Confocal fluorescent study of the fish blood flukes: the serotonergic elements and ultrastructure of the nervous system of adult Sanguinicola plehnae (Digenea: Sanguinicolidae)

The first data on the neurochemical and ultrastructural organisation of the nervous system of the fish blood fluke, suckerless adult Sanguinicola plehnae Warren et Bullard in Warren, Poddubnaya, Zhokhov, Reyda, Choudhury et Bullard, 2023 (Digenea: Aporocotylidae) from the circulatory system of pike, Esox lucius Linnaeus are presented. Based on 5-HT-IP staining, the simple, uniformly developed orthogonal pattern of S. plehnae nervous system is revealed. The ventral and dorsal nerve cords originate from the brain lobes, but the lateral nerve cords originate from anterior nerves at the level of the large serotonergic neurons. In addition, several pairs of such large 5-HT-IP neurons (22-23.5 µm in diameter) are revealed along the ventral nerve cords. Unusual spindle-shaped 5-HT-IP perikarya (7.8-19.8 µm in diameter) are observed along each ventral and lateral nerve cords. The neuroblasts and developing neurons are seen between neurites in S. plehnae along with neuron somata scattered around neuropil periphery, evidencing the renewal of neuron somata population in adult digeneans. The morphological variability of both the orthogonal pattern and neuron somata and types of neurovesicles in adult digeneans are discussed.