Mayflies: Gila mayfly (Lachlania dencyanna)
Profile prepared by Sarah Foltz Jordan, Celeste Mazzacano, Sarina Jepsen, Scott Hoffman Black and Julia Janicki, The Xerces Society for Invertebrate Conservation
Lachlania dencyanna is a highly unusual species of mayfly with rapid flight habits, atypical wing morphology, and a molting behavior that is unique among the Ephemeroptera (Edmunds et al., 1976, McCafferty et al., 1997). Known solely from a small area of the Gila River drainage system in Grant County, New Mexico, this narrowly endemic and sensitive species is in imminent danger of extinction. Lachlania dencyanna is threatened by numerous anthropogenic alterations to its habitat, including increased sedimentation, nutrient loading, as well as pollution resulting from recreational activities at Gila Cliff Dwellings National Monument and surrounding areas in the Gila National Forest. Cattle grazing is also increasing sediment and nutrients in this sensitive aquatic habitat. Additionally, this species is threatened by altered hydrological conditions and flow regimes, largely due to global climate change and increases in human water demand in an arid and increasingly populated region. These stressors, in combination with the species’ limited range, limited dispersal ability, and the inherent instability of small populations, collectively threaten this rare and remarkable species with extinction. Lachlania dencyanna should be given immediate protection under the Endangered Species Act.
Canada – Species at Risk Act: N/A Canada – provincial status: N/A Mexico: N/A USA – Endangered Species Act: N/A USA – state status: N/A NatureServe: G1 (Critically Imperiled; at high risk of range wide extinction or extirpation due to extreme rarity, rapidly declining numbers, or other factors) IUCN Red List: N/A
Lachlania dencyanna currently receives no federal protection.
Mayflies (order Ephemeroptera) are elongate, soft-bodied insects with large compound eyes and typically three caudal filaments (two cerci and one terminal filament) projecting from the end of the abdomen, although some species (including Lachlania dencyanna) lack the terminal filament and have just two caudal filaments. Since mayflies exhibit incomplete metamorphosis, the aquatic nymphs have many of the same features as the adult, differing mainly in the lack of wings. Mayfly adults generally have two pairs of wings: somewhat triangular forewings and much smaller hind wing (Waltz & Burian, 2008).
Lachlania dencyanna is a member of the family Oligoneuriidae, commonly known as the brush-legged mayflies. Both adults and nymphs in this family are among the most distinctive mayflies known, having diverged considerably from their nearest relatives. Members of the Oligoneuriidae family are readily recognized from other families by the highly reduced wing venation. They are further separated from other mayfly families by having exposed anterior abdominal gills, un-fringed (as opposed to fringed) margins on gills of abdominal segments 2 – 7, and, most notably, a double row of long setae on the inner margins of the foreleg femora and tibiae (Waltz & Burian, 2008). The Oligoneuriidae family is primarily pantropical, with just two genera in North America: Lachlania and Homoeoneuria (Edmunds et al., 1976). Lachlania nymphs are distinguished from Homoeoneuria and other Oligoneuriidae genera by several characteristics, including the flattened head with dorsal (as opposed to lateral) eyes; somewhat depressed (as opposed to stream-lined) body, claws present (as opposed to absent) on forelegs, and two (as opposed to three) caudal filaments. Lachlania adults are distinguished from Homoeoneuria and other genera in the family by the forewing venation (presence of R3 and IR3) and male genitalia (presence of genital forceps) (Edmunds et al., 1976).
The presence of mid-dorsal abdominal tubercles is unique to L. dencyanna nymphs, and will readily distinguish this species from all other known nymphs of Lachlania (Koss & Edmunds, 1970) . This species further differs from all other known Lachlania in the well-developed posterolateral projections of abdominal segment 9. Lachlania dencyanna is separated from L. saskatchewanensis, the only other Lachlania species in the United States, by the well-developed lateral projections of the thorax, the presence of lateral spines instead of hairs on the posterolateral abdominal projections, and the high density of short ventral abdominal spines. Mature nymphs of L. dencyanna are 15 to 17 mm in body length, with caudal filaments ranging from 10 to 12 mm in length.
