Ultrastructural features of aporocotylid blood flukes: The tegument and sensory receptors of Sanguinicola inermis Plehn, 1905 from the pike Esox lucius, with a comparative analysis of their traits within the Neodermata

This work represents the first ultrastructural study of the tegument of a freshwater teleost-infecting aporocotylid blood fluke and the first detailed observations of the surface sensory receptors of any aporocotylid. The work was carried out on Sanguinicola inermis from the branchial arteries of the pike, Esox lucius. Light and scanning electron microscopy observations failed to show the presence of protruding surface spines in S. inermis. Transmission electron microscopy studies revealed, deep beneath the distal cytoplasm of the tegument, sporadic, membrane-bound sarcoplasmic processes, each containing a single spine with the typical crystalline structure of digenean spines. On rare occasions, the spine may protrude above the body surface, but in such cases the distal tegumental cytoplasm is extremely flattened. This finding supports a unique, deep origin of aporocotylid spines when compared to those of other digenean families. This character may well prove to be an autapomorphy of the Aporocotylidae, and the position of the base of aporocotylid spines within sarcoplasmic processes supports their origin as a muscle derivative. Unlike other digenean groups, the distal region of the cytoplasmic processes, connecting sunken tegumental perikarya with the syncytial tegumental cytoplasm, is strengthened by peripheral microtubules. Three types of ciliated and three types of non-ciliated sensory receptors were distinguished in the tegument of S. inermis. One of the distinctive features of the ciliated receptors is the unusual position of the basal region of the cilium within a tegumental outgrowth. Such a position has previously been described for certain receptors types in larval and adult schistosomatids and larval diplostomids, suggesting that this feature, in adult worms, might prove to be an apomorphic trait for some or all groups of blood flukes.