In the spirit of Halloween, let’s take a look at a creepy but otherwise harmless ocean arthropod: the sea spider.
As was the case with the grandaddy longlegs (aka “harvestmen”) we discussed a couple years ago, despite their spidery looks, sea spiders are not actually spiders. They do belong to the subphylum Chelicerata like spiders and horseshoe crabs, but while land spiders are part of the class Arachnida, sea spiders are in the class Pycnogonida.
As chelicerates, both arachnids and pycnogonids bear fangs called chelicerae. However, apart from physical resemblance, that’s basically where the similarities between land and sea spiders end. Sea spiders don’t spin webs, they don’t poison their prey, and they don’t breathe.
Well, okay, I guess they do breathe, but they don’t have lungs, gills, or anything resembling a respiratory system. Because sea spiders have such a dramatically large surface area to body size ratio, it was once thought that they could absorb oxygen directly across their exoskeleton via diffusion. However, we now know that they take in water and subsequently oxygen through pores on their legs.
There’s actually a lot of wacky stuff going on with pycnogonid legs. For one, they’re incredibly large compared to the size of the sea spider’s body. In fact, sea spiders’ torsos are so tiny that they can’t fit all their organs inside, so their digestive and reproductive systems extend into their legs. Additionally, male sea spiders sport a fancy pair of legs called “ovigers” that they use for grooming themselves and carrying the female’s eggs around until the baby sea spiders hatch.
Sea spiders live throughout the world’s oceans and can grow especially large near the poles and at great depths. However, they tend to stay pretty small in the tropics. As adults, they mostly feed on sponges using their large proboscis. Most species also eat cnidarians like hydroids and anemones, and some consume seaweed, or other invertebrates. As larvae, sea spiders are often parasites of sponges and cnidarians.
Sea spiders can be hard to find due to their light coloration and small size in areas where you might actually come across them. However, if you do cross paths with a pycnogonid at the beach, don’t worry – sea spiders are completely harmless to humans.
Happy October everyone! Have a fantastically spooky Halloween, and remember – sea spiders won’t hurt you, land spiders are our friends, and the spider “skeletons” at Target are anatomically incorrect.