Systematics of aquatic beetles (Coleoptera): current state and future directions
2017; Wiley; Volume: 43; Issue: 1 Linguagem: Inglês
10.1111/syen.12270
ISSN1365-3113
Autores Tópico(s)Forest Ecology and Biodiversity Studies
ResumoWith more than 13 000 described species (Fig. 1, Table 1), water beetles are one of the most globally abundant groups of aquatic insects. Among insect orders, only Diptera has more aquatic taxa (though just as larvae) than Coleoptera, and the two largest water beetle families, Dytiscidae and Hydrophilidae, each have more species than either Ephemeroptera or Plecoptera. This rich diversity is not the result of a single aquatic invasion, but rather of repeated macroecological shifts from terrestrial habitats throughout more than 300 Ma of evolutionary history (Toussaint et al., 2017b). Consequently, water beetles do not form a single clade but are better described as an ecological guild distributed across at least 30 families in three of the four coleopteran suborders. Water beetles have been a popular study group among professional and amateur entomologists alike, even boasting their own international society, the UK-based Balfour-Browne Club. Because of their ecological sympatry, for centuries water beetle specialists have often collected and/or studied multiple families in this guild rather than limiting themselves to one particular lineage. The reasons for the relative popularity of aquatic beetles include: (i) their sheer abundance and broad distribution – they are easy to collect almost anywhere; (ii) their occasionally large size, with some adult species exceeding 5 cm in length, makes some species highly visible and some of these are even kept as pets; (iii) their diversity and often beautiful colour patterns, particularly evident in some diving beetles; and (iv) the breadth of their ecologies and behaviours – from incompressible plastrons in Elmidae to repeated aquatic/terrestrial habitat shifts in Hydrophilidae – ensure water beetles are tractable for a range of biological questions. Here, I attempt to summarize the current state-of-the-art of water beetle systematics, including a synthesis of recent literature on how well water beetle taxa are described, the degree to which we understand their evolutionary relationships, and in what ways they are being used as model systems in systematics. I also provide a 'horizon scan' of where water beetle systematics is going, and how its prominence and role in shaping our understanding of evolutionary biology will continue to increase. Due to being defined by ecology rather than a single clade, which groups fall under the aquatic beetle umbrella differ slightly depending on the specialist. For simplicity and the purposes of this review, I limit discussion to families within the five major groups in which the vast majority of species have adults and/or larvae occupying aquatic habitats: Myxophaga, 'Hydradephaga', Hydraenidae, Hydrophiloidea and the core aquatic Byrrhoidea (Dryopidae, Psephenidae, Lutrochidae, Elmidae). I have omitted a few groups that some workers might generally consider aquatic, in particular the Limnichidae and Scirtidae. Many additional terrestrial beetle families have taxa with significant aquatic modifications (e.g. Curculionidae, Chrysomelidae and Staphylinidae among others), but these are also not considered here. Similarly, a number of attempts have been made to partition water beetles into various categories based on the particulars of their life history (e.g. aquatic adults vs. aquatic larvae vs. aquatic adults and larvae). For a more detailed summary of how various water beetle groups are defined, and for a limited review of aquatic taxa in otherwise dominant terrestrial families, see Jäch (1998) and Jäch & Balke (2008). The taxonomy of aquatic beetles has long benefited from a dedicated and relatively large community of both professional and amateur entomologists. Consequently, although there is no question that thousands of new species of aquatic beetles remain to be discovered or described, they are among the better-known and best-catalogued groups of beetles. There are modern world catalogues for a large majority of aquatic beetle families, many of which are updated regularly (Table 1), notable exceptions being Gyrinidae and the smaller aquatic byrrhoid groups (Psephenidae, Dryopidae). Estimating the number of described species is a frequent thought experiment for taxonomists. In one recent attempt to estimate