The surprise mid-June heatwave not only wilted as much as a third of Monterey County's lettuce crop, it also caused a mass die-off of mussels all the way up the California coast.

Mussels were found dead, some cooked right in their shells while stuck to rocks, in spots all along the coast after the Northern California heatwave that lasted from June 8 to June 11, with some of the warmest temperatures recorded on the 10th and 11th. As the journal Bay Nature reports, the die-off was easily visible in tide pools at Bodega Head, along with bleached algae that had also baked in the heat. Also, the journal writes, "Further reports came in of die-offs around Bodega Bay at Dillon Beach and Pinnacle Gulch, at Sea Ranch, and at Kibesillah Hill north of Fort Bragg."

Bodega Marine Reserve research coordinator Jackie Sones wrote about the sight on her blog, saying, "A large percentage of the mussels were open and gaping, some were empty and some still had tissue inside." Sones walked the beach for a quarter mile and found more dead mussels all around the Bodega Head area.

Mussels live in tidal areas where they are not constantly submerged in water. Healthy mussels stay closed while gripping seaside rocks with strong threads — and just as you know a mussel is cooked, when you're cooking them, when it opens, dead mussels gape open in nature as well.

As NBC Bay Area reports, the temperature at Bodega Bay hit 86 degrees on June 10, a full 24 degrees above average, and it hit 81 on June 11, 19 degrees above average. In those conditions, at low tide, the mussels were exposed to unusually intense sun and heat over at least two days.

Bay Nature reports that it's nonetheless an unusual phenomenon. Kelp, seabirds, and mammals have been known to die off in warmer than usual ocean conditions, such as those caused by El Nino cycles. But, "University of British Columbia biologist Christopher Harley said it’s still quite rare to document marine plants and animals dying from hot air." During a similar heatwave event in 2004 in the same area of Bodega Bay, Harley had logged temperatures on the rocks where the mussels live and found that while the outside air was only 70 degrees, the temperature on the rocks themselves was 97 degrees, which was hot enough to kill the mussels.

While it can't be confirmed beyond a doubt whether heat caused the mussel die-off, or how extensive it was, the cause and effect appear pretty clear to marine biologists. As this 2017 story from PRI discusses, mussels are a perfect "canary in a coal mine" for climate change given how vulnerable they are to changes in ambient temperature. Because of their typically dark or black color, they absorb heat while sitting exposed at low tide, and they wait for the tide to come in to provide them with oxygen and the plankton they eat, and to cool them down.

As Northeastern University marine biologist Brian Helmuth put it, "You are sitting there in the blazing sun, you’re not going to be able to move. You can’t escape the heat, you can’t escape the sun, you can’t go into a crevice like something like a crab." He continued, "The closest thing I can think of to describe what that’s like, is, if you reached down into your chest cavity, you rip out your lungs, and you hold them above your head, and you hope to God that the wind blows because if it doesn’t you’re going to suffocate."

Photo: Jackie Sones