Sea spider
Marine arthropods with tiny bodies and enormous legs.
Sea spiders are marine arthropods of the class Pycnogonida, also called pycnogonids. Despite their name and slight resemblance, they are not spiders nor even arachnids; their traditional classification as members of chelicerates alongside horseshoe crabs and arachnids has regained wide support. They are notable for their often enormous walking legs relative to a reduced body, giving an 'all legs' or 'no body' appearance.
- field
- Marine arthropods (class Pycnogonida)
- known_for
- Extremely reduced body with long walking legs; unique proboscis; distribution of digestive and reproductive systems into leg diverticula
- leg span
- 1 mm to over 70 cm
- habitat
- Cosmopolitan in oceans; most are small in shallow depths, larger in Antarctic and deep waters
Lore & Background
Sea spiders have a body divided into three tagmata: cephalon (head), trunk, and abdomen. The cephalon includes a unique proboscis with three-fold symmetry and a Y-shaped mouth, an ocular tubercle with up to two pairs of simple eyes (median ocelli), and up to four pairs of appendages: chelifores, palps, ovigers, and first walking legs. The chelifores terminate in a pincer like chelicerae; palps are used for sensing; ovigers are used for cleaning and, in males, carrying offspring. The abdomen is unsegmented and vestigial in extant species, terminating with the anus.
Reader's Guide
Sea spiders are significant for their unusual morphology and evolutionary position. Their body is so reduced that the digestive and reproductive systems extend into branched diverticula within the legs. The homology of their appendages has been debated, but neuroanatomic and genetic evidence confirms a one-to-one alignment with the prosomal appendages of other chelicerates, though the leg pair order is mismatched. Their proboscis is unique among arthropods, and its homology remains enigmatic. The class includes the only extant order Pantopoda, with fossil species dating to the early or mid-Paleozoic. Polymerous species with five or six leg pairs exist in several families. Their cosmopolitan distribution and range in size from 1 mm to over 70 cm highlight their ecological diversity.
Did You Know?
- Sea spiders are not spiders or arachnids; they are chelicerates related to horseshoe crabs and arachnids.
- Their digestive and reproductive organs extend into branched diverticula within the legs because the body is too small to contain them.
- Some species have five or six pairs of walking legs, known as polymerous species.
- The proboscis has three-fold symmetry and a Y-shaped mouth, unique to pycnogonids.
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