Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, Université Claude Bernard Lyon I, 2 rue Raphaël Dubois, 69622, Villeurbanne, France.
Biol Rev Camb Philos Soc. 2013 May;88(2):406-26. doi: 10.1111/brv.12011. Epub 2012 Dec 24.
The cuticle of crustaceans bears numerous organs, of which the functions of many are unknown. One of these, the sensory dorsal organ (SDO), is present in a wide diversity of taxa. Here we critically review the variability, ultrastructure, distribution, and possible function of this enigmatic cuticular organ. Previous data are complemented by new observations on larvae and adults of various malacostracans. The SDO is composed of four sensors arranged as the corners of a square, the centre of which is occupied by a gland. Pores or pegs surrounding this central complex may also form part of the organ. The arrangement and the external aspect of the five main elements varies greatly, but this apparently has little impact on their ultrastructural organisation. The sensors and the gland are associated with a particularly thin cuticle. Each sensor contains four outer dendritic segments and the central gland is made of a single large cell. It is not yet known what this large cell secretes. The SDO is innervated from the tritocerebrum and therefore belongs to the third cephalic segment. A similar organ, here called the posterior SDO, has been repeatedly observed more posteriorly on the carapace. It resembles the SDO but has a greater number of sensors (usually six, but up to ten) apparently associated with only two outer dendritic segments. The SDO and the posterior SDO are known in the Eumalacostraca, the Hoplocarida, and the Phyllocarida. Some branchiopods also possess a 'dorsal organ' resembling both the SDO and the ion-transporting organ more typical of this group. This may indicate a common origin for these two functionally distinct groups of organs. New observations on the posterior SDO support the hypothesis that the SDO and the posterior SDO are homologous to the lattice organ complexes of the costracans. However, the relationship between the SDO and the dorsal cephalic hump of calanoid copepods remains unclear. No correlation can be demonstrated between the presence of a SDO and a particular ecological or biological trait. In fossils, the most convincing examples of SDO-like organs are found in some Late Cambrian arthropods from the Alum Shale of southern Sweden. They suggest that related organs might have been present in non-crustacean Cambrian arthropods. The distribution of the SDO and posterior SDO in extant and fossil crustaceans strongly suggests that these organs originated early in the history of the group, and are crucial to the functioning of these organisms. However, except for knowing that the sensors are chemoreceptors and that in a given organ a functional relationship probably exists between them and the gland, little is known about this function. The description of a SDO in freshwater carideans, which can be easily reared in a laboratory, opens the way for behavioural and physiological experiments to be undertaken that could prove crucial for the determination of this function.
甲壳动物的外骨骼上有许多器官，其中许多器官的功能尚不清楚。其中一个器官是感觉背器官（SDO），它存在于广泛的分类群中。本文对这个神秘的外骨骼器官的变异性、超微结构、分布和可能的功能进行了批判性的综述。以前的数据通过对各种软甲纲幼虫和成虫的新观察得到了补充。SDO 由四个传感器组成，排列成正方形的四个角，中央是一个腺体。围绕这个中央复合体的孔或钉也可能构成器官的一部分。五个主要元素的排列和外部外观变化很大，但这似乎对它们的超微结构组织影响不大。传感器和腺体与特别薄的外骨骼有关。每个传感器包含四个外部树突段，中央腺体由单个大细胞组成。目前还不知道这个大细胞分泌什么。SDO 由前脑三叶神经支配，因此属于第三个头节。在后甲上更靠后的位置，我们也观察到了一个类似的器官，这里称为后 SDO。它与 SDO 相似，但有更多的传感器（通常为六个，但多达十个），显然只与两个外部树突段有关。SDO 和后 SDO 存在于真软甲亚纲、端足目和叶足纲中。一些鳃足类也有一个“背器官”，类似于 SDO 和更典型的离子转运器官。这可能表明这两个功能截然不同的器官群具有共同的起源。后 SDO 的新观察结果支持这样的假设，即 SDO 和后 SDO 与外骨骼动物的晶格器官复合体同源。然而，SDO 与桡足类的头甲背隆之间的关系仍不清楚。不能证明 SDO 的存在与特定的生态或生物特征之间存在相关性。在化石中，最令人信服的 SDO 样器官的例子是在瑞典南部 Alum Shale 的一些晚寒武纪节肢动物中发现的。它们表明，相关的器官可能存在于非甲壳类寒武纪节肢动物中。SDO 和后 SDO 在现存和化石甲壳动物中的分布强烈表明，这些器官起源于该群体的早期，对这些生物的功能至关重要。然而，除了知道传感器是化学感受器，并且在给定的器官中，它们与腺体之间可能存在功能关系之外，对这种功能知之甚少。SDO 在淡水长臂虾类中的描述，这种虾类可以很容易地在实验室中饲养，为进行行为和生理实验开辟了道路，这些实验可能对确定这种功能至关重要。
