Character displacement and coexistence in mud snails (Hydrobiidae)

Summary Populations of coexisting and of allopatrically occurring species of hydrobiid snails (Hydrobia ulvae, H. neglecta, H. ventrosa and Potamopyrgus jenkinsi) have been studied in 90 localities within three different areas. When H. ventrosa coexists with H. ulvae they show character displacement, i.e., the average body size of the former is smaller and that of the latter is larger. When these species live alone they are of approximately the same size. It is shown here that the size ratio between the coexisting species usually found (1.3–1.5) allows stable coexistence based on food praticles size selectivity alone. Variation in the degree of character displacement from locality to locality is explained by different degrees of genetical isolation of the populations. Coexisting H. ventrosa and H. ulvae have shorter, more well-defined periods of reproduction than they do when they occur alone. H. neglecta is larger than coexisting H. ventrosa and smaller than coexisting H. ulvae. Due to the patchy distribution and the fluctuating populations of this species, and due to the fact that pure H. neglecta populations are rare, data on this species are difficult to interpret. Potamopyrgus jenkinsi shows a different food particle size selection than the Hydrobia spp. of identical sizes. Competitive interactions between P. jenkinsi and coexisting Hydrobia spp. are therefore probably weak. In accordance with this, P. jenkinsi does not show character displacement when coexisting with, e.g., H. ventrosa.The fact that the major study area, the Limfjord, is only 150 years old as a marine habitat as well as various more recent man-made changes of the coast line allow estimates of the time scale of the microevolutionary changes which lead to character displacement.     

THE EARLY LIFE-CYCLE STAGES OF HYDROBIA VENTROSA AND HYDROBIA NEGLECTA WITH OBSERVATIONS ON POTAMOPYRGUS JENKINSI

The egg mass of Hydrobia neglecta contains a single egg, whilethat of H. ventrosa contains up to three eggs. At hatching H.neglecta has a significantly greater shell length than H. ventrosaand the late embryos and young snails of the two species canbe separated according to the pigmentation of the head region.The young snails can also be separated on the basis of surfaceornamentation of the shell and comparisons with the closelyrelated H. ulvae and Potamopyrgus jenkinsi show how the basicpattern of shell sculpturing differs in the four species. Thesedifferences are discussed in relation to the mode of development. (Received 14 February 1980;     

Reproductive effort in Danish mudsnails (Hydrobiidae)

Summary The four species of Hydrobiid mudsnails found in Danish brackish water habitats are ranked with respect to reproductive effort. The biology of the species indicate that Hydrobia neglecta and Potamopyrgus jenkinsi, showing the highest reproductive effort are also more r-selected than the other species, H. ventrosa and H. ulvae.     

Tolerance and Resistance in Gastropod Mollusks Hydrobia ulvaeand H. ventrosafrom the White Sea to Abiotic Environmental Factors

Tolerance to salinity changes and resistance to desiccation, fresh water, and freezing were studied in two hydrobiid species from the White Sea, Hydrobia ulvae(Pennant) and H. ventrosaMontagu. It was shown that H. ventrosahas a greater tolerance to low salinities in the range of 6–10 but is less tolerant to high salinities (35–45). The interspecies differences in low salinity tolerance persisted after the acclimation of snails to 20, 16, and 12. A comparison of survival in fresh water and under desiccation conditions suggests that H. ulvaeis more resistant to these factors. In fresh water, LT₅₀was 30 and 60 days for H. ventrosaand H. ulvae, respectively. Under desiccation conditions, LT₅₀was 6 and 25 days for H. ventrosaand H. ulvae, respectively. At subzero temperatures, H. ventrosawas shown to be a superior survivor. Within the same species of mud snail, the detrimental effect of freezing depended on the salinity: survival decreased with decreasing salinity. These data suggest significant differences in the mechanisms of resistance and tolerance to abiotic environmental factors between the two species. Despite the partial overlapping of their ecological niches, the interspecies differences may play an important role in the distribution and spatial structure dynamics of coexisting populations of these species in the White Sea region.     

