Gorbushin A.M. 1996 The enigma of mud snail shell growth: asymmetrical competition or character displacement?
Oikos, 77, 85-92
. 
Abstract:
The growth rate and survival of Hydrobia ventrosa and H.ulvae
from the naturally co-existing populations were investigated when kept together
and separately in cages settled in the intertidal pool. Population density ranged
from 2800 to 22000 ind./m2. Density dependent mortality was not found. No influence
of related species on mortality was observed. However, growth variation of snails
suggested for strong interspecific competition. Despite large differences in
size of H.ulvae and young H.ventrosa used in experiments
(size ratio: initial - 1.8; final - 1.4), the effect of interspecific competition
was significant. Moreover interspecific competition was asymmetrical. At population
densities of 7000 - 14000 ind/m2 with H.ulvae present, H.ventrosa
grew slower than in single - species population. In contrast, H.ulvae
under the same conditions increased in growth rate in comparison to a single
- species population. The competition coefficient (a ), as a function of population
density based on growth rate of individuals was calculated. The proportion of
inter- and intraspecific competition in their joint influence on Hydrobia
growth rate depends on population density. This correlation was non-linear.
With population density close to natural H.ulvae growth is limited
by intraspecific competition. H.ventrosa growth is limited by
interspecific competition. It may be supposed that stable coexistence of these
two species is possible only in populations with dominating of H.ventrosa.
Moreover, the larger the proportion of H.ventrosa, the higher
should be the growth rate. In contrast the correlation between growth rate of
H.ulvae and proportion of this species should be negative.
The population density and presence of congener has a great impact
on shell size as can be shown after only three months of cages experiments.
The growth rate of the mud snails varies within the wide range of the norm response
on food availability. It contradicts the opinion of hereditary nature of character
displacement of H.ulvae and H.ventrosa when they
co-exist and allows to adopt an environment-based explanation of the phenomenon.
Materials and methods.
The investigations were carried out in the estuary of the Keret river (Chupa Bay region of Kandalaksha shore of White Sea). The Kandalaksha shore of White Sea mainly consists of rock and gravel. Large silt or sand beaches and salt marshes are very rare and usually exist in the innermost parts of bays and lagoons. The salinity in the open parts does not exceed 27 ‰ in bays and lagoons is usually lower because of significant fresh flow from the shore. This region is characterised by seasonal changes of temperature, salinity and light. The intertidal zone is covered by the ice for almost half a year (from the first half of November to second half of May). Ice melting in spring leads to a significant salinity decrease of the upper water layers, and as a result biological summer in the intertidal zone often begins only in the first part of June. Autumn frosts are possible already in early September. Already this brief description can give an impression of severe conditions at the marginal part of both H.ventrosa and H.ulvae areas.
The snails were kept in cages placed in the intertidal zone at the south-east end of Malyi Gorelyi island. The study site supports natural population of H.ulvae and H.ventrosa. The tidal amplitude is 1.3 m. The salinity depends on tidal state and season and ranged from 3 to 15 ‰. The overall Hydrobia density in 1991 - 1992 was 5000 -7000 ind./m². The sediment is muddy and anoxic below 1.0-1.5 cm.
The intensity of interspecific and intraspecific competition of H.ulvae and H.ventrosa was measured by shell growth and survival parameters during experiment using enclosed populations. Experimental animals were collected by sieving natural sediment through sieves which retain snails larger than 0.3 mm. These were sorted according to species and size in the laboratory. Equal-sized snails from the more numerous age group were taken for the experiment. The diameter of the last shell whorl was used as a size parameter. For experiments in 1991 H.ventrosa and H.ulvae with shell diameter of 1.56 mm (s.d. 0.21) and 1.88 mm (s.d. 0.16) respectively were used. The snails of both species have survived one winter and their age at the start of the experiments was around 8-10 months. In similar experiments in 1992 two age groups of H.ventrosa were used: snails that survived one winter (8 months old) with a last whorl diameter of 1.10 mm (s.d. 0.15) and snails that survived 2 winters (22 months old) with whorl diameter of 1.88 mm (s.d. 0.16); H.ulvae snails in the 1992 experiment had overwintered once and had a shell whorl diameter of 1.96 mm (s.d. 0.13) (10 months old).
