. Compare and contrast osmoregulation by freshwater
fishes and osmoregulation by marine fishes.
Osmoregulation is the process by which organisms maintain a balance of water and ions within their bodies, in relation to the external environment. Freshwater and marine fishes have different osmoregulatory mechanisms due to the differences in the osmotic conditions they face.
1. Environment:
- Freshwater fishes live in environments with lower salt concentration than their bodies. In contrast, marine fishes live in environments with higher salt concentration than their bodies.
2. Osmotic Gradient:
- Freshwater fishes are considered hypoosmotic to their environment, meaning their bodies have a higher concentration of ions than the surrounding water. They constantly gain water through their gills and skin due to osmosis, while continually losing ions.
- Marine fishes are considered hyperosmotic to their environment, as their bodies have a lower concentration of ions than the surrounding water. They continuously lose water through their gills and skin mainly through osmosis, while gaining ions.
3. Water Balance:
- Freshwater fishes primarily face the challenge of gaining water and losing ions. To prevent excessive intake of water, these fish have specialized cells in their gills called chloride cells that actively transport ions out of the body. They also produce a large amount of dilute urine to excrete excess water and maintain proper ion levels.
- Marine fishes need to balance water loss and ion gain. They drink seawater for hydration but have to deal with the high salt intake. They possess specialized glands (e.g., the rectal gland in sharks) that excrete excess salt through the rectum. They produce a small amount of concentrated urine to conserve water.
4. Ion Regulation:
- Freshwater fishes need to actively absorb ions from the environment. They have specialized cells in their gills that actively transport ions into the body against a concentration gradient. Some freshwater fish also intake ions through their food.
- Marine fishes actively lose ions to their environment. They have specialized cells in their gills that actively transport ions out of the body against a concentration gradient.
5. Adaptations:
- Freshwater fishes have evolved to have efficient mechanisms to avoid excess water uptake and maintain ion balance, such as the presence of specialized cells (chloride cells), high filtration rates in their kidneys, and low urine production.
- Marine fishes have adapted to minimize water loss and maximize salt retention. They have glands that excrete excess salt, specialized cells in their gills to reduce water loss, and concentrated urine production.
In summary, freshwater fishes focus on preventing water uptake and losing ions, while marine fishes focus on minimizing water loss and gaining ions. These adaptations allow them to osmoregulate in their respective environments effectively.