A live 'Kona crab' (Ranina ranina) in a Honolulu fish market is considered a rare speciality in Hawai'i.
There is just one native crab in Hawai‘i that is commercially harvested in the islands: the mysterious, poorly understood “Kona crab.” This elusive species remains one of Hawai‘i’s least exploited fisheries. It is rarely found in restaurants, and when it appears in specialty markets, it can sell for upwards of $100 for each crab.
Why is it so expensive and rarely available? HPU Associate Professors Brenden Holland, Ph.D. and Matt Iacchei, Ph.D., together with Master of Science in Marine Science student Jonah Sheridan, are working to address these questions and more. Holland states that the high cost is likely due to a few factors, including low demand and the challenges in catching Kona crabs, as they are not harvest in large numbers. The crab’s meat is described as sweet and delicate, like snow crab, and they have a flattened, squat red shell around the size of a Dungeness crab. The edible meat is mainly found beneath the hard main shell, while the claws and legs offer little meat.
Kona crabs are also known as “red frog” or “spanner” crabs in some regions and are found from the Indian Ocean throughout Southeast Asia to Hawai‘i, all representing a single species: Ranina ranina. Although the species is distributed across such a vast range, Kona crabs have long been a part of Hawai‘i’s natural marine environment. One of Holland’s hypotheses is that the R. ranina may not represent a single species at all, but rather a complex of different species spanning multiple countries and continents across two ocean basins.
“In biology, when you have something that doesn’t disperse very far but is found across a wide range, it’s often not just one species but a series of different species—and no one has studied the Kona crab this way before,” Holland said. “We are sequencing DNA from crabs across its range to confirm its taxonomy to determine whether we’ve been calling them by the right name.”
Understanding the crab’s taxonomy is not just an academic exercise—it has real implications for conservation and management. Taxonomy helps determine if a species found in one place has the same basic ecology as those found elsewhere, and often, species with such widespread distributions exhibit ecological differences in reproductive output, growth rates and diet. In addition to wide-ranging sampling of crabs, this project will also make molecular comparison of Kona crabs from multiple populations in the Hawaiian Islands. For harvested species, understanding population connectivity is crucial for understanding resiliency of regional populations and establishing effective fisheries regulations.
Though prized for its flavor, the Kona crab is difficult to find. Holland has visited many fish markets in Honolulu to inquire about the crab’s availability, sources, pricing, and popularity. Vendors report that they do not get them often, and when they do, they can be somewhat difficult to sell because of their high price and limited supply. There is a fishery off Kona—the namesake of the crab—but the catch there is also often small.
NOAA Fisheries has done a stock assessment of the Kona crab in Hawaii and has determined that it represents an underutilized fishery. Researchers are exploring whether this fishery could be sustainably expanded, but catching the crab presents significant challenges.
“A NOAA-funded project conducted by John Wiley at Poseidon Fisheries attached cameras to the Kona crab hoop nets a few years ago to observe the crabs’ behavior as they approached the nets and were caught. Surprisingly, the crabs were often torn apart by sharks, rays, and pufferfish when they arrived on the hoop nets. The crabs were unable to escape by burrowing into the sand because they were trapped on the nets. Many were observed being eaten alive. This may be one reason why it is difficult to catch a lot of crabs,” Holland concluded.
Not much is currently known about the Kona crab’s life cycle, population size, or reproductive habits. As Holland explains, understanding how these crabs move between locations is critical. “Connectivity between populations is important to understand. Can Kona crabs travel between two different locations, or will they stay in just one place?” Holland said. “Genetic techniques will tell us how readily they disperse based on the genetic diversity between populations.”
Holland and Sheridan are already hard at work collecting samples and beginning to analyze DNA sequences from Kona crabs across Hawai‘i. “We look at haplotype diversity and partitioning of variation between populations. We have samples from various locations across Hawai‘i, and we are working on getting samples from across the world as well,” Holland said.
The team of Sheridan, Holland and Iacchei is planning to apply for additional funding to support their work, including efforts to study the crabs in controlled environments.
Although the Kona crab is relatively unknown in Hawai‘i, it is in high demand in other regions. “Vietnam and Australia have a large demand for the Kona crab, and the market and annual tonnage is much higher than in Hawai‘i,” Holland said. “Locally, many people have never tried it or seen it in the markets, or on menus.”
Adding to the challenge, Kona crabs are typically sold live and stored on ice, unlike other crabs that are cooked and preserved for sale, extending shelf life. This makes it more difficult to keep them fresh and distribute them widely. But Holland believes that if more people become familiar with the Kona crab, it could become an important part of Hawai‘i’s diet—and help relieve pressure on other, more heavily fished species.
As research continues at HPU, Holland hopes to uncover the answers needed to support a sustainable Kona crab fishery in Hawai‘i.
“We want to continue to study the Kona crab in detail, to understand and get as much information about it as possible, and to encourage the fisheries to catch greater numbers of Kona crab so people can eat them as a sustainable food source,” Holland said.
