Pycnogonid Morphology
Intertidal sea spider from Australia

Pycnogonids have a very thin body, four pairs of walking legs (five or six pairs in some genera), a pair of chelicerae, a pair of pedipalps, and a pair of ovigerous legs or ovigers. Ovigerous legs, which are located between the pedipalps and the first pair of walking legs, are unique to pycnogonids. These appendages have a variety of functions including: grooming by both sexes (Cole, 1909; Hedgpeth, 1947; King, 1973; Brusca and Brusca, 1990; Bain, 1992; Ruppert and Barnes, 1994); food handling (Hoek, 1881; Cole, 1901b; B. Bain, personal observation); courtship, mating, and egg transfer from the female to the male (Cole, 1901a; Cole, 1901b; Cole, 1906; Hooper, 1980; Nakamura and Sekiguchi, 1980); and transport of eggs and larvae by the males (Hedgpeth, 1947; King, 1973; Brusca and Brusca, 1990; Bain, 1991; Bain, 1992; Ruppert and Barnes, 1994). In the more primitive pycnogonids, the last four segments of the ovigerous leg have one or more rows of compound spines. The shape and arrangement of these spines are species specific (Bain, 1992) and are very helpful in separating closely related species. A handful of pycnogonid species have either one (decapodous) or two (dodecapodous) extra pairs of walking legs. Hedgpeth (1947) attributes these extra legs to a polymorphism, but a more likely explanation is that they are just duplicated segments since many of the decapodous and dodecapodous species are otherwise very similar to octopodous forms found in the same area.

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