Invasive crayfish and their symbionts in the Greater London area: new data and the fate of Astacus leptodactylus in the Serpentine and Long Water Lakes.
The branchial cavities of four non-indigenous crayfish species resident in the Greater London area were examined for crustacean symbionts. Pacifastacus leniusculus from Cripsey Brook, Chipping Ongar, Essex, and the Serpentine Lake, Central London, both carried the symbiotic ostracod Uncinocythere occidentalis, which is indigenous to the western USA. Analysis of covariance showed that crayfish carapace length did have an effect on the abundance of entocytherid associates, whereas intensity did not seem to be dependent on whether the host was female or male. Another invasive entocytherid, Ankylocythere sinuosa, was found on the gills of Procambarus clarkii in Hampstead Heath, representing the first record of the species for northwestern Europe. Contrary to recent model-based predictions, A. sinuosa appears to survive British winters. Examination of the gills of Astacus leptodactylus from the Serpentine revealed the presence of two symbiotic copepods, Nitocra hibernica and Acanthocyclops sp., and a common but accidental association with the cladoceran Bosmina longirostris. The observation of N. hibernica on all three P. leniusculus specimens collected in 2010 suggests that the copepod switched crayfish hosts in the Serpentine following the decimation of the Turkish crayfish population after 2008. This is the first record of an indigenous copepod becoming associated with a non-indigenous crayfish, a case of unusual host switching having previously been reported only for branchiobdellidans. The deliberate introduction of signal crayfish in the Serpentine can probably be attributed to aquarists, 'well intentioned' individuals and/or people who have animal release as part of their religious practice. Serpentine signal crayfish that were tested for Aphanomyces astaci (crayfish plague) produced inconclusive results. The sudden collapse of the Turkish crayfish population in the Serpentine between 2008 and 2010 remains unexplained although a number of potentially causative factors are discussed. No crustacean symbionts were obtained from Orconectes virilis in the River Lee at Enfield, Middlesex.