Pycnogonida – They need no body!

This animal walked up my leg after a dive, and it reminded me of a harvestman for a second. To be honest I would never have expected to see one, because I thought that they would live in great depth only. I picked up a diaper from the reef, so I assume that this creature sat on it.

Like many times before, again there’s a lot of uncertainty about the classification. There is already a lot of uncertainty while only looking at them!

Usually when you see an animal with long legs, you would try to locate the front of it, just for orientation, and maybe a hint of where it moves next. Do that with a sea spider. One leg leads to another, there is almost nothing in between. The scientific name of the order Pantopoda means ‘legs only’, and this is what you see! When you think you found a belly, or abdomen, it is probably the trunk, which is much bigger than the rudimentary body. Wouldn’t you need to have a body? No, not really, all they need finds place in the legs. Sea spiders have 8 legs (few species have ten though, what is awkward), a proboscis, palps, chelicerae with claws, eyes, and an abdomen, but everything seems out of proportion. It is a bit like the clown game, where you compose a clown out of parts, with the size of these parts depending on the number of eyes on the dice.

Because of these abnormalities, which should just be seen as differences, sea spiders are separated from insects, crustaceans, and arachnids. They’re still arthropods, invertebrates, but you can’t call them spiders. They form the class Pycnogonida in the subphylum Chelicerata. Not much appears normal when it comes to sea spiders. Like seahorses, males carry the eggs, after they glued the eggs layed by the female together. They carry the eggs with their ovigers, extra arms under their body. It reminds me a bit of the pectines, which scorpions have as sense organs. Sea spiders prefer cold water (awkward!), but obviously also occur in warm waters.

Such a strange thing must be a rare animal you might think. So did I, but we counted 1,300 species of sea spiders so far! They are real, and there are more species than geckos. Some are tiny, like the one I had on my wetsuit. The Arctic sea spider, Decolopoda australis, is as large as the largest spiders when it comes to legspan. It hardly finds place on a plate. Still they are totally harmless.

Sea spiders feed on corals, sponges, molluscs, and other organisms. They have no venom and seem to move very slowly. Actually after writing about them, I’m pretty much fascinated, and will keep an eye open for them on the water! My sea spider wasn’t colorful, but it had dark banded legs. You can find some great photos of beautiful sea spiders on social media! #seaspider

This sea spider is about 15mm long