Centro de Biologia Marinha, Universidade de São Paulo, São Sebastião, São Paulo, Brazil.
PLoS One. 2010 Mar 22;5(3):e9654. doi: 10.1371/journal.pone.0009654.
Sea biscuits and sand dollars diverged from other irregular echinoids approximately 55 million years ago and rapidly dispersed to oceans worldwide. A series of morphological changes were associated with the occupation of sand beds such as flattening of the body, shortening of primary spines, multiplication of podia, and retention of the lantern of Aristotle into adulthood. To investigate the developmental basis of such morphological changes we documented the ontogeny of Clypeaster subdepressus. We obtained gametes from adult specimens by KCl injection and raised the embryos at 26 degrees C. Ciliated blastulae hatched 7.5 h after sperm entry. During gastrulation the archenteron elongated continuously while ectodermal red-pigmented cells migrated synchronously to the apical plate. Pluteus larvae began to feed in 3 d and were 20 d old at metamorphosis; starved larvae died 17 d after fertilization. Postlarval juveniles had neither mouth nor anus nor plates on the aboral side, except for the remnants of larval spicules, but their bilateral symmetry became evident after the resorption of larval tissues. Ossicles of the lantern were present and organized in 5 groups. Each group had 1 tooth, 2 demipyramids, and 2 epiphyses with a rotula in between. Early appendages consisted of 15 spines, 15 podia (2 types), and 5 sphaeridia. Podial types were distributed in accordance to Lovén's rule and the first podium of each ambulacrum was not encircled by the skeleton. Seven days after metamorphosis juveniles began to feed by rasping sand grains with the lantern. Juveniles survived in laboratory cultures for 9 months and died with wide, a single open sphaeridium per ambulacrum, aboral anus, and no differentiated food grooves or petaloids. Tracking the morphogenesis of early juveniles is a necessary step to elucidate the developmental mechanisms of echinoid growth and important groundwork to clarify homologies between irregular urchins.
海胆和沙钱大约在 5500 万年前与其他不规则海胆类群分化，并迅速扩散到世界各地的海洋中。一系列形态变化与在沙床中的生存有关，例如身体变平、初级刺变短、附肢增多以及亚里士多德氏灯保留到成年。为了研究这些形态变化的发育基础，我们记录了短腕海胆的个体发生。我们通过 KCl 注射从成年标本中获得配子，并将胚胎在 26°C 下培养。纤毛囊胚在精子进入后 7.5 小时孵化。在原肠胚形成过程中，肠腔持续伸长，同时外胚层红色色素细胞同步迁移到顶盘。浮游幼虫在 3 天开始进食，在变态时为 20 天大；饥饿的幼虫在受精后 17 天死亡。幼体在变态后没有口或肛门，也没有背面的板，除了幼虫刺的残余物，但它们的双侧对称性在幼虫组织吸收后变得明显。亚里士多德氏灯的骨片存在并组织成 5 组。每组有 1 颗牙、2 个半椎体和 2 个副骨片，中间有一个滑车。早期附肢由 15 根刺、15 个附肢（2 种类型）和 5 个球形体组成。附肢类型符合洛夫恩法则，每个步带的第一个附肢不是由骨骼环绕的。变态后 7 天，幼体开始用亚里士多德氏灯刮沙粒进食。幼体在实验室培养中存活了 9 个月，死亡时具有宽的、每个步带一个开放的球形体、背面肛门、没有分化的食物沟或花瓣状。追踪早期幼体的形态发生是阐明海胆生长发育机制的必要步骤，也是阐明不规则海胆同源性的重要基础工作。
