Hexagenia limbata

Geographic Range

Hexagenia limbata , a species of burrowing mayfly, sometimes called the giant mayfly, is native to the Nearctic region. It is widespread across the entirety of the United States, having been documented in every state except Arizona and Alaska. It is particularly prevalent in the Great Lakes region. Populations near Lake Erie have been the focus of much research and attention throughout the last few decades, as Hexagenia populations have disappeared and later re-established in the area. It is also found throughout Canada, its range extends to just south of Alaska and Nunavut. It is the most widespread burrowing mayfly species in North America.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Nymphs of Hexagenia limbata are aquatic and dig u-shaped burrows in the sediment at the bottom of lakes and streams in temperate habitats. The most suitable habitats for these nymphs have well-mixed, shallow water about 3 meters deep, which ensures the aeration of sediment and abundant detritus for food. Subimagos and adults are typically found in riparian habitats, close to the bodies of water from which they emerged. Subimagos are often found resting in trees and bushes on shore before molting to adults, while imagos can usually be found flying in swarms on shore.

• Habitat Regions
• temperate temperate
• terrestrial terrestrial
• freshwater freshwater

• Aquatic Biomes
• benthic benthic
• lakes and ponds
• rivers and streams

• Other Habitat Features
• riparian riparian

Physical Description

As their common name suggests, giant mayflies can grow rather large, from 8.7 to 27.3 mm in length. Like all adult mayflies, they are soft-bodied, with large forewings and a smaller pair of hindwings, which are held together above the body when at rest. Two hairlike tails extend from the abdomen and they have small antennae. Burrowing mayflies of family Ephemeridae can be distinguished by their 4-segmeted hind tarsi, as well as their wing venation, most notably an abrupt distal bend in the base of the M2 vein in the forewing. Imagos, the sexually mature adults, can be differentiated from subimagos by their clear wings. Female imagos have lighter yellow bodies and smaller eyes than males and are typically several millimeters longer than males. Female imagos can also be twice the mass of males. They have a large variety in coloration (anywhere from yellow to white to shades of brown), patterns, and size.

Nymphs of Hexagenia limbata are elongate, with a cylindrical body. They have 3 tail filaments extending from the end of the abdomen and gills line the outer edges of the abdomen. Nymphs also have modified mandibles with sclerotized tusks for burrowing. When they hatch, nymphs are 1 mm long and males can grow to lengths of 23 mm, while females grow to lengths of 30 mm. Later nymphal instars have dark, prominent wing pads. Eggs are 0.3 mm by 0.2 mm and ellipsoid in shape. They are a white color and transparent enough to see the embryo inside. Non-viable eggs are dark or black. Hexagenia limbata often inhabits the same area as another burrowing mayfly, Hexagenia rigida . Male imagos of these two mayfly species can be distinguished by their genitalia; H. limbata has penis lobes that hook, while H. rigida has penis lobes that are elongate and straight. Hexagenia limbata also has a dark band along the outer edge of the hind wing that H. rigida lacks. Otherwise, it can be tricky to differentiate between Hexagenia species.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• sexes colored or patterned differently

Development

Like all mayflies , Hexagenia limbata is hemimetabolous. It develops from an egg into a nymph, also called a naiad, with several instars, then molts into a sexually immature adult called a subimago or dun, then molts into a sexually mature adult called an imago or spinner. Eggs are laid in water by newly-mated females and either drop into the sediment or are dispersed by flowing water. Most eggs overwinter, as eggs of Hexagenia limbata can survive in cold temperatures (8 degrees Celsius) for over a year. Development and hatching depend on exposure to anoxia and water temperatures. After the surrounding water reaches the appropriate temperature, eggs hatch a couple days to 2 or 3 weeks later. Some eggs may hatch within a few weeks of oviposition, without overwintering. Nymphs that hatch early experience little growth and few molts before winter.

