Can Starfish Live In Freshwater?
Starfish or sea stars are fascinating marine invertebrates, primarily found in seashores or coral reefs. If you wonder if they also live in freshwater, we’ll talk all about that in this blog post. However, let’s start with a quick answer:
Starfish cannot live in freshwater due to their state of isotonic stability with the surrounding saltwater. This means that the same amount of water moves between the body tissues and saltwater, and they cannot prevent the accumulation of fluids in their cells in low salinity environments.
However, that certainly doesn’t tell the whole story. Below I’ll explain more about why starfish can’t live in freshwater and what happens to them if they’d be in freshwater. Furthermore I’ll explain if they can live in brackish water and what is their perfect environment.
Can starfish live in freshwater? (Explained)
Starfish can’t live in freshwater because of the changes in salinity. These animals live in a state of isotonic stability with the surrounding saltwater, which means that the same amount of water moves between body tissues and the saltwater.
Starfish bodies’ tissues, the cells, have a permeable membrane that allows water to flow in and out of the cell body, helping to soothe the cell’s vital needs. Based on chemical principles, the water will always flow to an area with a higher concentration of salts and minerals through a permeable membrane.
If you’re interested in reading more about how starfish breathe, check out my other blog post here: “How Do Starfish Breathe?”
If you throw a starfish into freshwater, the state between the starfish and the surrounding water is unstable. The cells of a starfish contain higher ions of salt and other minerals. Water invades cells, causing them to swell and eventually break. This kills cells as minerals try to diffuse into the water and dilute.
Starfish are invertebrates and very simple organisms, lacking bones, hearts, or brains. They don’t have the ability to push water out of cells actively. Vertebrates such as fish can do this because they have specialized organs like kidneys that help keep the body stable. This takes a lot of energy and a more complex organization of the body organs.
However, animals that live in saltwater usually cannot tolerate freshwater for long periods and will die if left for a long time. There are, of course, exceptions because some animals adapted to different environments. Animals that evolved can change their kidney chemistry to function in both salt and fresh water due to their life cycles or migration patterns.
Examples are bull sharks that live in oceans but also in freshwater rivers. Other examples of animals adapted for both fresh and salt waters are salmon, eels, Atlantic stingray, and Gulf sturgeon.
What happens if you put starfish in a freshwater?
As I mentioned before, when you put starfish in freshwater, the state between the starfish and the surrounding water will be unstable. This will cause the water to rush into the cells, swelling them and eventually breaking them. As a result, the starfish will die.
But what happens to the sea stars if the water salinity is low? The starfish Luidia clathrata is a starfish that adapted to a wide range of salinities. It lives in salinities ranging from 17 to 28%. Scientists observed its behavior in low salinity water of 17% and searched for effects on activity, feeding rate, absorption activity, and growth.
During the observation, the starfish decreased its activity and feeding rate. However, the absorption efficiency didn’t change largely. Its growth, indicated by body size and energy content of the body wall, reduced due to a decrease in feeding rate (energy consumption). The starfish reduced efficiency in utilizing the absorbed material and energy.
Low efficiency means that starfish would either require additional energy for maintenance or the energy is used inefficiently in low salinity. Therefore, L. clathrata was limited energetically in low salinities, but it lives and reproduces there.
Can starfish live in brackish water? (Explained)
No starfish can live in purely freshwaters. However, a few species adapted to low salinity waters called brackish water (8-30‰). In comparison, the usual range of ocean salinity ranges between 33‰ – and 37‰.
L. clathrata (grey sea star) that we discussed above was found even in salinities of 14‰. Nevertheless, it mostly lives around the coastlines of the western Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico.
Another species found in low salinity waters is Asterias rubens. This starfish lives in tropical waters but also in the Baltic Sea with a salinity of 15‰. This is the most common and familiar starfish in the north-east Atlantic so it’s also called “common starfish”.
Although some species live in low salinity waters, their activity is reduced due to energy consumption. The lack of an osmoregulation system probably explains why sea stars can’t survive in fresh or even in many brackish waters.
After discussing why starfish can’t live in freshwater, let’s now talk about their perfect habitat. Sea stars belong to the phylum Echinodermata, and like all species from this group, they’re exclusively marine animals. Starfish live in the Pacific, Indian, Atlantic, and even the Arctic Oceans.
There are about 1,500 recognized species of sea stars, and the largest concentration was found in the tropical Indo-Pacific region. However, we have discovered only 20% of the ocean so far, so who knows what’s hiding in the dark depths 😉
Starfish habitats range from tropical coral reefs, rocky shores, mud, tidal pools, and sand to kelp forests. They also live in seagrass meadows and the deep sea. They inhabit every zone, starting from the intertidal zone (seashores) to 20,000 ft (6000 m) depths.
Interestingly, for echinoderms, depths and pressure are usually not a barrier to their distribution because of the lack of gas pockets in the bodies. More potent barriers for them are the water temperature and salinity.
Starfish live on the ocean floors because of their feeding habits. They have a mouth underside their body, right in the center. They mainly feed on mollusks such as clams, mussels, and oysters, but some starfish species feed on corals. You can read more about it in my other blog post: “What Do Crown Of Thorns Starfish Eat?”
Like many sea animals, starfish prefer coral reefs due to the source of food and shelters where they can hide under corals or rocks. They can even locate reefs with their eyes by seeing large dark structures. This allows them not to go far from their homes.
- In Byrne, M., In O’Hara, T. D., CSIRO (Australia),, & Australian Biological Resources Study,. (2017). Australian echinoderms: Biology, ecology and evolution.
- Forcucci, Dave & Lawrence, John. (1986). Effect of low salinity on the activity, feeding, growth and absorption efficiency of Luidia clathrata (Echinodermata: Asteroidea). Marine Biology. 92. 315-321. 10.1007/BF00392671.
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