The cages were made of a nylon net with a mesh size 0.3 mm which was taut on a wire frame shape with a vertical side of 6 cm and a square base with sides of 25, 16, 12 or 10 cm. The basal areas were 1/16, 1/39, 1/69 or 1/100 m² respectively. In the 1991 experiments three snail densities were tested: 3200, 7800 and 20000 ind./m², i.e. each cage with base square of 1/16, 1/40 and 1/100 m² contained by 200 snails. In the 1992 experiments four population densities were tested: 2880, 7800, 13880 and 22000 ind./m², i.e. in the cages with base square of 1/16, 1/39, 1/69 and 1/100 m² 180, 200, 200 and 220 snails were placed respectively.
Thus for experiments in 1991 9 cages were used (Table 1). In
1992 12 cages were used (Table. 2).
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The cages were placed on the bottom of an intertidal pool (with depth 4 - 5 cm) at the mid tidal zone. Each cage was pushed 1 cm in the sediment. The silt was sieved through the upper side of the cage (roof) so that the sediment level inside the cages was equal to the surrounding sediment level. During the experiments visual control of cage's content was carried out at regular intervals through transparent net walls. The experiment started at the beginning of the snail's growth season (Table 3). The line of winter growth interruption served as a natural mark of the start of experiment.
At the end of the growth experiments snails were taken from the cages and fixed with 70% ethanol.
The following measurements were taken from the experimental snails
(basic method): shell diameter at the level of the
last winter growth interruption line (d0); shell diameter at the end of experiment
(d); and shell angle gain (
)
from the last winter interruption line (Gorbushin, 1993) to the rim of the shell
aperture. All measurements were taken under the stereo microscope using a standard
ocular - micrometer and ocular angle meter (originally constructed by the author).
For angle growth measurement the shell was vertically oriented so that collumelar
axis was perpendicular to the plane of measurements. The error of diameter measurement
did not exceed 0.05 mm; the angle gain was measured with p /12 accuracy. After
the measurements each snail was examined for trematode infection. In the following
study infected snails not were included in analysis because they differ
in growth rate from uninfected. The rate of infection did not exceed 12
%. In the following comparative analysis of snail's growth we have used only
shell angle gain due to greatest sensibility of this parameter. The average
shell angle gains of both species in the single species and co-existent cages
were subjected to comparative analyses using Student' test. The confidence intervals
for percents of surviving in each cage were computed on ArcSin transformed data.
The quantitative assessment of mutual effect of intra- and interspecific competition was done with using the competition coefficient (a ). Basic idea of the coefficient accounting the ratio of intra- and interspecies competition in general competitive influence stated in the Lotka - Volterra model (Begon,1989). For the purposes of present investigation the calculation way of the coefficient was modified, therefore it makes sense to dwell on it. First of all, as the parameter reflecting effect of competition I used a average growth rate of individuals but not a growth rate of population. Further, contrary to the primary source for calculation I used the valuation of the population density in the cages but not the number of the population.
In this I assumed that H.ulvae specimens equal in number
to Dcu would have the same impact on a shell growth of a single H.ventrosa
individual in a coexistent population as Dav - Dcv of H.ventrosa
specimens in a single species population (Fig.3, H.ventrosa, dotted lines, 1991).
In contrast H.ventrosa specimens equal in number to Dcv would
have the same impact on a shell growth of a single H.ulvae individual
in coexistent population as Dau - Dc of H.ulvae specimens in a
single species population (Fig.3, H.ulvae, dotted lines, 1991).
Thus, the coefficient of competition:
a uv = (Dav - Dcv) / Dcu;
a vu = (Dau - Dcu) / Dcv;
Here Dcv = q * Dc , Dcu = p * Dc : - respectively H.ventrosa
and H.ulvae density in coexistent cage; q,p - proportions of the both
species in the cage; q + p =1; p = q = 0.5;
Dc - total density in coexistent cage; Dav, Dau -
density in cage with single species.
Based on equal angle gain of a snail' shells from coexistent and
single species cages the Dc, Dav, Dau are estimated graphically (Fig.3,
dotted lines, 1991).
Results.
