Pycnogonid anatomy is a study in reduction. The body is a slim central rod, and the digestive caeca run out along each walking leg to absorb nutrients where the volume is. Reproductive organs follow the same pattern, and in many species gonopores open on the legs. There is no dedicated respiratory or excretory system; gas exchange occurs by diffusion across the body surface, which the high surface-to-volume ratio of long thin legs makes workable.
Males in most species carry the eggs. A specialised pair of appendages called ovigers holds the egg masses, often for weeks, until the larvae hatch. The arrangement is close to universal in the group.
Antarctic and deep-sea species reach far larger sizes than shallow tropical ones, a pattern known as polar gigantism, often linked to cold water holding more dissolved oxygen. Sea spiders' position in the arthropod tree has been debated for decades, with molecular evidence generally placing them as an early-branching chelicerate lineage.