Why won't they spawn?

If you go out and pop a bunch of mussels off the rock every month, and then dissect out the gonads separate from the rest of the body tissue, and then you dry those two blobs of tissue, and finally compare the mass of gonads versus the total body mass of the mussel (sans shell), you get a graph that looks like this. Now the impressive part is that not only are the mussels at this site 40% gonad tissue by weight, but they've been accumulating that tissue since April of 2005. They're coming up on two years between spawning events.

The moral of this story? If someone tries to convince you that Mytilus californianus dribbles out gametes all year around, you should kick them in the shin.