An 'opihi came to the Hawaiian archipelago, and evolved into three distinct species, each with specific characteristics built for their specific habitats.
You may not have thought much about 'opihi, but there's a lot to know about these limpets--besides that they are growing so rare due to overharvesting that they're priced like gold. That's because `opihi is a prized item at any Hawai`i gathering--a salty, crunchy condiment served raw, and delicious grilled with a little butter and garlic.
But we digress. Back to the science.
One thing: There's nothing to prevent their seagoing larvae from invading other shores, but Hawai`i's three edible `opihi are only found in Hawai`i.
Another: Since the original migrants could readily cross with others of their species, how and why did they evolve into three distinct species.
“Speciation seems highly improbable under these circumstances,” writes author Christopher E. Bird, of the Hawai`i Institute of Marine Biology at the University of Hawai`i. His paper is “Morphological and Behavioral Evidence for Adaptive Diversification of Sympatric Hawaiian Limpets (Cellana spp.)” The paper is in the journal Integrative and Comparative Biology: Integrative and Comparative Biology, pp. 1–8 doi:10.1093/icb/icr050
But they did evolve, and probably they did it, he argues, due to natural selection—the individuals found environments to which they could adapt, and they and their offspring stayed there.
Still, it seems odd that you can find three different species of 'opihi on the same slab of rock—the exarata or makaiauli baking in the sun above the tide; the sandwicensis, `alinalina or yellowfoot taking the brunt of the wave force at the interface between air and sea; and the talcosa or ko`ele fighting off marine predators below the surface.
Bird argues that those locations below to above the water define their ecological niches: “Overall, C. talcosa is regularly exposed to pelagic predatory fish, C. sandwicensis is subject to the greatest amount of wave energy and mechanical stress, and C. exarata is subject to the most extreme temperatures and desiccating conditions.”
The ko`ele is sometimes called the kneecap `opihi. It's much bigger than the others, and its shell can be very thick—perhaps a trait developed to protect it from marine snails and predatory fish.
In another just-published article, this one in Molecular Ecology, Bird teamed up with Brenden Holland, Brian Bowen and Robert Toonen, all of the University of Hawai'i. This paper is “Diversification of sympatric broadcast-spawning limpets (Cellana spp.) within the Hawaiian archipelago,” Molecular Ecology (2011) 20, 2128–2141 doi: 10.1111/j.1365-294X.2011.05081.x
They say genetic studies suggest that the limpets first arrived in the Hawaiian archipelago between 3.4 and 7.2 million years ago—back in a time when Kaua`i, Ni`ihau and Nihoa were young islands
They probably arrived from the vicinity of Japan as free-floating larvae, and the original migrant was probably an inhabitant of the high shoreline, like Cellana exarata. Their nearest relative outside Hawai`i are probably the Japanese limpets, Cellana nigrolineata, C. mazatlandica and C. grata.
As little as six years ago, when this article was written, those things weren't known. http://the.honoluluadvertiser.com/article/2005/Jun/01/ln/ln08p.html
The researchers found that the Hawaiian 'opihi technically can hybridize but rarely do. They also found that they have different ranges throughout the island chain. All three are found in the main Hawaiian Islands, but talcosa doesn't go beyond Kaua`i and Ni`ihau. Sandwicensis stops at the basalt bastion, La Perouse Pinnacle. And exarata extends to the northewestern most Hawaiian land that still has volcanic rock above the surface, Gardner Pinnacles.
To confuse things, there are a couple of other `opihi reported from Hawaii. One is the small false `opihi, `opihi `awa, or Siphonaria normalis, which isn't eaten and isn't closely related to the others. The other is sometimes called Cellana melanostoma, which characteristically has steep sided shells, but genetic tests suggest it's a form of exarata.
© Jan TenBruggencate 2011