Lachlania dencyanna adults are distinguished from other Lachlania species by wing venation. In particular, this species differs from L. saskatchewanensis by the greater number of crossveins in the forewing of L. dencyanna (Koss & Edmunds, 1970). Lachlania saskatchewanensis usually has a total of 3 crossveins in each forewing (5 is also possible, but less common, 25% or less), while L. dencyanna has a total of 5 to 14 crossveins in each forewing, with 8 to 14 being most common. L. saskatchewanensis has no more than 2 crossveins in the R1-R3 interspace, while L. dencyanna always has 3 or more crossveins in the R1-R3 interspace. The males of the two species are further distinguished as follows: the femur-tibia joints of the male meso- and metathoracic legs are pale in L. dencyanna, and brown in L. saskatchewanensis; the male sublateral abdominal sclerotized bars are poorly developed and visible dorsally in L. dencyanna, but well-developed and visible both laterally and dorsally in L. saskatchewanensis. The male genitalia are also distinctive for each species.
Mayflies are considered to be one of the most sensitive indicators of water quality in streams and are frequently used as sentinel organisms in biomonitoring, as they are among the first macroinvertebrates to disappear from systems impacted by physical habitat degradation and thermal and chemical pollution (Brittain, 1982; Menetrey et al., 2008; Rosenberg & Resh, 1993; Barbour et al., 1999). The larvae, in particular, have very narrow dissolved oxygen, pH, substrate, stream-size, and temperature requirements, making them especially vulnerable to eutrophication, sedimentation, nutrient loading, ambient water temperature changes, altered flow regimes, chemical pollutants, and other anthropogenic impacts on water quality (Brittain, 1982; Earl & Callaghan, 1998; Solimini et al., 2006; Menetrey et al., 2008; Bryce et al., 2010).
Mayfly eggs are usually deposited at the surface of the water, either in large clusters or a few eggs at a time. Although embryonic development usually takes just a few weeks, the eggs of most temperate species enter diapause, causing hatching to be delayed for approximately three to nine months. The majority of the mayfly life cycle is spent as an aquatic nymph. The length of the nymphal life stage varies greatly with temperature but is usually three to six months. During July, most nymphs appeared to be one to two weeks from emergence, although the observance of some individuals with darkened wing pads indicated that emergence was near or already occurring in a portion of the population. Mayfly nymphs actively feed and undergo numerous molts (ranging from ~12 to ~45) before emerging as a subimago.
The subimago is a winged, terrestrial, pre-adult stage that resembles the adult (imago) in most features, but is sexually immature and undergoes one additional molt into the sexually mature imago stage. The metamorphosis of Lachlania is unique among all insects; this genus undergoes a peculiar process of subimaginal molting to adults in which the subimaginal cuticle is shed from the body but retained on the wings, resulting in the retention of microtrichia on the wings. Although it has been hypothesized that the typical loss of wing microtrichia in the adult (as seen in other families of mayflies) is advantageous to flight due to reduced air friction, the Lachlania are among the fastest flying adult mayflies, with flight speed rivaling that of a horsefly. Adults fly upstream in a horizontal criss-crossing pattern, very atypical of mayflies. Subimago behavior in the Gila mayfly is also highly unusual, even with regard to others in its genus, in that the subimago sheds the exoskeleton and molts into an adult without alighting, in contrast with typical mayfly subimagos which perch on shoreline vegetation during the molting process.
Most adult mayflies live for two hours to three days.
Lachlania dencyanna is the only mayfly species endemic to New Mexico, where it is known from the upper Gila River drainage (McCafferty et al., 1997). Although many other mayfly species occur in the Gila River drainage, L. dencyanna stands out as the only one restricted to the upper Gila River drainage, with a distribution that does not include the lower, Arizona portion of the drainage, or any other drainage. In contrast, the only other Lachlania species in the United States (L. saskatchewanensis) is widespread, with a distribution stretching from Alberta and Saskatchewan south to Mexico, including Montana, Colorado, Nebraska, North Dakota, New Mexico, and Utah (Lugo-Ortiz & McCafferty, 1994; McCafferty et al. 1997; Guenther & McCafferty, 2005).
It is likely that this difficult-to-capture species still persists at the type locality at low numbers, although it has not been detected during the recent surveys . The species is also thought to occur in the relatively pristine wilderness between the two known localities (East Fork of the Gila River at junction with Gila River, 40 miles north of Silver City, state highway 527 and tributary to the Gila River, one mile south of Cliff. Grant County, New Mexico), although the number and abundance of populations are expected to have declined in recent years due to land use changes in the watershed and documented water-quality impairment in the stream.
