Introduction

The highly diverse phylum Rotifera includes more than 2000 species (Segers, 2008; Wallace et al., 2015). Moreover, the discovery in recent decades that several rotifer species are, in fact, complexes of cryptic species (Fontaneto, 2014) suggests that this diversity could be much higher. As defined by Pfenninger & Schwenk (2007), cryptic species ‘are two or more distinct species that were classified as a single species due to their morphological similarity’. The phenomenon of cryptic species is widespread throughout a broad range of taxonomic groups and has implications for biodiversity, conservation and fundamental evolutionary questions (Knowlton, 1993; Schonrogge et al., 2002). It is now clear that under the traditional morphological or typological species concept, morphological analyses may fail to discriminate species when intraspecific variation overlaps interspecific variation (e.g. Bernardo et al., 2007; Fontaneto et al., 2007a). Thus, molecular taxonomy is the key that has unlocked this cryptic diversity (Bickford et al., 2007). Nevertheless, for the recognition of cryptic species and the discovery of such hidden diversity, an integrative approach combining morphological, ecological and molecular data is highly valuable (Dayrat, 2005; Puillandre et al., 2012). Because the criteria for recognizing species differ among these separate approaches, different species concepts are applied, as these concepts are essential to the study of species in practice (Mayden, 1997).

Phylogenetically closely related species, mainly those with a recent history of independent evolution, tend to retain their ancestral ecological requirements (i.e. niche conservatism or phylogenetic niche retention) (Futuyma & Mitter, 1996; Webb, 2000; Violle et al., 2011). Thus, strong competitive interaction is expected to occur between cryptic species, favouring the competitive exclusion of all species but one (Hardin, 1960). However, species belonging to the same cryptic species complex are often in sympatry (e.g. Ortells et al., 2003; Leibold & McPeek, 2006; Nicholls & Racey, 2006; Wellborn & Cothran, 2007). This finding is surprising because competing species must respond differently to their abiotic and/or biotic environments to persist in a stable regime (Leibold, 1995; Chase & Leibold, 2003).

The aim of the present work is to review the evidence for ecological differentiation within cryptic species complexes in rotifers, and to provide a deeper understanding of the patterns and processes maintaining biological diversity in nature. We will focus on identifying mechanisms involved in ecological divergence and allowing competitor coexistence. To this end, we first survey the discovery of cryptic species complexes. We explore the mechanisms that played key roles in that discovery, and we consider whether evidence for ecological divergence exists in the cases considered. Second, we focus on the co-occurrence of species of the same complex in single localities, and we summarize the cases where the niche differentiation of co-occurring cryptic species has been addressed in an effort to detect the mechanisms responsible for coexistence. Our analysis of the current literature on cryptic rotifer species begins with the most recent review on the cryptic rotifer species (Fontaneto, 2014), and it also included all studies subsequently published on the topic. We double-checked for additional missing studies by screening all the papers found in ISI Web of Science and Google Scholar with the keyword ‘rotifer’ from 2002, the year of the first clear description of cryptic diversity in rotifers (Gómez et al., 2002).

Unmasking cryptic rotifer species and their ecological correlates

In Rotifera, the first evidence for cryptic speciation was found in the species Brachionus plicatilis Müller, 1786 by (Fu et al., 1991a, b). The next reliable information about other cryptic rotifer species was not reported until 12 years later, when Derry et al. (2003) confirmed the species status of two different morphs in Keratella cochlearis by analysing the genetic distance between them. At present, evidence for at least 42 cryptic rotifer species complexes is available (see references in Fontaneto, 2014; Obertegger et al., 2014; Kimpel et al., 2015). Most of the cryptic species complexes in rotifers have been discovered in the last 10 years (Fig. 1), and it has been found that bdelloids shows higher levels of cryptic diversity than monogononts (Fontaneto et al., 2012; Tang et al., 2014).

