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Wednesday, May 15, 2013

Along came a sea spider

In late April, I was looking through a microscope trying to photograph a local hydroid when I was distracted by something orange with long legs:

I zoomed in for a closer look:

This is a pycnogonid, or sea spider.  It's a little hard to make everything out at first, but I just had to post these pictures because this animal is so unusual.  

This sea spider is mostly transparent.  The orange lines are branches of the gut that extend throughout the body and into each leg.  (Somehow this patterning made the sea spider look like some sort of Halloween skeleton to me!)

The head is on the left side and if you look carefully you can see small silvery spots that are the eyes. [There are four eyes, but all four are not always visible.]  The two arching appendages on either side of the eyes are chelicerae — the presence of chelicerae tells you that sea spiders are related to horseshoe crabs, spiders, and scorpions.

After that, the body (or trunk) is narrow, but there are very long legs extending out to either side.  In the next photo, the sea spider has turned to face away from the camera and is now crawling along a branch of the hydroid.  Note the sharp curved claws at the tips of its legs for holding on (reminiscent of a sloth).

Close-up of leg and claw wrapped around hydroid branch:

I believe this sea spider had just eaten one of the hydroid polyps see the pink mass next to the sea spider in the following image.  The sea spider has a long proboscis that it uses to suck fluids out of its prey.

There are many other interesting facts to share about sea spiders, but I'll save those for another night. 

For now, since you've become familiar with what this sea spider looks like, can you find it in the photo below?  The transparent-and-orange coloration is effective camouflage against the backdrop of the hydroid, but I think you'll be able to find it if you search carefully.

The sea spider is in the lower right corner, at the lower end of the rightmost brown hydroid branch.

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