the actual diversity, Jäch & Balke (2008) suggested that water beetles were approximately 70% described (although their circumscription of water beetles differs slightly from that of this review). Bloom et al. (2014) used the taxonomic expertise of the authors to estimate the actual diversity of each tribe of Hydrophilidae, coming to a rough calculation of 4183 species for the family; coincidentally also equating to about 70% of the currently described diversity. Using taxonomic revision data and controlling for several variables such as body size and geography, Nilsson-Örtman & Nilsson (2010) used modelling methods to estimate the actual number of species of diving beetles to be 5405, which would mean that Dytiscidae is presently ∼80% described (Nilsson, 2016). Meanwhile, some smaller families such as Epimetopidae and Lutrochidae have more than doubled in size in the last few years alone (Perkins, 2012; Maier & Short, 2014) and it is likely that they remain substantially underdescribed. The families treated in this review have an approximate total diversity of ∼13 000 described species. Assuming that this represents in aggregate between 65 and 75% of the actual diversity, there are roughly between 17 000 and 20 000 water beetle species. This large amount of remaining unknown water beetle biodiversity is, however, far from evenly distributed among biogeographical regions. The Nearctic, accounting for just ∼8% of water beetle diversity (Jäch & Balke, 2008), is both the best known and also the most species-poor region. Although new species are still to be found, they are often either isolated/unusual endemics (e.g. stygobionts; Miller et al., 2009) or 'known unknowns' from previously unpublished theses (e.g. Hydrochus falsus Hellman in Worthington et al., 2016). At the opposite end of the spectrum is the Neotropics, particularly tropical South America; the region is both the most species-rich (Jäch & Balke, 2008) and likely the most undescribed. More than 300 new species have been described from the Neotropics in the last 10 years alone. The Palaearctic already is relatively high in species richness but much better described than the tropical regions. The Afrotropics and particularly the Oriental region are likely to have many new species awaiting discovery. I proffer that no group of Coleoptera has received as robust and sustained study into their phylogenetic relationships as water beetles. In the last quarter of a century, more than 200 studies have generated new hypotheses of relationships based on a wide range of morphological and molecular data (Table 2). Far from the taxonomic chaos that renders some beetle groups unapproachable, the higher-level classifications for most major lineages of water beetles have been established and tested with multiple lines of evidence. Few phylogenetic studies have focused on Myxophaga. Phylogenies examining interfamilial relationships using adult (Beutel, 1999) and larval characters (Beutel et al., 1999, but excluding Lepiceridae) have found a generally consistent branching pattern of (Lepiceridae + (Torridincollidae + (Hydroscaphidae + Sphaeriusiidae))). With the exception of a recent comprehensive molecular phylogeny of Hydroscaphidae (Short et al., 2015), no phylogenies have been published that substantively examine relationships within any family of Myxophaga. Various taxa of the suborder have been the subject of detailed morphological studies (e.g. Beutel, 1998; Anton & Beutel, 2006) that provide additional discussion of phylogenetic affinities of the group examined. The branching pattern of the hydradephagan families has received significant attention, although no strong consensus on interfamilial relationships has yet emerged. Most morphology-based studies in the last 10 years suggest Gryinidae is the earliest diverging adephagan lineage and would render Hydradephaga nonmonophyletic. Various molecular phylogenies ranging from single-gene studies using 18S rDNA to more comprehensive multigene analyses have found support for a monophyletic Hydradephaga (e.g. Shull et al., 2001; Ribera et al., 2002; McKenna et al., 2015, but see Maddison et al., 2009). Using mitochondrial genomes, López-López & Vogler (2017) did recover a monophyletic Hydradephaga, but with weak support. However, in the first use of phylogenomic analysis in beetles using Ultraconserved Elements (UCE), Baca et al. (2017a) recovered a paraphyletic Hydradephaga. Beutel et al. (2008), Maddison et al. (2009) and