Fig. 1
figure 1

The accumulated number of cryptic rotifer species complexes (A) and the accumulated number of potential cryptic rotifer species (B) since 1991

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The presence of morphological types also provided the first cue for the existence of cryptic species in the monogononts B. calyciflorus, K. quadrata and Lecane bulla (Kutikova & Fernando, 1995; Segers, 1995; Derry et al., 2003), and the bdelloids Adineta vaga, Macrotrachela quadricornifera and Rotaria rotatoria (Donner, 1965; Ricci, 2001). However, morphological differences were not always detected. In 78% of the cryptic species complexes now known, molecular marker differentiation was the first cue for the identification of cryptic species. In terms of the phylogenetic species concept, species are delimited on the basis of shifts in evolutionary rates in the branching patterns within and between species from molecular phylogenies (Cracraft, 1989; Fontaneto et al., 2015). For example, this is the case for the cryptic species complexes Testudinella clypeata (Leasi et al., 2013) and Abrochtha meselsoni/kingi (Birky Jr et al., 2011). In general, molecular markers have been useful in discovering cryptic species complexes and increasing the number of species belonging to these complexes, and in confirming the status of cryptic species after their recognition by other means, e.g. after the detection of slight phenotypic differences (e.g. Gómez et al., 2002; Gilbert & Walsh, 2005; Walsh et al., 2009; Fontaneto et al., 2011, Mills et al., this volume).

Because monogonont rotifers reproduce sexually during part of their life cycle (e.g. Wallace et al., 2015), species status can also be confirmed according to the biological species concept (Mayr, 1995), through evidence of reproductive isolation. Nevertheless, only a few studies have tested the reproductive isolation of cryptic rotifer species. These studies have investigated three species complexes: B. calyciflorus (Gilbert & Walsh, 2005; Hua-Bing & Yi-Long, 2008; Xiang et al., 2011), B. plicatilis (Ortells et al., 2000; Suatoni et al., 2006) and Epiphanes senta (Schröder & Walsh, 2007).

Adaptation to different environments or ecological niches can promote speciation (Schluter, 2001; Rundle & Nosil, 2005) and is critical to co-occurrence of closely related species in a region (via habitat partitioning) or in a single location (via stable coexistence). Hence, quantifying ecological niche divergence may be another useful criterion to delimitating the species status (Wiens & Graham, 2005). Here, the ecological species concept (Van Valen, 1976) is applied. In some cases, ecological differentiation has been shown to exist within cryptic rotifer species complexes. For example, the two species belonging to the Ascomorpha ovalis cryptic species complex occur at different altitudes (García-Morales & Elías-Gutiérrez, 2013). Several species of the B. calyciflorus complex show differential responses in relation to temperature and algal food concentration (Li et al., 2010). One of the two K. cochlearis cryptic species occurs in oligotrophic and cold habitats. The other species prefers eutrophic and warm habitats (Hillbricht-Ilkowska, 1983). In other cases, ecological characterization was not studied, but morphological divergence in traits putatively related to ecological divergence was observed. For example, differences in size and shape in the trophi could promote differences in resource preferences, as has been demonstrated in certain species that belong to the same cryptic species complex (Ciros-Pérez et al., 2001a). However, reports on this type of morphological feature are scarce.

By integrating both genetic and ecological approaches, several studies have recently explored the geographic distribution of rotifer complexes (Table 1). In some cases, the differential distribution of species has been related to ecological factors: climate differences in the B. calyciflorus complex (Xiang et al., 2011); salinity in the B. plicatilis complex (Gómez et al., 2002); total phosphorus concentration in the S. pectinata complex (Obertegger et al., 2012); and altitude, conductivity and silica concentration in the P. dolichoptera complex (Obertegger et al., 2014). All these studies have demonstrated the utility of combining ecological and phylogeographic information for understanding the factors that drive the distribution of cryptic species.

Table 1 Cryptic rotifer species complexes in which the geographic distribution of cryptic species has been explored
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Cryptic species co-occurrence and ecological differentiation

Co-occurrence and coexistence

Rotifer species do not seem to be an exception to the common observation that cryptic species can co-occur in the same locality (Bickford et al., 2007). In fact, the co-occurrence of at least two rotifer species of the same complex has been observed in nearly 60% of the complexes in the phylum (Fig. 2).

