Question

Some animals are osmoconformers, meaning that they maintain the tonicity/osmolarity of their body fluids equal to that of the external environment. Hydroids, a type of cnidarian, are osmoconformers. But they don't move and thus are adapted to the environment they live in. A) What would happen if you take a marine hydroid and transfer it into a freshwater aquarium? B) What would happen if you take a freshwater hydroid and transfer it into a saltwater aquarium? C) Sharks are extraordinary osmoconformers. Sharks store urea in their tissues and use it to match the tonicity of their tissues to that of the surrounding environment. Some sharks can move from saltwater to freshwater, hypothesize what changes will sharks do in order to adjust when moving from saltwater to freshwater and vice versa. Explain your answer in terms of tonicity and specify the direction of water movement and urea concentration.

Answer 1

a. If a marine hydroid is placed in a freshwater aquarium, its cells would rapidly absorb water and rupture. Marine fish are adapted to have body fluids isotonic to seawater. So, their system would be hypertonic to the new freshwater environment. Water would move by osmosis into the fish's cells and the intake of water into the cells would cause them to burst and potentially die.

b. If a freshwater hydroid is put in a saltwater aquarium, its cells would be hypotonic to the new saltwater environment. They would lose water and shrivel, which will damage the cells.

c. Shark’s body fluid concentrations conform to changes in seawater concentration. Their body fluids possess same concentration of ions as seawater, but they use different ions. The blood of sharks contains the organic compounds urea and trimethylamine oxide (TMAO). TMAO is used to stabilize proteins in the presence of high level of urea. Sharks are cartilaginous fish with a rectal gland to secrete salt and assist in osmoregulation. They excrete more urea in less concentrated salt water and fresh water to make the body fluid isotonic to that environment. In marine environment, it accumulates more urea in blood to remain at same concentration with respect to the surrounding sea water.