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Slater, L.M., W. Gaeuman, W. Cheng, G.H. Kruse, C. Habicht, and D. Pengilly. 2024. Molecular evaluation of the mating dynamics of snow crab (Chionoecetes opilio) in the Eastern Bering Sea. Ecology and Evolution 14:e70416.

This is a tale of two crabs: the snow crab Chionoecetes opilio and its lesser known congener, the Tanner crab C. bairdi. Both contribute to the highly prized commercial crab fisheries in the eastern Bering Sea – or EBS for short – that has brought these species to many a buffet and dinner table. Laura Slater, Ph.D. candidate at the University of Alaska Fairbanks, notes, “The EBS is the only location in the world where these two species naturally overlap, and this is where hybrids between the two species are found.” Although the two species have long been known to hybridize, just how much and what effect this could have on snow crab reproduction were unknown. 

Sometimes in science when a study is conducted, samples are archived for future use. This can happen for a number of reasons. Perhaps technology needed to address a research question has not yet been fully developed, or researchers are seeking collaborators with specific expertise, or funding has not yet been secured to do the work. In this case, female crab samples were collected and archived over a 10-year period to address particular research questions as part of a broader long-term study. Once funding was secured, Laura Slater and her research team began analyzing these samples and tackling these important crab reproduction questions:

• How often are male Tanner crab mating with female snow crab?
• Are hybrids the result of male snow crab mating with female Tanner crab, or male Tanner crab mating with female snow crab, or both?
• How many males are mating with each female snow crab?

Before we get into the results, let’s pause for a minute and explain the reproductive strategy of these crabs. As crabs grow and mature, they must shed their exoskeleton, a process known as molting. At some point, females will have their “terminal”, or last, molt when they stop growing and begin mating. Males, however, can begin mating prior to their terminal molt, but become more successful after their terminal molt to adulthood. During the mating process, males will deposit their sperm into the female’s seminal receptacles where they will be stored – sometimes for a year or more! – until the female fertilizes her eggs with them. So, having female crab samples can provide information on the mating history of those females. 

Various samples were analyzed for genetic data, including sperm stored in the female crab’s seminal receptacles, female crab body tissue, and fertilized embryos. As Laura explains, this was no easy task. “Although genetic markers have been well developed for many other fisheries species, this is not the case for crustaceans. Genetic marker evaluation for this population of snow crab was a large component of the project in terms of time and effort.” These obstacles were eventually overcome, yielding some interesting results. There were some differences among regions, but overall, researchers found that most snow crab mated only with a single male, specifically male snow crab. In fact, mating with Tanner crab (or hybrids) was only found to occur for about 2% of the snow crab females. 

So, the short answer is: Yes, they are indeed an unlikely pair. From a management perspective, this is welcome news, as hybridization likely doesn’t present a problem for the future of the snow crab stock. Further, projected shifts in the two species’ distributions as a result of climate change should make overlap and the possibility of hybridization even less. 

This doesn’t mean all the news is good, however. Given that female snow crab were typically only carrying sperm from a single male, even in subsequent mating seasons, researchers suspect that mating opportunities with adult males in the EBS are in short supply. Laura adds, “The EBS population may benefit from precautionary approach management that includes indicators of female reproductive potential and/or mating success, similar to that used for snow crab populations in other locations. Although females have the capacity to store sperm for future use when males may be unavailable, our findings suggest the EBS population often does not receive enough sperm during mating to fully benefit from this capability.” 

The next chapters in this crabby tale are still to come. Laura is currently working on analyzing other components of this long-term study focusing on snow crab reproductive potential and mating dynamics. These results will further add to what we know about snow crab reproduction and offer valuable insights to inform future management decisions for this species.

The full manuscript can be found and downloaded here (open access):
doi.org/10.1002/ece3.70416