Baca et al. (2017b) provided more detailed reviews on hypotheses of relationships among adephagan families. Most species-rich families of Hydradephaga have been the subject of phylogenetic studies in the last decade. Miller & Bergsten (2012) recently published a detailed total-evidence phylogeny of Gyrinidae based on morphology and five genes, and revised the classification of the family accordingly. Previous classifications of gyrinids were based principally on morphological works by Beutel (1989a,b, 1990) and Beutel & Roughley (1994). Still, additional recent studies have continued to revise this classification and illuminate our knowledge of whirligig beetle relationships (e.g. Gustafson & Miller, 2017). The phylogeny and classification of Noteridae was revised by Miller (2009) based on a cladistic analysis of morphological data. However, an expanded dataset (in both taxa and characters) using five genes negated that classification and supplanted it with a new one (Baca et al., 2017a). No molecular data has been applied to the evolution of Haliplidae, and the phylogenetic validity of some smaller genera is uncertain; phylogenies based on morphological data have been inferred for the family (Beutel & Ruhnau, 1990) as well as the genus Brychius (Mousseau & Roughley, 2007). By far the largest family, Dytiscidae has been subject to more phylogenetic studies than any other water beetle group (Table 2). Although there have been numerous and persistent efforts to resolve relationships within various genera, tribes, and subfamilies, the family as a whole has been surprisingly little studied. Using a combination of adult and immature characters and a single gene, Miller (2001a, 2003) made the first attempt to infer a family-wide phylogeny. Subsequently, Ribera et al. (2008) provided the first substantial multi-gene molecular phylogenetic estimate for the family. However, many of these other early larger-scale studies were confounded by incomplete sampling of major lineages or poor resolution/low support along the backbone of the tree. Miller & Bergsten (2014) provided a major attempt at a family-wide classification using a thoroughly sampled, six-gene dataset combined with adult morphology, which remains the most complete and robust estimate to date. Michat et al. (2017) provide an independent estimate of dytiscid phylogeny based solely on larval characters. The larval phylogeny largely agrees with Miller & Bergsten (2014) with regard to the monophyly of most tribes and subfamilies, though there are exceptions (e.g. Dytiscinae, Agabini, Hydroporini), and the relationships among subfamilies differs substantially (e.g. Laccophinae being a highly nested, late diverging lineage in Miller & Bergson but sister to the remaining Dytiscidae in Michat et al., 2017). Relationships among the families of Hydrophiloidea have been the subject of more than a dozen studies over the last 25 years (Table 1). Although a definitive resolution has yet to be reached, the monophyly of Hydrophiloidea, and that of its six constitute families is not in dispute. Short & Fikáček (2013) recently revised the classification of Hydrophilidae based on an analysis of six genes. Among the most significant changes were that the genera Horelophus and Horelophopis, previously presumed to be primitive early diverging lineages and considered their own subfamilies, actually were highly derived taxa nested within other tribes (the inclusion of the latter was supported independently by a concurrent morphological study (Minoshima et al., 2013)). Of the five smaller families, only the internal relationships of Helophoridae have been examined (Fikáček et al., 2012a,b). In a large two-gene study, McKenna et al. (2015) place the Hydrophiloidea as well as Hydraenidae in the context of staphyliniform evolution, being sister to Histeroidea and within Staphylinoidea respectively. The family Hydraenidae has never been the subject of a comprehensive cladistic analysis with the current classification (see Hansen, 1998) based largely on detailed morphological studies by Perkins (1980, 1997). The megadiverse genus Hydraena, with nearly 1000 described species, has been the subject of several recent molecular phylogenies to sort out this explosive diversity (Trizzino et al., 2011, 2013). The relationships between, and within, the families of aquatic Byrrhoidea are the least understood among water beetle lineages. How the families are positioned within Elateriformia, and