Fig. 2
figure 2

Pie chart of the percentages of cryptic rotifer species complexes in which the co-occurrence of species was observed, of complexes in which the co-occurrence was not observed and of complexes for which information is lacking

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As stated above, competition between cryptic species is expected to be strong, and their co-occurrence is paradoxical. However, what is actually observed in ordinary field surveys is co-occurrence, which does not necessarily imply (stable) coexistence. The co-occurrence of competing species may be a transient situation if the exclusion dynamics mediated by competitive interactions are slow (Leibold & McPeek, 2006). Alternatively, persistence may be permanent and stable if co-occurring competitors are protected from exclusion at a relevant ecological time scale. In that case, species are said to ‘coexist’ even if their population densities fluctuate and are not at any population density equilibrium (Chesson, 2000; Adler et al., 2007). Exclusion may be avoided if a stabilizing mechanism based on an ecological divergence exists. This principle implies that critical niche differences do exist despite the presence of a substantial niche overlap due to phylogenetic niche retention. A number of possible factors promote niche partitioning by competing species, e.g. differential use of the same resources, differential vulnerability to predation, frequency-dependent predation, habitat heterogeneity and environmental fluctuations (Tilman, 1982; Gendron, 1987; Begon et al., 1996; Grover, 1997; Amarasekare, 2000; Chesson, 2000; Tokeshi, 2009). In rotifers, information about the mechanisms that may promote the stable coexistence of cryptic species is restricted to the B. plicatilis complex. In this cryptic species complex, a series of studies, including laboratory experiments performed with populations co-occurring in the field, have provided insights into the factors that might act in natural populations.

The Brachionus plicatilis cryptic species complex

The B. plicatilis complex is currently the most diverse and most studied monogonont cryptic rotifer species complex (see above and Mills et al., this volume). Therefore, the accumulated information about the B. plicatilis complex and it’s the ubiquity of this complex in saline lakes (Fontaneto et al., 2006) make this complex an ideal subject for the study of factors that influence the co-occurrence of cryptic species. Six species of the B. plicatilis complex has been found inhabiting saline ponds in Spain (Gómez et al., 2002, 2007), and subsets of these six species commonly co-occur in these habitats (Ortells et al., 2003; Gómez, 2005). In the last 15 years, the coexistence of four of these species has been studied, B. plicatilis sensu stricto, B. ibericus (Ciros-Pérez et al., 2001a, b) and B. rotundiformis Tschugunoff, 1921 were studied in a first series of investigations, while B. plicatilis and B. manjavacas (Fontaneto et al., 2007a) were studied in a second series.

The first three species have been reported to appear together in the water column during extended time periods (Gómez et al., 1995; Ortells et al., 2003). Additionally, Gómez et al. (1995) and Ortells et al. (2003) found that they are involved in a seasonal succession in some Mediterranean ponds. Brachionus plicatilis and B. manjavacas are also frequently found co-occurring in Spanish inland ponds (Ortells et al., 2000; Gómez et al., 2002). A phylogeographic analysis suggests that they have co-occurred in this region since the time of the Pleistocene glaciations (Gómez et al., 2007), with partially parallel post-glaciation range expansions (Campillo et al., 2011). Additionally, paleolimnological analyses have suggested that B. plicatilis and B. manjavacas have co-occurred stably in the same pond at least for decades, and water column surveys have shown that their active periods in the water column can overlap temporally (Montero-Pau et al., 2011). All this evidence suggests that the co-occurrence of several species belonging to the B. plicatilis complex is not transient but that they would coexist in a stable manner, not necessarily associated with an equilibrium state. Thus, stabilizing mechanisms—and, hence, niche differentiation—should occur, even if the densities of these species in the water column are not at equilibrium.

Adaptation to biotic factors

In a series of studies, Ciros- Pérez et al. (2001a, 2004) and Lapesa et al. (2002, 2004) have reported that B. plicatilis, B. ibericus and B. rotundiformis differ in both resource use and vulnerability to predation. A study of exploitative competition among these three sympatric cryptic species feeding on two different sized microalgae showed that these species strongly competed but that food partitioning and the effect of short-term disturbances such as daily fluctuations in food availability could promote their coexistence (Ciros-Pérez et al., 2001a). The importance of predation in mediating the competitive interactions among these three species was experimentally studied by analysing the vulnerability of these species to invertebrate predators (Lapesa et al., 2002, 2004; Ciros-Pérez et al., 2004). Predation was found to be dependent on body size. The smaller the species, the higher its vulnerability to predation. This relationship may result in a trade-off between competitive ability and vulnerability to predation. For instance, when the smallest species, B. rotundiformis, is the superior competitor, the presence of an invertebrate predator protects the rotifer species that is the inferior competitor (Ciros-Pérez et al., 2001a, 2004; Lapesa et al., 2004). As these results were obtained in laboratory communities, the role of the differences in vulnerability to predation in natural populations remains unclear. Predation may be a factor promoting coexistence or, alternatively, promoting succession if the smallest species is the inferior competitor during a seasonal period. A caveat is that a single clone per species was used in each of these studies. Using multiclonal populations or replicating the study for several clones is desirable to achieve general conclusions. Nevertheless, these studies, by identifying factors that promote the coexistence of monoclonal populations of different species, are informative about what can be expected to occur in genetically diverse populations.

