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Shellfish Assessment Program - Other Research


The Search for the Red King Crab Pod

by Dr. Bradley Stevens

Photo of a red king crab pod
A 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.

Photo of a diver looking for red king crab pods

When my eyes 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 says sight.

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 hydrophone 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. "Let's 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 are slowly moving north. They 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 cannot move around much, and they can't eat for a week or two afterwards. In tanks at the Kodiak Laboratory, 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 is 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. 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 Maryland Eastern Shore Department of Natural Sciences.



Shellfish Assessment Program - OTHER RESEARCH




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