Different parasite species sharing the same intermediate host species may have similar or conflicting interests, depending
on whether they are at the same stage in their life-cycle or whether they share the same definitive host. In the New Zealand
cockle, Austrovenus stutchburyi, metacercariae of the digenean Meiogymnophallus sp. are positively associated with
metacercariae of Curtuteria australis. This relationship is found in different cockle samples, and is independent of cockle
shell size, which suggests that it is not merely the product of metacercariae accumulation over time. Both digenean species
have the same definitive host, oystercatchers. Metacercariae of C. australis manipulate the phenotype of cockles, impairing
the cockle's ability to burrow in the sediments. This makes the host more susceptible to oystercatcher predation. Thus
Meiogymnophallus sp. can benefit by associating with C. australis and may hitch a ride with the manipulator parasite. This
is supported by the finding that cockles impaired by C. australis and lying at the sediment surface harbour greater numbers
of Meiogymnophallus than buried cockles. A third digenean species, whose sporocysts are found in cockles and which is
not transmitted by predation, occurred only in surface cockles. Finally, a parasitic copepod with a direct life-cycle was
found evenly distributed among buried and surface cockles, independently of their metacercarial loads. These results show
that different parasite species do not use cockles in a random fashion, and that not all patterns of host use are consistent
with shared or conflicting interests among parasites.
Differential patterns of definitive host use by two fish acanthocephalans occurring in sympatry: Pomphorhynchus laevis and Pomphorhynchus tereticollis