The Distribution of Hydrobiids and the Effects of Sediment Characteristics on the Population Dynamics of Hydrobia ventrosa in a Coastal Region of the Southern Baltic

The composition of the hydrobiid fauna in a coastal water of the southern Baltic was studied in 1990/91 at three stations with different sediment characteristics (grain size, organic content). Four species were found (Hydrobia ventrosa, H. ulvae, H. neglecta and Potamopyrgus jenkinsi). H. ventrosa was dominant at all stations. The mean hydrobiid abundances were 4,000 (stn. C), 18,000 (stn. B) and 40,000 (stn. A) ind./m² respectively. Abundance was found to correlate with the organic content of the sediment. The reproduction and growth of H. ventrosa was studied. Two reproduction phases were observed, one in summer and the other in late autumn. The autumn reproduction peaks were staggered owing to the different growth rates at the various stations. Possible correlations between the differences in shell size of H. ventrosa at the various stations and both sediment characteristics and abundance are discussed.     

The effect of larval trematodes on the survival rates of two species of mud snails (hydrobiidae) experimentally exposed to desiccation,freezing and anoxia

Digenetic trematodes are widespread among mud snails (Hydrobiidae) living in coastal lagoons and estuaries, but knowledge is generally lacking on their impact on these host organisms. We examined the survival rates of infected and non-infected experimental populations of two mud snail species,Hydrobia ventrosa (Montagu) andHydrobia neglecta Muus, exposed to desiccation, freezing and anoxia in the laboratory. Our experiments indicated that non-infected groups of both species had similar survival rates after being subjected to desiccation and anoxia, whereasH. ventrosa survived freezing better thanH. neglecta. However, infected groups ofH. neglecta specimens subjected to desiccation showed significantly lower survival rates than non-infected groups. Infected and non-infected snails of both species exposed to freezing and anoxia exhibited similar survival rates. The possible mechanisms by which parasites influence their hosts are discussed. It is unlikely that the parasites in the present case mediate the coexistence of the twoHydrobia-species, because the snail with the highest reproductive effort-H. neglecta-showed lower infection rates in situ than its congenerH. ventrosa.     

Evidence for competition between mudsnails (Hydrodiidae): a field experiment

The paper describes an experimental investigation of competition between Hydrobia ulvae and H. ventrosa using enclosed populations at a site at which the species coexist naturally. lntraspecific competition is more intense than interspecific competition and may have a regulatory influence on snail densities. Other experimental studies of competition between mudsnails are reviewed and found to infer strongly the occurrence of competition in natural field populations. However, features of the species' field distributions previously attributed to interspecific competition can be more convincingly explained by other processes.     

Character displacement in style length between pollinator-sharing Clerodendrum trichotomum and C. izuinsulare (Verbenaceae)

Competition for pollination may occur between pollinator-sharing sympatric plants and this may cause character displacement of their floral traits. We examined this possibility by comparing flower morphology of the sympatric population of Clerodendrum trichotomum and its co-flowering congener, C. izuinsulare, with that of the allopatric populations. The two species were visited in common by such insects as diurnal hawkmoths, bees, swallowtails and nocturnal hawkmoths, and were pollinated nocturnally as well as diurnally. Interspecific pollen transfer can occur by sharing pollinators; however, they did not hybridize when artificially pollinated. Flower size, including stamen and style lengths, is larger in C. trichotomum with an overlap in range. The style of C. izuinsulare in the sympatric population was significantly shorter than that in the allopatric population, while there was no significant difference in style length between the allopatric and the sympatric C. trichotomum. This seems to facilitate avoidance of interspecific pollen transfer in the sympatric population.Ken Inoue was killed in an accident on July 28, 2003, during his research trip in Sakhalin.     

Directional mitochondrial introgression and character displacement due to reproductive interference in two closely related Pterostichus ground beetle species

Reproductive interference due to interspecific hybridization can lead to character displacement among related species with overlapping ranges. However, no studies have examined which reproductive traits are most important in reducing reproductive interference. We conducted molecular analyses of two nuclear genes (28S and Wingless) and a mitochondrial gene (COI) from two closely related ground beetle species, Pterostichus thunbergi and Pterostichus habui (Coleoptera: Carabidae), with overlapping distributions. In addition, we examined four reproductive traits (body size, organ morphologies of intromittent and non‐intromittent male genital organs, and female reproductive period) in sympatric and allopatric habitats. We compared male genital morphology using geometric morphometric analysis. The species determined by morphology were classified into separate groups based on the phylogenetic tree constructed by the nuclear gene (Wingless). However, according to the mitochondrial genes examined, P. thunbergi was not monophyletic, whereas at the sympatric sites, these species formed a monophyletic clade. This incongruence suggests that interspecific hybridization and subsequent mitochondrial introgression from P. habui to P. thunbergi have occurred. Concerning genital morphology, both of the intromittent and nonintromittent organs of P. thunbergi differed more from P. habui at the sympatric sites than between allopatric sites, suggesting reproductive character displacement. Pterostichus thunbergi, which likely arrived in P. habui habitat in small numbers, would have experienced stronger selection pressures than P. habui.     

