In a dedicated clean lab at UC Santa Cruz, researchers carefully ground 59 black abalone shells into powder and painstakingly recovered fragments of ancient DNA—a technique no one had ever attempted with mollusks before. What they found inside those shells rewrites the story of one of California's most tragic ecological collapses, and offers an unexpected path toward recovery.
Black abalone once carpeted the rocky shores of California by the millions, sustaining Indigenous peoples along the coast for thousands of years and anchoring a thriving 20th-century commercial fishery. Then, in 1985, withering syndrome—a bacterial illness—appeared along the Southern California coast. Within just a few years, it killed roughly 99% of black abalone across their range. The species was added to the U.S. Endangered Species List in 2009, and today's critically endangered survivors cling to a handful of intertidal sites along California and Mexico's coasts.
Scientists expected the genetic aftermath to be devastating. When Brock Wooldridge and his team at the UC Santa Cruz Genomics Institute compared pre-collapse genomes from their ancient shells to genomes from modern survivors, they anticipated seeing the classic warning signs of a species in severe trouble: drastically reduced genetic diversity, mounting inbreeding, and growing isolation between populations. They saw none of that. Genetic diversity, inbreeding levels, and population structure all looked nearly identical before and after the disease outbreak—a finding published in the Proceedings of the National Academy of Sciences.
The stability likely reflects the short timeframe since the collapse occurred, only 10 abalone generations ago. Computer simulations confirmed this pattern and issued a measured warning: if populations stagnate rather than rebound, signs of genomic erosion could still emerge in generations ahead. But the simulations also revealed something crucial—even modest, sustained recovery appears to be enough to prevent that future decline. The window for action is open.
Most remarkably, researchers spotted signs of natural selection in genes involved in immune defense, hinting that surviving abalone may be evolving resistance to the very disease that nearly wiped them out. The shells that told this story came from museum collections, archaeological sites, and donations from the Amah Mutsun Tribal Band, whose ancestors harvested abalone for generations. One shell from a 1,500-year-old Indigenous site yielded an unusually complete genome, demonstrating that archaeological sites along coastal California—many rich with shells—represent a "seriously underutilized resource" for understanding the past.
These findings arrive as state and federal agencies begin moving black abalone between sites to rebuild populations, a strategy known as translocation. The genetic data suggests that approach has real promise. Because today's abalone still carry most of the genetic diversity from their pre-collapse populations, recovery efforts need not start from scratch. "If managed correctly, we have a meaningful chance of avoiding the genetic issues that plague other threatened species," Wooldridge said. The ancient DNA tells us that California's black abalone are not a lost cause—they are, instead, a species with a genuine chance to return.