‘O’ group mullet, Chelon labrosus. were experimentally infected with Cryptocotyle lingua (Heterophyidae) by tail dip in a suspension of cercariae. Metacercariae were excised after 1 and 24 hours and prepared for TEM and post-embedding immunogold labelling. Antisera to cercariae of C. lingua were raised in adult mullet by natural infection via the skin and by intra-peritoneal injection of sonicate. The membrane-bound vesicles within the syncytial lining of the metacercarial excretory vesicle were found to be intensely antigenic with both antisera: the epidermal secretory bodies type 5 within the cystogenousglands gave a positive response. Penetration gland contents were not found to be antigenic with either antiserum. Discharge of the membrane-bound vesicles coinciding with both the reorganization of the lining of the metacercarial excretory vesicle and with cyst wall formation appears to be of significance in the initiation of the host immune response. That the term ‘excretory vesicle’ in Digenea may be a misnomer is discussed in the light of current information regarding the wide range of functions attributed to this structure.

In an earlier study it was shown that Cryptocotyle lingua cercariae, matured in Littorina littorea from a polluted marine lagoon, displayed slower horizontal swimming rates, and reduced longevity compared to cercariae released by periwinkles from a cleaner environment. This work investigated whether the pollution-induced reduction in swimming rates was due to an inefficient swimming action or the adoption of a less direct swimming path. In addition, cercariae from L. littorea that had been transferred from an ‘unpolluted’ to a ‘polluted’ site for 1 month provided information on the speed with which pollutants affect cercariae through their intermediate hosts. Results indicated that, in general, horizontal swimming rates were reduced due to slower swimming rather than disorientation and longer swimming pathways. Effects of host transplantation to a polluted site were clearly evident after 1 month. Evidence suggested that the pollutants accumulated by the cercariae via their first intermediate host affected the neuromusculature associated with swimming performance rather than sensory structures. Bearing in mind the reduced viability of C. lingua cercariae in polluted sites it is assumed that high prevalence of this digenean in gastropods (at such sites) must be due to their continual introduction by infected birds attracted to these habitats from other areas.

Effects of heavy metal pollution on the cercariae of the marine trematode Cryptocotyle lingua (Creplin) were studied by measuring horizontal swimming rate (HSR) and longevity. These factors are important for transmission to the next host, a fish. Cercariae released by Littorina littorea (L.) collected from polluted and unpolluted sites were compared. Both HSR and longevity were significantly reduced in cercariae from the polluted environment. Cercarial quality was therefore reduced, directly or indirectly, by development within a metal-accumulating host. Cercariae released by hosts from a clean environment were subjected to nominal concentrations of 2 and 3 mg/l copper, 1 and 2 mg/l zinc, 2 and 5 mg/l iron and 2 and 4 mg/l manganese in artificial seawater. In all cases the HSRs and longevity were reduced. The effect was more pronounced in the higher concentrations. The significant HSR tests indicate that the absorption and effect of metals occurred within 1 min. The cercarial tegument, specialized for absorption in endoparasitic environments, is possibly responsible. Cercariae may therefore be excellent indicator organisms for pollution. The pollution-induced reductions in cercarial quality seem capable of producing transmission failure. Heavy metal pollution could therefore alter parasite populations and communities.

To study the infection dynamics of metacercariae of the digenean Cryptocotyle lingua, wild living Atlantic cod, Gadus morhua, were caged for 18 months close to the shore. Here they were exposed to naturally occurring transmission stages of the parasite. First, both the abundance and the variance to mean ratio of metacercariae increased, but during the second half of the study the abundance levelled out, and the variance to mean ratio showed a significant decrease. Host mortality was negligible throughout the study. Based on the relationship between pigment spots and metacercariae observed by skin digestion, there was no indication of density-dependent parasite mortality. We conclude that the infrapopulations of metacercariae on the caged cod probably were regulated by density-dependent host responses acting against the cercariae.