Wildlife of the Week!

Today we look into the tide pools and along the rocky shores of this Pacific coast to discuss our next species for “Wildlife of the Week”. A common name for a collection of similar species, the blue mussel (Mytilus sp.) is an important and familiar member of Salish Sea ecosystems. Although mussels may be famous for their uses as food, their interesting ecological lifestyle should certainly not be overlooked! The blue mussel is often found surrounded by barnacles as in this image; can you identify the type of barnacles too? Share with us in the comments below! Read on below to learn more about blue mussels.

The blue mussel is also referred to as the common blue mussel and/or bay mussel. The proper species assignments for blue mussels includes M. trossulus, M. edulis and M. galloprovincialis. These species look nearly identical to the eye, and must be determined through genetic means. 

Mussel colonies form major components of their habitats. This group of mussels is identified by their slightly rounded and tapered shells (two) attached at their bases, each between 7-11cm long. This shell set-up groups blue mussels (and all mussels) with other “bivalves”. This is a group that can essentially be explained as having two hinge shells. The blue mussel’s shells are usually blueish, black or brown/tan. These shells have a shiny/waxy outer appearance and curved lines/bands that run perpendicular to their length. Often the interior of the mussel cannot be seen, due to the closed nature of the two shells. It is important not to attempt to disturb mussels or other intertidal animals when encountered. 

Blue mussels are understood to generally range from Alaska to northern Mexico, however there exists hypotheses that different species may prefer slightly different latitudes as well as subtle habitat differences (e.g. intertidal zone vs docks). Interestingly enough, at least one of the three above-mentioned species is found on most coastlines of the world. This being said, it is important to understand a portion of this is not due to naturally-occurring species introductions. With the strong and tactical “byssal threads” that extend from their bodies, mussels can attach firmly to hard substrates or surfaces. For blue mussels specifically, these surfaces very often include docks, pilings and rocky intertidal sites within relatively calm areas. Multiple individuals can form dense covers on the ground or other surfaces. 

Blue mussels, along with many other invertebrates, utilize a unique method of feeding while underwater; they “filter-feed”. Filter-feeding is accomplished by the mussel running water through a system within its mantle cavity (inside the shell). From this point, particles of plankton or organic matter can be selected and used. It is also suspected that mussels feed on kelp as well, via any fragments drifting in the water. 

Blue mussels reproduce through external fertilization. Spawning is brought upon by environmental cues. In Puget Sound, and even some other areas, blue mussels begin this process in the spring and/or summer. The fertilized eggs will transition into a larval stage where they become plankton. They will drift in the currents, travelling at the mercy of them, until finally settling to the substrate after 1-2 months. Despite not being able to swim against the current, the blue mussels can still have some role in their movement with their byssal threads. 

As mentioned above, the three blue mussel species common to the Pacific regions are quite near identical. With this being said, identification also becomes difficult as the species are able to hybridize with each other. A fourth species may also confound I.D.; it is the California mussel (Mytilus californianus). This mussel is not a blue mussel and usually produces a larger and thicker shell.

Mussels can perform fascinating feats to escape snail predators, with simple use of their byssal threads. When predation is attempted, the mussel can reach out with its foot and attach the predator to another surface using its threads, sometimes killing the predator after time. Mussels are also at risk from predation by sea stars, gulls, anemones, ducks, crows and crabs, to name a few. The tough shells and the strong anchorage helps mussels to avoid this predation. 

Mussels are extremely famous for their use as human food. All three species can be consumed (not to forget shellfish consumption restrictions, other collection/fishing legalities, etc.), however the introduced M. galloprovincialis was brought into Puget Sound for aquaculture purposes, as well as unintentionally through ship ballasts.  M. edulis is also utilized very commonly for commercial aquaculture. It is supposed that these two are introduced to Pacific regions, compared to the presumed native species M. trossulus. 

There is clearly much more we can discover about this interesting group of mussels. Try your luck and see if you can answer some mussel trivia below! 

1. True or False? Blue mussel colonies can number into the thousands. (ANS: True). 
2. Which Phylum do mussels belong to? (ANS: Mollusca). 
3. True or False? Fish may also feed on mussels. (ANS: True). 

BONUS CHALLENGE: 

What is the maximum water pumping rate that a blue mussel can maintain for feeding? 

Up to 1 litre/ hour/ gram of body weight. Wow! 

Article Authored By: Alexa D., B.Sc./ Five Star Whale Watching. 

To learn more about this amazing invertebrate, visit our References below!

Biodiversity of the Central Coast.

Memorial University.

Slater Museum of Natural History: University of Puget Sound.