I do not like spiders. I know, I know. They’re important for catching insects like mosquitos and they serve as an important food source for other animals, like bats. But I still don’t like them. They just have too many legs.

So imagine my horror when I learned about sea spiders. There are over 1,300 known species of sea spiders that live in oceans across the world. Some of them are super tiny, with only 1 mm (0.04 in) long legs and muscles that are a single cell. The biggest has legs that are 70 cm (2.3 ft). That is house cat-sized. Nowhere is safe, not even the ocean.

I tried to get over my uncomfortableness with sea spiders by researching them. Here’s what I found:

• Not really spiders
  • Contrary to what their name would suggest, sea spiders are not actually spiders. They are arthropods (invertebrates with an exoskeleton, segmented body, and paired, jointed appendages), but they are not arachnids like spiders. Sea spiders are instead pycnogonids. Still, sea spiders seem to be closer to true spiders than other arthropods like insects and crustaceans. Some genetic analyses suggest that sea spiders may even be the sister group to other arthropods, or their closest relatives on the evolutionary tree.
• Daddy’s on baby duty
  • Male sea spiders are actually great fathers, a trait that is rare among arthropods. Almost all sea spider species show paternal care. After the female lays her eggs, the male will fertilize them and attach them to his body. He has a pair of special legs called “ovigerous legs” where the eggs are clustered. The male will continue to carry around these egg clusters (often as many as 1,000 eggs on each leg!) until they hatch in a few weeks or months. Some species of sea spiders will even continue to take care of the larvae until they are a bit more developed.
• All legs
  • Speaking of legs: sea spiders are basically all leg. They have between four and six pairs of legs, and basically no body to speak of. All of the important organs are housed in their legs. For instance, their intestines have pouches that go all the way to the tip of their legs. They also have no real respiratory system. Instead, sea spiders “breathe” through holes in their legs, with oxygen diffusing into them from the ocean. This oxygen is carried through the body in hemolymph (invertebrate “blood”). While there is a small heart in the sea spider body, it only pumps hemolymph in the body and the part of the legs closest to the body. Hemolymph in the legs circulates as a result of the peristaltic movement (the same type of movement when your throat swallows) of the guts inside the legs.

• Proboscis
  • Legs are not the only important body part sea spiders have: they also have a proboscis. This proboscis is just like the ones butterflies have – a long, tubular mouthpart. But instead of flowers, sea spiders use their proboscis to suck the insides out of their prey. Basically, they will puncture the skin of soft invertebrates like sponges and sea anemones, send in some digestive enzymes to liquify things, and suck it all up. In some cases (aka, when the prey is very big), sea spiders take on a parasitic instead of a predatory role. They won’t kill the prey, just weaken them. For instance, one group of researchers from the Monterey Bay Aquarium Research Institute found sea spiders sucking out the juice from pom-pom anemone tentacles. This didn’t kill them, just left them “wilted-looking – but still alive.“
• Polar gigantism
  • I might be able to handle the tiny sea spiders. But I struggle with the big ones. Why are some sea spiders so big? The simple answer is a phenomenon called “polar gigantism.” Species of marine animals like sponges, worms, and sea spiders living in cold water are much, much larger than their temperate and tropical counterparts. Okay, but WHY are cold-adapted species bigger? The prevailing hypothesis is the oxygen-temperature hypothesis. Cold water is more oxygen-rich, as the oxygen molecules are packed together tightly. Cold-adapted animals also tend to have a much slower metabolism. Putting it together, you have an animal that doesn’t need as much oxygen in water that is chock-full of the stuff. This lets animals grow huge without the metabolic constraints animals in warmer climates have.

• Turning themselves into Swiss cheese
  • The large Antarctic sea spiders have another trick that may have allowed them to grow so big: they have more holes. Remember how sea spiders breathe through their legs? One study found that large sea spiders have a greater density of leg holes than smaller sea spiders. This likely lets them get more oxygen in general and compensates for their hugeness. And it has implications for global warming, too. Scientists were originally concerned that the giant Antarctic sea spiders wouldn’t be able to handle warming temperatures, since warming temperatures would decrease the amount of available oxygen in the water. They appear to have figured out a way around this issue by just adding more holes in their skin so they can still get plenty of oxygen.

---

The good news is that sea spiders are completely harmless. Apparently they spend a lot of time grooming and taking care of their legs (makes sense because they are literally all leg). Sea spiders also don’t have venom and don’t really bite. They just walk / swim around looking for food and mates.

I still don’t like them. I am perfectly content to continue admiring them from a distance.

Resources

Monterey Bay Aquarium Research Institute

Science News for Students

The New York Times

NewScientist

All You Need Is Biology