Navigation within host tissues: cues for orientation of Diplostomum spathaceum (Trematoda) in fish towards veins, head and eye.

Cercariae of Diplostomum spathaceum penetrate the skin of fish, and then migrate along blood vessels and tissues towards the head and the eye-lens. We studied their orientation behaviour in tail fins of guppies and in chemical concentration gradients within agar-filled choice chambers. In fins, they entered veins and orientated cranially, independent of the blood flow and living cells. In choice chambers, they were attracted by a small molecular fraction of fish serum, D-glucose (at 1, 10, and 1000 microM), D-mannose, D-maltotriose and Cl-ions, whereas D-glucosamine repelled them (even at 1.0 nM). Amino acids were not attractive, but arginine in tetrapeptides attracted at concentrations as low as 1 microm and melatonin at 0.4-4.3 PM. We suggest a preliminary model for the behaviour of diplostomula in fish fins and attracting (+) or repelling (-) host cues: (1) migration towards deeper skin layers and avoidance of skin surface, cues: Cl-ions (+ and -), glucose (+), glucosamine (-), light radiation (-); (2) orientation in cranial direction, cue: Cl-ions (+); (3) localization of blood vessels, cues: glucose (+), arginine-residues (+); (4) localization of the retina, cue: melatonin (+). A comparison with the navigation mechanisms of tissue-migrating schistosomules and hookworm larvae reveals an enormous diversity of strategies.