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Enigma of mud snails shell growth: asymmetrical
competition or character displacement?
Hydrobiid snails belong to the most important deposit feeding invertebrates
of Northern European estuaries. In Scandinavian waters four species
are found: Potamopyrgus jenkinsi, and the three Hydrobia
spp., H.ventrosa Montagu, H.neglecta Muus and
H.ulvae (Pennant) (Muus,1967). In the White Sea only two
species occure: H.ulvae and H.ventrosa (Gorbushin,1992).
The species are ecologically and morphologically very similar (Muus,1963;
1967; Charrill & James,1985; Gorbushin,1992). Field observations
and laboratory experiments (Fenchel,1975b; Fenchel & Kofoed,
1976; Charrill & James, 1987b) indicate that where more than
one species occurs together in the same habitat, they compete and
evidence for three potential effects of such interspecific competition
exists. Fenchel (1975a,b) has shown that morphological differences
(shell size) between Hydrobia species in the Limfjord, Denmark
are larger in coexisting populations than in single-species ones.
The mud snails feed upon microorganisms associated with sediment
particles (Lopez & Levinton 1978). Fenchel (1975b) and Fenchel
& Kofoed (1976) suppose that competitive interactions relate
to the size of particles ingested and have led to evolution of larger
H.ulvae in sympatry than allopatry and conversely smaller
H.ventrosa in sympatry than allopatry. They suggested that
character displacement with regard to body size was a result of
co-evolution of competitors, thereby permitting H.ventrosa
and H.ulvae to co-exist by utilising different parts of a
common resource (most probably food). However, character displacement
as a result of co-evolution is notoriously easier to postulate than
to demonstrate conclusively (Levinton,1982). Hylleberg (1986) and
Cherrill & James (1987a) failed to find any evidence for hereditary
nature of the phenomenon in other mixed Danish populations and in
East Anglia. Cherrill & James (1987a) believe that differences
in character states attributed to the process of character displacement
may result from a number of other causes. Environmental conditions
at sympatric and allopatric sites may act differentially on the
size of H.ulvae and H.ventrosa. The conditions that
prevail at sites supporting both H.ulvae and H.ventrosa
appear to lead to increased body size of H.ulvae irrespective
of the presence of H.ventrosa. This assumption is confirmed
by results of Saloniemi's (1993) study at the Finnish coast of the
Baltic Sea. The results give an environment-based explanation for
the character displacement pattern and shows the need for critical
evaluation of the evidence for character displacement in Hydrobia
snails in other populations as well.
The character used to measure displacement
in Hydrobia species is difference in shell size. However,
there have been no studies of Hydrobia growth in connection
with this problem. Information however exists (Rothschild &
Rothschild,1939) that shell growth (at least in H.ulvae)
is a very sensitive process which is strongly dependent for example
on environmental conditions and parasitic sterilisation by trematodes.
In this paper I present data on the influence of different population
densities and presence of congeners on growth rate and survival
of H.ventrosa and H.ulvae in cages established at
a site at which the species coexist. PDF-684kb
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