J . Grudemo, T. Bohlin J. Exp. Mar. Biol. Ecol. 253 2000 115 –127
117
therefore how competition within and between the species affects growth and thereby possibly body size. Indeed, competition over food within and among Hydrobia species
seems to be important in nature Fenchel, 1975b; Fenchel and Kofoed, 1976; Hylleberg, 1986; Levinton, 1985; Cherril and James, 1987b; Morrisey, 1987; Gorbushin, 1996.
Morrisey 1987 found that growth of H
. ulvae decreased rapidly due to intraspecific
22
competition up to a population density of 10–20 000 m , and then more slowly.
22
Growth stopped entirely at 50 000 m . Fenchel and Kofoed 1976 suggested from
experiments that intra- and interspecific competition were of the same strength in H .
ulvae and H . ventrosa, whereas Cherril and James 1987b concluded, from a field
experiment, that intraspecific competition was stronger than interspecific. Gorbushin 1996 measured growth in the field and found that competition between H
. ulvae and H
. ventrosa was asymmetrical with H. ulvae as the stronger competitor. Thus, although these three studies indicate that the two species do compete, the conclusions concerning
the effect of this are conflicting. However, none of them satisfied the demands for replication and unconfounded design of the competition levels Underwood, 1986, so
the conclusions should be interpreted cautiously. Our second objective was therefore estimate the effect of intra- and interspecific competition on growth in these species
using an experimental set-up specifically designed for this purpose.
The observation that abundance and body size are correlated to sediment quality, together with the evidence for competition under natural conditions, leads to three
questions of interest, not previously investigated experimentally: does sediment type affect the outcome of competition between the species? Is the predominance of H
. ventrosa on finer substrates caused by a greater competitive ability on such sediments.
And, is H . ulvae the better competitor on coarser substrates?
To investigate these questions further we conducted an experiment where the main purpose was to test the following hypotheses:
1. both species grow faster on fine-grained sediment than on coarse-grained Chatfield, 1972; Fish and Fish, 1974; Morrisey, 1990;
2. intra- and interspecific competition is of the same strength Fenchel, 1975b; 3. intraspecific competition is stronger than interspecific in both species Cherril and
James, 1987b 4. competition is asymmetric, with H
. ulvae as the stronger competitor Gorbushin, 1996;
5. the effect of interspecific competition is dependent of sediment type. In the experiment, we used shell growth of young individuals as the response variable.
We thereby assumed that growth is a predictor of fitness of non-reproductive individuals.
2. Materials and methods
2.1. Experimental set-up During 2 months, from middle August to middle October 1995, we studied the growth
of small individuals of H . ulvae and H. ventrosa under different treatments. The
118 J
. Grudemo, T. Bohlin J. Exp. Mar. Biol. Ecol. 253 2000 115 –127
¨ ¨
¨ experiment was performed at Tjarno Marine Biological Laboratory, nearby Stromstad at
the Swedish West coast. Snails were placed on a 1-cm thick sediment layer in a plastic
2
cup with a bottom area of 13 cm . The cups were placed in a climate chamber with a light regime 16:8 h light dark cycle and in running sea water 30‰ salinity that could
reach the inside of the cups through a net that served to keep the snails within the cups
2
aperture size, 0.8 cm ; mesh size, 0.5 mm. To make sure that water was changed, all water in the cups was drained and refilled once every second day. The water was taken
from 30 m depth in the nearby Koster fjord. The temperature of the water was ambient and decreased gradually from 158C in the beginning of the experiment and 108C in the
end. The cups were lighted by fluorescent tubes with appropriate light for algae growth.
2.2. Sediments and snails ¨
All snails and sediments were collected in the almost atidal area around Stromstad at the Swedish West Coast Grudemo and Johannesson, 1999. In each plastic cup, we put
one randomly chosen marked snail from one of the species. This snail was used for growth measurement and will further be called the target snail. Most cups also contained
other snails, used as competitors Fig. 1. Before and after growth, we measured the length of the target snails from the apex to the anterior part of the margin to the nearest
0.04 mm with an ocular micrometer. At the start of the experiment, the size of the target H
. ulvae individuals were 1.5860.09 mm n 5 240, mean6S.D.. The size target H. ventrosa individuals were 1.5960.11 mm n 5 240. As a measure of the growth rate of
these, we used the absolute length increment, which is a relevant growth estimate when, as in this experiment, the target individuals are of equal initial size. The size of the
competitors of both species was 2.6–3.0 mm, thus 1.6 to 1.9 times larger than the target snails.
