Experienced moms stick together <> at least among red king crabs. In Alaskan waters, each mother crab carries thousands of eggs while making her way with other females to the muddy bottoms where new crabs hatch. At journey's end they release their clutch upcurrent of protective nursery grounds that will give their young the best chance of survival.
Researchers Pete Cummiskey and Eric Munk spend a lot of time swimming after red king crabs in Women's Bay on Kodiak Island several days a week and all year round to learn about hatching, molting and mating in this commercially important species. Their work and that of other scientists at the National Marine Fisheries Service will lend insight on the basic biology underpinning the ups and downs of red king crab populations.
First-time moms spend their early years in shallow nursery grounds of Southeast Alaska, Kodiak Island, and northeast Bristol Bay. Young crabs are extremely vulnerable to predators and try to stay hidden in protective habitats that include hydroids (which look like tiny evergreens) and other three-dimensional structures. During the day juveniles pile on top of each other to rest in a great ball of 2,000 to 3,000 crabs known as a "pod," which may afford protection from hungry fish.
Cummiskey and Munk follow the pod by tagging a few crabs in each group. Later, they track the pod's movements by listening to electronic beeps from the tags. The young crabs grow to maturity over six or seven years, shedding their shells frequently along the way. Each time the crabs molt, they lose their electronic tag and must be found again during the scientists' next dive.
Adult crabs also form pods to regroup after long nights of clam hunting. In some regions experienced mother crabs, known as "multiparous" females, form their own separate pod. These older females are larger and carry more eggs than first-time mothers, and are responsible for producing the majority of the next generation. This group continues to forage while carrying eggs, but doesn't wander as far offshore as first-time mothersor males.
Red king crab are also caught incidentally as bycatch in commercial trawlers fishing for sole, and some researchers speculate that podding behavior makes crab more vulnerable to the nets. In 2002, National Marine Fisheries Service observers that count the catch on fishing boats found more than 100,000 red king crabs caught incidentally by trawling operations. In comparison, other fisheries in the North Pacific had a bycatch of only 27,500 red king crabs.
In addition, multiparous mother crabs have historically concentrated on one of the most popular trawling areas in the Bering Sea, between western Unimak Island and Port Moller. Experienced moms retreat to these inner bay waters each spring, just as their precious cargo begins to stir. When the time is ripe, mom opens her abdominal pouch and tiny larvae come out swimming, piling out by the thousands for as long as a week's time. Even as they embark on their journey, the mother crab gives them an advantage by releasing her offspring in hatching grounds upstream of the nursery habitat.
Continuously tracking the pod gives Cummiskey and Munk the opportunity to learn about hatching and molting through direct observation. Once freed of her progeny, the mother crab pulls away from her skin in a familiar ritual of molting and renewal. For the rest of the year, older male and female crabs often keep to themselves. But when the babies have gone and females begin to molt, potential fathers draw near, perhaps attracted to chemical smells released during molting.
Individual males may even help the female by holding on to the old exoskeleton while she steps out of it into newly softened skin. The pair will then grasp each other's claws and begin a new batch of baby crabs. Once their clutches are restocked with fertilized eggs, mother crabs reunite with each other and get back to the daily grind of digging up clams.
Margot Stiles is a marine biologist based at AMCC.
Giant pods make it easier to follow crabs and study their behavior, but they also make it difficult to count their populations. Research trawls give fairly straightforward counts of crab density if individuals are spaced evenly across the seafloor. However, podding crabs make this estimate less clear-cut because periodically a trawl net collides with a pod and comes up on deck packed with crabs. The next sample may find very few crab, leaving scientists with the formidable challenge of combining these numbers in a realistic count to determine fishermen's catch limits.
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