Biology Reference
In-Depth Information
Figure 2.2. The polyopisthocotylean monogenean Eurysorchis australis from the gill
arches, pseudobranchs and mouth cavity of Seriolella brama in New Zealand. Most
species of this group have clamps consisting of two valves that can grasp gill filaments
(see Figure
2.3
), but this species is attached by means of flat sucker-like clamps to flat
surfaces. From Rohde, Roubal, and Hewitt (
1980
). Reprinted by permission of the
Royal Society of New Zealand.
and cannot live elsewhere. It is unlikely that any of the species interact
with the others because of their spatial segregation, but it cannot be
excluded that Eurysorchis australis, which has a sucker apparently second-
arily modified from a clamp-like one as in Neogrubea seriolellae, has been
forced into a new microhabitat away from the gill filaments by competi-
tion in the past (''the ghost of competition past'', Connell
1980
), for
which assumption, however, there is no evidence. Syncoelium, on the
other hand, one of the very few trematodes (if not the only one) infecting
the gills and the only one using its sucker for attachment to the spines of
the gill arches, must have invaded the gills secondarily in the past, coming
from the digestive tract where most trematodes live. There is no reason to
assume that at any stage in the evolutionary past it had to compete with
other species in the new microhabitat (unless, of course, competing
species have become extinct, an assumption without basis). In other
words, Syncoelium very likely associated individualistically with the gill
community of parasites. Also, it cannot be excluded that other species
trying to enter the community in the past have been ''rejected'' because
