Host-parasite relationships of the common goby, Pomatoschistus microps

Pomatoschistus microps at Salthouse Point, Burry Inlet, Wales return to the estuary and spawn during June, the 1-group adults then disappear and presumably die but the developing juveniles become heavily parasitized with overdispersed, innocuous, encysted digenean metacercariae namely Cryptocotyle jejuna and C.lingua in subcutaneous muscle, C.concava (Heterophydae), in the body cavity, Labratrema minimus (Bucephalidae), in the liver, Timoniella imbutiforme and T.praeterita (Acanthostoidae) in subcutaneous muscle. Host weight, condition and other growth parameters do not appear to be affected by these parasites. However, declining rates of heterophyid infections after August suggest that host defence responses or hyperparasitism may kill some parasites. Gobies leave the estuary after November and, as suggested by increasing C.lingua and L.minimus infections, may migrate to overwinter near West Cross, Swansea Bay. Sixteen allozyme loci were resolved in these fish, using gel electrophoresis based on a sample of 575 fish. Five of these loci are polymorphic namely LDH, IDH, PGM-1, PGM-2 and PGI. The mean heterozygosity per locus is 0.06 - 0.004 and individual heterozygosity is not usually correlated with host growth parameters but is usually negatively correlated with parasite burden. The relationship is statistically significant, however, only with C.jejuna in September, the sole parasite species which elicits an intense melanized host defence response namely 'black spot' disease. The possible reasons for this are discussed in detail. In Pomatoschistus minutus at Oldbury, Severn Estuary, very lightly parasitized with the larvae of Ligula intestinalis (Cestoda: Pseudophyllidea) and Hysterothylacium sp. (Nematoda: Anisakidae), five of seventeen loci are polymorphic. The mean heterozygosity per locus is 0.03 - 0.014. The results provide equivocal support for the parasite version of the Red Queen hypothesis which predicts that the most heavily parasitized populations will have the highest mean heterozygosity and, within populations, individuals with least parasites will have highest heterozygosity.