Salinity effects on competition between cryptic species of the nematode Rhabditis (Pellioditis) marina

Behind the morphological similarity of many species, a substantial hidden genetic diversity can be found. This cryptic diversity has been well documented in the marine nematode, Rhabditis (Pellioditis) marina, where several cryptic species occur sympatrically. Despite the growing knowledge about its cryptic diversity, little is known about potential differences in the ecology and functional roles of the different species. In this experiment four cryptic species of Rhabditis (Pellioditis) marina were reared together, starting from identical initial abundances at two different salinity concentrations to investigate differences in their environmental preferences and how they may affect the outcome of their competitive interactions. Every fifth day of the experiment one third of the adults was removed from the population for analyzing the genetic composition of the population; adult and juvenile population dynamics were also examined. The first results show a difference in population dynamics dependent on the salinity. At low salinity, a population crash occurred after 15 days in all the populations. Only half of the populations recovered from this bottleneck and reached higher abundances than before. Genetic analyses revealed that the composition of these populations consists solely of individuals of one cryptic species. This effect was totally absent in the populations reared at higher salinity. Two explanations are possible: (1) the different cryptic species have different survival rates at different salinities and (2) the competitive interactions between them differ at different salinities. We are currently identifying the sampled adults using a restriction fragment analysis to assess these explanations and to elucidate the effect of salinity on the coexistence of cryptic species of Rhabditis (Pellioditis) marina

Salinity effects on competition between cryptic species of the nematode Rhabditis (Pellioditis) marina

Behind the morphological similarity of many species, a substantial hidden genetic diversity can be found. This hidden or cryptic diversity leads to a significantly higher biodiversity in ecosystems than previously regarded and is important for estimating total diversity on earth. It has been well documented in the marine nematode, Rhabditis (Pellioditis) marina (Derycke et al., 2008), where several cryptic species occur sympatrically (Pm I, Pm II, Pm III and Pm IV). This coexistence challenges traditional ecological competition theory, stating that competition will be most severe between closely related species, rendering coexistence unlikely. Niche partitioning, where species coexistence is explained by differences in phenotype is a possible way to achieve such coexistence. Closely related sympatric species, despite only minor differences in morphology, can display different preferences for abiotic conditions. So if cryptic species differ in their response to ecological heterogeneities, their coexistence may be facilitated and the outcome of competition will depend on fluctuations in the abiotic environment. Studying these preferences and more broadly the ecology of cryptic species can lead to a better insight in the origin of cryptic speciation and the geographical distribution of these cryptic species. Rhabditis (Pellioditis) marina is a common bacterivore associated with decomposing macro-algae in the littoral zone of coastal and estuarine environments. One of the possible factors shaping the distribution patterns and coexistence of the cryptic species is salinity, one of the most conspicuous environmental factors causing heterogeneity in tidal environments. The effect of salinity on the coexistence of cryptic species was studied in a lab experiment, where population structure between competition cultures (with equal abundances of four cryptic species together) and monospecific cultures (where the four cryptic species can not affect each other’s population development) was compared at two different salinities. The results showed an effect of competition and salinity on the population structure with (1) an increased time-averaged abundance of Pm III at the low salinity in monospecific cultures, (2) a negative effect of competition on the abundance of two cryptic species (Pm II and Pm IV) and (3) stronger competition at lower salinity. Lower salinity has mostly an intensifying effect on the outcome of the competitive interactions and besides competition, complete exclusion (Pm IV) and facilitative interactions occurred (Pm I and Pm III at high salinity). These results show that changes in the abiotic environment (in casu salinity) can shift the outcome of competition between and hence the possibility of the coexistence of different cryptic species of Rhabditis (Pellioditis) marina

Salinity effects on competition between cryptic species of the nematode Rhabditis (Pellioditis) marina

Behind the morphological similarity of many species, a substantial hidden genetic diversity can be found. This cryptic diversity has been well documented in the marine nematode, Rhabditis (Pellioditis) marina, where several cryptic species occur sympatrically. Despite the growing knowledge about its cryptic diversity, little is known about potential differences in the ecology and functional roles of the different species. In this experiment four cryptic species of Rhabditis (Pellioditis) marina were reared together, starting from identical initial abundances at two different salinity concentrations to investigate differences in their environmental preferences and how they may affect the outcome of their competitive interactions. Every fifth day of the experiment one third of the adults was removed from the population for analyzing the genetic composition of the population; adult and juvenile population dynamics were also examined. The first results show a difference in population dynamics dependent on the salinity. At low salinity, a population crash occurred after 15 days in all the populations. Only half of the populations recovered from this bottleneck and reached higher abundances than before. Genetic analyses revealed that the composition of these populations consists solely of individuals of one cryptic species. This effect was totally absent in the populations reared at higher salinity. Two explanations are possible: (1) the different cryptic species have different survival rates at different salinities and (2) the competitive interactions between them differ at different salinities. We are currently identifying the sampled adults using a restriction fragment analysis to assess these explanations and to elucidate the effect of salinity on the coexistence of cryptic species of Rhabditis (Pellioditis) marina