Increased haemolymph osmolality suggests a new route for behavioural manipulation of Talorchestia quoyana (Amphipoda: Talitridae) by its mermithid parasite

1. Hairworms (phylum Nematomorpha) and mermithid nematodes (phylum Nematoda) have independently evolved almost identical life cycles: both parasites cause their terrestrial arthropod hosts to seek water, required for their aquatic free-living adult stage. Hairworms achieve this by altering host neurotransmitters, but the mechanisms used by mermithids remain unknown. 2. The physiological effects of the mermithid nematode Thaumamermis zealandica on its host, the supralittoral talitrid amphipod Talorchestia quoyana, are investigated. The parasite develops in the haemocoel, and induces the host to burrow more deeply than healthy amphipods and the adult T. zelandica emerges from the host into moist sand at these greater depths. 3. Parasitized amphipods had higher haemolymph osmolality than unparasitized amphipods. There was no difference in haemolymph Na, K or Mg concentrations between parasitized and unparasitized amphipods. 4. Water content did not differ between parasitized and unparasitized amphipods. Lipid reserves were lower in parasitized male amphipods than in unparasitized males; there was no difference among females. 5. Increase of host haemolymph osmolality by T. zealandica could induce ‘thirst’, explaining why parasitized amphipods seek water-saturated sand. This mechanism appears more parsimonious than that used by nematomorphs to achieve the same change in host behaviour.