Differences in body size seem to explain the divergence in biotic niche between B. plicatilis, B. ibericus and B. rotundiformis. They may affect the ecological niche, especially in aquatic systems (Werner & Gilliam, 1984), as body size has implications for predation susceptibility and competitive ability. Brachionus plicatilis, B. ibericus and B. rotundiformis differ markedly in their average lorica length: B. plicatilis is 35 and 50% larger than B. ibericus and B. rotundiformis, respectively, and B. ibericus is 23% larger than B. rotundiformis (Serra et al., 1998; Ciros-Pérez et al., 2001b). These species also differ slightly in lorica shape (Ciros-Pérez et al., 2001b). In contrast, B. plicatilis and B. manjavacas are a special case of extreme similarity (Fontaneto et al., 2007a), and although the former is on average 6% longer than the latter, their length ranges largely overlap, and molecular identification is required to discriminate between the two species (Campillo et al., 2005).

Accordingly, the coexistence of B. plicatilis and B. manjavacas is unlikely to be explained by ecological differentiation based on biotic factors. In a recent experimental study, Gabaldón et al. (2013) found that the clearance rates on two different microalgae and the ability to withstand various starvation periods were very similar in both rotifer species. Additionally, these authors found that the susceptibility of B. plicatilis and B. manjavacas to predation by the copepod Arctodiaptomus salinus was virtually identical. On the basis of data gathered on tolerance to different starvation periods after birth, susceptibility to different predation regimes and clearance rates on two microalgae, these authors estimated that biotic niche overlap between B. plicatilis and B. manjavacas was very high (i.e. 0.78; range 0–1) and that significant niche differentiation was only found for one axis (i.e. the response to 24 h of starvation after birth). This niche overlap in biotic axes might reflect the principle that both differentiations in resource use and predation vulnerability operate via morphological differentiation in some rotifer taxa so that a high overlap between B. plicatilis and B. manjavacas might be expected.

Differential response to abiotic factors

Interestingly, the ponds inhabited by species of the B. plicatilis complex in the Iberian Peninsula are shallow, with low spatial heterogeneity but high temporal —e.g. seasonal variability (Comín et al., 1992; Rodriguez-Puebla et al., 1998). These temporal changes may make it possible to achieve some seasonal specialization relative to abiotic factors such as temperature and salinity.

As mentioned above, B. plicatilis, B. ibericus and B. rotundiformis underwent seasonal succession in some ponds. For instance, in Poza Sur pond (Eastern Spain), B. ibericus and B. rotundiformis co-occur from late spring to summer. Brachionus rotundiformis is dominant for most of the summer and may be the only species to persist at the end of summer. Brachionus rotundiformis is then replaced by B. plicatilis throughout the winter and until the spring, when it is displaced by the other two cryptic species. The relative frequencies of the three species have been shown to be correlated with temperature and salinity, and the succession of the three species can be explained on the basis of their tolerance to these abiotic factors (Gómez et al., 1995). Further laboratory experiments tested the dynamics of monoclonal populations of each species in response to salinity and temperature. The results of these experiments suggested that specialization in relation to these factors occurred and was consistent with the seasonal distribution of these species in nature (Gómez et al., 1997): B. plicatilis was shown to be a euryhaline species with higher performance at low temperatures than the other two species, B. ibericus was found to be better adapted to low salinity and high temperature, and B. rotundiformis was found to perform better at high salinity and high temperature. However, as stated above, broad co-occurrence periods is found in the wild, and salinity and temperature tolerance ranges consistently overlapped despite differential preferences.

The response of B. plicatilis and B. manjavacas to abiotic factors was also addressed in a series of studies. Using clones isolated from eight lakes where both species coexist, Montero-Pau et al. (2011) experimentally estimated the response of growth rate to salinity—a highly variable environmental parameter in the Mediterranean saline lakes where these species dwell. They found that these two species, despite the overlap in their salinity tolerances, differed in their optimal salinity. Brachionus plicatilis grew better than B. manjavacas at low salinity, while the opposite held for B. manjavacas. Therefore, these authors suggested a role for salinity fluctuations in the coexistence of both species.

