Monday, June 23, 2008

Why I HATE spider crabs!

I know, I know “hate” is a strong word, but I can’t think of a more appropriate way to describe my invertebrate nemesis. Spider crabs may be responsible for more failed eelgrass plantings under my watch than any other cause. OK, maybe not ANY other cause as we did really mess up on the site selection thing in the 90’s, but it’s got to be up there…

Above is a picture of a spider crab (Libinia emarginata) eating one of our transplants at the Sag Harbor test plot site. It isn’t bad enough the we can never seem to get plants to survive in Sag Harbor (I’ll write about this soon), now we have crabs yanking out plants as fast as we can plant them! Maybe this has been the problem all along?

In this case I just happened upon the unsuspecting culprit as he casually munched on one of our shoots like he was eating a celery stick filled with peanut butter. He couldn’t have started more than a minute or two before I got in the water.

In the past we have witnessed spider crabs uprooting and dislodging transplants as they always seem to show up right after we plant. They must be attracted to the good smells coming from the disturbed sediments. In most cases they rip out a few shoots along one edge of the plot and back in amid the shelter of the new patch. I can hardly blame them as there is literally nothing else on the bottom at most of these sites (see my “Oasis Effect” comment).

The typical sign of crab damage is a shoot cut cleanly off at a 10-30% angle. This kind of damage is common and it does not seem to harm the plants too much if the trimming is minimal. Occasionally, we do see more crushing type wounds and these are likely caused by crabs as well.

I must say that it is almost (do I dare say it) cute when the young "decorator" spider crabs cut off SMALL pieces of shoot (thus the use of the term “cute”) to add to the forest on their back like a sniper customizing his ghillie suite to blend in with the surrounding landscape.

This time the damage was clearly catastrophic to the shoot and there was no chance of recovery. After I took a couple shots with the camera I grabbed the crab and decided that he was going to be sacrificed in the name of science to see how much he actually consumed.

Once back in the boat a fatal blow to the carapace allowed me to dig through the goo that is a crabs innards. I was somewhat surprised to find that most, if not all, of the eelgrass sections were still lined up at or near the crabs mouth parts.

Later that day, back at the lab, I photographed what came out of the crab and was surprised to find a small leaf tip. Since the shoot that he was eating when I found him was not missing any tips, this obviously came from another shoot. Through my frustration I had a warm feeling knowing that there was one less multiple offender in Peconic Bay.


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