Filamentous algae developed simultaneously in and outside
the cages. This fact is evidence for normal light regime inside cages and suggests
normal conditions for development of diatoms which constitute a considerable
part of Hydrobiids diet. Equal levels of sediment were found inside and outside
cages and thus the processes of water flow and organic matter accumulation inside
and outside cages were similar. Comparatively low mortality of snails during
the investigation was observed. H.ulvae and H.ventrosa survival in cages are
shown on Fig. 2. It could be noticed that snail survival in cages was above
60%. It should be emphasised strongly that these experiments did not show increased
mortality as a result of enhanced intra- or interspecific densities.
 |
| Fig.2. The survival as a function of density. Rectangle - single species population; circle - coexistent population. D1,D2,D3,D1',D2',D3',D4' : - density of cage populations are expressed as an average density between initial and final densities of cage population. Bars - 95 % confidence interval. |
The results of growth experiments for different population densities are shown on Fig.3. Shell growth is expressed as an average angle gain throughout the experiment. The differences in angle gain of H.ventrosa in experiments of 1991 and 1992 are caused by different initial size of snails. For assessing the results of interspecific competition it is necessary to have knowledge of intraspecific competition effects. The total results of intraspecific competition in H.ulvae and H.ventrosa are similar: snail growth decreased as population density increased.
Significant differences in an average shell gain were found
in the 1991 experiments under conditions of coexistent and single species populations
with middle population densities close to natural ones. In coexistent populations
H.ventrosa grew slower and H.ulvae grew
faster than in cages where species were kept separately. Significant differences
in average shell gain of H.ulvae in conditions of coexistent
and single species populations with low and high densities were not found.
| H.ventrosa | H.ulvae | ||
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1991 | 
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1992 | 
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| Fig.3.
The average angle gain of shell in cages -
(N whorls) as a function of density in conditions of coexistent and single
species populations; Continuous line - single species cage population; Dashed
line -coexistent cage population; Bars - 95 % confidence interval; * - significant
differences are found. On the pictures of 1991 year the examples of estimation
of Dc , Dav and Dau are shown (dotted lines). These estimates
have used for calculation of both a uv and a vu (see the text)
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| Fig. 4. Competition coefficient (a ) as a function of coexistent cage population density ( Dc ); Calculated on result of experiments held in years A) 1991, B) 1992. a vu1 - adult H.ventrosa ; a vu2 - juvenile H.ventrosa . a 12 < 1 means that species 2 produces less inhibitory influence on the shell growth of species 1 than species 1 on itself. a 12 > 1 means that inhibitory influence of species 2 on the shell growth of species 1 is expressed to greater degree than inhibitory influence of species 1 on itself. | |||
A similar pattern was found in the 1992 experiments. H.ventrosa
grew slower in conditions of middle population densities and presence of H.ulvae
than when it exists in a single species population. In contrast, H.ulvae in
combination with H.ventrosa had enhanced growth rate in comparison with growth
rate in conditions of single species existence. Thus H.ulvae and H.ventrosa
were apparently subjected to interspecific competition, moreover, the former
of the two is a superior competitor. The intensity of interspecific competition
however differs with different population densities (Fig. 4). For example, H.ventrosa
is not subjected to competition from H.ulvae and the competition coefficient
is equal to 1 when population density was 3000 - 3500 ind./m2 in coexistent
populations with a species ratio of 1:1. Apparently, influences of intraspecific
and interspecific competition were equal for this population density. Further
rising of density led to a rapid increase the intensity of competition which
was maximal with density values close to natural for the given habitat. Then
interspecific competition decreased as density was raised and with population
density of 18000 - 20000 ind./m2 the competition coefficient again tends to
1. This was true only for H.ulvae and juvenile H.ventrosa and can be explained
by increased intraspecific competition the intensity of which becomes equal
to interspecific competition intensity for both species. However, adults of
H.ventrosa were subjected to a slightly different type of interspecific competition
influence at high population densities (18000 -20000 ind./m2) where the effect
of interspecific competition exceeded intraspecific competition.
H.ventrosa 's growth at a population density lower than 3000 - 3500 ind./m2
deserves special attention. At this density H.ventrosa grew larger in H.ulvae
' presence than in a single-species' population. Apparently population density
in these cages was so low that competition was reduced to a intraspecific interaction.
Discussion
Before proceeding I would like to present the working definition of competition: the competition is such interaction between individuals which is elicited by a similar demands on a limited resource and which causes to decrease of survival, growth rate and (or) reproduction of competing individuals.
My experiments have not shown an impact of either interspecific or intraspecific competition on survival of H.ulvae and H.ventrosa. This fact seems to contradict the results of Fenchel's (1976) experiments. However, there are at least two possible explanations for this. Firstly, H.ulvae's longevity in White Sea populations is at least one year more than in Danish populations (Gorbushin, 1993). Secondly, growth experiments in cages in situ are preferable to laboratory experiments. The lack of recruitment in experimental cages results from use of immature snails in these experiments. An exception was with part of the H.ventrosa population in the 1992 experiment. Mature individuals constituted half of the H.ventrosa cage populations and probably bred. However, during this year in the natural H.ventrosa population that the production of recruitment was not observed.
