Oregon Institute of Marine Biology, University of Oregon, Charleston, OR 97420, USA.
Front Zool. 2010 Dec 2;7(1):30. doi: 10.1186/1742-9994-7-30.
The nemertean pilidium is one of the most notable planktotrophic larval types among marine invertebrates. The juvenile forms inside the larva from a series of isolated rudiments, called the imaginal discs. The development culminates in catastrophic metamorphosis, in which the larval body is consumed by the juvenile worm. Although the pilidium was first described in 1847, and is commonly found among marine plankton, there is not a single complete description of its development. The few published studies of pilidial development are based on observations of typically unidentified larvae opportunistically collected from plankton at various developmental stages.
The development of Micrura alaskensis, a common Northwest Pacific coast intertidal nemertean, is described from fertilization to metamorphosis. A staging scheme is proposed based on characteristic developmental milestones. Three pairs of imaginal discs develop as invaginations of larval epidermis. The cephalic discs invaginate from the larval epidermis above the ciliated band, while the cerebral organ discs and the trunk discs invaginate below the ciliated band. All paired imaginal disc invaginations are closely associated with different portions of the larval ciliated band. In addition, two unpaired rudiments contribute to the juvenile - the proboscis rudiment and the dorsal rudiment, which do not develop as invaginations. A pair of thick-walled esophageal pouches previously thought to represent nephridial rudiments give rise to the juvenile foregut. Branched rudiments of protonephridia, and their efferent ducts are also described. Larval and juvenile serotonergic nervous systems are briefly described. Development of the juvenile is completed by 5-8 weeks at 11-15 degrees C. During the rapid metamorphosis the juvenile emerges from and devours the larva.
This study is the first description of pilidial development from fertilization to metamorphosis in a single species. It is illustrated with photomicrographs of live larvae, diagrams, confocal images, and videos. The findings are discussed in the context of previously published accounts of pilidial development, with which they disagree on several accounts. The results described here indicate a different number, origin and fate of various juvenile rudiments. The proposed staging scheme will be useful in subsequent studies of pilidial development.
纽形动物的担轮幼虫是海洋无脊椎动物中最显著的浮游幼虫类型之一。幼虫体内的幼体由一系列孤立的原基发育而来，称为成虫盘。发育的最终结果是灾难性的变态，幼虫的身体被幼体虫吃掉。尽管担轮幼虫于 1847 年首次被描述，并且在海洋浮游生物中很常见，但没有对其发育过程的单一完整描述。少数关于担轮幼虫发育的已发表研究基于对在不同发育阶段从浮游生物中偶然收集的通常未识别幼虫的观察。
描述了阿拉斯加沿岸常见的潮间带纽形动物 Micrura alaskensis 的发育过程，从受精到变态。根据特征性发育里程碑提出了一个分期方案。三对成虫盘作为幼虫表皮的内陷而发育。头盘从有纤毛带上方的幼虫表皮内陷，而脑器官盘和体盘从有纤毛带下方内陷。所有成对的成虫盘内陷都与幼虫纤毛带的不同部分密切相关。此外，两个不成对的原基有助于幼体的形成——吻原基和背原基，它们不会内陷。一对以前被认为代表肾原基的厚壁食管囊形成幼体前肠。分支的原肾管原基及其排泄管也被描述。幼虫和幼体的 5-羟色胺能神经系统也被简要描述。在 11-15 摄氏度下，5-8 周内完成幼体发育。在快速变态过程中，幼体从幼虫中出现并吞噬幼虫。
本研究首次描述了单一物种从受精到变态的担轮幼虫发育过程。它用活幼虫的照片、图表、共聚焦图像和视频进行说明。研究结果在以前发表的担轮幼虫发育的论述中进行了讨论，在几个方面与它们存在分歧。这里描述的结果表明，各种幼体原基的数量、起源和命运不同。提出的分期方案将有助于随后对担轮幼虫发育的研究。
