Rickettsiales‐like organisms (RLOs) are intracellular bacteria that have been reported to be symbionts of over 60 species of bivalves and marine gastropod molluscs of aquaculture importance around the world. Morphologically, some of these prokaryotes are similar to bacteria from the order Rickettsiales, and in most cases, they form intracytoplasmic inclusions similar to those observed in bacteria of the family Anaplasmataceae. Genetically, only Candidatus Xenohaliotis californiensis (CXc), a parasite of several abalone species, has been classified as a member of the order Rickettsiales. The other two characterized RLOs are distantly related; one belongs to the same class (α‐proteobacteria), and the other belongs to the γ‐proteobacteria. Hyperparasitism of RLOs by bacteriophages has been reported in several species of mollusc and appears to have a negative effect on RLOs. The intracellular lifestyle of RLOs has been associated with cell alterations including hypertrophy and compression of the cytoplasmic organelles, and in most cases, no host reaction is observed by histology. In some cases, a high infection intensity of RLOs has been related to dysfunction of the affected organs that can result in disease, and in other cases, high burdens of RLOs have been associated with mortality, but most of these cases have yet to be confirmed. To date, only CXc has been confirmed as the aetiological agent of a chronic and potentially lethal disease of several abalone species known as withering syndrome. This disease is listed by the World Organization of Animal Health. With the ever‐increasing interest in preserving fisheries and expanding the aquaculture production of mollusc species, more efforts should be focused on the characterization and study of RLOs to elucidate the type of symbiosis they maintain with their host and determine when they are acting as true pathogens that should be subjected to control in aquaculture. This review provides an overview of the literature on RLOs that infect bivalves and gastropods of aquaculture importance, and we discuss the most important aspects of these unique prokaryotes.

中文翻译：

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双壳类和海洋腹足类动物中的类立克次体微生物：综述

类立克次体细菌（RLOs）是细胞内细菌，据报道是全世界60多种对双壳类和海洋腹足纲软体动物具有重要水产养殖意义的共生体。形态上，一些原核生物的类似于从订单菌立克次氏体，在大多数情况下，它们形成类似于那些在家庭中观察到细菌胞浆内夹杂物的无形体科。从基因上讲，只有Candidatus Xenohaliotis californiensis（C Xc）是几种鲍鱼的寄生虫，被列为立克次体的成员。其他两个具有特征的RLO距离很远。一个属于同一类别（α-变形杆菌），另一个属于γ-变形细菌。在几种软体动物中已经报道了通过噬菌体引起的RLO的超寄生现象，并且似乎对RLO产生了负面影响。RLO的细胞内生活方式与细胞改变有关，包括肥大和细胞质细胞器受压，在大多数情况下，组织学未观察到宿主反应。在某些情况下，RLO的高感染强度与受影响器官的功能障碍有关，可能导致疾病，而在其他情况下，RLO的高负担与死亡率相关，但其中大多数病例尚未得到证实。 。迄今为止，只有CXc已被确认为几种称为枯萎综合症的鲍鱼物种的慢性和潜在致死性疾病的病因。该疾病被世界动物卫生组织列出。随着对保护渔业和扩大软体动物种类的水产养殖兴趣的日益增长，应将更多的精力集中在对RLO的表征和研究上，以阐明它们与宿主保持的共生类型，并确定它们何时才是真正的病原体。应在水产养殖中加以控制。这篇综述提供了有关感染双壳类和腹足类对水产养殖具有重要意义的RLO文献的概述，并且我们讨论了这些独特的原核生物的最重要方面。