Pennington J Timothy, Chia Fu-Shiang
Biol Bull. 1985 Oct;169(2):391-396. doi: 10.2307/1541490.
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.
扭转现象发生在腹足纲软体动物中，表现为贝壳和内脏相对于担轮幼虫的头部和足部发生180°的扭转。加斯特唐（1928年、1929年）提出，扭转起到幼虫防御的作用，即担轮幼虫先将头部缩回壳内，然后用足部和厣封闭壳口，这一观点后来被广泛接受。然而，当我们将皱纹盘鲍担轮幼虫扭转前和扭转后的幼虫作为猎物提供给来自四个门的七种浮游捕食者物种时，只有在一种情况下，扭转幼虫的被捕食率有所降低。因此，扭转似乎并没有起到防御作用，这表明其他选择压力可能在原始腹足纲动物中维持了这一特征。