As the experiments have demonstrated the growth rates of H.ulvae
and H.ventrosa vary with the presence of congener since the two
species compete. As would be expected from theory the intensity of competition
between H.ulvae and H.ventrosa depends on their
population densities. More unexpected are results indicating a significant asymmetry
in interspecific competition. H.ulvae is the superior competitor
in conditions of density close to natural and suppresses H.ventrosa's
growth whereas its own growth is unaffected. In contrast, H.ventrosa's
influence on H.ulvae's growth is less than that of H.ulvae
on itself. However in coexistent populations H.ventrosa and H.ulvae
can potentially have population densities and species proportions that the influence
of intra- and interspecific competition will be equalised. Even more, at low
densities H.ventrosa can obtain competitive release when in competition
with H.ulvae and in such a case intraspecific competition becomes
more important. Hence the impact of interspecific competition on growth rate
is possible only within certain limits of population densities.
 |
| Fig.5.
Space of possible realization of niche
(SPRN); SPRN of H.ulvae (U) and SPRN of H.ventrosa (V); filled
areas - realized niches; A - the combination of adult H.ventrosa and H.ulvae; B - the combination of juvenile H.ventrosa and H.ulvae 1 - low population density. Resource consumption takes place in area of optimum. Interspecific competition impact is smaller than that of intraspecific competition or interspecific competition influence is absent. 2 - middle (natural) population density. Realized niches overlap, interspecific competition impact is more significant that the impact of intraspecific one. The realized niche of H.ulvae is overlapping the optimal point of H.ventrosa' s consumption. 3 - high population density. SPRN is exhausted. Interspecific competition effect is equal to intraspecific competition effect in case of H.ulvae and juvenile H.ventrosa combination (B). The first effect is higher for combination of H.ulvae and adult H.ventrosa (A) |
The reasons for asymmetrical competition are difficult to interpret. It could be explained as a result of purposeful competitor suppression by H.ulvae (for instance, by secretion of substances which make the food unsuitable for another species). However in my opinion, there are another possible explanation. The limits of the realized niche can apparently change with changing environmental factors. However the changing of the realized niche can take place only within the space of possible realization of niche (SPRN) which is limited on one hand with the fundamental niche limits and on other hand with the habitat peculiarities (the necessity to introduce this conception becomes clear if we will admit that realised niche in the locality fluctuates in depending on environmental changing). The availability of resources at different points inside this space (SPRN) is not even. The consumption of abundant resources is assumed to be near optimal point. Once resource deficiency occurs consumption moves to less available regions within SPRN. It is assumed that the SPRN of H.ventrosa in food resource aspect (i) is smaller than that of H.ulvae and (ii) is absolutely (in adults) or mostly (in juvenile snails) included within the SPRN of H.ulvae. As this takes place the SPRN of H.ulvae overlaps the optimal point of the SPRN of H.ventrosa (Fig.5). The such relationship can explain (i) the significant asymmetry of competition; (ii) the absence of interspecific competition at low population densities, when intraspecific competition is noticeable and (iii) equal effects of both kinds of competition when population densities are very high and very low.
What are the mechanisms that ensure stable existence of H.ventrosa in coexistent populations with the competitive dominant H.ulvae. Firstly it is possible that even when in "robbing" competition H.ventrosa nevertheless has sufficient resources left for effective reproduction as a result of energy relocation from somatic growth to reproduction. It is obvious that a change in somatic growth rate of competitors only indicates some degree of competitive overlap and by not the final result of competition on population level. This view is indirectly confirmed by the fact that during experiments no density dependent mortality of H.ventrosa occurred.
Another possible mechanism is an asymmetrical proportion of two species in a coexistent population. The smaller the proportion of H.ulvae in a population, the higher is the probability of stable coexistence. It is likely that this occurred in coexistent Hydrobiids populations studied by T.Fenchel (1975a). The analysis of published results show that in 76 % of coexistent H.ulvae and H.ventrosa populations studied the latter species is dominates in number. Cherrill, James (1985) in 6 studied populations found H.ventrosa density was larger and in 1 population was equal to H.ulvae density. In my study area H.ulvae constituted only 34 % of the co-existent population.
