The great escape: the tale of a sea urchin

May 5, 2009

collecting urchinsThe sea urchin experiment (Study 6)  is now up and running so I thought now would be a good time to fill you in on what we have done.

Luckily, even with the fjord frozen we were able to get enough sea urchins for our experiment. The divers had made a big hole in the ice near the old pier outside the marine laboratory, and there happened to be urchins on the seabed below the hole ( not all the experiments have been so lucky and are waiting for the sea ice to melt!).

urchins in holding ank

The urchins were brought in by the divers in big plastic tubs. They had to keep the collected urchins in bags hanging through the icehole until the moment they were leaving when they were transferred to the transporting tubs. This is because it is so cold that the water in the tub freezes very quickly and would kill the urchins. Even leaving it to the last minute the water looked like a giant slush puppy by the time it was handed over to us back at the laboratory. We put the urchins into holding tanks supplied with fresh seawater, and with some kelp- a type of seaweed that the urchins eat.

Steve putting the urchins in their potsThen on Saturday the mission to begin the experiment began. It took three of us working together; I used a net to fish the urchins out of the holding tanks. I then weighed them and passed them to Karen, who measured their diameter. Steve then took the urchins through to the main experiment room and randomly put them into the exposure pots. We were quite pleased with our efforts and the whole thing took about three hours. We went off to dinner quite satisfied with our work.Karen measuring the urchins

After dinner we popped back to the lab and something didn’t look quite right. . . .


. . . . some of the urchins were making a break for it and climbing out of their pots! By removing them from the kelp and holding tank walls we had activated the urchin’s climbing instinct. Because in nature if the urchins are no longer attached to the rock or kelp, it usually means they have been knocked off by perhaps wave action, or even a passing iceberg, their instinct is to then climb to regain a similar position to the one they had before. However when it comes to our little holding pots this is not such a good move!making a break for it

Our first attempt to keep them in the pots was to put netting in the pot as a barrier to put them off climbing out. However when we came in the next morning again some of the more determined urchins had climbed out! So after breakfast we left Steve to devise a new ‘containment strategy’ which involved placing plastic trays on top of the containers to keep them in. Steve was gone for about an hour and a half and Karen was a little concerned he had come across a polar bear and was stuck in the lab, but no, just battling the wills of thew urchins!  There was some debate over dinner as Helen and Steve  thought the urchins wouldn’t be able to move the tray. I was less convinced- but maybe I was just a pessimist. Back in Plymouth I had made lovely little containers for some crabs and come in several times to find them all running riot around the tank-  and I was sure that the urchins could be just as determined! Sure enough the next morning more had escaped. netted in!As we went around returning them to their pots, and finding it all quite funny, it was decided to use elastic bands to keep netting in place on top of the pots. Steve was flying out that day so the task fell to myself, Helen and Karen. It started well but putting your hands in water that is zero decrees Celsius (this is the temperature freshwater freezes but because of the salt in seawater it does not freeze until it is around minis 2 degrees Celsius) soon meant our hands became clumsy and the elastic bands seemed to have  a life of their own!

Our efforts paid off and this morning there were no escapees, all were still in their pots eating away the small discs of seaweed we put in for them to eat. So for now things are quiet on the urchin front until next Sunday when we do our first sampling. We will also be doing some behavioural measurements by timing how long it takes the urchin to turn the right way up after we turn them upside down in a tank- this will give us some idea of how changing temperature and ocean acidification may affect their ability to move- but if the events so far are an indication it seems they can still climb just fine!


One comment

  1. I love it when a plan comes together!!!!

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