Sea Spider

No, though it is a close-ish relative and the resemblance is not accidental. Sea spiders make up their own arthropod class, Pycnogonida, and they sit near the base of the chelicerate branch of the arthropod tree — the branch that also contains spiders, scorpions, mites, and horseshoe crabs. So they are cousins of true spiders rather than members of the spider order, and they split off very early, more than half a billion years ago. They share the chelicerate features of clawed mouthparts (chelifores) and a body divided into a front and rear section, but their eight (sometimes ten or twelve) walking legs, tiny trunk, sucking proboscis, and egg-carrying ovigers are their own.

In a sea spider the trunk is so narrow and short that there is simply not enough room inside it for full-sized organs, so evolution has pushed them outward. The gut is not a single tube confined to the body; it sends a long blind branch out into each leg, almost to the tip, so that digestion and nutrient absorption happen throughout the limbs. In females, the ovaries likewise extend down the legs, and the eggs mature there, which is why a gravid female's legs look swollen. There is no proper heart driving a closed circulation; instead, waves of contraction passing along the gut slosh the body fluid (haemolymph) around, distributing oxygen and nutrients. Oxygen itself is not carried by a respiratory pigment or taken up by gills but diffuses directly through the enormous surface area of the thin legs.

Sea spiders in the shallow tropics are usually tiny, a few millimetres across, but around Antarctica and in the cold deep sea some species have leg spans of 25 to 50 centimetres, the size of a dinner plate. This is an example of "polar gigantism," a pattern seen in several groups of marine invertebrates. The leading explanation is oxygen — cold water holds much more dissolved oxygen than warm water, and cold-adapted animals also have slower metabolisms, so an animal that relies on oxygen simply diffusing across its body surface (as sea spiders do) can afford to grow much larger in cold, oxygen-rich water before its interior tissues start to run short. Warming seas are therefore a specific concern for these giants.

No. Sea spiders have no venom, no biting jaws capable of hurting a human, and no interest in anything larger than a small soft invertebrate. They are slow, delicate animals that most people never notice even when snorkelling or rock-pooling over them, because they look like bits of algae or hydroid and move very little. Divers and aquarists sometimes find them clinging to anemones and soft corals, which they feed on, and in a reef aquarium a sea spider that has arrived on live rock can be a minor nuisance to prized corals, but it poses no risk to people. Handling one risks injuring the spider far more than the handler.