The properties of the sediments used were determined with the method in Buchanan 1984. The weight of the silt-clay fraction , 64 mm as a proportion of the weight of
all fractions smaller than 1 mm was used to characterise the sediment. On the basis of ¨
this, we divided the 16 sediments collected in the area around Stromstad at the Swedish West coast, into two sediment types, one fine-grained with eight sediments with
45.6621.9 mean6S.D. silt and clay fractions , 64 mm, and one coarse-grained eight sediments with 4.461.1 mean6S.D. silt and clay. Before the start of the
experiment, we picked out most of the snails from each of the 16 sediments and froze them for 3 days to kill undiscovered individuals. After melting, an unfrozen, well-picked
sample of each sediment was added to restore the native microalgal flora.
2.3. Competition treatments We used five levels of competition in the experiment Fig. 1: level 1, no competition
22
1 target snail of H . ulvae or H. ventrosa: population density, 770 m
; level 2, medium competition from H
. ulvae one target snail 1 seven H. ulvae competitor snails:
22
total population density 6160 m ; level 3, high competition from H
. ulvae one target
22
snail 1 23 H . ulvae competitor snails: total population density 18 480 m
; level 4,
J .
Grudemo ,
T .
Bohlin J
. Exp
. Mar
. Biol
. Ecol
. 253
2000 115
– 127
119 Fig. 1. Sketch of the competition treatments. In five of them, the studied species was H
. ulvae upper row, d, and in five H. ventrosa lower row, j. The growth of the target individual filled symbol was estimated under the densities given in the figure s, H
. ulvae; h, H. ventrosa.
120 J
. Grudemo, T. Bohlin J. Exp. Mar. Biol. Ecol. 253 2000 115 –127
medium competition from H . ventrosa one target snail 1 seven H. ventrosa competitor
22
snails: density 6160 m ; and level 5, high competition from H
. ventrosa one target
22
snail 1 23 H .ventrosa competitor snails: density 18 480 m
. These figures were chosen to represent low, intermediate and high population density and competition
Fenchel, 1975a; Morrisey, 1987. 2.4. Statistical analyses
Growth of the target snails was analysed with a four-factor analysis of variance ANOVA. In this, there were three orthogonal factors, Species SP, a fixed factor with
two levels H . ulvae and H. ventrosa; Competition C, a fixed factor with five levels
see above; and Sediment type ST, a fixed factor with two levels sandy or silty sediment, see above. The fourth factor was a random Bay B factor with eight levels,
nested under Sediment type ST. That is, eight sediments came from bays with coarse-grained sediment and eight from bays with fine-grained sediments.
This experimental design resulted in an ANOVA with the following linear model: x
5 m 1 SP 1 Cj 1 ST 1 BST
1 SP 3 C
ijklm i
k lk
ij
1 SP 3 ST 1 SP 3 BST 1 C 3 ST
ik ilk
jk
1 C 3 BST 1 SP 3 C 3 ST
jlk ijk
1 SP 3 C 3 BST 1 e
1
ijlk ijklm
where x is the growth of a specific target individual,
m is the growth averaged over
ijklm
all treatments, and e is the residual.
ijklm
We used three replications of each combination of treatments, resulting in a total number of 480 target snails. To obtain appropriate interactions Hurlbert and White,
1993, and to reduce the heterogeneity in variances, we used the logarithm of growth lngrowth11 in the ANOVA. Degrees of freedom and expected mean sum of squares
were calculated with the method in Underwood 1997, pp. 364–369. To obtain a balanced statistical analysis, we replaced dead snails with the average growth of the
survivors within the same treatment, and we accordingly reduced the degrees of freedom in the residual Underwood, 1997. Student–Newman–Keul’s SNK test was used as
post-hoc test to unveil differences among levels of the significant factors and interac- tions. If the relationship between growth and level of competition is similar between
species, then the interaction term SP3C will not be significant. If, on the other hand, intra- or interspecific competition is more important, the interaction effect will become
large.
3. Results
