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Libellulidae Classification Essay

WHAT YOU NEED TO KNOW: In general, dragonflies are truly amazing animals. They’ve had pretty much the same anatomy since prehistoric times. Their bulging eyes provide them with near-perfect vision in a 360-degree sweep around them. This vision supplements their hyper-speedy reaction time that helps them as they hunt their prey. An aerodynamic design makes flight effortless and, with a stiff wind behind them, they can reach speeds of up to 75 miles an hour. Further, dragonflies are sheathed in a chitin armor that protects their delicate innards.

The most obvious difference between Scarlet Darter Dragonflies and nearly every other dragonfly are their color. Most dragonflies are green, but the Scarlet Darter is – you guessed it – scarlet red. This coloration extends across the entire body – their legs, head, eyes and even the veins in their wings are red. This light, highly reflective color helps the dragonfly avoid overheating during its daytime activity.

You may have heard of the term helicopter parents, well; Scarlet Darters give a completely new meaning to the term! These momma dragonflies often lay their eggs while in mid-flight. Yes, you read that correctly! They literally bomb-drop their eggs, letting them plop into the water. Once submerged, the eggs incubate underwater. Upon hatching, the larvae stay below the surface for about one year before emerging as full-grown dragonflies.

Beyond its interesting behaviors and vibrant appearance, the Scarlet Darter is also an animal that shows how climate change affects wildlife. For centuries, the Scarlet Darter’s range kept it in Africa around the borders of Mediterranean Sea, and it was only a rare sight in Europe. Now it is common there, and countries in Northern Europe report large populations of these insects. That, of course, is great news for the Scarlet Darter, but its expanding range may be hurting other species.

How so? As the Scarlet Darter enters a new region, it has skills that allow it to prey on insect species that have no defenses against it. This advantage could put prey insects at risk for extinction. Further, it’s now in competition with the predators that are native to the areas it is now entering. With an extra predator in the area, the food supply may run low and the native predators are put at risk. Both these issues (and many other concerns) are why scientists see climate change as a major threat.


Family-Level Relationships

Over the last 100 years, the family-level taxonomy of these species has been shuffled and reshuffled as new data, methods, or theories have been discovered. The available family-group names applied have been relatively stable, but the various groupings of genera and species have not. It would perhaps be instructive for the student of the history of freshwater malacology to review the arc of progress that has led us from the arrangement proposed by Charles Torrey Simpson (1900, 1914) to that listed in Table 19.1. However, for those interested in freshwater mollusks, it should be sufficient to point out that (1) pre-cladistic systems were based upon flawed methodologies, either upon authoritarian essays or studies making no distinction between synapomorphy and plesiomorphy (derived vs. ancestral homologies); and (2) cladistic analyses have been reasonably consistent with regard to the patterns of phylogeny recovered. We will dwell upon the current system of the Nearctic Unionoidea. A thorough review of the history of freshwater mussel classification is available elsewhere (Roe and Hoeh, 2003).

Globally, the Unionidae is the largest family of freshwater bivalves with more than 670 currently recognized species, making it among the largest bivalve families in marine as well as freshwater environments (Graf and Cummings, 2007). Unionids possess parasitic glochidia and their larvae are brooded in either the lateral (outer) pair of ctenidial demibranchs or in all four. The principle diagnostic character for the family is the presence of a supra-anal aperture (Graf and Cummings, 2006). The North American unionid assemblage is currently estimated at ∼300 species (Williams et al., 1993; Graf and Cummings, 2007; Bogan, 2008; Bogan and Roe, 2008).

The North American Unionidae is represented by two subfamilies: Unioninae and Ambleminae (Figure 19.2) (Graf, 2002b; Graf and Cummings, 2006). The Unioninae is Holarctic in distribution, with only a few species spreading south into Central America, Africa, or southeastern Asia. The subfamily is diagnosed by large (200–380 μm), subtriangular, hooked-type glochidia. One tribe of the Unioninae, the Unionini (e.g., Unio), is strictly Old World in distribution. The other tribe, the Anodontini, is Holarctic in distribution, with representatives on both sides of the continental divide in North America (Graf and Cummings, 2007). Anodontines are generally without schizodont pseudocardinal hinge teeth, and the laterals, if present, tend to be rudimentary. However, some taxa (e.g., Lasmigona) have secondarily derived hinge teeth. Species are bradytictic (long-term brooding), and the marsupial demibranchs bear lateral, accessory water-tubes and have ventral margins enhanced with additional tissue to allow for greater expansion (Graf and Foighil, 2000). The area from the Arctic south to Mexico is inhabited by 44 species of the Anodontini (Unioninae) (Graf and Cummings, 2007).

