Brackish water occurs when the freshwater from a river or lake meets the salty seawater of an ocean body. One key difference between these types of habitats is salt content, or the salinity of the water. Salinity is measured in a unit called parts per thousand (ppt), which refers to the concentration or percentage of salt in the water. For example, one ppt is the equivalent of one ounce of salt for every 1,000 ounces of water. While fresh water has an average salinity of less than 0.5 ppt, seawater generally ranges between 35-38 ppt. Brackish water, however, can range in salinity anywhere from 0.5 to about 30 ppt.

While most aquatic animals have adapted to live in either freshwater or saltwater environments, some unique animal species thrive in these fresh and salty estuaries, mangroves and marshes. Here are just a few of my favorite brackish water buddies:

Crocodiles

While alligators primarily live in freshwater, they have been known to adapt to brackish water environments like their cousin—the crocodile. In fact, the Florida Everglades is the only known environment where alligators and crocodiles coexist. However, crocodiles are better suited for brackish environments thanks to their lingual salt glands. These salt-excreting glands live directly under a crocodile’s tongue and help them better manage their electrolytes.

Mudskippers

These fish are the perfect examples of adaptation. Many species of mudskippers are amphibious and spend much of their time on land, which is ideal if you’re setting up residence in brackish water. The name is derived from the ability of the mudskipper to use its body and tail to skip and hop about on land. Mudskippers are highly adaptive to varying levels of salinity.

Bull Sharks

Unlike most other sharks, bull sharks can venture out of saltwater habitats and handle brackish or even fresh water. They spend a lot of time on tropical shorelines, but have been spotted inland in rivers and tributaries, including in the Amazon River. Bull sharks are diadromous which means they can migrate between salt water and fresh water easily. This is because some of the organs responsible for their excretion of excess salt (kidneys, liver, gills and rectal gland) decrease activity depending on their environment. Bull sharks travel to freshwater areas to spawn and juveniles stay in brackish nursery areas where this is less predation risk.

Oyster

Oysters are hard-working animals that are found both in the ocean and in brackish environments. As adults, oysters filter 25-50 gallons of water each day and take on the salinity of the water in which they reside. This is why you’ll find some places are famous for their sweet or salty oysters. Oysters in brackish environments tend to be sweeter since they are filtering less salt than oysters in brinier ocean water.

Manatees

Manatees can thrive in both freshwater and saltwater, making brackish environments no problem. While they can’t drink saltwater per se, manatee kidneys are specially adapted to help filter out excess salt—just in case. Manatees prefer slow-moving rivers and coastal areas, sticking mostly to mangroves where seagrass and other vegetation is thriving. They are vegetarians after all, even though they might sneak a small fish or occasional invertebrate.

But manatees are in trouble. 2021 was the deadliest year on record for manatees living in the warm coastal waters off Florida. According to state officials, the major cause is starvation due to the loss of seagrass beds. You can take action by asking the United States Department of Interior to put the manatee back on the endangered species list.

With voices like yours, we can help protect manatees and other beloved creatures that inhabit brackish environments. What is your favorite brackish water animal?