.
Sawfish have evolved to have a highly sensitive rostrum with the ability to better detect electrical fields. What both of these studies suggest is that yes, a reason why there are so few sharks and rays that inhabit freshwater is directly related to the limited value of electroreception in fresh water.
Klimley, A.P. (2013). The biology of sharks and rays. The University of Chicago Press: Chicago, IL. Pgs 84-85, 86-89, 213.
Praebel, K. (2019).
S.P. Collin, (2010). Electroreception in vertebrates and invertebrates. Retrieved from: Encyclopedia of Animal Behavior.
Wueringer, B.E., Squire Jnr., L., Kajiura, S.M., Tibbetts, I.R., Hart, N.S., Collin, S.P. (2012). Electric field detection in sawfish and shovelnose rays.