Hydraena Kugelann, 1794

Perkins, Philip D., 2011, New records and description of fifty-four new species of aquatic beetles in the genus Hydraena Kugelann from South America (Coleoptera: Hydraenidae), Zootaxa 3074, pp. 1-198 : 13-14

Hydraena Kugelann, 1794

Hydraena Kugelann, 1794: 578 (type species Hydraena riparia Kugelann 1794: 579 ).

The speciose, cosmopolitan genus Hydraena is well defined by the following autapomorphic characteristics: 1) presence of a labral-mandibular interlocking device ( Perkins 1989), 2) mentum with an acute median projection anteriorly ( Perkins 1989), 3) various complex sensilla ( Perkins 1997), and 4) specialized features of the exocrine secretion delivery system ( Perkins 1997). There is general agreement among current workers that Hydraena is monophyletic, based on these characters (for example, Jäch et al. 2000). The South American species meet these generic criteria.

The division of Hydraena into subgenera is currently a matter of uncertainty. Some groupings of species into large putative monophyletic groups is more certain, but the basal relationships among the major clades, based on recently published DNA analysis, are not well supported ( Ribera et al. 2011: 508). The morphological cladistic analysis of Jäch et al. (2000) concluded that "the great phylogenetic alternative" to Hydraena (s. str.) had been found in Hydraena (Hydraenopsis) . At least some of the South American species of Hydraena are most probably related to species placed by Jäch et al. in Hydraena (Hydraenopsis) . However, the DNA study by Ribera et al. included only one unidentified South American species (from the famous Tobogan locality), probably of the Marginicollis Group. More DNA studies are needed, especially of the unusual Paeminosa , Curvosa , and Multispina Groups, to better understand the relationships of South American Hydraena . For now, I prefer not to formally assign species to subgenera until more results of DNA analysis are published, and it is shown that all species are monophyletic, and that " Hydraenopsis " is definitely not nested within what is now considered Hydraena (s. str.).

Hydraena is perhaps the most speciose water beetle genus ( Perkins 1997, Jäch & Balke 2008, Perkins 2011). Including the current contribution, the number of described species becomes about 900, but many, many more undescribed species reside in museums ( Jäch & Balke 2008). The tremendous "success" of Hydraena appears to be, in large part, a result of the evolution of an exocrine secretion delivery system (ESDS), which is a combination of specialized cuticular structures, exocrine glands, and behavior that together function as a microbial defense system ( Perkins 1997). About 50 genera are currently recognized for the family. The number of species is quite modest in all but three of these genera, and only these three have an ESDS: Hydraena , Ochthebius , and Limnebius . Hydraena , with perhaps 1000+ species, is the largest and qualifies to be regarded as hyperdiverse (sensu Wilson 2003). The small size of these water beetles certainly contributes to the ESDS diversity equation, through effectively limited dispersal capabilities. Many other groups of insects are tiny, and disperse poorly, but are not hyperdiverse.