even if they are themselves reciprocally monophyletic, remains unclear. In the most robust study to date, Kundrata et al. (2016) inferred the phylogeny of Byrrhoidea using four genes. In their analyses, neither Psephenidae nor the subfamilies of Elmidae (Elminae and Larainae) were monophyletic. However, their taxon sampling within each family was limited, and many relationships, especially those among families, were not supported statistically. No comprehensive phylogeny for Dryopidae or Elmidae has yet been undertaken. The phylogeny of Psephenidae was inferred based on adult morphology (Lee et al., 2007), and a new classification proposed. Recently, Jäch et al. (2016) elevated Protelmini (a small group of about six described species from the Afrotropics and Neotropics) from a tribe of Elmidae to its own family, Protelmidae. In proposing this new family, however, no phylogeny or other evidence was offered to support this change. Because there are not yet any reviewable data for the justification of this new family, I have treated it as part of Elmidae for the purpose of this review (similarly, a figure of an elmid phylogeny depicted in Kodada et al. (2016) is based on unpublished studies for which the data, taxon sampling, and clade support cannot be evaluated). Aquatic beetles have a rich fossil record due to strongly sclerotized bodies and predilection for habitats such as lakes and marshes that enhances preservation (Smith, 2000). In addition to providing important insights on character evolution, the robust fossil record provides critical information for time-calibration of phylogenies and consequently supports aquatic beetles as a model system in evolutionary biology. Although compression fossils are prevalent, amber inclusions dating back to the Late Cretaceous are not uncommon (e.g. Kirejtshuk, 2009). The last decade in particular has seen a plethora of new fossils come to light and – of equal importance – reappraisals and clarifications of century-old unreliable descriptions (e.g. in Hydrophiloidea, Fikáček et al., 2010; see also Ponomarenko & Prokin, 2015, for a general review). The majority of extant aquatic beetle families are now known in the fossil record. Even the very rare and obscure suborder Myxophaga is now documented in several Cretaceous-aged deposits (e.g. Kirejtshuk, 2009; Cai et al., 2012), including magnificently preserved specimens of Lepiceridae from Burmese amber (Kirejtshuk & Poinar, 2006; Ge et al., 2010; Jaloszynski et al., 2017) – a family now restricted to the northern Neotropics. Hydradephaga, including some now-extinct families (e.g. Coptoclavidae) allied to Gyrinidae and Hygrobiidae, have a fossil record of both larval and adult forms extending back to at least the Early Triassic (Beutel et al., 2013; Ponomarenko & Prokin, 2015). Fossils of the family Haliplidae have been rare but are known as far back as the lower Cretaceous (Prokin & Ponomarenko 2013, Prokop et al. 2004). Diving beetle fossils attributable to extant clades have been described from Baltic and Dominican amber, including representatives of Copelatinae (Copelatus aphroditae Balke, 2003; Copelatus predaveterus Miller, 2003; Miller & Blake, 2003), Hydroporinae (Hydroporus carstengroehni Balke et al., 2010), and Agabinae (Hydrotrupes prometheus Gomez & Damgaard, 2014). The families Noteridae, Amphizoidae and Meruidae lack described fossils; none are attributed to the Aspidytidae, although the extinct family Liadytidae is extremely similar and may prove to be confamilial. See Prokin et al. (2013) and Gomez & Damgaard (2014) for recent summaries of fossil Hydradephaga. The fossil record of Hydrophiloidea has been extensively expanded in recent years (Fikáček et al., 2010, 2012a, 2014, 2017; Fikáček & Engel, 2011) and now extends back 150 Ma to the Jurassic. Some early descriptions have been shown to be misattributed to the lineage or lack characters that would allow unambiguous attribution, underscoring the need for critical review of historical records. Fossils for all six hydrophiloid families except Epimetopidae have been described, though the few fossils currently assigned to Georissidae are of doubtful attribution (M. Fikáček, personal communication). The staphylinoid family Hydraenidae is putatively known from a variety of compression fossils dating back to the lower Jurassic (Ponomarenko, 1977; Ponomarenko & Prokin, 2015). Recently, Yamamoto et al. (2017) described a spectacularly preserved fossil from Cretaceous