A third possible mechanism is sharing of habitat space. Barnes (1991) reports that in most of the East Anglia lagoons with more than one species, for example, the different species are either found in different regions of the shared lagoon or occupy different microhabitats. A very similar situation is described in the White Sea (Gorbushin,1992).
It is known that by means of character displacement, interspecific competition may be relaxed in nature. Having shown that when coexisting the mean size of H.ulvae becomes larger and H.ventrosa smaller. Fenchel (1975b) assumed that by feeding on food particles of different diameter these species avoid competition and are able therefore to found stable coexistent populations. However, as was marked by Hylleberg (1976) "there is an unsolved problem in this system based on particle sizes". A stable population means a stable size - age structure of a population, in which young (and small) individuals dominate in number in comparison with adult (and large). In juveniles the sizes of Hydrobia spp. overlap and young, small H.ulvae have to compete with a large number of H.ventrosa of the same size until it reaches a size allows it to feed with larger particles. It is important to note that just this peculiarity of biology distinguishes the system of the competing mud snails from other ones having more or less well documented evidences on relation between morphological size and size of the food particles. Indeed all of the known me and best documented examples are shown on birds (Lack, 1947; Grant, 1981; 1983; 1986;) and ants (Davidson, 1978). It is noticeably that these species have a strong pronounced parental care. Of course in such systems the youngest overlapping in size individuals are divided spatially and do not compete directly but only through the parents. The size differentiation (adult size) may relieves exploitation competition in such case. From my point of view the peculiarities of the mud snails biology do not allow to use this possibility.
It seems natural to assume that coexistence of mud snails is more probable when size-age structure of a H.ulvae population is disturbed and unstable. But such disturbances in H.ulvae reproduction do not immediately lead to its extinction from a coexistent population. H.ulvae lives twice as long as H.ventrosa (Chatfield,1972; Siegismund, 1982; Barnes,1990; Gorbushin,1993) and this fact apparently gives an additional reserve for population's survival under such circumstances. Unfortunately, age dynamics of H.ulvae and H.ventrosa have not been studied and I could not check this possibility.
One of the most important results of my experiments is that despite large differences in size of H.ulvae and young H.ventrosa in experiments of 1992 (size ratio: initial -1.8; final in coexistent populations -1.4, final in single species populations -1.3), the effect of interspecies competition was significant. The other principle result may well be relevant to the debate surrounding Fenchel's description of character displacement where the inheritance of character displacement is the most controversial point. If it is accepted that the character displacement of Hydrobia species is hereditable phenomenon we should find a similar mean growth rate under same density independently of congener presence. In the experiments just the another situation occurs. The growth rate of the mud snails simply varies within the wide range of the norm response on food availability.
Character displacement observed in coexistent populations of H.ulvae and H.ventrosa is most probably not a result of the species's co-evolution. The same result could arise faster by another mechanism. The experiment by Rothschild and Rothschild (1939) showed influence of environmental factors on H.ulvae shell's growth. After a breeding in laboratory containers for a year snails grown in smaller containers were smaller than ones in the larger containers. As can be seen from my results population density and presence (or absence) of competitor of the related species has a great impact on shell size as can be shown after only three months of cages experiments.
Saloniemi (1993) showed that H.ulvae mean size has a high positive correlation with the proportion and abundance of H.ventrosa in co-existent populations. The causes of such a correlation become clear, if one takes into account that H.ulvae growth depends more on the intensity of intraspecific competition than on the intensity of interspecific one. It could be assumed that in habitats suitable for the coexistence of these two species (shallow, sheltered, lagoon-like habitats) environmental factors favour the intensive growth of H.ulvae's individuals but don't favour the abundance of this species. The differences in the reproductive strategies between marine and lagoonal H.ulvae's populations studied by Barnes (1988, 1990) confirm this assumption. Under low intensity of intraspecific competition in such habitats H.ulvae grows faster than in single species populations in more typical (more open) marine habitats with high population density.
An obvious deficiency of biotopic space, short summers and considerable environmental instability are peculiarities which distinguish White Sea hydrobiid habitats from those such as British and Denmark waters. Undoubtedly competitive relations in this area are more intense and the result of competition (with large asymmetry) perhaps appears to be more dramatic for H.ventrosa. It is likely that the absence of H.neglecta in the White Sea region is a result of a competitive "sandwich" formed by H.ventrosa and H.ulvae!
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