Sea Spiders!

As if there wasn't enough to worry about while swimming in the ocean, a new threat emerges; SEA SPIDERS!!!  They swim, they crawl, they can grow a leg span of over two and a half feet! They have a proboscis with which to suck your precious life fluids...  

Well - actually they are mostly less than 1/2 inch across, tremendously slow, and only a threat if you are a tiny soft bodied invertebrate.

Pycnogonida (PIK-nuh-GON-uh-duh) is the Class of Arthropoda to which all sea spiders belong. They also belong to the sub-phylum Chelicerata, which means that they share common ancestry with true spiders, scorpions, and horseshoe crabs.  In addition to their chelicera (pointed mouth parts particular to this group), most species possess four pairs of legs.

The thorax comprises the bulk of the body, though it pales in comparison to the legs.  The body is so small that it looks as though its only purpose is to connect the legs.  The minimalism of the body and the slender appearance of the legs has given way to these discoveries:

The chest of a pycnogonid is so small there isn't room for lungs, so they breath by diffusion.

The legs of a pycnogonid are so thin, each leg only has one muscle cell surrounded by connective tissue.

The digestive tract of a pycnogonid runs into its legs because there isn't room in there body.

Pycnogonids are soo thin, they make stick-bugs look fat (not a fact).

Seriously, these emaciated looking arthropods are well adapted to surviving in a wide range of environments extending from the poles to the tropics.  Most species are found in shallow water, but some species can be found at depths as low as 7000 meters.  They have also been around longer than sharks.  One species of pycnogonida , Haliestes dasos, is estimated to have lived 425 million years ago.

Pycnogonid image by L. Weaver

The best part about pycnogonida is that their diet does not include terrestrial mammals, like you, me, and Lassie.  Most pycnogonids are parasitic predators whose feeding is more reminiscent of a mosquito than that of a true spider.  The majority use their proboscis to feed upon soft-bodied invertebrates like sponges, anemones, hydroids, polychaete worms, and bryozoans.  

Generally, pycnogonids develop a preference for a single prey item based on what they ate as juveniles.  However, studies show that one species in the Mediterainian will change its diet according to the season.  The pycnogonid Ammothella longipes will prey upon polychaete worms when their numbers abound in the spring and summer.  When polychaete populations decline in the winter, A. longipes feeds upon detritus, dead organic mater. 

Pycnogonid image by L. Weaver

So if you're swimming in the ocean and you see eight slender segmented legs flailing about, remember that they're not real spiders, and they are not after your life fluids, but possibly your detritus.

Aloha,

Leon

If you are interested in learning more about pycnogonids, please check out the following links:
Pycnogonida (Sea Spiders) http://www.earthlife.net/chelicerata/pycnogonida.html  

Fossil sea spiders thrill experts http://news.bbc.co.uk/2/hi/science/nature/6948161.stm 

Early sea spider flashes pincers http://news.bbc.co.uk/2/hi/science/nature/3756614.stm  

Bamber, R.N., El Nagar, A. (Eds) (2013). Pycnobase: World Pycnogonida Database. Available online at http://www.marinespecies.org/pycnobase/ accessed on 2013-05-03

Soler-Membrives, Anna; Rossi, Sergio; Munilla, Tomás.  "Feeding ecology of Ammothella longipes (Arthropoda: Pycnogonida) in the Mediterranean Sea: A fatty acid biomarker approach" Estuarine, Coastal and Shelf Science. Volume 92, Issue 4, 20 May 2011, Pages 588–597. Accessed 2013-05-03 http://dx.doi.org/10.1016/j.bbr.2011.03.031