Burmese amber, and provided a checklist of pre-Quaternary fossils for the family. Although most aquatic byrrhoid families are known in the fossil record, the oldest known representatives (to date) are comparatively young, with Elmidae and Psephenidae known from the Eocene, and Dryopidae and Lutrochidae from the Oligocene (Ponomarenko & Prokin, 2015). A potentially older elmid from Cretaceous Spanish Amber was removed from the family (Bukejs et al., 2015; see also for a review of Baltic Amber Elmidae). The status of fossil Psephenidae are reviewed in Wedmann et al. (2011), whereas those in Elmidae are discussed in Jäch et al. (2016). At higher taxonomic levels, our knowledge of aquatic beetle larvae is fairly robust, with the larvae even of the newest families Aspidytidae and Meruidae having been described (Alarie & Bilton, 2005; Alarie et al., 2011b). Of the nearly 23 families considered in this review, the confirmed or putative larva of at least one representative of each have been described, as well as most of the subfamilies and higher-level diversity of each lineage. The development of molecular techniques to associate life stages (such as DNA barcoding) has been a boon to specialists eager to describe phylogenetically or taxonomically important water beetle larvae whose identity would have otherwise been tentative at best (e.g. Curiel & Morrone, 2012; Minoshima et al., 2015). Indeed, water beetles were early test-subjects of such methods (Miller et al., 2005), and remain a focal group for exploring the opportunities and limitations of DNA barcoding (e.g. Bergsten et al., 2012; Baselga et al., 2013). The development of increasingly robust regional DNA barcode databases (Hendrich et al., 2015) has made accurate identifications (at least to some level of taxonomic resolution) of unknown larvae more feasible, which also has implications for aquatic bioassessment (Sweeney et al., 2011). However, these databases often focus on what are already the most well-known and well-studied regions, and any widespread use of the method in the more diverse tropical regions remains purely theoretical. Consequently, sequencing both the target immatures alongside a battery of potential adults remains necessary in most cases. The larvae of most myxophagan genera are described (10 of 13). Within Hydroscaphidae, the larvae of the Neotropical genera Confossa and Yara remain unknown, as does the Malagasy Incoltorrida (Torridincolidae). Lawrence et al. (2013) putatively assigned a curious larva from Panama to the family Lepiceridae (whose larvae were unknown previously), although lacking association with adults, rearing, or molecular data. Few recent references are available, and some older references (e.g. Reichardt, 1974) remain definitive for some taxa. Overviews can be found in Lawrence & Reichardt (1991) and Beutel & Vanin (2016). Within Hydradephaga, most subfamilies and tribes have the immature stages described for at least one taxon. Notable exceptions include Spanglerogyrus and Heterogyrus among Gyrinidae and the early-diverging noterid genus Notomicrus. Of the 11 subfamilies and 19 tribes recognized currently within Dytiscidae, the larvae for only one subfamily, Hydrodytinae, remain unknown (Miller & Bergsten, 2016). Michat et al. (2017) provide a comprehensive review and discussion of dytiscid larvae in a phylogenetic framework, as well as paper and online keys to the larvae of the subfamilies and tribes of the diving beetles of the world. The monumental work by Larson et al. (2000) is also a good source of larval keys and illustrations, albeit focused on North America. Similarly, larvae have been described for representatives of all families, subfamilies and tribes of Hydrophiloidea, and despite continued progress, our knowledge at the genus and species level remains substantially underdeveloped (Archangelsky et al., 2016). Of the four currently recognized subfamilies of Hydraenidae, most larval descriptions are from the more species-rich and widespread Hydraeninae and Ochthebiinae. The larvae of Orchymontiinae was described for the first time by Delgado & Palma (2004), but immature stages remain unknown for the Afrotropical endemic Prosthetopinae. The larvae of aquatic byrrhoids perhaps are best known due to their frequent collection and their value in biomonitoring. Indeed, Psephenidae is probably better known as larvae than as adults. Dryopidae remain an exception: of the 33 genera, larval descriptions exist for