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Inhabitants of Oregon's Tidepools: California Mussels

Attached to the rocks and to each other, California mussels offer different kinds of habitats.

California mussels are bivalves (clam-like mollusks with two shells) that are about twice as long as wide, and nearly round in cross-section through the middle.  These mussels have a blunt, rounded point at the bottom end, and are curved and somewhat flattened at the top end.  Heavy, curved growth rings lay out from bottom to top of the shell, intersected by length-wise, radiating ridges; the hinge between the two shells is on one side, about a quarter of the way up from the bottom.  Older, larger shells are heavy and tough.  Overall, the shell is dark blue, but wears down on the raised parts (on some of the ridges and some places where the rings and ridges intersect) to a honey-tan, making the shell look vaguely plaid.  Usually, the older part of the shell, at the blunt bottom, wears down to expose light blue under the surface and sometimes even exposes the white mother-of-pearl beneath.  Occasionally, the blue-black edge of the animal can be seen between the shells.California mussels produce a liquid adhesive from a gland opposite the hinge; a special grove in the mussel’s foot directs the adhesive to connect to something hard. Exposure to the seawater makes the adhesive solidify as the foot is retracted, and the mussel tightens down the line to keep firmly in place.

Attached together, the mussels form beds at a characteristic level in the intertidal. Many other animals and some seaweeds attach to the mussel shells, including barnacles, calcareous tubeworms, and sea palms.

Since the mussels are bluntly pointed on the bottom, there’s some space between them even when the sides are packed together.  The spaces under and between the mussels are a haven for many animals, protecting them from crashing surf at high tides and drying sun at low tides, as well as protecting them from many predators.

Take care to not dislodge the mussels and expose the life beneath.

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