A new finding about the mating habits of crabs shows that, as elsewhere in the animal kingdom, being a big, dominant male has its advantages and its disadvantages.
The older males with the big claws get the females, suggests the research of Hans Laufer, a professor of developmental biology at the University of Connecticut.
But Laufer also points out that the crabs that have the most sexual success also are the ones most likely to end up on a seafood platter.
The research suggests that present practices in commercial fishing for species such as snow crab could eventually endanger crab populations because fishermen take the big males most valuable for reproduction, throwing back smaller crabs that may not be ready to reproduce.
Laufer’s work is with the spider crab, a common crustacean of the Connecticut coast that is not much sought after. But the spider crab is very similar in lifestyle to two species that are commercially fished: the snow crab and the sheep crab.
The big, old crabs — they can cover a dinner plate — constitute the upper crust among their crustacean colleagues. Laufer says scientists believe that if the younger, smaller males live long enough, they will one day attain the same status.
“These are social animals,” he says. “They have a caste system. They seem to have a pecking order as chickens in a barnyard do, with the very large males being the dominant ones, the ones with the largest claws and the largest bodies.”
“There is a lot of evidence that attaining this relatively larger claw is important in reproductive behavior,” says M. John Tremblay, snow crab biologist with Canada’s Department of Fisheries and Oceans. A male snow crab, he noted, may clutch a female for as long as three weeks.
“Male dominance gone awry, you might say,” he says.
Laufer says a big pair of claws serves three purposes. It helps attract females; it helps the male grab and turn over the female during mating; and it helps to ward off other males during and
after mating.
Dominant male spider crabs, which can weigh about a pound, are easily twice the size of females. Smaller male crabs, about the same size as females, are capable of reproduction but have a difficult time of it. Instead of a comparatively athletic flip-flop maneuver of the female — Laufer has a videotape of this — the process becomes something like an interminable wrestling match.
The pace on the ocean floor is leisurely at this time of year, compared with the warmer months, when crab reproduction is practically a never-ending process.
“In nature, they go from spring to fall,” Laufer says. “The females lay a batch of eggs every 20 days.”
The spider crab is common on the Atlantic coast. Fishermen regard them as pests that steal bait from lobster pots.
Unlike the snow crab, which is hard to study because it lives in very cold, deep waters, the spider crab is a good subject for scientists. Laufer gets his live specimens from the crews that conduct environmental monitoring at the Millstone nuclear power complex in Waterford.
One of the interesting findings of Laufer’s study is that it’s not enough for a crab to be big to succeed with the females; it must be mature as well. Males that have only recently grown to full size are large enough to manipulate the female easily, but Laufer has found that, at least with spider crabs, they just don’t have the desire yet.
“They will run away from a female, or just sit there when a female comes along,” he says. “This was a surprise.”
So that leaves the big, older males to carry on — if they haven’t been plucked from the ocean floor.
Canadian fishermen land 10 to 20 tons of snow crab each year, much of the catch shipped to Japan.
“If you are going to conserve or preserve the species, you are going to have to take some of the biology into account,” says Laufer, whose research is funded by a grant of about $50,000 a year through the Sea Grant College Program of the National Oceanic and Atmospheric Administration, an agency in the U.S. Commerce Department.
For example, he says, at some point policy-makers may want to consider regulations governing minimum size to ensure long-term protection of the species. Governments may want to make sure that some percentage of breeding males are tossed back.
Laufer says his findings also could help with the hatchery rearing of crabs, in which the young are raised in controlled conditions — reducing the heavy mortality in the wild — so that more reach the size at which they can be harvested.
In California, some fishermen hauling up sheep crabs tear off the large claws and toss the crabs back, figuring the claws will regenerate.
Laufer is all but certain this will cause problems before long.
A small crab will regenerate a claw when it molts. But Laufer says he believes the big, mature males have molted for the last time. “They have essentially emasculated these males because, even if they regenerated, they would never regenerate a large claw,” Laufer says.
Tremblay says the question of crab maturity — whether these crabs stop shedding their shells — is still debated among scientists. A group of Canadian scientists recently found evidence
that snow crabs continue to molt.
Laufer says he is proposing further research into that question. But he believes strongly that commercial fishermen should be thinking about the long-term health of the species.
“We’re interfering with evolution,” he says. “Evolution has determined that the fittest animal will fertilize the eggs and perpetuate the genes. You take those good genes out, you’re going to let secondary guys take over.