Interspecific body size differentiation in species assemblages of the carabid subgenus Ohomopterus in Japan

Geographic variation and interspecific differentiation in body size (body length) were analyzed for 15 species of the carabid subgenus Ohomopterus (genus Carabus; Coleoptera, Carabidae) in Japan. Local species assemblages of this subgenus consist of up to 5 species of different size classes. These beetles exhibited sexual dimorphism in body size where females are larger than males, except Carabus uenoi, in which the male and female sizes were equivalent, possibly because of the exaggerated male genitalia. In 9 of 15 species, there was a positive correlation between mean body size and annual mean temperature of habitat, representing the converse of Bergmann's rule. However, in some cases this correlation does not hold over the range of a species because of regional differences. When allopatric and sympatric populations were compared, allopatric populations of Carabus albrechti and C. japonicus had larger bodies than sympatric populations. These intraspecific differences may have resulted from character displacement. In each local assemblage with 2 or more species, there was little interspecific overlap of body size, although the body size ratio between two species with adjacent body sizes seldom showed strict constancy. The mean size ratio between 2 adjacent species in an assemblage was reduced with the number of species, whereas the size ratio of the largest to smallest species in an assemblage increased with the number of species (i.e., the expansion of body size range). These results indicate that the body size of Ohomopterus species may have evolved in response to both climatic conditions and interspecific interactions. Because each species or species group represents the same size class over the distribution range and similar-sized species are parapatric or allopatric, the interspecific segregation in body size in local assemblages may have resulted mainly from a size assortment process during colonization. Received: June 8, 2000 / Accepted: October 10, 2000     

Character displacement in sailfin mollies, <Emphasis Type="Italic">Poecilia latipinna</Emphasis>: allozymes and behavior

Synopsis We analyzed variation in allozymes and mating preferences in 12 populations across much of the range of the sailfin molly, Poecilia latipinna. Sailfin mollies can be sympatric with its sexual parasite Amazon mollies, P. formosa. Amazon mollies must co-exist and mate with bisexual males of closely related species (including sailfin mollies) to induce embryogenesis but inheritance is strictly maternal. Where sailfin and Amazon mollies are sympatric there is evidence of reproductive character displacement as males show a significantly stronger mating preference for sailfin molly females over Amazon mollies compared to preferences of males from allopatric populations. From the allozyme data we found a moderate amount of genetic variation across all populations but this variation did not reveal significant partitioning between sympatric and allopatric populations. Additionally, we found no evidence for isolation by distance as genetic distance was not significantly correlated with geographic distance. While allozyme variation also did not significantly correlate with male mating preferences, there was a significant correlation between male mating preferences and geographic distance. This correlation between mating preferences and geographic distance may have arisen from coevolution with Amazon mollies resulting in reproductive character displacement. Taken together, the distribution of genetic and behavioral variation among sympatric and allopatric populations suggests that behavioral evolution has outpaced evolution at the allozyme loci we examined in P. latipinna.     

DISTRIBUTION AND PRODUCTION OF THREE HYDROBIA SPECIES (GASTROPODA: HYDROBIIDAE) IN A SHALLOW COASTAL LAGOON IN THE BAY OF CADIZ, SPAIN

The abundance, life span, growth and production of the mud snailsHydrobia minoricensis, H. ulvae and H. ventrosa in a semi-naturallagoon system were studied by taking monthly samples at threesites during 1991 and 1992. The most abundant species, H. minoricensisoccurred at mean densities of 12834 to 26264 snails m^–2(10.7 to 25.8g dry weigh m^–2), depending on the site.The least abundant species, H. ulvae, occurred at mean densitiesof 185 to 353 snails m^–2 (3.2 to 2.2g dry weight m^–2).The numerical abundance and biomass of the three Hydrobia specieswere positively related to the biomass of benthic macroalgae(P<0.01). Although H. ulvae egg capsules were recorded throughoutthe year, newly hatched snailsof this species were not observed,in contrast to the other two species. The early spring and summercohorts of H. minoricensis and H. ventrosa seemed to be themost numerous. The average life spans of these two species wereestimated to be about 18 and 13 months respectively. Annualproduction estimates for the whole lagoon system were 29.0 (6.3),5.5 (0.8) and 5.2 (1.0)g dry weight (ash-free dry weight) m^–2yr^–1 for H. minoricensis, H. ulvae and H. ventrosa respectively.The annual P/B ratio was about 2 for H. minoricensis and H.ventrosa. (Received 5 July 1994; accepted 5 October 1994)     

