Large mussel bed on exposed rock

Why do mussels only live in a narrow vertical band

Visitors will often notice that mussels seem to only live in a narrow vertical band typically within just a few feet. They do not live high up in the intertidal zone and also are not normally seen below a certain level. This distribution of marine life can be defined as a pattern in tide pool ecology.

Too understand this pattern more and find out why this pattern occurs we must first understand a little more about the ecology of mussels. Mussels feed by capturing small food particles from the water. Mussels will pump water through their body cavities and filter out the small bits of food and plankton. Mussels are also sessile in they cannot move ( or only move a small distance ) once they have settled. Mussels need a constant supply of water where they can pump large volumes of water that contain food. The higher up in the intertidal zone, the less water is available. The less water means less food. The reason mussels don’t survive well in the high intertidal zone is this limited supply of food. This explains the reasons for the upper boundary of this pattern but what about the lower boundary.

Mussels prefer to live submerged as much as possible. The lower they can live leads to more water and more food. The availability of food is clearly not a factor. Drying out by lack of water is also not an issue the lower they live in the water column. Wave action does not seem to be a factor in that the wave action would generally be less the lower in the water column. Other physical factors also don’t seem to be the reason. The reason must be a biological factor that prevents mussels from living lower in the tidal zone. Sea stars are the primary predators for mussels. Sea stars can devour vast numbers of mussels in a short amount of time. Sea stars are the reason why mussels don’t survive for long below a certain level because they are eaten by sea stars and a few other animals.

This is a good explanation but there is one problem. Why wouldn’t the Sea stars eat all the mussels? The reason is related to the Ecology of Sea Stars. Sea stars however do not like to venture too far away from water or they will dry out and die. Sea stars will only climb high enough where they can safely return to water or where there is enough water and moisture for them to survive. Sea Stars will only consume mussels up to a certain point and then stop because there is not enough water for them to survive.
Posted in Patterns.

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