Sea urchins and their relatives are grouped together taxonomically in the class Echinoidea (phylum Echinodermata). Echinoids exhibit remarkable variation in their pentaradially symmetric body shape phenotypes, from spheroid tests (skeletons) in sea urchins to discoid tests in sand dollars. Researchers have speculated on the morphological evolutionary transitions among these phenotypes without reaching consensus. We designed and performed an experiment in which sea urchins grew under a geometric constraint manifested through restricted vertical spatial dimension physically limiting growth in height. Individuals in this treatment group became flatter relative to individuals in a control group after 8 weeks. These findings may be considered as instantiating phenotypic accommodation, an underappreciated phenomenon that may constitute the first step in novel phenotype evolution. We present preliminary speculations relating phenotypically accommodated changes in shape and morphological evolutionary relationships among echinoids. By demonstrating a plausible initiating mechanism in a morphological transition, we hope to contribute to theoretical discussions about phenotypic evolution, emphasizing the role that physical factors play in changing complex biological systems.

中文翻译：

---

在几何约束下生长的海胆中的表型住宿

海胆及其近亲在分类学上归入棘皮动物纲（棘皮动物门）。从海胆的球体测试（骨骼）到沙钱的盘状测试，海胆类动物的五角对称体形表型表现出显着的变化。研究人员对这些表型之间的形态进化转变进行了推测，但尚未达成共识。我们设计并进行了一项实验，在该实验中，海胆在几何约束下生长，该几何约束通过受限的垂直空间尺寸在物理上限制了高度的生长。8 周后，该治疗组中的个体相对于对照组中的个体变得更平坦。这些发现可以被认为是表型调节的实例化，一种被低估的现象，可能构成新表型进化的第一步。我们提出了初步推测，涉及海胆类动物之间形状和形态进化关系的表型适应变化。通过展示形态转变中一个合理的启动机制，我们希望为关于表型进化的理论讨论做出贡献，强调物理因素在改变复杂生物系统中的作用。