Biology Reference
In-Depth Information
australiensis harbors up to six in the stomach and large digestive ducts.
There is simply not sufficient space for larger numbers of parasites in
Cerithium (as well as Peristernia). In other words, it is likely that intraspe-
cific competition for space and food limits parasite numbers. Both
Cerithium and Peristernia are also hosts to several species of digenean
trematodes, some occurring at high prevalences of infection. The popu-
lation dynamics of snails and parasites were studied over several years at
Heron Island, Great Barrier Reef. Over a period of presumably unfavour-
able conditions between January 1971 and April 1972, there was a strong
decrease in the relative numbers of infected Cerithium and Peristernia.
Snails with double infections (Digenea plus Lobatostoma) disappeared
first, i.e., the co-occurrence of other parasite species affected the survival
chances of the aspidogastrean. Infection with Lobatostoma did not affect
the relative numbers of egg-producing snails during the period of high
prevalence of infection.
Snails that are suitable hosts were spread all around the Heron Island,
but only one small flat-bottomed area, ''Shark Bay'', had snail populations
with a high prevalence of infection. The reason is as follows: schools of
juvenile dart follow the incoming tide and feed on large numbers of snails,
indicated by the many shell fragments scattered over the sea bottom at
Shark Bay. At the same time, worm eggs are shed which are eaten by the
snails. In other regions, many of which are inhabited by the same snail
species, few snails are infected or none at all, because the sea floor is too
uneven or too sandy, or the wave action is too strong, for any significant
level of infection of snails to occur. In other words, the local distribution
of the parasite is not determined by resource availability, but is the
consequence of interactions between the physical environment, the
hosts and the infective stages of the parasite. Future research should
attempt, in line with the autecological paradigm, to quantify these inter-
actions. Which abiotic factors determine the distribution of the snail and
fish hosts, under which conditions can infective stages of the parasites
within the eggs survive long enough to facilitate infection of the snails,
and how do movements of snails in different localities affect the like-
lihood of being eaten by the fish?
Comparison of the species, and coevolution
Looking back at and comparing the two species, we find distinct differ-
ences. Each is adapted to its particular host and has evolved morphological
adaptations and life cycles ensuring transmission. In Lobatostoma manteri,
