Blue Zoo: Sea Spiders

Featuring one amazing marine animal per week.

Sea spiders usually have eight legs, which are long and attached to a small body, just like many true spiders. However, sea spiders are not the same as the spiders we’re familiar with on land (arachnids). And although sea spiders are arthropods that live in the ocean, they are not crustaceans, such as crabs or shrimp. 

Sea spiders instead form a taxonomic class of their own called Pycnogonida (sometimes it’s called Pantopoda, instead). All pycnogonids are found in the ocean, and not in terrestrial or freshwater habitats.

A sea spider sitting on a gorgonian sea fan. Photo by Derek Keats via Flickr, Creative Commons License. 

Sea spiders can be found in all oceans and most marine habitats. In remote polar or deep sea regions, they can reach large sizes with a leg span of more than a foot (30cm) wide. However, most sea spiders you could encounter are much smaller, ranging from 1 to 10 millimeters. As a result, even though they’re common, pycnogonids are very easy to miss.

A larger deep-sea species of pycnogonid. Photo by Scott C. France via Wikimedia Commons, Creative Commons License. 

Most species are bottom-dwelling carnivores. They live in seafloor habitats and feed on animals like corals, sponges, and worms. There are also some sea spiders that live as parasites on mollusks, or that scavenge for detritus or algae.

Photo by Steve Childs via Wikimedia Commons, Creative Commons License.

Because the body of most sea spiders is so small, some of their organs have to extend into the legs where there is more room. Also because they’re so small, they don’t need a respiration system; instead they simply breathe using diffusion across the body’s surface. 

Photo by NOAA/Monterey Bay Aquarium Research Institute via Wikimedia Commons, Public Domain.

Pycnogonids engage in external fertilization through multiple reproductive openings. Oddly enough, the female’s ovaries are in the femurs of her legs. Once she releases the eggs, they are fertilized. The male secretes a glue to hold the egg masses together under his legs and he broods them until they hatch. Other than that, little is known about pycnogonid reproduction – for example, some species seem to go through a courting ritual but that process is still poorly understood.

So the next time you’re tidepooling or checking out the growth on a pier, take a closer look – you just might find a little sea spider living there!

Small sea spiders crawling on a bryozoan in a tidepool habitat. Photo by Jerry Kirkhart via Flickr, Creative Commons License. 

Primary source:
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7th ed. Brooks Cole/Thomson, Belmont, CA.

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