In the Bering Sea, once-rare crabs have multiplied at an astonishing rate: there are now more than 13 times as many.
These animals are hybrids. They result from the crossbreeding of two closely related species, the snow crab and the Tanner crab. Researchers identify them in particular by the shape of their mouths, which differs from that of either parent. For a long time, they accounted for only a small share of the crabs observed off Alaska.
A snow-Tanner hybrid crab captured during bottom trawling conducted in 2026 in the eastern Bering Sea. Hybrid crabs display intermediate morphological characteristics, such as a curved mouth, compared with the parent species, the snow crab and the Tanner crab.
Credit: NOAA Fisheries / Leah Zacher
Everything changed after the marine heatwave of 2018 and 2019. In this usually cold region, the water temperature remained abnormally high. Snow crabs were particularly affected. Their population collapsed, with billions of individuals disappearing.
A few years later, scientists observed the opposite phenomenon in hybrid crabs. Their numbers had increased more than 13-fold. In 2025, they accounted for 41.7% of males of commercially harvestable size according to fishing criteria, compared with less than 10% previously.
At first glance, one might think that warmer water encouraged more crossbreeding between the two species. However, the analyses by the American researchers do not support this explanation. They did not show that adults of the two species were meeting more often to reproduce, nor that the heat directly benefited the hybrids.
Close-up view of the face of a Tanner crab (Chionoecetes bairdi), a species related to the snow crab.
Photo: Wikimedia Commons.
The explanation is more likely linked to the disappearance of snow crabs. They were so numerous that they occupied much of the available space and shared the same food resources. They could also eat smaller crabs. Unable to tolerate the warming of their environment, their collapse would have left more food and space for the survivors, including hybrids and Tanner crabs.
The researchers used statistical models to examine this chain reaction. According to their calculations, the indirect effects linked to the disappearance of snow crabs would explain about 76% of the observed increase in Tanner crabs and hybrids combined.
This situation now poses a problem for fishers and authorities. Hybrids can be caught along with snow crabs, but fishing regulations did not anticipate such a significant presence. The quantities that can be harvested without threatening populations therefore need to be reassessed.
This story shows that a marine heatwave does not merely directly affect sensitive species. By causing some animals to disappear, it can also benefit their neighbors, sometimes in completely unexpected ways.