Novel Ultrastructural Features of Clinostomum cutaneum in Nile tilapia revealed by Scanning Electron Microscopy

Abstract

The parasitic trematode Clinostomum presents a huge challenge to freshwater fish species, particularly Nile tilapia (Oreochromis niloticus), which plays a vital role in both aquaculture and wild fisheries. In a parasitological survey of 157 Nile tilapia samples from fish farms in the Upper Tana River Region, Kenya, 17.2% of the samples dissected and screened for parasites were found to be infected with Clinostomum metacercariae, encysted in the buccal cavity, skin and gills. This study used an integrative identification approach that combined traditional morphological techniques with ultrastructural analysis using scanning electron microscopy (SEM) and molecular methods targeting ribosomal regions (ITS1, 5.8S, ITS2, and 28S). The Clinostomum metacercariae were identified both morphologically and molecularly as C. cutaneum and C. phalacrocoracis. SEM revealed some new ultrastructural features specific to C. cutaneum. One of these is an excretory pore surrounded by minute, spiny papillae, a feature which had not been reported in previous studies. Another finding we observed was the presence of an everted cirrus in the metacercariae, a feature seen in the adult stage of C. cutaneum. However, in our samples, the cirrus lacked basal papillae, showing that there is morphological variation between the adult and metacercarial stages. Additionally, the tegumental area around the genital pore was surrounded by dome-shaped papillae. Our findings, therefore, provide new taxonomic features for the metacercariae of C. cutaneum and demonstrate the value of SEM as a complementary tool for more precise parasite identification and species differentiation.