Biomechanics of Locomotion in Antarctic Sea Spiders (Pycnogonida)

Eight‐legged locomotion within chelicerates and other arthropod groups allows a diverse array of maneuvers (e.g. variable speeds; forwards, sideways and backwards walking; low turning radius turns) on uneven substrate. During straight path locomotion, an alternating tetrapod gait cycle (R1, R3, L2, L4 to L1, L3, R2, R4) is characteristic in terrestrial eight‐legged species. However, far less is known about octopod locomotion in aquatic or marine arthropods which, like their terrestrial relatives, face highly uneven and variable substrates, but also live in a highly resistant fluid medium. While aboard the RV Laurence M. Gould , we investigated the walking kinematics of Antarctic sea spiders (Pycnogonida) in the genera Pallenopsis , Colossendeis , Nymphon , and Austropallene collected from the Southern Ocean near the Antarctic Peninsula. In all groups, absolute and relative walking speeds were comparatively slow, with absolute walking speed ranging from 0.05 to 0.2 cm/sec, and relative walking speed ranging from 0.07 to 0.11 body lengths/sec. While footfall patterns sometimes approximated an alternating tetrapod gait, the gait pattern was variable and often non‐symmetrical, with leg pairs commonly transitioning from out of phase to in phase within the same individual. Extremely cold temperatures, low relative muscle mass, and a resistant fluid medium likely contribute to the extremely slow walking speeds of sea spiders, while their variable gait patterns compared to those of terrestrial spiders suggest that aquatic and terrestrial habitats impose different selective pressures on central pattern generators controlling leg movements and walking. Support or Funding Information A.R.M. and S.P.R. were supported by research grant ANT‐1043670 by the US National Science Foundation (NSF). K.M.H. was supported by research grant ANT‐1043745 by the NSF.