Long-forgotten research from the 1940s helped reveal that a mussel bed near Dillon Beach is just as healthy today as it was 80 years ago—even as subtle changes in ocean temperatures have altered the mix of invertebrate species that call it home.
In 2019, scientists at the University of California, Davis, stumbled upon an unpublished, typewritten manuscript in the school’s library written by two graduate students just before the U.S. entered World War II.
“It was like unearthing a time capsule,” said Emily Longman, who led the modern study as a Ph.D. candidate at Davis. “Their report gave us an incredible glimpse into how coastal ecosystems have evolved over time.”
Intrigued, Dr. Longman and her adviser, Eric Sanford, decided to resurvey the same 2.5-by-3-foot section of rocky shoreline, using the original report’s grainy photographs, scrupulous species inventories and hand-drawn maps as their guide.
“The 1941 report was about as good as you can get for historical data,” Dr. Longman told the Light. “Many older studies don’t meet today’s experimental design standards.”
The researchers replicated the original report’s methods. They carefully peeled back the dense layer of inky black mussels, extracting the invertebrates within the quadrat before identifying and counting each specimen in the laboratory. Their findings, published this month in the journal Scientific Reports, confirmed that the mussels remain a dominant and thriving “foundation” species in the rocky intertidal zone.
Much like coral reefs or kelp forests, mussel beds provide vital habitat for myriad smaller organisms—miniature invertebrates like snails, anemones, barnacles, nudibranchs, limpets and crabs—that find shelter and breeding grounds among the tangle of blue-black shells and byssal threads.
As immobile organisms permanently anchored to intertidal rocks where land and sea temperatures are steadily rising, mussels are particularly vulnerable to environmental shifts. In Southern California, entire mussel beds are vanishing as ocean waters warm—so Dr. Longman and her team expected to see signs of decline in Northern California, too.
But as her fieldwork progressed and the worms, chitons, mussels and snails in her collection buckets grew heavier, she realized their hypothesis was wrong. By the end of the summer, Dr. Longman had recorded nine more species living in the mussel bed at Dillon Beach than the original survey had found in 1941.
Yet the study also revealed shifts in species composition that mirror wider patterns linked to climate change: even though biodiversity has increased, the relative abundance has tilted away from cooler-water species once common in Northern California and toward warm-adapted species more typical of Southern California. Those include the California horse mussel (Modiolus carpenter) and a chiton known as Mopalia lionata. Such changes are consistent with slowly rising ocean temperatures documented in nearby Bodega Bay since the 1950s.
As Dr. Longman and Dr. Sanford spent countless hours studying this band of tidal life, they often joked about how much easier it would be if the original researchers—Harvey Fisher and Milton Hildebrand—could lend a hand. “We’re waking up at 4:30 a.m., staring through microscopes until we’re cross-eyed,” Dr. Longman said. “We kept saying ‘We could really use four sets of hands.’”
Then came a twist: Just as they were nearing the end of their survey, they discovered that Dr. Hildebrand was not only alive at 101 but living in Davis. After completing his fieldwork in Dillon Beach, Dr. Hildebrand had been drafted into the Army’s 10th Mountain Division, tasked with rooting out Nazi forces at Germany’s southern border. After the war, he returned to academia and emerged as a respected zoologist—and the mussel survey was all but forgotten.
When Dr. Longman and Dr. Sanford met with him just before his death in 2020, they brought along his typewritten report, which concluded with the hope that the survey might someday prove useful to future researchers revisiting the same patch of rock.