Across the Hawaiian archipelago, ‘opihi appear to be growing flatter and darker to evade our grasp
‘Opihi are limpets native to Hawaii and they are, by all accounts, delicious. Picking them out from the wave-pummeled lava rocks is so hazardous however, that indigenous Hawaiians call it the “fish of death”.
The limpet must be worth all the danger because Hawaiians have taken that risk for a millennium, collecting ‘opihi for food, and to fashion their shells into jewellery and sharp-edged tools.
New research suggests that the long history of harvesting appears to be having a distinct effect on ‘opihi evolution – they’re becoming harder to pluck off the shoreline. Unfortunately, this amazing bit of human-proofing may now be making them more vulnerable to climate change.
Across the Hawaiian Islands, ‘opihi shells vary from flat to tall and from light to dark. Biologist Ashley Hamilton 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, 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 – 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.
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 – darker shells absorb more light from the sun, heating up the shellfish inside. Flatter ‘opihi may not be able to produce as many eggs as taller individuals either, which have more room for ovaries under their shells.
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.