Hickman C S, Hadfield M G
Department of Integrative Biology and Museum of Paleontology, University of California, Berkeley 94720, USA.
Biol Bull. 2001 Jun;200(3):257-60. doi: 10.2307/1543507.
The causes and effects of ontogenetic torsion in gastropods have been debated intensely for more than a century (1-19). Occurring rapidly and very early in development, torsion figures prominently in shaping both the larval and adult body plans. We show that mechanical explanations of the ontogenetic event that invoke contraction of larval retractor muscles are inadequate to explain the observed consequences in some gastropods. The classic mechanical explanation of Crofts (4, 5) and subsequent refinements of her explanation have been based on species with rigid larval shell properties (18, 19) that cannot be extrapolated to all gastropods. We present visual evidence of the lack of rigidity of the uncalcified larval shell in a basal trochid gastropod, Margarites pupillus (Gould), and provide photographic confirmation of our prediction that larval retractor muscle contraction is insufficient to produce more than local deformation or dimpling at the site of muscle insertion. These findings do not refute muscular contraction as a primary cause of ontogenetic torsion in gastropods that calcify their larval shells prior to the onset of torsion, nor do they refute the monophyly of torsion. They do, however, suggest that torsion may be a loosely constrained developmental process with multiple pathways to the more constrained end result (20, 21).
腹足纲动物个体发育过程中扭转现象的成因及影响已被激烈争论了一个多世纪（参考文献1 - 19）。扭转在发育过程中迅速且很早就发生，在塑造幼虫和成虫的身体结构方面起着重要作用。我们发现，用幼虫牵缩肌收缩来解释这一个体发育事件的力学理论，不足以解释某些腹足纲动物中观察到的结果。克罗夫茨（4, 5）的经典力学解释以及后来对其解释的完善，都是基于具有坚硬幼虫壳特性的物种（18, 19），而这些特性不能外推到所有腹足纲动物。我们展示了一种基眼目腹足纲动物——瞳孔珍珠螺（玛格丽特·普皮勒斯，古尔德命名）未钙化幼虫壳缺乏刚性的视觉证据，并提供了照片证实我们的预测：幼虫牵缩肌收缩不足以在肌肉插入部位产生超过局部变形或凹陷的效果。这些发现并不反驳肌肉收缩是在扭转开始前就钙化其幼虫壳的腹足纲动物个体发育扭转的主要原因，也不反驳扭转的单系性。然而，它们确实表明，扭转可能是一个约束较松的发育过程，有多种途径通向约束更强的最终结果（20, 21）。
