Differences in time until dispersal between cryptic species of a marine nematode species complex

Temperature-driven dynamics: unraveling the impact of climate change on cryptic species interactions within the Litoditis marina complex

Anthropogenic climate change and the associated increase in sea temperatures are projected to greatly impact marine ecosystems. Temperature variation can influence the interactions between species, leading to cascading effects on the abundance, diversity and composition of communities. Such changes in community structure can have consequences on ecosystem stability, processes and the services it provides. Therefore, it is important to better understand the role of species interactions in the development of communities and how they are influenced by environmental factors like temperature. The coexistence of closely related cryptic species, with significant biological and ecological differences, makes this even more complex. This study investigated the effect of temperature on species growth and both intra- and interspecific interactions of three species within the free-living nematode Litoditis marina complex. To achieve this, closed microcosm experiments were conducted on the L. marina species Pm I, Pm III and Pm IV in monoculture and combined cultures at two temperature treatments of 15 °C and 20 °C. A population model was constructed to elucidate and quantify the effects of intra- and interspecific interactions on nematode populations. The relative competitive abilities of the investigated species were quantified using the Modern Coexistence Theory (MCT) framework. Temperature had strong and disparate effects on the population growth of the distinct L. marina species. This indicates temperature could play an important role in the distribution of these cryptic species. Both competitive and facilitative interactions were observed in the experiments. Temperature affected both the type and the strength of the species interactions, suggesting a change in temperature could impact the coexistence of these closely related species, alter community dynamics and consequently affect ecosystem processes and services.

Population density, bottom-up and top-down control as an interactive triplet to trigger dispersal

Dispersal reflects the trade-offs between the cost of a change in habitat and the fitness benefits conferred by that change. Many factors trigger the dispersal of animals, but in field studies they are typically not controllable; consequently, they are mostly studied in the laboratory, where their single and interactive effects on dispersal can be investigated. We tested whether three fundamental factors, population density as well as bottom-up and top-down control, influence the emigration of the nematode Caenorhabditis elegans. Nematode movement was observed in experiments conducted in two-chamber arenas in which these factors were manipulated. The results showed that both decreasing food availability and increasing population density had a positive influence on nematode dispersal. The presence of the predatory flatworm Polycelis tenuis did not consistently affect dispersal but worked as an amplifier when linked with population density with respect to certain food-supply levels. Our study indicates that nematode dispersal on small scales is non-random; rather, the worms’ ability to perceive environmental information leads to a context-dependent decision by individuals to leave or stay in a patch. The further use of nematodes to gain insights into both the triggers that initiate dispersal, and the traits of dispersing individuals will improve the modeling of animal behavior in changing and spatial heterogenous landscapes.

Experimental selection of Marylynnia puncticaudata (Cyatholaimidae, Nematoda) and effects of organic enrichment

Meiobenthic nematodes are well-known bioindicators in aquatic ecosystem health programs. However, the explored taxa are still limited and practically devoted to the community level. The present study provided a new method of experimental isolation of a species from a pristine nematofauna. In our method, the nematofauna faced two types of sediment, namely, the leaves of Posidonia oceanica and shells of Mytilus galloprovincialis, under controlled laboratory conditions, and several changes in species composition occurred through gradual selection of the most adaptable nematode taxa to the new environments, which were previously defaunated. We used the selected nematode taxon, Marylynnia puncticaudata (Cyatholaimidae), to examine the possible effects of organic enrichment, and the results clearly showed that the body size of the nematodes significantly increased and they became fat when after enrichment using a powder made of marine agar (1200 mg l^-1) and cuticles of Crangon crangon (900 mg l^-1), but their relative body growth showed no discernible changes.

Living apart-together: Microhabitat differentiation of cryptic nematode species in a saltmarsh habitat

Coexistence of highly similar species is at odds with ecological theory of competition; coexistence, then, requires stabilizing mechanisms such as differences in ecological niche. In the bacterivore nematode Litoditis marina species complex, which occurs associated with macro-algae, four cryptic lineages (Pm I-IV) co-occur in the field along the south-western coast and estuaries of The Netherlands. Here we investigate the temporal and/or spatial niche differentiation in their natural environment using a qPCR-based detection and relative quantification method. We collected different algal species (i.e. two Fucus species and Ulva sp.) and separated algal structures (i.e. receptacula, thalli, non-fertile tips and bladders) at different sampling months and times (i.e. twice per sampling month), to examine differences in microhabitat use between coexisting L. marina species. Results demonstrate that the cryptic species composition varied among different algal species and algal structures, which was also subject to temporal shifts. Pm I dominated on Fucus spp., Pm II showed dominance on Ulva sp., while Pm III overall had the lowest frequencies. Microhabitat partitioning was most pronounced between the two cryptic species which had similar microbiomes (Pm I and Pm II), and less so between the two species which had significantly different microbiomes (Pm I and Pm III), suggesting that species which share the same microhabitats may avoid competition through resource partitioning. The interplay of microhabitat differentiation and temporal dynamics among the cryptic species of L. marina implies that there is a complex interaction between biotic components and abiotic factors which contributes to their coexistence in the field.

