The Search for the Red King Crab Pod
On a blustery day in January, I sit in an inflatable boat in my drysuit,
shrugging off the cold wind. We are bouncing around in Womens Bay, a few
miles south of Kodiak. Pete Cummiskey, wearing a pair of headphones,
listens intently to the clicks and bleeps picked up by the hydrophone
hanging off the side of the boat. He is listening for the signal from an
ultrasonic tag glued to the back of a king crab, somewhere below us. After
turning the hydrophone in an arc, he smiles, a sign that he has heard the
right signal. He starts up the engine, and we motor over another 50 feet,
where he cuts the motor and listens again.
"That’s good," he says, "drop the anchor, we’re
right on top of them".
I do as he says, dropping the anchor and chain overboard. It hits the
bottom about 30 feet down, and I tie off the line to the boat.
"Last week there were about 3,000 crab here," he says,
"and at least three of them have tags on them. Only a few percent had
molted last week, but they might have all done it by now. When we get to
the bottom, I’ll home in on the signal with the underwater hydrophone
and we’ll find the tag. But if the crab it was on has molted already,
the tag may just be sitting on the bottom. In that case, I’ll bring it
up and put it back in the boat."
"Why not just put it in your goodie bag?" I ask.
"Because I’ll keep hearing it on the hydrophone, and it will
cover up the signal from the other tags. One time I left a recovered tag
in my bag from a previous dive, and I spent the whole dive trying to find
it, when it was in my bag all the time." Like a dog chasing it’s
tail, I thought.
We put on the rest of our scuba gear, slip on our masks, then lean back
and splash into the water. It’s cold, about 40 degrees F. I squeeze all
the excess air out of my suit until my head is underwater, and swim
towards the bow of the boat. Finding the anchor line, we follow it down to
the bottom at 30 feet.
The visibility is about 8-10 feet, which is pretty decent for Womens
Bay. I spend the first minute fussing with my gear, getting my buoyancy
adjusted, clearing my mask, and equalizing the pressure in my ears. My
head is so cold it hurts, and my eyes are full of tears.
When they finally clear up, I look around in amazement. The bottom is
littered with king crab shells. I have never seen anything quite like it.
I can only see about 20 in the small circle of visibility around me, but
as we swim, we pass over many more. They are scattered about every couple
of feet. Some are in piles of two or three. Most are upside down. There
must be hundreds of them in the short distance we swam. It’s an
incredible sight.
January is the winter molting season for king crab. Because crabs have
a hard outer shell, or exoskeleton, they do not grow in a linear fashion
like most other animals. In order to grow, they must shed their shells in
a process called molting. This is a complicated physiological process. A
month prior to molting, the crab secretes enzymes which help to separate
the epidermis (a skin layer that secretes the shell) from the exoskeleton.
Over the next few weeks, the crab gradually retracts all of its body
parts from the outer shell by a few millimeters, while it begins to
secrete a new shell beneath the old one. If we pull off one of the small
mouthparts from the crab, and place it under a microscope, you can see the
new shell beginning to retract from the old one.
About 24 hours before the crab molts, it starts absorbing water, which
causes it to swell up and burst out of its shell like a kernel of popcorn.
In less than ten minutes, it backs out of the shell, leaving behind all of
it’s legs, mouthparts, antennae, eyestalks, the pharynx and stomach
lining, and gill coverings.
To say this is an amazing process is a major understatement, and it has
been the focus of research by many scientists for decades. Molting takes
place about 15-20 times in the life of a crab. A king crab may molt six
times in its first year, four in its second, two or three times in it’s
third, and after that, perhaps only once a year. After sexual maturity,
the females continue to molt annually, while intermolt periods become
continuously longer for the males, so that they only molt once every few
years or so.
Pete has been following this group of king crabs since they were about
two years old. They are about 5 years old now, and the females are just
reaching maturity. After this molt, they will mate for the first time.
Pete stops and points the sonar gun as he turns in a circle and listens
for a signal. In a moment, he looks at me and points, telling me he is
heading toward the tag. I follow him as he swims along. Soon he is
pointing the gun down towards the mud, and I know he is close to the tag.
I start turning over crab shells to look, but he finds it before I do.
The tag is about the size of my thumb, and is glued to the back of a
crab shell with epoxy. Pete attached this tag last winter, and it has been
telling him the location of the crab ever since. But now the crab has
molted, and left it’s old shell behind, along with the tag. Pete points
up toward the surface, and we both ascend slowly.
When we get to the surface, he swims over to the boat and tosses the
tag into it.
"Lets go back down and see if we can find some live crab" he
says. We descend to the bottom again, and spend the next thirty minutes
looking for crab. We only find two live crab in that time, but we must
have seen thousands of molted shells. After a while we swim back to the
boat, and climb in.
I should have scanned a larger area", Pete says, "The
crab have been foraging in this area for about six months, and slowly
moving north. The were just south of here last week, so they’re probably
up near the Lash Dock by now."
In a period of about two weeks, almost all 3,000 crab have molted. Why
do they molt in such close synchrony? There must be a biological reason
for it. It probably helps them to reduce the likelihood of being eaten.
After molting, the crabs are very soft and vulnerable. They can’t move
around much, and they can’t eat for a week or two afterwards.
In tanks at the Kodiak Fisheries Research Center, they get along fine
together during most of the year, but if one molts in the tank, the others
will eat it. However, if they molt simultaneously, then all the crabs are
vulnerable at the same time, so that no crab can eat the others.
But how do they all know when to molt? There is probably some
physiological signal, like a stretch receptor, that tells the crab that
its shell is too full, and it’s time to molt. But it is unlikely that
all crabs reach that point at the same time. Do they wait for some
environmental signal? Do they somehow communicate with each other? The answer is...
probably both.
Lobsters can be induced to molt by changing day length. Perhaps the king
crabs perceive the lengthening days of spring as the signal that it’s
time to molt. Another possibility is chemical communication. As the first
few crabs start to molt, they release chemicals into the water from the
molting process. Crabs have very sensitive smell receptors, and can detect
these chemicals. That may signal the other crabs to begin molting
simultaneously.
Pete starts up the engine and motors us North for a few hundred yards,
then listens to the hydrophone again, but does not hear any signal. The
wind is getting stronger, and we are both cold, so we fire up the motor
and head back to shore.
The next day, Pete found the group of crab a quarter mile south of
where we had been looking. Most of them were wearing their bright red new
shells. We had just missed them.
Written by Dr. Bradley G. Stevens, now faculty at the University of Massachusetts Dartmouth School for Marine Science and Technology (SMAST).
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