The primary threats to this species are identified as recreational activity and livestock grazing, both of which contribute to aluminum pollution in the water and lead to widespread habitat degradation that threatens the survival of this species. Dispersal limitations, the inherent vulnerability of small populations to stochastic events, and global climate change in this region pose additional threats to the continued existence of this species.
The East Fork of the Gila River has been on the Clean Water Act list of impaired waters due to high levels of aluminum from 1996 to 2002 (EPA, 2010; NMED, 2002). Both “chronic” ( 87 µg/L) and “acute” (750 µg/L) concentrations of aluminum have been recorded (NMED, 2002), and the most recent report for this watershed continues to list the inability of this river to support high quality coldwater fishery due to high levels of aluminum (EPA, 2010). Aluminum in the water is highly toxic to aquatic insects at concentrations as low as 400 µg/L (Kegley et al., 2009), and is known to alter the contents of structural lipids, deteriorate membrane structures, accumulate in tissues, decrease successful egg hatch, and increase adult mortality in mayflies and other aquatic insects (Tabak & Gibbs, 1991; Regerand et al., 2005; Kegley et al., 2009).
The habitat conditions and water quality requirements of the Gila mayfly are threatened by intense recreational use at the type locality and surrounding area within the Gila National Forest. The East Fork Gila River type locality for this species is located at Grapevine campground, a popular and very heavily used Forest Service dispersed campground with approximately 20 sites. Recreational activities at this site may adversely affect L. dencyanna habitat in numerous ways, including increased erosion and sedimentation from foot, bike, car, and OHV traffic; runoff of pollutants from roads and ORV trails; introduction of bacteria and excess nutrients from dog and horse waste; manipulation and alteration of stream flow by swimmers; and the trampling of streamside riparian habitat by campers, hikers, rafters, and fishermen.
Livestock grazing is a common non-point source of pollution in this region, and both the East Fork Gila River type locality and the “Cliff locality” are threatened by grazing-related habitat impairment. Livestock grazing can degrade water quality and negatively impact aquatic macro-invertebrate communities in several ways: trampling riparian vegetation; consuming streamside vegetation and downcutting the riparian buffer; defecating and urinating on stream banks or in the channel; and increasing sedimentation due to removal of riparian vegetation and direct damage to banks and channel from trampling and wallowing. In places where cattle are not excluded from the stream bed, eutrophication and erosion are especially significant threats. Intensive livestock grazing has been shown to result in loss of biodiversity, disruption of biological communities, and dramatic alteration of terrestrial and aquatic communities (Fleischner, 1994; Agouridis et al., 2005).
Barriers to dispersal are also a threat. Mayfly dispersal to new habitat occurs primarily by means of larval drift downstream of an existing population. Even in species which fly relatively well, such as the Gila mayfly, adults are often restricted in distribution to stream reaches within or adjacent to their stream of origin (Brittain, 1982; Hynes, 1970). The current, impaired habitat conditions in the Gila River downstream of the known range of this species, and the Middle Fork and East Fork of the River, upstream (EPA, 2010), may limit the Gila mayfly’s ability to both inhabit these rivers and to use them as vehicles to colonize or re-colonize other apparently suitable tributaries. The Gila mayfly may thus be confined to a much smaller set of stream reaches than historically. Dispersal potential is of particular importance for this species, since dispersal is likely associated with the long-term persistence of freshwater taxa, and may be a predictor of a species’ ability to withstand global climate change.
Like most mayflies, L. dencyanna requires a narrow set of environmental conditions to survive, including clean, rapidly flowing, well-oxygenated water and a substrate composed of rocks, leaves and other vegetation, and free of heavy siltation.
Due to the multiple threats faced by this species, its small population size, restricted distribution, isolation, and the likelihood that it will be driven to extinction, the the Gila mayfly should be listed as an endangered species. Necessary actions to conserve the Gila mayfly include restricting the recreational use and human activity of the areas in its range distribution; monitoring and protecting the wildlife habitat from damage due to livestock grazing and restoring grazed areas; and improving the water quality of the rivers.
Further research into life history, ecology, and current distribution of the Gila Mayfly would be valuable.
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