I've been monitoring floating docks at MacMillan Wharf in Provincetown for a few years now, and each season has been slightly different and yet broadly similar regarding the associations of different species. One of the abundant species in the marina is the common blue mussel, Mytilus edulis. It is abundant under the floats and variable in number on the sides depending on competition with other species and cleaning activities by marina personnel. Each Spring, cleaned float surfaces provide opportunities for larval settlement.
Mytilus normally lives on rocky shores in the intertidal zone attached to rocks and other hard substrates by strong, somewhat elastic structures called byssal threads. These threads are secreted by byssal glands located in the foot of the mussel. The mussels are firmly attached, but they have the ability to detach and reattach to the substrate allowing them to reposition themselves relative to their neighbors. They are usually found clumping together on wave-washed rocks, which helps hold the mussels firmly on the rocks against the force of the waves.
When the mussel larva first settles, it first secretes a thin shell and then develops an elongated foot with byssal glands. If the substrate is suitable, it will metamorphoses into a juvenile form and attach byssal threads. This attachment is a prerequisite for the foundation of the blue mussel population. On the sides of the Provincetown docks, large numbers of mussels will often settle on a clean surface and form masses of juvenile mussels which are striking because most of the individuals are about the same size, indicating that they settled around the same period of time. These juvenile beds are apparently seasonal, because the beds do not mature over winter, and new beds of young mussels are seen the next Spring.
Mussels can move
slowly by extending a byssal thread, using it as an anchor and then shortening
it. A thread is formed by the foot by creating a vacuum at the contact site and secreting a foamy mixture of proteins into the formed chamber, producing sticky threads about the size of a human hair.
Mussels and ascidians frequent colonize together on floating docks, ropes, and fishing gear. Ascidians compete for substrate, limiting colonization by mussels, and colonial ascidians will grow over the surface of the shell, limiting growth and food supply. Co-colonization and competition of ascidians with mussels has had an impact on mussel aquaculture throughout the northeast.
Juvenile Mussels Beds on the Sides of Floating Docks
Mussel beds of young Mytilus edulis on the sides of floating docks at MacMillan Wharf, Provincetown, August, 2013. Individual mussels averaged about 1-1.5 cm long. In July, the mussels were covered by the colonial ascidian Diplosoma listerianum (which was extremely abundant), but most mussels were clean in August. The soft ascidians were presumable removed by predation, an idea support by the observation in August of torn sections of Diplosoma colonies pulled off the substrate. In these photos, small colonies of orange Botylloides violaceus and green-grey Diplosoma can be seen at the water surface, and two small colonies of Botryllus schlosseri (star tunicates) can seen deeper in the water in the top photo. (Images in this post were taken with a 14 megapixel camera and can be enlarged without losing detail by zooming into the photo).
Mixed Floating Dock Communities of Mussels, Colonial Asicidans, and Algae
Botrylloides violaceus, green algae (Ulva), red algae (Neosiphonia harveyi), bryozoans (Bugula neritina), and other ascidians (Botryllus schlosseri, Diplosoma listerianum, Didemnum vexillum) form a colorful blend of species along with mussels. Green-gray areas in top photo are Diplosoma. Milky white areas in both photos are probably Didemnum.
Mussels and Orange Colonial Ascidians Cohabitate Hanging Ropes
Outside the juvenile beds, mussels grow and mature in small groups along with ascidians, especially Botrylloides violaceus. Growth can become so extensive that the colony forms orange "hanging gardens."
LINKS:
A Snails Oddessy: Learn about Mussels. Anchoring.
Van Winkle, W. Effect of environmental factors on byssal thread formation. Marine Biology 7: 143-148, 1970.
Lane, DJW, AR Beaumont, and JR Hunter. Byssus drifting and the drifting threads of the young post-larval mussel Mytilus edulis. Marine Biology 84: 301-308, 1985.
