So, good news and bad news for those of you afraid of spiders. Good news is, this creature isn’t a spider because spiders can’t breathe underwater, so you’re safe from arachnids if you want to spend more time in the ocean. (Well, one species of spider actually can live underwater by trapping air around its abdomen—sorry, I’m really bad at good news.) The bad news is what while the sea spider may not be an actual spider, it looks like the ghost of a spider you once killed. Which might actually be worse and ... totally scientific?

The 1,300 known species of sea spider are truly ancient animals that as far as scientists can tell aren’t closely related to any extant species, spiders or otherwise. At the moment, though, they’re lumped in the group that holds spiders and horseshoe crabs. They have such tiny abdomens that their guts extend into their legs. Their genitals are there in the limbs too, which makes mating ... interesting. And like sea horses, it’s the males that carry the young.

Sea spiders live in both deep and shallow seas around the world, but all are carnivores, through and through. They have claw-like mouthparts known as chelicerae, which spiders also have, suggesting they may belong to the same group (appropriately enough called Chelicerata). The actual feeding happens through a proboscis, a sort of tube that can be longer than the rest of the sea spider’s abdomen in some species.

“They feed generally on things that don't move, like sponges and corals, but also slow-moving things like worms or sea slugs,” says marine biologist Claudia Arango of the Queensland Museum in Australia. “What they do is they've got very sharp jaws at the tip of that tube, the proboscis, so they pierce the prey and start sucking out fluids.”

The deep and shallows are of course worlds apart as far as habitats go, so sea spider species have adapted accordingly. To find food in the blackness, the blind deep-sea varieties likely sniff out their prey's chemical cues, while their shallow-water peers have four simple eyes. In the shallows, they also tend to be more colorful than in the deep sea, since in the darkness, flashy colors won’t do you no good nohow.

The sea spider Nymphon grossipes, which really got short-changed on the whole name thing. Alexander Semenov / Science Source

Their body plans, too, are wonderfully adapted for their environments. The long-legged species are built like the tripods from The War of the Worlds—but with fewer legs and death rays—because sediment down in the deep can be soft and unstable, so lankier limbs keep it from sinking up to its abdomen. In the shallows, sea spiders tend to be stouter, with thicker, shorter legs that help them better hold on in rough waters, not to mention avoid shattering into a shower of limbs.

Leg Genitals and Other Adventures in Sea Spider Sex

What the many species of sea spider can agree on, though, is how to have sex: namely, very acrobatically. Both males and females have genital pores in their legs, males on just their last two pairs and females on every single limb. When a couple comes together the spindly male crawls on top of the spindly female. “So basically he climbs up and walks all over the female and then starts trying to go under the female so that both the pores come in contact,” says Arango. “The female would be standing totally normal while the male would be upside down, clinging on the female.”

Pseudopallene harrisi, from Australia. Note the claw-like chelicerae. It's fashionably colorful not because it's from Australia, but because that's par for the course for shallow-water sea spiders. Claudia Arango

When the female releases her eggs, the male combines them with his sperm and bundles them together in balls. These he holds with special appendages, known as baby björns ovigers, and “he carries the eggs all the way until they hatch, and sometimes even later you can see fathers carrying the babies,” says Arango. “It's quite a heavy load sometimes.”

Inevitably, though, the larvae must set out on their own, and some species won’t just float at the mercy of the currents. They’ll invade the bodies of other creatures on the seafloor, including bivalves, burrowing into their flesh and feeding on them and eventually killing them. Others invade the bodies of coral, stealing the nutrients that algae produce for them.

A sea spider male carries eggs with specialized limbs called ovigers. Claudia Arango

Still other species go after creatures called hydroids. One particular sea spider (.pdf) parasitizes the hydroid Tubularia larynx, a sort of small tube with frilly pink polyps, which grows en masse on rocks. The male sea spider will carry his young to fields of these hydroids and release them. Amid plenty of food, the young invade the hydroids’ tissues, feeding and feeding and growing and growing before erupting out of their dead hosts.

File Sea Spiders Under: It's Complicated

It should be clear by now that sea spiders aren’t like any other creature on Earth—not by a long shot. I mean, the body plan alone is out of control. The daddy long legs is a lanky little thing, but the sea spider has so simplified its body plan that really its abdomen is little more than a joint for its legs, forcing its organs to flow into its limbs. Its heart is exceedingly simple, and because it lacks gills, it seems to absorb oxygen through its cuticle. And it has that bizarre proboscis, plus the males have those unique specialized arms used to hold eggs.

It all adds up to large-scale befuddlement for the folks studying them, and accordingly it's a matter of controversy where exactly sea spiders fall in the tree of life. But Arango has an idea. “When you look at the morphology of the sea spiders, apart from a superficial resemblance to spiders, there's lots of things that are unique,” says Arango. “In terms of giving them a place in the classification the best thing we can do, based on DNA mostly, is keep them at the base of the chelicerates."

Anoplodactylus evansi may be Australian, but it looks a lot like a St. Louis Blues fan. Claudia Arango

The problem is that sea spiders are a very, very old group of animals that likely diverged early on in the evolution of arthropods. I’m talking more than 500 million years ago, not all that long after the appearance of the first creatures we’d even recognize as animals. But for all that time on Earth, their fossils are scant, so tracing their lineage is a nightmare. And it doesn’t help that anatomically, they’re like nothing else living on this planet.

The larger problem is that classification can be messy. Hell, scientists have a hard time agreeing on how to define what a species is, for Pete’s sake. And really, animals didn’t evolve for humans to place in a perfect order. Don’t get me wrong, taxonomy is indispensable to biology. It’s just that it’s messy out there, and the sea spider is a great reminder of that.

Also, another great reminder: You aren’t safe anywhere from spiders or spider ghosts.

Browse the full Absurd Creature of the Week archive here. Know of an animal you want me to write about? Are you a scientist studying a bizarre creature? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.