only about seven. This discrepancy arises from most known larvae of the family being terrestrial, and rarely collected by aquatic insect specialists. Water beetles, although defined by their affinity for aquatic ways of life, occupy a broad array of habitats, and have shifted secondarily back to their terrestrial roots (either as adults, larvae or both) on multiple occasions. Many aquatic habitats are definable and discrete in space and time, a luxury more difficult to find in terrestrial systems. This ecological variability coupled with repeated, parallel transitions has positioned water beetles as a premier study group for questions related to dispersal, ecological speciation, and diversification rates (e.g. Ribera et al., 2008; Bloom et al., 2014). The development of comprehensive online specimen and fieldwork databases also has helped anchor our knowledge of water beetle ecology (e.g. Collections Resources for Aquatic Coleoptera (CReAC): http://creac.kubiodiversityinstitute.org/collections/). Jäch & Balke (2008) provide a general synopsis of water beetle habitats and the different categories into which they may be divided. Two particularly remarkable recent findings relating to the ecology of aquatic beetles have been the subterranean radiation of diving beetles in Australia, and the unveiling of a poorly understood and little-known guild of hygropetric water beetles. These discoveries of hundreds of new species and many new lineages show how much remains unknown about water beetle communities. Subterranean aquatic habitats such as underground cave and aquifer systems have long been known to host unique water beetle communities. Stygobionts have been described from a variety of water beetle families including Elmidae, Dryopidae, Noteridae, Hydrophilidae and Dytiscidae. However, the majority of these lineages are known from one or a few species. This changed dramatically with the discovery of a vast community of diving beetles living in calcrete aquifers in Western Australia (e.g. Watts & Humphreys, 2003, 2006). Already about 100 species have been described in little more than a decade, and many more await description. As with hygropetric habitats, the existence of these communities was known but not the extent of their phylogenetic diversity or species richness. Phylogenies reveal many of these taxa to be merely derived members of otherwise surface-dwelling lineages (Leys et al., 2003; Kanda et al., 2016). This may not be unexpected, yet it highlights the dramatic and perhaps very rapid morphological changes that take place when lineages enter a new adaptive zone. Water beetles from hygropetric habitats such as waterfalls and rock seepages generally were considered novelties or aberrant lineages rather than major radiations of phylogenetic significance in their own right. That changed with the discovery of two previously unrecognised beetle families from seeps, namely Aspidytidae from South Africa and China (Ribera et al., 2002a; Balke et al., 2005), and Meruidae from Venezuela (Spangler & Steiner, 2005). Since then, the first hygropetric representatives of Lutrochidae (Maier & Short, 2013) and Noteridae (Miller, 2009) have been discovered in South America. New seepage genera have been described in Dytiscidae (e.g. Fontidessus Miller & Spangler 2008, Spanglerodessus Miller & García 2011, Petrodessus Miller, 2012), Hydrophilidae (e.g. Tobochares Short & García 2007, Radicitus Short & García 2014) and Hydroscaphidae (Confossa, Short et al., 2015). In addition, hygropetric lineages known from few taxa have been found to be substantially underdescribed (e.g. Perkins 2005, 2006; Short, 2010; Short & Garcia, 2010; Clarkson & Short, 2012; Bilton, 2016). These discoveries have only driven additional fieldwork in these habitats which in turn have illuminated yet more previously unrecognized hygropetric communities. It is likely hundreds more hygropetric species will be described in the coming decade, shifting the habitat from its once marginal status to one that is more mainstream. Having accurate accounting of the diversity within water beetle lineages is critical to facilitating and accelerating both taxonomic and evolutionary research within the group. The water beetle community is tantalizingly close to having a completely catalogued fauna. Most water beetle families have modern catalogues which are updated on a regular basis. Efforts to compile taxonomic information in the few remaining linages (Gyrinidae, Dryopidae, Psephenidae) should be completed, and those that have already been completed should continue to be maintained. A drawback is that none of these catalogues are expressly digital, or allow for real-time updates, corrections or additions. Consolidating these catalogues into a single, online platform should be the goal. We need modern molecular phylogenies integrated with morphological data for all lineages. In group after group, mounting molecular support for nontraditional relationships has required reconsideration of the morphology and classification of families considered stable (e.g. the separation of the terrestrial and aquatic hydrophilids into different subfamilies, or the very derived nature of Pronoterus within Noteridae; Short & Fikáček, 2013; Baca et al., 2017a). Often, discrepancies between morphological and molecular datasets have not been due to a difference in the quality of characters, but instead are due to inaccurate and subjective assessments of homology. In the former, extensive ecology-driven evolution both within and between aquatic and terrestrial habitats has, unsurprisingly, resulting in repeated and convergent evolution of easily observable morphological syndromes. These homoplasious characters often were used as diagnostic characters to assign taxa various groups, which led to artificial classifications. The families of aquatic byrrhoids, particularly Elmidae and Dryopidae, uniquely lack a comprehensive modern phylogenetic hypothesis. Much of the higher-level relationships of Hydraenidae remain untested and given that both groups have substantial ecological variability, major classification changes are likely. Many ambitious surveys that focussed in total or in part on water beetles have been carried out in the last quarter of a century, including those in China (Jäch & Ji, 2003), Venezuela, Madagascar, Costa Rica, New Caledonia (Jäch & Balke, 2010), United Arab Emirates (e.g. Van Harten, 2010) and New Guinea. Many, especially those in the Oriental and Neotropical regions, only served to illuminate our ignorance of the biodiversity of these regions rather than finish the job of documenting their faunas. From a geographical standpoint, whereas many areas warrant and deserve inventory work, tropical South America and the zone of tropical Asia including and bounded by southern China–India–Malaysia likely will yield the highest percentage and volume of new taxa and lineages. Ecologically, focussing on undersampled sampled habitats such as seepages, underground waters and, to a lesser degree, the margins of river and streams, will likely yield new discoveries regardless of their geographical location. As aquatic beetles become increasing used as models in evolutionary biology and conservation, democratizing the ability to identify them takes on increasing importance. Although regional treatments and generic revisions exist to a certain extent for some groups, the recent publication of Miller & Bergsten's (2016) comprehensive resource on diving beetle biology and identification put into sharp relief the varying degrees to which water beetle identification is feasible. Of the 10 water beetle families with more than three described genera, modern global genus-level keys now exist for just half: Dytiscidae (Miller & Bergsten, 2016), Noteridae (Miller, 2009), Gyrinidae (Miller & Bergsten, 2012), Hydroscaphidae (Short et al., 2015) and Psephenidae (Lee et al., 2007). We still need comprehensive resources for the identification of Elmidae, Dryopidae, Hydraenidae and Hydrophilidae. I am grateful to Pete Cranston whose suggestion sowed the seed for this review as well as his subsequent critiques. I am indebted to Grey Gustafson and Crystal Maier for sharing their data on the species richness of the Gyrinidae and Lutrochidae, respectively. Stephen Baca, David Bilton, Jennifer Girón, Grey Gustafson, Martin Fikáček and Ignacio Ribera graciously provided valuable feedback on early drafts of this manuscript. I thank Michael Balke (Aspidytidae, Hygrobiidae), Martin Fikáček (Epimetopidae, Helophoridae), Guy Hanley (Amphizoidae), David Maddison (Lepiceridae), K.B. Makapob (Georissidae, Hdyrochidae), Harold Schillhammer (Meruidae) and Udo Schmidt (Dryopidae, Elimdae, Psephenidae, Hydraenidae, Spercheidae, Haliplidae, Gyrinidae, Dytiscidae) for use of their beetle images. This work was supported in part by National Science Foundation award DEB-1453452.
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