Character displacement in giant rhinoceros beetles

I quantitatively analyzed male morphology of two closely related rhinoceros beetles species (Chalcosoma caucasus F. and Chalcosoma atlas L.) in 12 allopatric and seven sympatric locations throughout Southeast Asia. The qualitative features and the magnitude of intraspecific variation of each species were unaltered between allopatric and sympatric locations. Across allopatric locations, body size, horn size, dimorphic dimension, and genitalia size nearly completely overlapped between C. caucasus and C. atlas. Yet, in all sympatric locations, the differences between the two species in these characters were highly significant. While the enlarged difference between the two species in body size in sympatry could be attributed to habitat differentiation, that in genitalia size far exceeded what was expected from the general body-size displacement. These results indicate that morphological character displacement in sympatry was most complete in sexual organs. This may account for the process of existing species conserving themselves as integrated units by avoiding interspecific competition and enhancing reproductive isolation.     

Effects of latitude and allopatry on body size variation in European water shrews

We studied the intra- and interspecific size variability of 271 water shrewsNeomys fodiens (Pennant, 1771) andN. anomalus Cabrera, 1907 from seven sample sites along a latitudinal transect from Bosnia and Herzegovina to Poland.Neomys anomalus was the only water shrew in three Dinaride karst fields, while it was sympatric with N.fodiens in remaining sites. The first principal component scores (PC1; 72.2% of variance explained), derived from principal components analysis of 13 cranial, mandibular and dental measurements, were used as the size factor. One-way ANOVA detected significant interpopulation variation in both species; intraspecific variation, however, was much more pronounced inN. anomalus. No latitudinal size pattern was found in N. fodiens (r = −0.42, p = 0.58), while mean PC1 scores correlated significantly and negatively with latitude inN. anomalus (r = −0.92, p = 0.004). Therefore, along a north to south transect,N. anomalus converged in size towards N. fodiens, which suggests that the former species occupies increasingly more aquatic habitats in the same direction. Individuals from allopatric populations ofN. anomalus from Slovenia and Bosnia and Herzegovina were, on average, larger than sympatric conspecific populations from the same latitudinal zone, which is consistent with the hypothesis of character displacement.     

Character displacement drives floral variation in Pelargonium (Geraniaceae) communities

Interactions between plant community members are an underexplored driver of angiosperm floral variation. We investigate character displacement as a potential contributor to floral variation in Pelargonium communities. Pelargoniums all place pollen on the ventral sides of their pollinators, potentially leading to interspecific pollen transfer (IPT) in sympatry. We show that the positions of pollen placement and receipt are determined by anther and style exsertion lengths. Using field experiments, we demonstrate that heterospecific species experience higher IPT if they have similar style lengths than when they have greater style length differences. Using crosses, we show that IPT has negative consequences on seed set. In combination, these results suggest that character displacement in style length is likely to reduce IPT and increase female fitness in sympatry. Patterns of style length variation across 29 different Pelargonium communities suggest that character displacement has occurred in multiple communities. Furthermore, analyses using a wide-ranging species pair show that style lengths are more different between sympatric populations than they are between allopatric populations. In addition to pollinators as agents of floral divergence, this study suggests that variation in Pelargonium community structure has driven style length variation through character displacement.     

Effects of sediment type and intra- and interspecific competition on growth rate of the marine snails Hydrobia ulvae and Hydrobia ventrosa

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.     

