Scientists have long warned about the potentially devastating impact of marine heatwaves, and just a few years ago, the impact of the warming waters potentially led snow crab numbers to dramatically decrease. The consequences of this may have led to rare hybrids between two crab species, and fishermen in the Bering Sea in 2025 caught unusually large numbers of them.

Hybrid breeding between snow crabs (Chionoecetes opilio) and tanner crabs (Chionoecetes bairdi). had occurred before, and the tanner crabs were once quite rare, but their numbers have risen sharply since the snow crab decline. 

Monitoring of the eastern Bering Sea ecosystem has taken place since 1982, and scientists at the US National Oceanic and Atmospheric Administration (NOAA) have tracked commercially important species, as well as monitoring changes in marine life.

Before the heatwave, snow crab populations had an estimated population of about eight billion individuals. By 2021, that estimate had fallen to one billion.

Winter sea ice reduced, and temperatures increased in the cold layer of water normally found at the seabed. Changes in the crabs’ environment contributed to widespread starvation. The impact was so devastating that Alaska introduced its first snow crab fishing closure response. This lasted approximately two years.

Hybrid snow crabs had made up to 10% of the commercially sized males, but by 2025, almost 50% were hybrids. This was estimated to be 13.4 times higher than the historical average. NOAA researchers used computer modelling to examine how the relationship may have changed between the two species following the marine heat wave. The results indicated reduced competition for food but also a potential reduction in hybrid predation. Researchers believe the decline in sea ice triggered the snow crab collapse, which benefited and increased the number of hybrids.

Scientists are still trying to determine the reproductive capabilities of these hybrids to see whether they can reproduce successfully and for there to be a self-sustaining population. Should the hybrids be unable to reproduce then it means, their presence will depend on the two parent species continuing to breed. There is also a question over how these hybrids might impact the ecosystem and whether it might influence the recovery process of the snow crabs.

These findings illustrate how marine heat waves can lead to consequences beyond the immediate loss of marine animals. Marine heatwaves can alter the vital conditions on which so many marine species depend.

In this case, a sudden increase in hybrid crabs appears to be connected to the collapse of another population following extreme warming. For as long as ocean temperatures continue to rise, more consequences like this and unexpected shifts in marine life could occur.

Photo: NOAA Ocean Exploration, 2023 Shakedown + EXPRESS West Coast Exploration