Surface ultrastructure of plerocercoids of Bombycirhynchus sphyraenaicum (Pintner, 1930) (Cestoda: Trypanorhyncha)

Light microscopy studies have previously shown that Bombycirhynchus sphyraenaicum is an exceptional trypanorhynch cestode, characterised by a poeciloacanthous armature and two enormous both-ridia, which overlap parts of the pars bulbosa, a character combination unique within the trypanorhynchs. Plerocercoids of B. sphyraenaicum from the fish Lates calcarifer (Centropomidae) were investigated by scanning and transmission electron microscopy. Results revealed that the tegument of the anterior margin of the distal bothridial surface bears three kinds of microtriche; palmate microtriches, 8 μm high, with seven to eight digitiform processes; equalized filamentous microtriches, 5-6 μm long, with cap and base each forming 50% of the length, borne on undulations beneath the palmate microtriches, and cap-dominated filamentous microtriches, 5-6 μm long, with the cap forming 75% of the length, borne on the apex of putative sensory papillae. Integumental connections link the bases of the palmate microtriches, forming transverse girdles around the worm, which may serve to coordinate traction. At regular intervals between the palmate microtriches are papillae, covered with filamentous microtriches extending above the palmate microtriches. A cilium emerges from a bulb at the apex of each papilla; other structures in the bulb include an electron-dense cuff, and two electron dense collars. The posterior part of the bulb tapers and passes into the tegumental cytoplasm. These adorned papillae, observed for the first time in trypanorhynch cestodes, are identified as putative mechanoreceptors.