FRANCESCO ROSSI
AMAZONAS
might be fed by turbid streams and/or occur in areas
with limestone or other sources of dissolved minerals.
They are further restricted to special areas of these lakes
where hydrologic conditions favor development and
persistence of the lake ball form.
While distributed across a vast area, known sites
for Marimo Balls and A. linnaei are of disjunct, patchy,
and sparse distribution. The highest concentration of
historical and existing habitats is in Northern Europe,
and there are also concentrations of sites in Japan and
Iceland. Just a few locations are known from northern
North America. The small number of sites listed in
Siberia, an area with tens of thousands of glacial lakes,
probably indicates less sampling effort. In Europe,
Marimo Ball colonies and the attached form of A. linnaei
have been impacted negatively by environmental change
(more about this below) but are known to currently
survive in Scotland, Estonia, the Netherlands, Sweden,
Ireland, Germany, and Ukraine.
Aegagropila linnaei also grows as a mat adhered to
stones or the shells of freshwater bivalves, but occurrence
of the spherical lake ball form is prevalent in broad, shal-
low sandy bays with pronounced wave action. Any small
fragment can form a new Marimo Ball in the right con-
ditions. As the piece is rolled about in the sand, filaments
respond to abrasion at their tips with radial branching.
With continued wave action, the sphere shape develops
as the filaments create a dense, cloth-like mat. This is
very different from the slimy, snot-ball texture of most
freshwater algae. The exclusion of light by the dense fila-
ment growth limits the thickness of the mat, both in the
ball form and the attached mat form, to 2 inches (5 cm).
Marimo Balls greater than 4 inches (10 cm) in diameter
are thus hollow at their centers.Most freshwater algae have annual cycles of growth
and pass the less favorable winter months as spores or
other dormant phases, but A. linnaei is a perennial alga.
Both the mat and the lake ball persist and grow for years,
even decades. The ball grows slowly, accruing just a few
millimeters of diameter each year. In ecological terms, you
could describe A. linnaei as a K-selected organism with low
fecundity and long life, whereas more typical freshwater
algae are r-selected with fast growth and rapid reproduc-
tion. The largest Marimos can reach a size of more than
12 inches (30 cm) and are near perfect spheres. How old
might such an extraordinary lake ball be?
A third growth habit—unconsolidated, free-floating
filaments—can occur in still other conditions, or may
be found drifting among colonies of Marimo Balls. The
filament form might be a way for A. linnaei to occupy
areas of silty lake bottom where neither the ball nor the
mat form can grow. With favorable conditions, such as
dispersal to a shallow sandy area with wave action, the
free-floating filament form can begin to transition to the
Marimo Ball form.
The forces that might have driven evolution of the A.
linnaei lake ball form have inspired vigorous speculation.
Although morphologies, behaviors, and interactions in
nature are not always evolutionarily adaptive, the lake
ball growth habit probably helps A. linnaei to establish
and persist in its native habitats. The shallow, sandy bays
where Marimo Balls develop do not provide the hard sur-Lake Ulemiste in Estonia, where Marimo Balls are formed naturally
in the clear, shallow waters. Such cold lakes, in northern Europe and
Japan, typically have sandy bottoms and the perfect set of wave actions
to cause the A. linnei alga to roll into perfect spheres.