Sea Spider

Class Pycnogonida — the sea spiders — contains about 1,300 species worldwide and is an entirely marine arthropod lineage. Despite the common name, sea spiders are NOT true spiders (class Arachnida) and NOT crustaceans (class Malacostraca etc.); they are a separate ancient arthropod class with uncertain placement in the arthropod phylogeny (most analyses suggest they are sister to all other Chelicerata, including arachnids and horseshoe crabs). Sea spiders are characterized by an extremely reduced trunk (so reduced that the body is essentially just a connector between the head and the leg attachments) and unusually long thin legs. The reduced body cavity cannot accommodate internal organs at typical scales, so the gut, gonads, and most organ systems extend OUT into the legs themselves — branching gut diverticula and reproductive tissues fill the leg lumens. Sea spiders have no respiratory organs (no gills, no book lungs); they exchange gases by simple diffusion across the cuticle, which works because the body is so thin (the surface-to-volume ratio is high enough that ambient seawater oxygen perfuses the tissue directly). This respiratory simplification explains the most remarkable feature of polar sea spiders: 'polar gigantism' — Antarctic and Arctic deep-sea species reach leg spans of up to 70 cm (Colossendeis spp.), among the largest arthropods in the cold-ocean fauna. Sea spiders feed by impaling soft-bodied invertebrates (sea anemones, hydroids, sponges, bryozoans, sea slugs) with a long snorkel-like proboscis and sucking out the partially-digested tissue. Many species also brood their eggs externally — males carry the eggs glued to specialized 'ovigerous legs' (a third pair of legs unique to males) — making sea spiders one of the few groups with paternal egg care.