09 Mar UCN–SECOS Research Suggests Strengthening Management of the Purple Rock Crab After Discovering Low Genetic Diversity

A novel study by the Universidad Católica del Norte and the Instituto Milenio en Socio-Ecología Costera (SECOS) revealed that the purple rock crab shows minimal genetic differences along Chile’s coastline. Researchers propose incorporating these findings into the species’ management as a benthic resource, since it appears to form essentially a single large population across the coast, which could make it more vulnerable to fishing pressure and environmental change.

An emblematic crustacean widely distributed from Ecuador to southern Chile, the purple rock crab (Homalaspis plana) is a benthic resource with an average annual catch of about 137 tons in Chile over the past 30 years, making it an important component of local economies and artisanal fisheries.

Despite its importance for hundreds of fishing coves and coastal communities—and its widespread use in Chilean cuisine—there had been no formal assessments of the genetic structure of its populations along the coast until now. Combined with relatively weak control and management mechanisms for the species, this raises concerns about the long-term sustainability of its harvest.

In a recent study published in the journal Global Ecology and Conservation, a team of researchers from the Universidad Católica del Norte and SECOS set out to fill this information gap.

“We discovered that the genetic diversity of the purple rock crab is low compared to other crab species globally—it falls within the lowest quartile relative to other species,” explains Pilar Haye, researcher at the Department of Marine Biology at the Universidad Católica del Norte and associate director at SECOS. “This could potentially make it more vulnerable to fishing pressure or environmental change, since genetic diversity is the source of populations’ adaptive capacity,” she notes.

For the study, the team analyzed 222 purple rock crab specimens collected along nearly 3,500 kilometers of coastline, from Iquique in northern Chile to Los Molinos, Los Ríos, Chile in the south. Researchers used mitochondrial genetic markers (COI) and nuclear microsatellites, tools that allow scientists to evaluate both genetic diversity and the degree of connectivity between populations.

“We used different molecular markers, such as mitochondrial COI—excellent for assessing diversity—and microsatellites, which are ideal for determining whether there are groupings or genetic differentiation. This allowed us to obtain an integrative view of different evolutionary processes,” Haye explains.

At the same time, the study found that at a broad geographic scale the populations are genetically connected along the entire Chilean coast, forming what scientists call a “panmictic” population—that is, one without major regional genetic divisions.

However, beneath this apparent connectivity, the analyses revealed a key phenomenon: many local populations depend heavily on their own recruitment, meaning that most of the new crabs arriving in an area originate from that same area rather than from distant regions.

“In the study we suggest that the low diversity may be due to sustained fishing pressure and also to a particular characteristic: most individuals at a site originate locally, while only a small proportion come from distant areas—even though the species has larvae that remain in the water column for long periods. This species likely has specific larval behaviors that have not yet been studied,” the researcher explains.

Implications for Artisanal Fisheries

Although at the national scale the species appears to form a single large population, the study warns that local populations may be far more fragile than landing statistics suggest. If a fishing cove or specific area loses its reproductive individuals, replenishment from other areas may be limited, increasing the risk of silent local collapses.

Nicolás Segovia, data manager, SECOS researcher, and co-author of the study, elaborates:

“The management approach for these organisms—common for all crab species, not only the purple rock crab—comes from a decree issued more than 35 years ago. It requires the release of egg-bearing females (ovigerous females) and sets a minimum size of 120 mm of cephalothorax. This results in a constant, sustained, and non-random extraction of individuals with certain characteristics, favoring females and smaller individuals, which could reduce genetic diversity,” he explains.

As a result, the current fishing practices combined with the study’s findings are particularly relevant in a context of climate change and sustained fishing pressure, where genetic diversity is crucial for species to adapt and recover from environmental and human disturbances.

Finally, the group of authors emphasizes that integrating genetic information together with fisheries, social, biological, and ecological aspects into fisheries management provides practical tools to ensure the long-term sustainability of a resource that supports local economies and coastal traditions across much of Chile.