Reproductive character displacement and potential underlying drivers in a species‐rich and florally diverse lineage of tropical angiosperms (Ruellia; Acanthaceae)

Reproductive character displacement is a pattern whereby sympatric lineages diverge more in reproductive character morphology than allopatric lineages. This pattern has been observed in many plant species, but comparably few have sought to disentangle underlying mechanisms. Here, in a diverse lineage of Neotropical plants (Ruellia; Acanthaceae), we present evidence of reproductive character displacement in a macroevolutionary framework (i.e., among species) and document mechanistic underpinnings. In a series of interspecific hand pollinations in a controlled glasshouse environment, we found that crosses between species that differed more in overall flower size, particularly in style length, were significantly less likely to produce viable seeds. Further, species pairs that failed to set seed were more likely to have sympatric distributions in nature. Competition for pollinators and reinforcement to avoid costly interspecific mating could both result in these patterns and are not mutually exclusive processes. Our results add to growing evidence that reproductive character displacement contributes to exceptional floral diversity of angiosperms.     

Post-pollination reproductive isolation between diurnally and nocturnally flowering daylilies, Hemerocallis fulva and Hemerocallis citrina

To examine whether floral and post-pollination isolation develops independently or not, we conducted a crossing experiment between Hemerocallis fulva and Hemerocallis citrina that shows large floral divergence adapted for diurnal and nocturnal pollinators that have been believed to be fully cross-fertile. Flowers of the two species from sympatric populations were hand-pollinated with conspecific pollen from the same population (control), interspecific pollen from the same area (sympatric cross), and interspecific pollen from the different area (allopatric cross). After capsule dehiscence, the fruit set, seed set per fruit and seed set per flower were determined among three cross categories. The seed sets per flower were 32 and 77% lower in sympatric and allopatric crosses than in the control when H. fulva was the pollen recipient. There was no difference in three reproductive measures among the cross categories when H. citrina was the pollen recipient. This finding indicates that post-pollination isolation does exist between H. fulva and H. citrina, although it is partial, asymmetric, and weakened in sympatry. Our result suggests that floral and post-pollination isolation may develop independently, and reinforcement may not be a general phenomenon in plants.     

Location-specific sympatric morphological divergence as a possible response to species interactions in West Virginia Plethodon salamander communities

1. The competitive interactions of closely related species have long been considered important determinants of community composition and a major cause of phenotypic diversification. However, while patterns such as character displacement are well documented, less is known about how local adaptation influences diversifying selection from interspecific competition. 2. We examined body size and head shape variation among allopatric and sympatric populations of two salamander species, the widespread Plethodon cinereus and the geographically restricted P. nettingi. We quantified morphology from 724 individuals from 20 geographical localities throughout the range of P. nettingi. 3. Plethodon nettingi was more robust in cranial morphology relative to P. cinereus, and sympatric localities were more robust relative to allopatric localities. Additionally, there was significantly greater sympatric head shape divergence between species relative to allopatric communities, and sympatric localities of P. cinereus exhibited greater morphological variation than sympatric P. nettingi. 4. The sympatric morphological divergence and increase in cranial robustness of one species (P. nettingi) were similar to observations in other Plethodon communities, and were consistent with the hypothesis of interspecific competition. These findings suggest that interspecific competition in Plethodon may play an important role in phenotypic diversification in this group. 5. The increase in among-population variance in sympatric P. cinereus suggests a species-specific response to divergent natural selection that is influenced in part by other factors. We hypothesize that enhanced morphological flexibility and ecological tolerance allow P. cinereus to more rapidly adapt to local environmental conditions, and initial differences among populations have allowed the evolutionary response of P. cinereus to vary across replicate sympatric locations, resulting in distinct evolutionary trajectories of morphological change.     

Aedeagal Divergence in Sympatric Populations of Two Sibling Species of Cactophilic Drosophila (Diptera: Drosophilidae): Evidence of Character Displacement?

Aedeagal morphology of two sibling cactophilic species, Drosophila buzzatii Patterson & Wheeler and Drosophila koepferae Fontdevila & Wasserman, was analyzed in nine allopatric and three sympatric locations throughout South America. Morphological differences were detected for both aedeagus size and shape between sympatric and allopatric populations of D. buzzatii, despite the significant variability within both groups. Populations of D. buzzatii sympatric with D. koepferae displayed smaller aedeagus than the allopatric ones as well as more differentiated aedeagus shape. The shape differences were non-allometric and mainly consisted in a change of curvature of the dorsal margin of the aedeagus being more pronounced in males from populations sympatric with D. koepferae. It is concluded that aedeagal morphology presented some degree of character displacement in both size and shape in populations of D. buzzatii in sympatry with D. koepferae. These results might suggest the existence of mechanisms of interspecific recognition and hybridization prevention between these species that include the morphology of the male genitalia.