What is rectal gland?

Rectal gland is an organ found in the rectum of certain marine elasmobranchs like dogfish, skates and rays. It aids osmoregulation and helps them survive in the hyperosmotic environment of the sea. The rectal gland functions by actively transporting excess salt from the blood into the rectal lumen. This helps the fish to retain their bodily water and maintain a stable internal osmotic balance. Here are some additional details about the rectal gland:

- Salt Secretion: The rectal gland contains specialised epithelial cells with a high concentration of mitochondria, which provide energy for the active transport process. These cells actively secrete sodium and chloride ions into the rectal lumen against their concentration gradients, creating a hypertonic fluid.

- Water Reabsorption: The remaining water from the rectal fluid is reabsorbed back into the bloodstream through a process called osmosis, driven by the concentration gradient created by the secreted salt.

- Role in Osmoregulation: Elasmobranchs have evolved the rectal gland to help them cope with the challenges of living in a hypertonic environment. By excreting excess salt while conserving water, they can maintain proper internal fluid balance and avoid dehydration.

- Location: The rectal gland is located in the posterior region of the digestive system, just before the anus. It may consist of a single, centralised gland or several scattered glandular tubules embedded within the rectal tissue.

- Adaptive Significance: The rectal gland is an important adaptation for elasmobranchs that inhabit marine environments. It enables them to thrive in the presence of high external salinity, which can be lethal to other aquatic organisms with less efficient osmoregulatory mechanisms.

In summary, the rectal gland in marine elasmobranchs plays a vital role in regulating salt and water balance, allowing them to survive in their high-salinity surroundings and maintain optimal internal osmotic conditions.