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# Osmoregulation **A) Osmoconformers and Osmoregulators in the Marine Ecosystem** The average salinity of the sea is 3.5 percent. The salt concentration in the body fluids of cartilaginous fish, crustaceans, and echinoderms is largely consistent with the surrounding seawater. They are isometric t...

# Osmoregulation **A) Osmoconformers and Osmoregulators in the Marine Ecosystem** The average salinity of the sea is 3.5 percent. The salt concentration in the body fluids of cartilaginous fish, crustaceans, and echinoderms is largely consistent with the surrounding seawater. They are isometric to their environment. These organisms are referred to as osmoconformers. Fish with bone skeletons, however, have a lower salt concentration in their body fluids than the surrounding seawater; they are hypotonic. This results in water constantly being lost to the environment by osmosis. Water intake compensates for this loss. Dissolved salts are actively expelled through the gills or kidneys. These specific regulatory mechanisms ensure the organisms maintain a constant salt concentration in their cells. These organisms are referred to as osmoregulators. Osmoregulators have the ability to maintain internal conditions, regardless of environmental changes, by closely regulating their internal environment; however, this constant maintenance is energetically demanding. Seabirds are also osmoregulators as well, excreting excess salt through special glands. **B) Water Fleas (Gammarus) in the Baltic Sea** Gammarus species are common in the Baltic Sea. Salinity in the Baltic Sea varies between 0.3% and 1.9%. **Questions and Answers** 1. **Define the terms "osmoconformers" and "osmoregulators."** Osmoconformers have internal salt concentrations similar to their environment whereas osmoregulators regulate internal salt concentrations to remain constant while maintaining a stable internal environment despite environmental changes. 2. **Explain why marine animals with body fluids containing less salt than surrounding seawater tend to lose water to the surrounding environment. Describe how they regulate their water and salt balance.** Animals with lower internal salinity compared to the surrounding seawater will lose water to the environment via osmosis. They regulate their water and salt balance through a combination of processes. These include actively taking in water from the surrounding environment, actively expelling excess salt from their bodies, and actively retaining water and salts depending on the organism and its specific needs. 3. **Based on text B and figure 1, determine whether Gammarus is an osmoconformers or an osmoregulator.** Based on the provided text and figure, Gammarus is an osmoregulator. 4. **Explain why it is beneficial for mussels and other organisms living in the intertidal zone to be osmoconformers.** Being an osmoconformer in the intertidal zone offers advantages to these organisms by allowing their internal salinity to fluctuate with the changing salinity of the seawater. This is in contrast to osmoregulators who would need to exert metabolic energy to maintain their own internal salinity against the changes in the environment.