Walking leg regeneration in the sea spider Nymphon brevirostre Hodge, 1863 (Pycnogonida)

Regeneration is widespread across all animal taxa, but patterns of its distribution and key factors determining regeneration capabilities stay enigmatic. A comparative approach could shed light on the problem, but its efficacy is limited by the fact that data is only available on a few species from derived taxa. Pycnogonida are nested basally within the Chelicerata. They can shed and replace their walking legs and have a high regeneration capacity. In this work, we carried careful observation on leg appendotomy and regeneration processes in a sea spider under laboratory settings. The limb structure and in vivo observation reveal autotomy as the most likely appendotomy mechanism. High regeneration capabilities were ascertained: an anatomically normal but small leg appeared in a single molting cycle and the full functionality regained in 2-3 cycles. Wound closure after appendotomy in N. brevirostre primarily relies on hemolymph coagulation, which apparently differs from both xiphosurans and crustaceans. Regeneration is provided by proliferation in the leg cutpiece. Regenerative morphogenesis resembles the normal ontogenetic morphogenesis of a walking leg, but accelerated. Unlike in most arthropods, in N. brevirostre, regeneration does not necessarily correspond to the molting cycle, inferring a plesiomorphic state.