The subfamily Ambleminae is much more diverse than the Unioninae in North America (253 species) (Graf and Cummings, 2007). The subfamily may be endemic to North America, although the phylogenetic relationships among New and Old World unionid lineages have thus far been approached with too few taxa and characteristics to derive any meaningful conclusions (Claassen, 1994). Based upon the current evidence, only a single amblemine species occurs west of the Rocky Mountains; the remaining species are found east of the Continental Divide. Gonidea angulata in the Pacific Basins has been argued to share a more recent common ancestor with certain eastern Asian taxa than any of the eastern Nearctic tribes, but this has not been supported by molecular analysis (Graf, 2002b). The four eastern tribes of the East (Amblemini, Quadrulini, Pleurobemini, and Lampsilini) have consistently been recovered as monophyletic, and the clade has been informally named the “Amblemini Tribe group.” The group is diagnosed by the presence of complete (i.e., imperforate) septa dividing the interlamellar spaces of the demibranchs. The species of the Ambleminae possess either the plesiomorphic unhooked or the axe-head-type glochidia, are either tachytictic or bradytictic, and may brood their larvae in either all four demibranchs or only the outer (lateral) pair of demibranchs (Graf and Cummings, 2006). These characteristics associated with reproduction and brooding were given great weight in historical arrangements of the Unionoida (Ortmann, 1912; Heard and Guckert, 1971; Davis, 1984; Lydeard et al., 1996; Graf and Foighil, 2000). Because nearly the full range of global variation was observed within the North American assemblage, it has been assumed that the worldwide Unionidae could be accommodated in the system derived for the North American taxa—thus, the source of the modern confusion over the relationships between New and Old World mussels. While these reproductive characteristics have been shown to be misleading for diagnosing the intergeneric relationships of the Ambleminae (indeed, only the Lampsilini has been well characterized by morphological characteristics), they are conservative among congeneric species (Graf and Cummings, 2006). This, however, may simply be due to the over-emphasis of reproductive characteristics in our current generic definitions, and forthcoming revisions based upon combined molecular and morphological evidence may render this generalization inaccurate (Campbell et al., 2005).

The interrelationships among the lineages of the Amblemini Tribe group are equivocal, and we have left the tribes unresolved in the phylogeny in Figure 19.2. The Amblemini, Quadrulin, and Pleurobemini are recognized largely by molecular characteristics (Davis, 1984; Lydeard et al., 1996; Campbell et al., 2005). The Lampsilini (122 Nearctic species + 28 in Mesoamerica), in contrast, is well diagnosed by a suite of morphological and behavioral characteristics (Graf and Foighil, 2000). The marsupium is restricted to only a portion of the outer demibranchs. Among the core lampsilines, it is restricted to a posterior section, it is capable of great expansion, and the shells of the females are more inflated as well (i.e., sexually dimorphic). In the most derived genera of the tribe, there are also a number of adaptations for host attraction, including larvae packaged into conglutinates or super-conglutinates and mantle lures mimicking host prey items (Zanatta and Murphy, 2006; Barnhart et al., 2008). While these sex-oriented traits are a boon for those making an argument for conserving the Nearctic freshwater mussel assemblage, most unionids (worldwide) apparently lack elaborate host attraction adaptations. However, new discoveries of interesting variations on the typical model are still being discovered (Vicentini, 2005; Barnhart et al., 2008). Among the other three amblemine tribes, the taxonomic and morphological diversity is generally lower than seen among the lampsilines, and the genera have been based largely upon shell characteristics and the number of brooding demibranchs (Ortmann, 1912; Simpson, 1914). Those tetragenous lineages with shell sculpturing are generally placed in either the Amblemini (3 species), or Quadrulini (26 species), depending upon their mitochondrial lineage. The Pleurobemini (100 species) are generally ectobranchous with only a few genera brooding in all four demibranchs (Graf and Foighil, 2000; Graf and Cummings, 2007). Existing hypotheses of the interrelationships among the members of the Amblemini Tribe group remain to be robustly tested.

The sister group to the Unionidae is the Margaritiferidae (Roe and Hoeh, 2003; Graf and Cummings, 2006). Margaritiferids differ from unionid species in lacking any kind of posterior mantle fusion, the diaphragm dividing the infrabranchial from the suprabranchial chamber is grossly incomplete, and the interlamellar spaces of the ctenidia are not divided by vertical septa (Graf and Cummings, 2006). There is variation among the species with regard to the development of interlamellar junctions (Smith, 2001). The Margaritiferidae has a patchy, Holarctic distribution (including southeastern Asia), with only about a dozen species worldwide (Ziuganov et al., 1994; Smith, 2001; Huff et al., 2004; Graf and Cummings, 2007). Based upon the morphological simplicity exhibited, the family may represent living fossils from the early diversification of the Unionoida. However, an equally parsimonious interpretation is that margaritiferids are derived and degenerate (Graf, 2002b; Graf and Cummings, 2006).

Together, the Unionidae and Margaritiferidae form one of two superfamilies of the Unionoida: the Unionoidea (Graf and Cummings, 2006). Historically, the Hyriidae of South America and Australasia were grouped with the Unionoidea based upon the shared characteristic of parasitic glochidia (Parodiz and Bonetto, 1963). However, more recent phylogenetic work (utilizing larval and adult morphology as well as molecular characteristics) has suggested that glochidia are the primitive larval condition among the Unionoida (i.e., not useful for forming groups among unionoids), and hyriids were recovered as part of a clade found on the southern continents (superfamily Etherioidea) (Graf, 2000; Graf and Cummings, 2006). Other etherioidean lineages have parasitic larvae known as lasidia (Wächtler et al., 2001). Based upon current taxonomy and fossil distributions, it is possible that the extant lineages of the Unionoidea first arose on the northern continents (Laurentia) following the breakup of Pangaea early in the Mesozoic. Subsequently, as fragments of the southern supercontinent (Gondwana) have reconnected with the north, unionoideans have spread south into Africa and India, for example (Graf, 2000).

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