GASTROPOD TORSION: A TEST OF GARSTANG'S HYPOTHESIS

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Abstract
Torsion occurs in gastropod molluscs as a 180° twisting of the shell and viscera relative to the head and foot of the veliger larva. Garstang (1928, 1929) proposed, and it has since become widely accepted, that torsion functions as a larval defense by allowing veligers to first pull the head into the shell, then sealing the aperture with the foot and operculum. However, when we offered pre-torsional and torted larvae of the abalone Haliotis kamtschatkana Jonas as prey to seven planktonic predator species from four phyla, in only one case was rate of predation reduced on torted larvae. It therefore appears that torsion does not function defensively, indicating that other selective pressures probably maintained this trait in primitive gastropods.