Mitogenomics reveals high synteny and long evolutionary histories of sympatric cryptic nematode species

Species with seemingly identical morphology but with distinct genetic differences are abundant in the marine environment and frequently co-occur in the same habitat. Such cryptic species are typically delineated using a limited number of mitochondrial and/or nuclear marker genes, which do not yield information on gene order and gene content of the genomes under consideration. We used next-generation sequencing to study the composition of the mitochondrial genomes of four sympatrically distributed cryptic species of the Litoditis marina species complex (PmI, PmII, PmIII, and PmIV). The ecology, biology, and natural occurrence of these four species are well known, but the evolutionary processes behind this cryptic speciation remain largely unknown. The gene order of the mitochondrial genomes of the four species was conserved, but differences in genome length, gene length, and codon usage were observed. The atp8 gene was lacking in all four species. Phylogenetic analyses confirm that PmI and PmIV are sister species and that PmIII diverged earliest. The most recent common ancestor of the four cryptic species was estimated to have diverged 16 MYA. Synonymous mutations outnumbered nonsynonymous changes in all protein-encoding genes, with the Complex IV genes (coxI-III) experiencing the strongest purifying selection. Our mitogenomic results show that morphologically similar species can have long evolutionary histories and that PmIII has several differences in genetic makeup compared to the three other species, which may explain why it is better adapted to higher temperatures than the other species.

Differences in life-histories refute ecological equivalence of cryptic species and provide clues to the origin of bathyal Halomonhystera (Nematoda)

The discovery of morphologically very similar but genetically distinct species complicates a proper understanding of the link between biodiversity and ecosystem functioning. Cryptic species have been frequently observed to co-occur and are thus expected to be ecological equivalent. The marine nematode Halomonhystera disjuncta contains five cryptic species (GD1-5) that co-occur in the Westerschelde estuary. In this study, we investigated the effect of three abiotic factors (salinity, temperature and sulphide) on life-history traits of three cryptic H. disjuncta species (GD1-3). Our results show that temperature had the most profound influence on all life-cycle parameters compared to a smaller effect of salinity. Life-history traits of closely related cryptic species were differentially affected by temperature, salinity and presence of sulphides which shows that cryptic H. disjuncta species are not ecologically equivalent. Our results further revealed that GD1 had the highest tolerance to a combination of sulphides, high salinities and low temperatures. The close phylogenetic position of GD1 to Halomonhystera hermesi, the dominant species in sulphidic sediments of the Håkon Mosby mud volcano (Barent Sea, 1280 m depth), indicates that both species share a recent common ancestor. Differential life-history responses to environmental changes among cryptic species may have crucial consequences for our perception on ecosystem functioning and coexistence of cryptic species.

Rhythmic natural selection over intertidal and brackish water genotypes: simple formulations for testing hypothesis

Tidal cycles at benthic habitats induce a set of periodic environmental changes in variables like salinity, temperature and sediment water content which are able to stress benthic organisms. Consequently, a natural selection temporally correlated with tides affects the fitness of genotypes (wi) depending on their adaptation degree. Classic population genetics demonstrate that (1) rhythmic wi is more restrictive than equivalent spatial variations to preserve genetic variance, and (2) mean fitness of the population (w¯) does not have to be enhanced by genetic variance (σ(2)w). The present study develops a simple replicator dynamics-based model of continuous selection, where wi of multiple asexual genotypes fluctuates as a sinusoid. The amplitude of w was set as 0.5 (1-wmin), whereas the ratio of tide period to generation time (h) was defined. Overall, the model shows that if h>1, then the success of an advantageous genotype is exposed to randomness, and w¯ may decrease over generations. In contrast, if h<1 the success is deterministic, is limiting co-dominance, and only depends on wmin. The amount of different genotypes buffers the decay of σ(2)w and hence increases cohesiveness. Finally, the reliability of the model is analyzed for a set of target intertidal and brackish water organisms.