Things that go bump in the night: the sea spiders look a fright

Eyes Under Puget Sound - Critter of the Month

You can run, but you can’t hide from this month’s spooky Halloween-themed critter! Break out your broomstick, because sea spiders are a horrifying reminder that no one is safe in the dark murky waters of Puget Sound...MWAHAHAHA!

Trick or treat?

Kingdom, Animalia; Phylum, Arthropoda; Class, Chelicerata; Class, Pycnogonida
Sea spiders have the segmented bodies and hard exoskeleton characteristic of all arthropods, including crustaceans, insects, and spiders. However, the name “sea spider” is confusing — dare we say, tricky — because sea spiders aren’t actually spiders at all. As one researcher put it, “They’re about as closely related to terrestrial spiders as a seahorse is to a horse.” Sea spiders are in their own special class called the Pycnogonida, while true spiders belong in the class Arachnida.

Where the wild things are

Chunky six legged creature shows as white on a brown rocky surface.

Pycnogonum stearnsi, Stearns' sea spider. Photo courtesy of Dave Cowles,  

There are a ghastly 1,300 species of sea spiders worldwide, and they are exclusively marine, found in every ocean and at every depth. We see multiple sea spider species here in Puget Sound, but we don’t normally encounter them during our sediment sampling because we don’t sample in the rocky areas they prefer. They like to hide in rock crevices or hang out on colonial animals like hydroids and bryozoans, which are typically found attached to hard substrates.

Closeup of creature that looks like it is made from lumpy tan sticks.

The dorsal

Most sea spiders are small (the largest Puget Sound species maxes out at a couple of centimeters in diameter), but others can be the size of a dinner plate (sometimes over 20 inches). Don’t start screaming in horror just yet — you won’t ever encounter these giant creepy crawlies unless you are diving in the polar oceans of the world, where the eerily large Arctic and Antarctic species live.


Don’t let the bed bugs bite

Although sea spiders might haunt the dreams of arachnophobes, they are actually harmless to humans, and lack the fur and fangs that make terrestrial spiders so scary. The sea spider’s body is small in comparison to its four to six pairs of long, prominent legs (think, daddy long legs rather than tarantula). The body is composed of two main sections: the cephalothorax (head/upper body), and the smaller abdomen (lower body region). Located on the cephalothorax is a lump called the ocular tubercle, with four simple eyes on the top. 

Translucent creature, like a many-legged stick bug with labels: chelifore, proboscis, ocular tubercle

Close-up of the cephalothorax of Nymphon sp.

Sea spiders are incredibly slow, so they feed primarily on immobile prey: invertebrates such as sponges, anemones, cnidarians, and bryozoans. They also lack the complex mouthparts that most arthropods have, so they prefer soft-bodied animals, sucking the juices up through a long straw-like proboscis located on the underside of the cephalothorax. Some species also have a pair of chelifores with tiny claws for tearing off small pieces of food.

Blood and guts

There isn’t much room to spare in a sea spider’s thin body, so it’s convenient that they have no need for lungs, gills, or respiratory organs. Instead, they have magical, multifunctional legs which provide a large surface area for collecting oxygen through diffusion.

Greenish spider-like creature with bunny-ear looking head and red

Anoplodactylus viridintestinalis has a bright green digestive tract

Surprisingly enough, digestion and circulation also take place in the legs, with digestive fluid and blood flowing easily though these amazing tube-like appendages. Blood is pushed through the legs by the gut, not the heart, which is too small and weak to pump blood to the sea spider’s extremities. The sea spider is the only animal in existence to have a gut that works to pump blood - now that’s a gut-wrenching bloody good show!

Father knows best

Two sea spiders: top one has a couple of shorter front legs labeled, “ovigers.” The bottom one holds whitish lumps, presumably eggs.

Top: Close-up of the underside of a male Nyphon sp., showing the ovigers. Bottom:  A male Anoplodactylus viridintestinalis clutches egg masses.

The abnormal activities of sea spider legs don’t stop at respiration, digestion, and blood flow. Reproductive organs are also located on the legs, and this is where the female grows her eggs. After a brief courtship, the male scoops up the fertilized eggs and takes care of them as they mature, clutching the egg masses with a special pair of legs called ovigers.

Critter of the Month

Our benthic taxonomists, Dany and Angela, are scientists who identify and count the benthic (sediment-dwelling) organisms in our samples as part of our Marine Sediment Monitoring Program. We track the numbers and types of species we see in order to understand the health of Puget Sound and detect changes over time.

Dany and Angela share their discoveries by bringing us a Benthic Critter of the Month. These posts will give you a peek into the life of Puget Sound’s least-known inhabitants. We’ll share details on identification, habitat, life history, and the role each critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.