Fróðskaparrit - 01.01.2008, Page 162

160
PARASITES OF THREE-SPINED STICKLEBACK, CASTEROSTEUS ACULEATUS L.
FROM FAROE ISLANDS
tempt was made to cultivate or section the
infected places, thus allowing further identi-
fication.
DISCUSSION
The Faroe Islands were a center of glaciation
during the Pleistocene and were entirely
glaciated (Johansen, 1975). Based upon sed-
imentation data deglaciation probably oc-
curred prior to 10400-10000 BP (Wastegárd
et al„ 2001). Thus the stickleback popula-
tions and their parasites described above
may have arrived as early as, or just prior to,
this time period. Ongoing genetic and mor-
phologic research (Makinen, 2007) with
three-spined stickleback populations has re-
vealed that the sample from Rituvik does
not fit the pattern of other marine samples
and is quite distinct from other Scandinavian
populations (Baltic Sea, North Sea and Bar-
ents Sea). Faroe sticklebacks in freshwater,
also show a high degree of isolation, perhaps
sub-speciation or even speciation, according
to Friðriksson (2005). This diversity in the
host population has been labeled as a phy-
logenetic raceme model (Bell and Foster,
1994), in which a core of marine populations
has diverged repeatedly into many freshwa-
ter populations.
One might expect that dramatic differ-
entiation in host populations would be re-
flected in high parasite speciation due to the
isolation of small populations with subse-
quent genetic drift accompanied by strong
local selection. Such does not appear to be
the case. None of the parasites found here
are unique to the Faroe Island populations
of Gasterosteus aculeatus (Barber, 2007). For
those species utilizing a bird as a final host,
such as Schistocephalus solidus, the ease with
which the final hosts (in this case larid gulls)
transverse long distances to nest and find
food provides an explanation of why specia-
tion has not occurred regularly in such
groups.
Flowever, Trichodina domerguei, T. tenui-
dens, Gyrodactylus sp. and Thersitina gas-
terostei all have direct life cycles. Also, with
the possible exception of T. gasterostei, this
group has a much shorter generation time
than its host. Such situations have been cited
as leading to speciation in other fish para-
sites including Gyrocotyle spp. (Bristow,
1992), Cestoda (Renaud et al„ 1990) and Di-
genea (Gibson and Køie, 1991). In each of
the papers mentioned, isolation due to gla-
ciation, was cited as the mechanism for gen-
erating a new parasite species, without host
speciation occurring. In Gibson and Køie
(1991) the host species was Gasterosteus ac-
uleatus.
The difference between Faroese fresh-
water and marine biota seems very distinct.
Thersitinagasterorstei is only found in marine
samples and Schistocephalus solidus has been
observed only in freshwater samples. Ther-
sitina seems to be restriced to open sea lo-
calities (Rituvík and Kráargil) because the
sticklebacks in the sheltered locality Kaldbak
were not infected. Schistocephalus solidus
also showed a somewhat varied pattern of
infestation by locality, and there are clear/
significant differences seen in different years
(p< 0.01).
Overall, the biodiversity of the parasites
of sticklebacks in the Faroes is low, with a
maximum of 4 species per population. It is
also quite even by species count with the
minimum being 3. The total number of par-
asite species recorded world-wide for Gas-