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Biology 10, Lesson 50 – Estuaries

Estuaries: the transition point of river and sea.  The incredible phenomenon is not only mesmerizing to watch, but it is also home to hundreds of water animals and plants.  In this essay, I am going to talk about what life is like for inhabitants of an estuary.

Estuaries are considered to be on the edge of an aquatic biome, which is the scientific name of aquatic habitats.

Several plants and animals are able to thrive in the ecosystem an estuary creates.  Some examples are “microflora like algae, and macroflora, such as seaweeds, marsh grasses, and mangrove trees (only in the tropics).”[i]  Organisms that live in these areas must learn how to adapt to the salinity in the water and the swell of the tide.

An example of these organisms are Blue Crabs.

Blue Crabs need different levels of salinity throughout their life, making them a compatible species for an estuary. Adult female Blue Crabs have a high-salinity preference, while adult male Blue Crabs have a low-salinity preference, causing them to live in different areas of the estuary.  However, during mating season their preferences overlap, allowing them to reproduce with one another.  Once the process is complete, the female crab will go offshore to lay her eggs in the high-salinity environment.  These larvae, known as zoeae, will live in this area until they develop into megalops (young crabs), which is when they will return to the estuary.

Once returning to the estuary, they will do something known as vertical migration, which is swimming up and down the estuary based on the tides and light they are receiving.  Eventually, they will settle down in shallow water banks to “feed and gain protection from predators.”[ii]

As you can see, the estuary is an incredible ecosystem but not an easy one to live in.  Every plant and animal must be able to adapt to the constant change of salinity and tides.  From the example of the Blue Crabs, I am sure you can see how the salinity levels can affect the behaviour and life of an animal in an estuary.

Thanks for reading!

[i] “Aquatic: The Wild Blue Yonder.” The World’s Biomes. Accessed 9 Sep 2022.

[ii] NOAA. “Adaptions to Life in the Estuary.” National Ocean Service. Accessed 11 Sep 2022.


Author: sophiaelahirpc

10th Grade student in the Ron Paul Curriculum. Full-time teen writer living in Singapore.

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