LMU Munich, Biocenter, Planegg-Martinsried, Germany.
SNSB, Bavarian State Collection of Zoology, München, Germany.
J Morphol. 2023 Nov;284(11):e21653. doi: 10.1002/jmor.21653.
The freshwater slugs of the genus Acochlidium (Heterobranchia, Gastropoda, and Acochlidimorpha) are peculiar, one to two centimeter sized animals found only in small coastal rivers and streams of Southeast Asian and Western Pacific islands. When first described by Bücking, the author observed a branching "net of dendritic vessels connected to the heart," which he assumed to have replaced the original gastropod gill. In the present study, we compare the renopericardial systems of four Acochlidium species in microanatomical, histological and ultrastructural detail and identify where exactly the enigmatic, subepidermal "dorsal vessels" connect to the renopericardial system to examine if they can really function as a gill. Acochlidium have elaborate renopericardial systems compared to their ancestrally marine and also freshwater relatives. The primary site of ultrafiltration is the epicardium of the atrium with podocytes as usual for gastropods. The "dorsal vessels" in Acochlidium are extensions of the outer epithelium of the pericardial cavity and represent true vessels, that is, coelomatic channels, having an endothelium with podocytes. Hence, they considerably enlarge the site of ultrafiltration increasing the pericardial surface. "Dorsal vessels" in Acochlidium are therefore not homologous to externally similar morphological structures in Sacoglossa (marine panpulmonate slugs and snails). The multiplication of renopericardioducts in Acochlidium is a unique feature within Mollusca that enhances the negative pressure necessary for ultrafiltration in the thin, tube-like dorsal vessels and as a consequence the transport of primary urine from the pericardium to the kidney. The circulatory and excretory systems in Acochlidium are adaptations to a lifestyle in their freshwater environment in which snail bodies are hyposmotic and accrue considerable influx of surplus water into the body, which needs to be expelled.
淡水豆螺属（腹足纲、后鳃亚纲和豆螺目）的淡水豆螺是一种奇特的动物，体长一到两厘米，仅分布于东南亚和西太平洋岛屿的小型沿海河流和溪流中。在 Bücking 首次描述这些豆螺时，他观察到一个分支的“树突状血管网与心脏相连”，他认为这些血管取代了原来的腹足纲鳃。在本研究中，我们比较了四个淡水豆螺物种的肾心包系统的微观解剖学、组织学和超微结构细节，并确定了神秘的皮下“背血管”与肾心包系统的确切连接位置，以检验它们是否真的可以作为鳃发挥作用。与它们的远古海洋和淡水近亲相比，淡水豆螺具有复杂的肾心包系统。超滤的主要部位是心房的心耳，通常具有足细胞，这是腹足动物的特征。淡水豆螺的“背血管”是心包腔外胚层的延伸，代表真正的血管，即体腔通道，具有内皮细胞和足细胞。因此，它们极大地增加了超滤的部位，增加了心包表面。淡水豆螺的“背血管”因此与 Sacoglossa（海洋肺螺类和蜗牛）中外部相似的形态结构没有同源性。淡水豆螺中肾心包管的倍增是软体动物中一个独特的特征，增强了在薄的管状背血管中进行超滤所需的负压，并因此促进了从心包到肾脏的原尿的运输。淡水豆螺的循环和排泄系统是对其在淡水中生活方式的适应，在这种生活方式中，蜗牛的身体是低渗的，会有大量的多余水分进入体内，需要排出。
