With respect to extracellular fluid, animals in marine environments either osmoconform (have the same osmotic concentration) to seawater (1000 mOsm; Figure 4) or osmoregulate at a lower osmotic concentration (usually 300-400 mOsm). In addition, they either ionoconform with respect to their extracellular fluid, having the same ionic composition as seawater, or they ionoregulate and maintain different ionic concentrations. Animals that conform to seawater do not have to overcome the problem of continual osmotic loss of body water to and gain of ions from the environment, but high ion concentrations adversely influence cellular metabolic processes. Most marine invertebrates osmoconform and ionocon-form to seawater, but a few osmo- and ionoregulate.
1200 1000 800 600 400 200 0
600 r tt
400 0 o
Osmoconformer Osmoconformer Osmoregulator ionoconformer ionoregulator ionoregulator
Figure 4 Patterns of extracellular ion and osmotic regulation in vertebrate animals.
Hagfish are the only vertebrates to both osmo- and ionoconform. Marine bony fish both osmo- and ionoregulate. Marine elasmobranchs osmoconform at 1000 mOsm but ionoregulate at about 600m0sm; urea and trimethylamine oxide (TMAO; which counteracts the negative effects of the urea on proteins) make up most of the 400 mOsm osmotic gap between the ions and seawater (Figure 4).
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