Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 59

We noted in Chapter 1 that salinity in the open ocean does not vary enough to be a major determining factor in the distribution of oceanic species, in contrast to estuaries where large drops in salinity occur over the course of a few kilometers and species compositions change accordingly. In the open ocean, salinity varies little over large oceanic areas, with a low from about 33‰ (in near‐surface waters of the Northeastern Pacific for example) to about 38‰ in the Red Sea and Mediterranean.

Marine species exhibit three basic strategies with respect to salinity. The first, typical of the invertebrates, is a strategy of osmoconformity. Virtually all marine invertebrates have a total internal ionic concentration that is nearly identical to that of seawater. As a consequence, they are in osmotic balance with their external environment and the need to regulate internal salinity is minimal. Regulation is usually restricted to ions such as magnesium that have potential for interfering with the function of nerves or muscle.

The second strategy, typified by the bony fishes, is one of hypo‐regulation. The prefix hypo is from the Greek meaning “under” or “below” (e.g. hypo‐dermic = under the skin). A hypo‐regulator keeps its internal salinity well below that of seawater. Bony, or teleost, fishes like tunas, sardines, and swordfishes have internal salinities 45–60% that of seawater. They maintain their low internal salinities using the suite of mechanisms briefly described below. However, it is important to remember that relative to the large difference between a fish’s internal salinity and that in the open ocean (e.g. 16 vs. 35‰), the small changes in oceanic salinity (typically <1‰) encountered by a pelagic fish during its lifetime are a trivial matter.

From a physiological perspective, one of a marine fish’s greatest problems is the loss of its body water. The basic laws of diffusion are such that ions and water, when left to their own devices, will move across biological membranes until their concentrations are equal on both sides of the membrane. For our purposes here, we can think of a fish’s skin as a membrane or barrier that is fighting the natural tendency of the ions in seawater to move into the fish, and the water inside of the fish to move out. Unfortunately for the fish, no skin is perfectly impermeable. Further, in order to breathe, the fishes must extract oxygen from seawater using their gills. In order to function effectively, the physical barrier between blood and seawater at the gill must be very thin indeed. Penultimately, fishes need to open their mouths to feed, which provides yet another chink in the armor. The final insult is that fishes cannot produce a concentrated urine like a kangaroo rat, or for that matter even like a human. The best that they can produce is a urine that is about the same salinity as the blood, so they do not lose much ground with their excretory system, but they do not gain any either.

Fishes replenish the water they lose through their excretory system and the ions they gain through their skin by drinking seawater! Water is extracted as it passes along the fish’s gut, along with some ions, but the great majority of the ions are allowed to pass right through. The excess ions that are taken up by the gut and that diffuse inward are excreted at the gills using cells specific for that purpose. Ironically, despite the fact that the blood–seawater barrier is thinnest at the gills, no unwanted ions are taken up there due to the tight‐junctions in the cells of the respiratory epithelium. The ion‐balancing act of bony fishes is portrayed in Figure 2.25.

The third osmoregulatory strategy exhibited by marine species is shown by the elasmobranchs: the sharks, skates, and rays. It is also employed by the ratfishes (chimaeras) and the ancient lobe‐finned fish, the coelacanth. Admittedly, only a very few representatives of those groups are the “small swimmers” that are the focus of this book, but their water‐balancing strategy is an important one in the marine system and is included here for completeness.

The third strategy is much like a combination of those used by the invertebrates and the teleosts. The internal salt concentration of the sharks and their kin is about half that of seawater, similar to that of the bony fishes. However, the difference in osmotic concentration between seawater and shark blood is made up by organic osmolytes: urea and trimethyl amine oxide, or TMAO (Figure 2.26). In fact, sharks and kin are slightly hyperosmotic to seawater; that is, they are at a slightly higher ionic concentration than seawater due to the urea and TMAO, which means that water tends to flow in! Urea and TMAO naturally tend to flow out, so the integument of the cartilaginous fishes is highly impermeable to their loss. In addition to using the same mechanisms employed by the bony fishes, i.e. excretion of ions at the gills, elasmobranchs have a rectal gland that can secrete a highly concentrated salt solution to aid in maintaining salt balance.

Figure 2.25 Osmoregulation in teleosts. Balance of water and ion concentration in the body of marine fishes through the processes of diffusion and active transport.

Figure 2.26 Osmoregulation in elasmobranchs. The osmolytes urea (a) and trimethylamine oxide (b) used by cartilaginous fishes to regulate osmotic balance in the blood.