Journal of Experimental Marine Biology and Ecology 253 2000 115–127
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Effects of sediment type and intra- and interspecific competition on growth rate of the marine snails Hydrobia
ulvae and Hydrobia ventrosa
Johan Grudemo , Torgny Bohlin
¨ ¨
Department of Zoology , Goteborg University, Box 463, SE-405 30 Goteborg, Sweden
Received 21 January 2000; received in revised form 22 May 2000; accepted 30 June 2000
Abstract
The combined effects of sediment grain size and intra- and interspecific competition on growth rate of Hydrobia ulvae and H
. ventrosa were investigated in a 2-month laboratory experiment. Individuals of H
. ulvae grew faster than H. ventrosa at all competition intensities. Growth rate did not differ between fine- and coarse-grained sediments in any of the species. This suggests that
other factors than the sediment quality cause the often observed size differences between Hydrobia snails from these sediment types in nature. Competition from 1.6 to 1.9 times larger competitors
affected growth of the smaller target snails. Competition between the species was asymmetrical and, in both species, per capita effect of competition was stronger from H
. ulvae than from H. ventrosa, regardless of sediment type. From growth, Lotka-Volterra competition coefficients were
estimated to 0.2 from H . ventrosa on H. ulvae and 1.8 from H. ulvae on H. ventrosa. Since
competition between the species seems to be independent of sediment grain size, the shift in numerical dominance in nature, from H
. ulvae on coarse-grained sediment to H. ventrosa on fine-grained, could not be explained with different competitive ability in different environments.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Competition; Hydrobia; Sediment; Growth rate; Character displacement; Experimental design
1. Introduction
The relation between body size and competition among closely related species has been a recurrent subject during the last decades, ever since the influential papers by
Brown and Wilson 1956 and Hutchinson 1959. The size in sympatry and allopatry of
Corresponding author. Fax: 146-31-416-729. E-mail address
: johan.grudemozool.gu.se J. Grudemo. 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
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. Grudemo, T. Bohlin J. Exp. Mar. Biol. Ecol. 253 2000 115 –127
the two marine mud-snails Hydrobia ulvae Pennant and H . ventrosa Montagu, which
are common on littoral muddy sediments, and which partly coexist in this seemingly homogeneous habitat, is one example. Fenchel 1975b found that the two species were
of similar size when living apart from each other in allopatric localities, but that H .
ulvae was larger than H . ventrosa when together in sympatric localities. A similar size
segregation was also found in Sweden Grudemo and Johannesson, 1999, although H .
ventrosa tended to be larger than H . ulvae in allopatry. In contrast, other studies from
Denmark Hylleberg, 1986 and England Cherril and James, 1987a showed no size difference between allopatry and sympatry.
Fenchel 1975b proposed that the reason for the observed size pattern was character displacement, caused by body size-dependent food size preference, with the same
relation in both species. Hence, the two species were proposed to be affected by limiting similarity for body size, which has led to genetically based divergence in size when
coexisting. These size relations in Hydrobia are among the most widely cited to support or illustrate character displacement.
Other studies, however, have stressed the possibility of non-genetic environmental causes salinity, sediment quality, water movements, etc. to this pattern Hylleberg,
1986; Cherril and James, 1987a; Saloniemi, 1993. H . ulvae is often larger on
fine-grained sediment than on coarse-grained Chatfield, 1972; Fish and Fish, 1974; Morrisey, 1990; Grudemo and Johannesson, 1999; but see Barnes 1988 for a divergent
observation. H . ventrosa is less well studied; at the Swedish West coast they are,
however, like H . ulvae, generally larger on fine-grained than on coarse-grained sediment
Grudemo and Johannesson, 1999. It is thus possible that size differences might be caused by differences in sediment type, or by conditions correlated to this factor, such as
water movements Barnes, 1988. In addition, the distribution of the species is correlated to sediment type, at least at the Swedish West coast, where H
. ulvae dominates on coarse-grained sediments and H
. ventrosa on fine-grained Grudemo and Johannesson, 1999. It can therefore not be excluded that the combined effects of the environment on
the distribution and body size may be an alternative to the character displacement explanation.
In this study we attempted to explore this possibility further. Since growth has a role in determining body size, a first question was if differences in sediment type including
differences in grain size, water content, organic content and other differences lead to differences in growth rate. This might explain why both species generally are larger on
silty sediments. Such differences may occur either if food production or the efficiency to utilise this depends on sediment type. For example, food production may be higher on
fine-grained sediments due to larger particle surface Newell, 1965. Previous studies on the effect of sediment type on the growth of Hydrobia species give no solid information.
For H
. totteni, Levinton 1982 found no difference in growth between one fine-grained and one coarse-grained sediment. Unfortunately, the experiment was unreplicated and
his results thereby circumstantial. Morrisey 1990 studied growth of H . ulvae on
sediments of different particle size in the field with conflicting results, partly because of poor replication. In addition, his study was conducted in the field, where sediment
structure is correlated with other factors, and the sediment effect thereby not possible to estimate.
Growth may also depend on population density. A second question of concern was
J . Grudemo, T. Bohlin J. Exp. Mar. Biol. Ecol. 253 2000 115 –127
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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
