Occurrence and nutritional relationships of four ectosymbiotes of the freshwater crayfish Cherax dispar Riek and Cherax punctatus Clark (Crustacea : Decapoda) in Queensland

Abstract

The occurrence and nutritional relationships of a suite of four ectosymbiotes from the freshwater crayfishes, C. dispar and C. punctatus in Queensland have been studied. The suite comprises the temnocephalid flatworms Temnocephala minor and Craspedella spenceri, the rhabdocoel flatworm Didymorchis cherapsis and the polychaete Stratiodrilus novaehollandiae. Occurrence of all four symbiotes appears to be independent of host species, host size and the presence of other members of the suite. As crayfish become scarcer in cooler, drier weather, worm numbers per host decline. The largest ectosymbiote, T. minor, lives on the external body surface and appears to be more susceptible to dry conditions than do the other symbiotes from within the branchial chamber. This species has no nutritional relationships with the other three species and uses its host primarily as a feeding platform from which it captures annelids and small arthropods; there is some evidence of supplementary opportunistic ectocommensalism. S. novaehollandiae feeds on the microflora of the branchial chamber; its eggs may be eaten by C. spenceri but this species feeds predominantly on co-symbiotic protozoa. D. cherapsis, in contrast, feeds mainly on the eggs and has small crustaceans as its secondary food resource. S. novaehollandiae beyond the egg stage is not attacked by either flatworm. The digestive physiology and food reserves of all four species conform to the pattern known for related ectosymbiotic and free-living species.