Exposed by low tide and bereft of a cooling breeze, the mollusks overheated to the point of cooking.
Context is everything when it comes to cooked mussels. In a bowl, served in a white wine-garlic sauce with a crusty baguette for dipping, they're a good thing. On a beach, however, still adhered to rocks, is not where you want to encounter cooked mussels.
This is precisely what marine researcher Jackie Sones saw in mid-June when visiting Bodega Bay in northern California – "scores of dead mussels on the rocks, their shells gaping and scorched, their meats thoroughly cooked." The unfortunate mollusks had succumbed to temperatures that were unusually hot for that time of year.On June 11, it was 75F/24C outside, and the breeze that usually sweeps in off the sea ceased as well. Marine ecologist Brian Helmuth is cited in Bay Nature:
"On a 75 degree Fahrenheit day, the tissues inside a marine creature glued to a rock out of the water might rise to 105 degrees. The animals try to vent the heat building up inside of them but can’t without a breeze to carry it away. The mussels’ black shells trap even more heat. 'They were just literally cooking out there,' Helmuth said. 'Unfortunately this was the worst possible time.'"
What made this situation so unusual is that the heatwave occurred early in the summer season, when tides shift in the late morning and early afternoon. This exposes the mussels to more direct sunlight than they'd normally have later in the year, when tides shift in the early morning or late at night, reducing the danger for tide pool dwellers.
As Eric Simons writes in Bay Nature,
"The more unusual early-season heat waves there are, the greater the chance they line up with those mid-day low tides, the harder it gets for mussels. Future die-offs could rewrite the ecology of California’s rocky shoreline, where mussels are a foundation species that hundreds of other animals depend on."
It's an alarming reminder of how present climate change already is; it's no longer a nebulous prediction for the future. In Simons' words, it's also indicative of the fragility of so many marine creatures, and how "a lot of ecosystems exist really close to the edge of what they can tolerate."