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Gastronomes covet ‘opihi—tasty limpets native to Hawai‘i. Picking them from the surface of the wave-pummeled lava rocks where they cling, however, is so hazardous that Indigenous Hawaiians have a saying about it: “he iʻa make ka ‘opihi”—the ‘opihi is a fish of death.
Hawaiians have taken that risk for a millennium, collecting ‘opihi for food, and to fashion their shells into jewelry and sharp-edged tools. And as new research suggests, that long history of harvesting appears to be having a distinct effect on ‘opihi evolution: they’re becoming even harder to pluck off the shoreline. Unfortunately, their human-proofing adaptations may be making them more vulnerable to climate change.
Across the Hawaiian Islands, ‘opihi shells vary from flat to tall and from light to dark. Ashley Hamilton, a biologist at Texas A&M University-Corpus Christi and the study’s lead author, thinks people’s gusto for ‘opihi is behind where their varied colors and shapes appear.
Hamilton and her colleagues initially speculated that ‘opihi with lighter, taller shells would have an advantage in the hot Hawaiian sun because they would be better at reflecting sunlight and dissipating heat. Instead, after collecting 402 ‘opihi from across the archipelago and measuring their shapes and colors, the researchers found that flat, dark limpets dominate on the main Hawaiian Islands. These ‘opihi, the scientists believe, are better camouflaged against lava rocks and harder to dislodge.
This points to an evolutionary trade-off, Hamilton says: on the six main Hawaiian islands, the pressure on ‘opihi to avoid hungry humans may have been greater than the pressure to stay cool.
Further supporting that interpretation is the fact that more than 1,000 kilometers away, on the uninhabited Northwestern Hawaiian Islands, ‘opihi are predominantly light and tall. Driven by human influence, Hamilton says, these two types of ‘opihi may be in the process of becoming different species.
“There’s an intimate relationship here between Hawaiians that are relying on ‘opihi for food and ‘opihi that are living with their new primary predator: humans,” says Chris Bird, a biologist at Texas A&M University-Corpus Christi and coauthor of the study. “Usually this story ends in extinction. But here it’s ended in potentially the formation of a new species.”
Hunting and fishing have been linked to evolutionary changes in wild animals before, like the shrinking horns of trophy rams and the dwindling body sizes of overexploited fish. But Andrew Hendry, an evolutionary biologist at McGill University in Quebec who was not involved in the research, says more work is needed to confirm the relationship between human harvesting and the limpet’s morphing shells. If it’s confirmed, he says, this is the first case he’s seen in which harvesting by Indigenous people led to physical changes in an organism.
Hamilton says confirmation could come by measuring ‘opihi shells from centuries-old middens to chart their evolution as humans showed up and see how the shells “match up to what’s present on those islands today.”
All this shell tweaking, however, may have backed the ‘opihi into an evolutionary corner. As climate change warms the islands, flatter, darker animals are at a disadvantage. Flatter ‘opihi may not be able to produce as many eggs as taller individuals, which have more room for ovaries under their shells, Bird says. Darker shells also absorb more light from the sun, heating up the shellfish inside.
To protect them for future generations, Hamilton suggests people leave the taller, more heat-tolerant ‘opihi alone and focus on harvesting the dark, flat ones—exactly what the limpets may have evolved to avoid.