Long-term dynamics: life-history traits and diapause

In experimental studies, the persistence or exclusion of competing rotifer species is usually assessed by monitoring the population dynamics in the water column during the active phase of the organisms and by investigating population densities (Rothhaupt, 1988; Sarma et al., 1996; Kirk, 2002; Stelzer, 2006). However, this approach might be misleading if success in the water column is poorly correlated with success in diapausing egg production, viability and hatching because the fate of temporary populations depends upon these between-year fitness components. In particular, the short-term exclusion (i.e. in the water column) of active stages does not necessarily involve species exclusion. The framework provided by the theory of coexistence in fluctuating environments (Chesson & Huntly, 1989, 1997; Chesson, 2000) stresses these points. Surprisingly, an approach exploring rotifer species competition and considering the complete rotifer lifecycle has been adopted only for B. plicatilis and B. manjavacas.

Gabaldón et al. (2015c), using multiclonal populations, experimentally estimated the growth rates and investment in sexual reproduction of B. plicatilis and B. manjavacas under a wide range of salinities (from 5 to 60 g/l). The measured performance varied with salinity, following the expected patterns, in accordance with a previous study by Montero-Pau et al. (2011). Moreover, B. plicatilis exhibited a higher investment in sex than B. manjavacas at low salinity (i.e. 10–20 g/l). It was suggested that these differences play a role in the coexistence of both species in a fluctuating environment. Using multiclonal laboratory populations, Gabaldón & Carmona (2015) measured the population density thresholds for sex initiation and the mixis ratios in multiclonal laboratory populations of both species. The findings confirmed species-specific differences in sexual pattern by showing that B. plicatilis had both a higher propensity to initiate sex (i.e. a lower density threshold) and a higher sex investment ratio than B. manjavacas. These authors also found, using a life-table approach, that fertilized females of B. manjavacas produced more diapausing eggs and had higher relative allocation per diapausing egg than those of B. plicatilis. The hatching and degradation patterns of diapausing eggs also differed between species (Gabaldón et al., 2015c). Although the hatching rates in both species were similar after 17 days of incubation under hatching induction conditions, B. plicatilis showed an extended hatching pattern for diapausing egg, while most diapausing eggs of B. manjavacas hatched synchronously during the second day. Furthermore, most diapausing eggs of B. plicatilis remained without a detectable deterioration after 180 days of incubation. In contrast, those of B. manjavacas began to degrade during the first weeks of the experiment. After a year, the degradation percentages of the diapausing eggs were 21 and 75% for B. plicatilis and B. manjavacas, respectively (Gabaldón et al., 2015c).

Long-term dynamics: coexistence vs. exclusion

Based on their results, summarized above, Gabaldón et al. (2015c) and Gabaldón & Carmona (2015) hypothesized that B. plicatilis acts as an opportunistic species in the habitats where both species co-occur, in contrast to B. manjavacas. During the short favourable periods of relatively low salinity, B. plicatilis would invest a large fraction of its growth potential in sexual reproduction. This strategy would result in the rapid production of a large number of diapausing eggs during a short-time period. Notice that by investing highly in sex, the B. plicatilis population would decrease its growth rate (Snell, 1987), weakening its competitive ability in the water column. Nevertheless, in the next favourable season, the hatchlings of these diapausing eggs produced during such a short and intense sexual phase might allow B. plicatilis to again colonize the water column and thus ensure long-term coexistence with B. manjavacas. As low-salinity periods, suitable for B. plicatilis are most likely rather short and unpredictable, and the production of durable diapausing eggs with an exploratory hatching time distribution might be relatively advantageous for this species. The studies performed by Gabaldón et al. (2015a, b), based on single-species experiments, allowed these authors to make predictions about the output of competitive dynamics between B. plicatilis and B. manjavacas. Gabaldón et al. (2015a) monitored the experimental competitive dynamics of multiclonal populations under different constant- and fluctuating-salinity regimes, using diapausing egg density instead of individual density as the ‘state variable’. The rationale for their approach was that these authors were interested in long-term (among growing season) dynamics. Six growing seasons, which simulated periods of habitat suitability, were experimentally produced under different salinity regimes. These favourable periods were separated by gaps unsuitable conditions. Each suitable period in which the active population is growing was initiated by the hatchlings of diapausing eggs produced in the previous active growth period (Gabaldón et al., 2015a). In some treatments, the unhatched eggs were removed before the growing season began. In other treatments unhatched eggs were kept in the culture, thus creating a diapausing egg bank. Under these circumstances, the delayed hatching of diapausing eggs might contribute to the population. The exclusion of one or the other species was always found under all studied dynamic regimes, indicating that these species compete intensively. At a constant salinity, the victorious species was well predicted from the a priori information on growth rates and sex investment. Brachionus plicatilis consistently excluded B. manjavacas from the dynamics when salinity was low and was excluded when salinity was high. Species co-occurrence under fluctuating salinity persisted longer than in constant salinity, and the species that was excluded varied under the same regime. These findings show empirically that fluctuating salinity favours the co-occurrence of both rotifer species. In some fluctuating-salinity treatments, the dominance of one species over the other (i.e. the competitive outcomes) differed between replicates, suggesting that these species have similar competitive abilities. This result agrees with the finding that the species have similar fitness (Gabaldón et al., 2015c). Additionally, the presence of a diapausing egg bank seemed to extend the time for species co-occurrence. The clones used in these studies were all founded from samples taken in Salobrejo Lake, a locality where these populations co-occur. Thus, the results of Gabaldón and co-workers might not be applicable to other populations if local adaptation has occurred (Campillo et al., 2010). Nevertheless, the Salobrejo population can be considered to represent a case study for investigating the conditions for stable coexistence.

