How mussels quickly release their tethers to the shore
A mussel attaches to a rock with its byssus, colloquially known as a beard, in Ocean Beach in California.
Brocken Inaglory, CC BY-SA 3.0
The threads that mussels use to attach themselves to rocky seashores are special. Extraordinarily strong for their size, the threads are also self-healing and able to stick to wet surfaces—a rare combination of properties that has made them a special case study for chemical engineers trying to make effective underwater glues (see the Quick Study by Bruce Lee, Physics Today, November 2022, page 62 ). Yet, for all their sticking power, the mussels can let go of the cluster of threads, more formally known as the byssus, at a moment’s notice. New research by Jenaes Sivasundarampillai and Matthew Harrington of McGill University and their colleagues sheds light on how mussels might achieve the unusual combination of strong hold and quick release.
Built by the mussel out of proteins, the threads of the byssus adhere to a hard surface and connect back to the mussel through a single stem root that is held by the mussel’s foot. As uncovered in the new study, a closer look reveals that the byssus stem unfolds from its cylindrical structure into dozens of thin sheets, only 2–3 microns thick. In the mussel’s foot, the sheets interweave with billions of cilia, which look like a dense carpet of tiny fingerlike structures. Like two phone books with their pages interwoven (see the Quick Study by Kari Dalnoki-Veress, Thomas Salez, and Frédéric Restagno, Physics Today, June 2016, page 74 ), the abundance of weak connections between the cilia and the sheets of the byssus combine to form a strong bond.
Research from the 1960s and 1970s had shown that cilia in a different part of the mussel—its gills—change their oscillation speed when exposed to the neurotransmitters serotonin and dopamine. Serotonin triggers faster movement of the cilia; dopamine slows them down. After the lab’s nanoscale imaging had uncovered the role of cilia in the mussel–byssus connection, Sivasundarampillai, a graduate student in chemistry, wondered if those neurotransmitters might also play a part in the hold-and-release mechanism.
Light-microscopy image of a stained section of the byssus stem root, which consists of thin wavy sheets surrounded by cilia (red). The root is embedded in the living tissue of the mussel (light blue).
Jenaes Sivasundarampillai
When Sivasundarampillai injected serotonin around the mussel’s byssus, “it popped right out,” says Harrington, the principal investigator. The effects on the mussels’ grip scale with dosage—at a high enough dose of serotonin, the lab couldn’t even measure the strength of the connection, because it would just fall apart. With a high enough dose of dopamine, the cilia were immobilized, and the bond strength almost quadrupled, resisting 20 newtons—about 4.5 pounds of force. The researchers believe that the cilium’s movement disrupts the weak bonds, allowing the byssus to slide out with ease.
Harrington hopes those insights into living–nonliving interfaces from mussels could help engineers working on the design of biomedical implants. Making safe, durable, and reversible connections between soft living tissue and hard materials poses unique challenges that will require creative solutions. The mussel byssus has already inspired new approaches to building strong, self-healing polymers. (J. Sivasundarampillai et al., Science 382, 829, 2023 .)