Addressing biodiversity shortfalls in meiofauna
Introduction
Small animals belonging to the meiofauna (here broadly defined as organisms smaller than 0.5 mm; Giere, 2009) are in the threshold of the optical resolution for routine identification, and our knowledge on their biodiversity is unfortunately still scarce compared to other groups of animals due to the inherent difficulties of working with microscopic organisms (Appeltans et al., 2012). Technological advances in optical and electronic microscopy provide us with the possibility to improve the quality and quantity of morphological data among meiofaunal groups, applying it for taxonomic descriptions (Boaden, 1963, Clausen, 1967, Di Domenico et al., 2013, Hummon, 1966, Martínez et al., 2013, Sterrer, 1998, Todaro, 2012), systematic classifications (Sørensen et al., 2015, Martínez et al., 2015, Sánchez et al., 2016), and studies on comparative morphology (Kirsteuer, 1976, Tyler and Hooge, 2004). In parallel, advances in molecular tools have dramatically improved our understanding of deep phylogenetic relationships within meiofaunal groups (Cannon et al., 2016, Dunn et al., 2008), including the distribution of meiofauna at different spatial scales (Curini-Galletti et al., 2012, Fonseca et al., 2014a, Scarpa et al., 2015), and of the potential speciation processes (Derycke et al., 2005, Fontaneto and Barraclough, 2015). A large number of molecular techniques can be applied in meiofaunal studies. Molecular data for meiofauna can be obtained from single individuals in the form of short target sequences (barcodes; Fontaneto et al., 2015) to that of entire genomes and transcriptomes (Bemm et al., 2016, Flot et al., 2013). In ecology, the use of dual stable isotopes and fatty acid analyses has allowed us to understand the role of meiofauna in the food web (De Troch et al., 2012, Guilini et al., 2013, Iken et al., 2001). In summary, we now have the power to scrutinize meiofauna in relation to species identity, phylogenetic position, trophic position, and ecological requirements.
The objective of this review is to use the framework of the seven global shortfalls in biodiversity knowledge suggested by Hortal et al. (2015) to identify how to address our lack of knowledge on meiofauna for: species identity, species distribution, species abundance, biological traits, evolutionary history, biological interactions, and environmental requirements (Table 1). Tackling these shortfalls in meiofauna is challenging given the large number of known and potential unknown meiofauna species (Appeltans et al., 2012). The integrated use of various methodologies could allow researchers to improve our knowledge on meiofaunal biodiversity, answering interesting questions in order to have a better understanding of the general rules governing biodiversity in these small animals, especially in marine ecosystems (Zeppilli et al., 2015, Schratzberger and Ingels, 2017). Now being at the omic-, big-data-, and conservation-oriented era, the identification of the theoretical framework to organize different complementary methodologies will help focus our efforts to enhance our predictive power to address the biodiversity shortfalls throughout the meiofauna.
Section snippets
Current theoretical framework merging ecology and evolution
In the past, ecology and evolution were often studied in parallel, as studies in the two fields usually addressed different questions (Mouquet et al., 2012). These studies were under the assumptions that evolutionary processes require a long time to be detected, whereas ecological processes may happen at small temporal and spatial scales. Yet, the legacy of evolutionary history is printed in current ecological patterns (Cavender-Bares et al., 2009). The field of evolutionary ecology addresses
Morphology
Detailed acquisition of morphological structures is a requirement for species identification and for understand the ecological adaptations of small taxa (Artois et al., 2011). Small body size and apparent morphological stasis in meiofauna may mask the actual complexity of the group (Vinther, 2015). Thus, addressing morphological diversity in meiofauna with high-quality morphological data would allow targeting the ‘Linnean’ shortfall by describing species, the ‘Raunkiaeran’ shortfall by defining
General challenges
The methodologies described above have the potential to obtain information at the level of individuals, populations, and even whole communities. Nevertheless, among the methodological techniques discussed above, there is often a trade-off between enhancing taxonomic resolution and sample size (Fig. 2). This means that while one technique may be better suited at the individual level, others are less precise, working optimally at the population level. On top of that, each type of data set is
Conclusion
Given the inherent difficulties of proposing a unique sampling and preserving protocol that is ideal for studying morphology, molecular biology, ecology, physiology, evolution, etc., meiofaunal studies addressing biodiversity shortfalls should be more open to data sharing and grounded on a clear theoretical framework to maximize the inclusion of different approaches. With the current amount of information that we are able to obtain from meiofauna organisms and with the promise of metabarcoding
Acknowledgments
We thank the organizing committee of the 16th International Meiofauna Conference for supporting this study and promoting stimulating discussions during the conference. M.D. thanks the support from FAPESP process 13/04358-7. We thank Brett Gonzalez and two anonymous reviewers for the constructive comments. [SW]
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