Question

When a salmon or other teleost fish migrates from seawater into freshwater, what are all the changes that take place or are likely to take place in its patterns of water-salt physiology?

Short answer

When a salmon or other teleost fish migrates from seawater to freshwater, it will experience significant changes in water-salt physiology. To combating dehydration in seawater, fish drink large volumes, absorb salts, and excrete them as concentrated urine. When they move to freshwater, they stop drinking, excrete excess water through dilute urine, and absorb and conserve as much salt as possible, often from their food and directly through their gills.

Step-by-step solution

Understand Osmoregulation in Seawater

In seawater, since the salt concentration is greater outside the body, fish usually lose water through their gills and skin due to osmosis, a process where water diffuses across a membrane from a region of lower salt concentration to a region of higher concentration. To counteract this, they drink large volumes of seawater, absorb the salts through their gills and excrete them as concentrated urine.

Understand Osmoregulation in Freshwater

On migrating to freshwater, fish face a reverse situation. Here, the salt concentration inside their body is higher than the outside environment. This causes them to gain water continuously through their skin and gills. The excess water is excreted as very dilute urine. In addition, the fish obtain salts, which are scarce in freshwater, from their food and actively absorb salts through their gills.

Detail the Changes

Upon entering freshwater from seawater, the fish needs to adjust its osmoregulation. It stops drinking water since it is now gaining too much water by osmosis and starts to produce large volumes of dilute urine to get rid of this excess. Its gills work to actively absorb salts from the environment, and it will also rely more on dietary salts. The fish now needs to preserve the salts in its body as much as possible while excreting excess water.

Key concepts

Teleost Fish Physiology

Teleost fish, such as salmon, exhibit complex physiological mechanisms to survive in different aquatic environments. These fish are known for their ability to regulate their internal water and salt balance, a process called osmoregulation. This ability is vital for their survival as they transition between freshwater and seawater environments, which have drastically different salinities.

In order to maintain homeostasis, teleost fish have developed specialized organs and cellular structures. Their gills play a crucial role in both gas exchange and ion regulation, acting like a two-way street for ions to move in and out of the fish's body. The kidneys of teleost fish are also adapted to produce urine of varying concentrations, depending on the external salinity of the water they inhabit. Furthermore, teleost fish are equipped with special cells called chloride cells that are essential for salt regulation, allowing them to expel excess salts when in seawater or conserve salts when in freshwater.

Understanding the physiology behind teleost fish is key to comprehending their remarkable adaptability and ability to thrive in diverse aquatic habitats.

Water-Salt Balance

The water-salt balance in fish is a delicate interplay of environmental conditions and biological processes that ensure survival and physiological function. In saline environments like seawater, teleost fish must combat the natural tendency to lose water and gain salt due to higher external salt concentrations. This is achieved through the active intake of seawater, the use of specialized gill cells to excrete excess salts, and the production of concentrated urine to minimize water loss.

On the other hand, in freshwater environments, a fish's internal environment has a higher salt concentration than its surroundings, causing it to gain water and lose salt. To address this, fish do not drink water but instead produce large volumes of dilute urine to eliminate excess water, while retaining as much salt as possible. Salts are absorbed from the environment via gills and through dietary intake.

Maintaining this water-salt balance is crucial as it impacts metabolic function, muscular activity, and overall health. If the balance is disrupted, it can lead to dehydration or swelling, both of which can be detrimental to a fish's health.

Freshwater Adaptation

Adapting to a freshwater environment is a significant challenge for fish that are usually associated with saline waters. In freshwater, the fish encounters a scenario where its body's salt concentration is higher compared to the surrounding water. This results in water continuously entering the body through osmosis. To adapt, these fish needs to stop drinking water and begin excreting large volumes of dilute urine to get rid of the excess water.

Moreover, teleost fish employ their gills extensively, switching their role from salt excretion to salt absorption. Their gill cells actively absorb essential ions and salts from the water to make up for the salts not freely available in freshwater. This function is so vital that, along with the ingestion of salts from food, it supports the fish's salt requirements to maintain homeostasis.

These adaptations are crucial for survival in a low-salt environment, ensuring that the fish can thrive even when external conditions change drastically.

Seawater Adaptation

When a fish moves into a seawater environment, it faces an opposite issue compared to freshwater. Here, the external salt concentration is substantially higher than that inside the fish. The primary challenge is to prevent dehydration due to water loss through osmosis. To overcome this, fish drink the surrounding seawater in large amounts to maintain their water balance. This involves an efficient regulatory system where the absorbed salt is expelled through specialized chloride cells in the gills.

The kidneys of seawater-adapting fish are adept at producing highly concentrated urine, minimizing the loss of water and maximizing the excretion of excess salts. This ability demonstrates an astonishingly precise control over water-salt regulation, allowing the fish to avoid dehydration while maintaining enough salt within its body for cellular functions.

Through these adaptations, teleost fish can successfully inhabit seawater, harnessing physiological mechanisms that ensure they remain hydrated and their internal ionic concentrations are balanced.