Fróðskaparrit - 01.01.2004, Síða 303

FESKVATNS SVAMPAR (PORIFERA: SPONGILLIDAE) I FØROYUM
301
measure 70-115 |jm in length (average 92 |jm). The spiny
gemmoscleres measure 50-90 pm in length (average 68
pm).
Geographical distribution: S. lacustris is widely
recorded from North America, Asia and Europe, mainly
in the cold-teniperate parts.
Discussion
It is surprising that freshwater sponges
have not been recorded from more locali-
ties on the Faroes; the most reasonable ex-
planation is that they are mostly small and
not very obvious, and even if a number of
investigations have been carried through,
also in recent years, nobody looked for
them. The two species recorded, Racekiela
ryderi and Spongilla lacustris, are those
that could be expected when considering
records elsewhere frorn near-coast locali-
ties and small islands off the coast (Annan-
dale, 1908; Stephens, 1920; Waterston and
Lyster, 1979; Økland and Økland; 1996).
A third species, Ephydatia muelleri (Lie-
berkiihn, 1851) could occur in some places
since it has been found scattered in coastal
areas of western Norway (Økland and Øk-
land, 1996), and in Iceland (Tendal, 1976)
and southern Greenland (Tendal, unpubl.).
E. muelleri seems not to tolerate pH lower
than 5.7 and avoids localities with low cal-
cium concentration (Jewell, 1939; Økland
and Økland, 1996).
The characters of the Faroese material
fall well within the known large variation
of the two species (Arndt, 1928; Stephens,
1920; Penney and Racek, 1968; Poirrier,
1977; Økland and Økland, 1989; Pronzato
and Manconi, 2001).
Because R. tyderi has a wider geographi-
cal distribution range and larger ecological
tolerance in eastern USA than in western
Europe, it is considered an American spe-
cies in origin. Since the discovery in Eu-
rope several theories have been offered to
explain the amphiatlantic distribution, both
with respect to the time ol'the presumed ar-
rival from America, and as to the mechanism
behind it (Hanitsch, 1895, summarized in
Økland and Økland, 1989). Although later
reintroduction might have occurred, the hy-
potheses claiming R. ryderi to be a recent
invader in Europe can in principle be ruled
out after the find of postglacial, 8,560 years
old, gemmules of the species in a former
freshwater lake in the Faroes.
Ideas about ice free refugia in the Faroes
during the last ice-age (Spiirck, 1934) are
not supported since, until about 10,000
years ago, the Faroes were, except for the
highest peaks, covered by an own ice-cap
(Jørgensen and Rasmussen, 1986), and no
traces of a local freshwater fauna of that
age or older have been recorded.
R. lyderi reached the islands soon after
the deglaciation. Experience, allhough with
other species of fresh-water sponges, from
other Scandinavian areas indicates that
such fast colonization of “empty” areas is
a general phenomenon (Wesenberg-Lund,
1896; Arndt, 1932a, b; Økland and Økland,
1996).
Where the species came from and by
what means are open questions. Consider-
ing the extent of the still existing ice-caps
and the harsh climate at ihe time northwest
of the Faroes it seems most likely that the
relationships are with more southern popu-
lations, maybe in Ireiand. The spreading
device must have been the gemmules which