Mussel Zonation Patterns in the Intertidal


In the rocky intertidal in the Gulf of Maine recruitment of Mytilus edulis larvae is high and the larvae show no differential settlement (in primary or secondary settlement) (Gilg and Hilbish 2000). So why do we see a zonation pattern in mussels? The distribution of organisms in the intertidal is thought to be controlled by two factors: the physical limitation of the species setting its upper limit and the biological interactions setting its lower limit. Mussels must be submerged in water in order to feed because they are filter feeders. Their darker colored shells also probably attracts a lot of sun, which might be why you often find mussel beds hiding under the shade of a big mass of Ascophyllum (this spot also stays moist during low tide, making it a good spot for avoiding desiccation). These characteristics of Mytilus are physical limitations that prevent it from surviving too high in the intertidal where water is most infrequent. The lower intertidal is nice and moist, making it a desirable place for Mytilus to stay, except that there are an abundance of predators living there, including Aterias sp (sea stars), Nucella (Dogwhelk snail), and Carcinus (green crab). Asterias and Nucella are much more moisture-limited and can not venture to high in the intertidal (another example of a physical limitation setting an upper limit), but they definitely do their share of feasting on Mytilus in the low intertidal. This leaves the mid-intertidal as the most likely place for Mytilus to survive, which is where we usually find them (see the results of my experiment, Fig 1) (Menge and Branch 2001).


In the mid-upper intertidal the presence of barnacles enhances the settlement of mussels by providing a rough-textured surface on which mussels show higher preference for and survivorship. When mussel recruitment and growth rates are high the mussels actually take over and suffocate the barnacles. Thus, another example of a biological interaction limiting the lower limit of a species. In this case though, the biological interaction is competition for space, not predation. (Bertness 1999)




Asterias after finishing a snack!                     Adult mussels settled within a barnacle bed


Asterias eats a mussel by suctioning onto the shell and prying it open. Then the sea star essentially turns its stomach inside out, releasing gastral digestive fluids which break down the mussel's meat which it will ingest once it is broken down.




Back to Mytilus page                                                      An  Asterias relaxes after a hearty mussel meal


The Rocky Intertidal





                                                                   2002 Laura Brentner, for educational purposes only