The variety of life-history traits estimated experimentally for B. plicatilis and B. manjavacas allowed Gabaldón et al. (2015b) to parameterize and simulate a computer model specifically oriented to address the long-term competitive dynamics of these species, including the whole rotifer life cycle and the possibility of environmental (i.e. salinity) variation. The agreement between theoretical and experimental results was high for constant environments. Moreover, simulations supported the idea that coexistence is promoted by both salinity fluctuations and the existence of durable diapausing egg banks. The theoretical dynamics are consistent with the notion that B. plicatilis exhibits a strategy of escape from the water column to a refuge in the sediment, with subsequent colonization of the habitat during future windows of opportunity (i.e. suitable low-salinity periods). Nevertheless, field observations demonstrated the coexistence of these two rotifers, and computer simulations show the same for life-history parameters estimated in the laboratory, where laboratory experiments, although informative, failed to find such coexistence. Logistic constrains limit experimental designs to specific ranges of conditions, but these ranges might be not the relevant ones in nature. Perhaps more important, laboratory population sizes are much smaller than natural rotifer population sizes or computer-simulated population sizes. Thus, demographic stochasticity might have an important effect on laboratory population extinctions, whereas this factor is most likely irrelevant to competitive dynamics in natural populations (Gabaldón et al., 2015a).

Conclusions

The advent of molecular taxonomy has revealed substantial cryptic diversity in the phylum Rotifera. Nevertheless, other approaches have proven useful in recognizing the occurrence of cryptic species complexes. Frequently, when investigation, a divergence in molecular markers correlates with more or less subtle morphological, behavioural and physiological differences, and, in Monogononta, with total or partial reproductive isolation. Additionally, ecological differentiation has also been reported. Up to now, the B. plicatilis species complex is the cryptic complex for which a multi-fold approach has been most thoroughly developed.

The co-occurrence of cryptic species is not rare in rotifers. Such co-occurrence, if it persists, most likely implies stable coexistence and therefore demands ecological differentiation. Such differentiation can arise in several ways (Table 2). Ecological studies focused on the B. plicatilis species complex show that a differential response to biotic factors can emerge when species differ in body size. Body size affects competitive ability and vulnerability to predation. In species, there are extremely similar in size, niche differentiation in biotic axes seems to be very small, and the competitive abilities of the species are also very similar. In this case, the coexistence of cryptic species is favoured by differences in the response to fluctuating physical conditions and by differences in life-history traits that are mutually constrained. Fluctuating conditions and temporarily suitable habitats, on the one hand, and life-history traits associated with diapause, on the other hand, seem to play a role in relation to competitive dynamics.

Table 2 Ecological features (i.e. biotic and abiotic factors) and life-history traits studied in several species of the B. plicatilis complex
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Studies involving the B. plicatilis species complex show that cryptic rotifer species are an excellent model for investigating coexistence mechanisms in phylogenetically closely related species, a crucial issue for understanding the species diversity that a system can harbour. Nevertheless, future rotifer research is needed to obtain a more general and complete picture in this field.