Pycnogonids really are fascinating animals and they deserve more attention. There’s a short news article on sea spiders that mentions their odd life style and their taxonomic awkwardness.
For over 100 years, scientists have been puzzling over how exactly to classify sea spiders or pycnogonids.
They crawl along the bottom of the sea floor, sometimes more than 6000 to 7000 metres down, where they live in the dark, feeding on slow-moving soft-bodied sponges and sea slugs.
The creatures are segmented and have an exoskeleton, which makes them an arthropod, the same grouping as crustaceans, insects, centipedes and spiders.
But they also have a very strange collection of features, including a unique feeding structure.
“They have a proboscis that’s like a straw that they insert into the animals and suck out the juices,” says Arango.
Such features make it difficult to fit them into any of the known groups of arthropods.
“They look like spiders, but they are not real spiders,” says Arango. “It’s been very hard to place them in a position within the tree of life.”
They really are hard to place—I’ve reviewed two articles on that subject, one that places them with the anomalocarids and another that groups them more conventionally, with the chelicerates (I’m going with that last one right now—patterns of Hox expression trump interpretations of innervation patterns, I think.)