I spent time last week driving up the coast, checking out all the beaches and birding spots I have recently been avoiding as I try my best to socially distance. My first stop was Wells Harbor. I love this place. It is a beautiful access point to the Webhannet estuary, and I have a long history of bringing students down to the dock to hang off the edge and learn about the diversity of marine life that attach to that artificial border between land and sea. On this particular trip I was watching the blue mussels (Mytilus edulis): how they cling to the dock, shoulder to shoulder with green sea lettuce and other seaweeds. Their shells agape, filtering plankton and detritus (dead organic material) out of the water column, mussels seem innocuous but play an incredibly important role in our coastal ecosystems.
Blue mussels are foundation species. Foundation species are organisms that play a critical role in structuring a community. Corals and beaver are foundation species for obvious reasons. Mussels are foundation species because their presence influences the diversity of intertidal habitats-when mussels are abundant there are higher numbers of other intertidal species. If you think about it, having masses of mussels attached to intertidal rocks creates a reef—a complex environment that provides shelter for some species and novel attachment sites for others.
In addition to their role as foundation species, mussels are filter feeders-they clean the ocean around them-filtering out plankton (their food) bacteria, heavy metals and toxins (I have a friend who refuses to eat all filter feeders for this reason).
Watching the mussels packed in along the edge of the dock I had a hard time remembering that these bivalves can move if they want to. They have a slender, brown foot that they use to hold onto surfaces, but the more permanent attachment is via a strong thread-like anchor called the byssal thread. These are very cool-the mussel secretes the tread as a liquid from a gland near the foot, the threads then harden upon contact with water. Byssal threads bind a colony of mussels together–they tend to be gregarious and often grow in huge dense colonies. This, however, can be their undoing as overcrowding can result in mortality – as the mussels stack up the underlying mussels are often starved or suffocated causing the entire colony to detach and get washed away.
Hang your head over a dock and watch mussels feed. It is absolutely mesmerizing, hypnotic, meditative. If you look carefully you’ll see two short siphons (tubes) with wavy edges extending from the inside of each mussel-these are the incurrent and excurrent siphons, they bring in water, from which the mussel extracts its planktonic food and expels wastes. If a mussel doesn’t like its location, it can move by severing its byssal threads, extending its foot and inching along to a better locale.
Sadly blue mussel populations are in decline in the Gulf of Maine as a result of a combination of factors; warming ocean temperatures, increased harvesting by humans, and predation by invasive species. For example, since I moved back to Maine in the mid-1990s the invasive Asian shore crab (Hemigrapsus sanguineus) has also moved in. One study in Long Island Sound found that Asian shore crabs accounted for as much as 25% of the blue mussel mortality at the site studied.
Human harvesting has also increased. Maine and Massachusetts account for the majority of wild blue mussel harvesting on the East Coast. When I moved back to Maine in the early 1990s I could go down to the coast and harvest large numbers of mussels. Now, the abundance isn’t there, I don’t harvest because I feel like I might be removing the last mussels from a given location. Human harvesting coupled with warming ocean temperatures might be enough to extirpate blue mussels from the Gulf of Maine.
Most studies of biodiversity decline indicate that we are in the midst of the sixth mass extinction, an extinction event comparable to the one that saw the end of the dinosaurs. This is the first mass extinction to be caused by a single species (humans). I find this hard to comprehend, I feel like if we’re in a mass extinction animals and plants should be dropping dead all around us on a daily basis. I am usually able to convince myself that it really isn’t that bad….and then I personally experience something like the decline of the mussels and realize that it is.