On hot summer days, we find them trying to stay cool and moist underneath the seaweed. They cling tenaciously to the rocks with their golden threads spindling out like remarkably strong anchors. There used to be literally buckets of mussels along the Maine coast, but in recent years they have been less prevalent. Now, if you want to buy them for dinner, you’re more likely to buy farmed mussels than wild harvested.  

Blue mussels (Mytilus edulis) have lovely blue shells as their name suggests. The blue is on a thin outer layer of the shell called the periostracum that covers over the otherwise pearly white shell. This is what you see inside of the shell. These lovely blue shells can be found throughout the North Atlantic coast. Other closely related Mytilus species can be found throughout the world. Even in northern Maine, you can find Mytilus trossulus, a similar but less meaty species.  

Aside from their blue color, one of the defining characteristics of these mussels is the way they form into clumps, sticking together with their byssal threads. These clumps can be composed of just a few mussels or sometimes larger groups that are all stuck together. Not only does this stick the mussels together, but it provides sticking points for lots of other marine life. Seaweeds attach their holdfasts to the shells as do small hangers-on like barnacles and lady slippers, making the shells often look more speckled than smooth.  

Creating habitat is just one of the ways that blue mussels are critical components of the intertidal ecosystem. They also help to filter the water and rid it of bacteria and other pollutants. They’re not just tasty to humans, but are also an important food source for predatory snails that drill into their shells, sea birds that crunch them with their beaks or drop them onto rocks, and sea stars that pry them open and slurp up their insides. This has given blue mussels the designation of being a keystone species – one that, if taken out of the system, will cause others to disappear much like if you take the top piece out of an arch the whole structure comes tumbling down.  

Blue mussels’ tastiness has been part of the reason for their decline. Human harvest of wild mussels used to be a lucrative business. At its peak in 2003, around 25 million pounds of whole product were harvested and the fishery was valued at over $4 million. Now, the harvest is closer to 9 million pounds and valued at $2 million. Humans aren’t the only ones that were making too big a dent in the mussel population, though. The invasion of European green crabs into Maine’s intertidal resulted in massive munching of blue mussels. These nasty predators don’t have many predators themselves, resulting in far too many in the population which in turn ate loads of shellfish of all kinds.  

Aside from predation, another factor at play is changing ocean conditions. The Gulf of Maine is warming more quickly than 99% of the world’s oceans. For a species that favors cooler waters, this is not helpful. Warmer waters also mean changing circulation patterns. Blue mussels depend on larvae coming from the southern part of their range and with the Gulf Stream pushing further south and moving against them as they try to move back north to settle into Maine’s intertidal areas, they have a tough time. Another challenge is the change in the chemistry of the water which has become more acidic. This acidic water eats away at the shells and can cause pitting on the surface that you can sometimes see on shells on the beach.  

While it is discouraging to see these iconic species in fewer numbers in the wild, there is some great work being done to grow them via aquaculture. Like many shellfish, mussels don’t need much aside from cool nutrient-filled water and a surface to attach to in order to grow well. Rafts and ropes are typically used in floating mussel farms. In some cases, they are even being grown along with native kelp species that can help improve conditions by taking up Carbon dioxide and thus leaving the water less acidic. This is known as the “halo” effect and is being studied by several scientists including those at the Bigelow Lab in Boothbay to determine the best way to co-grow these species. 

There is hope that these “farmed” but native species could help replenish the wild population. Blue mussels are critical components of the intertidal ecosystem that don’t just have lovely shells, but also serve many important functions from habitat to filtration – and, of course, as food.