Nymphs undergo as many as 30 molts and development time can take anywhere from 14 to 22 months. Hexagenia limbata traditionally has a two year life cycle, the bulk of which is spent as a nymph, though in colder regions these mayflies may have a 3 or 4 year life cycle. Nymphal development time is temperature dependent. The final nymphal instar swims to the surface and molts into a sexually immature subimago. Because nymphal development time varies greatly, the period of subimago emergence can occur over an extended time, anywhere from spring to early fall. Subimago emergence also depends on temperature and does not occur until water temperatures have reached 20 degrees Celsius. The subimago moves on shore where it rests for 24 to 48 hours. One final molt produces the adult imago. The imago lives for up to two days, in which time it must find a mate and breed, before dying. Peak emergence of adults is during June and July, though they can emerge anytime from late spring to early fall. The life history of Hexagenia limbata is complicated by protracted emergence, multiple cohorts, delayed hatching of eggs, and a wide variability of growth in individuals from the same egg mass.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

After molting from a sexually immature subimago to a sexually mature imago, males and females of Hexagenia limbata have one or two days to find a mate. Males form giant mating swarms just before sunset. Females fly through the swarm and are grabbed by males out of the air. They mate in mid-air for a brief time, with the male clinging on to the female, before separating. Males likely die shortly after mating, while females move on to oviposit their newly fertilized eggs. Due to their short lifespan, these mayflies are monogamous and only mate once. Peak mating season is June and July, though some adults may emerge before or after this period in much smaller numbers.

• Mating System
• monogamous monogamous

After mating, female Hexagenia limbata gather in swarms for an oviposition flight that occurs after sunset. These swarms may serve to decrease predation. The females fly out over the lake or stream and fly back and forth about 10 to 20 feet above the surface for a few minutes, before plummeting to the surface. Most hit the surface hard and flutter around and lift their abdomen to deposit their eggs in the water. If they cannot right themselves, which most cannot, they drown shortly after oviposition. Others manage to land gracefully on the surface of the water, deposit a few eggs, then rise back into the air to repeat the process. Occasionally, females have been observed dropping their eggs into the water from several feet above the surface.

Female body size is positively correlated with fecundity, each female contains 4,000 eggs on average, though this can range from 2000 to 7000. Due to their short life, they are semelparous, as they only lay one batch of eggs and die shortly after. Eggs may be dispersed by flowing water after oviposition. When strong wings are present, these female swarms have been observed being pushed ashore by the wind, then ovipositing their eggs under street lights rather than in the water. This may be because they are attracted to light, or because the streetlights may mimic moonlight reflected on water. Eggs that are deposited on cement are no longer viable, as they are prone to desiccation. Artificial insemination of eggs has been achieved in the laboratory by placing the male genitalia to an unfertilized egg mass, though the hatching rate is not as successful as natural insemination.

• Key Reproductive Features
• semelparous semelparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

Females of Hexagenia limbata provide provisioning in the eggs, and lay the eggs in a body of water that will provide a suitable habitat for the eggs and nymphs to grow and develop. Since adults live only for a day or two, they do not provide any further parental care.

• Parental Investment
• pre-hatching/birth
  • provisioning
    • female

Lifespan/Longevity

Hexagenia limbata typically has a 2 year life cycle, though it can be longer in colder regions and shorter in warmer regions. Development time of nymphs takes 14 to 22 months and the subimago stage usually lasts 24 to 48 hours. After molting into a sexually mature imago, they have an incredibly short adult lifespan, living only 1 or 2 days. One laboratory was able to keep a female alive for 8 days. Stormy weather can cause high mortality in emerging mayflies by drowning them if waves are rough or too high, or if wind speeds are too high.

Behavior

Nymphs of Hexagenia limbata can bury themselves up to 10 cm deep in the sand and sediment. They may share burrows and often aggregate together when brought into laboratories. While in their burrows, nymphs beat and wave their gills to create a water current through the burrow. This current not only allows for a high oxygen concentration for the purpose of respiration, it also sorts sediment particles and draws food towards their mouths. Their gills are in motion about 75% of the time and they can beat their gills for hours at a time. Nymphs are almost constantly in motion, and are active throughout the day and night. They push sediment around with their head and forelegs, switch directions in the burrow, and rub their gills together, likely to clean them. Subimagos emerge during the day, usually the morning and afternoon, and can typically be found resting on shore. Adults are active both during the day and at night, throughout their short lifespan. Adults gather in large swarms, both non-mating swarms, as well as mating swarms of males and oviposition swarms of females. They are not strong fliers and usually depend on wind currents.

• Key Behaviors
• flies
• diurnal diurnal
• nocturnal nocturnal
• motile motile
• sedentary sedentary
• social social

Home Range

Adults are poor fliers and often have to rely on winds to move them ashore after emerging from the water. They have been recorded traveling about 1.2 km on average away from their emergence site. Nymphs mainly remain in burrows.

Communication and Perception

Adult mayflies have large eyes and can perceive their environment and others visually. They are also attracted to light. However, nymphs are photophobic and actively stay away from light, which is not typically an issue in their underwater burrows.

• Communication Channels
• visual visual

• Perception Channels
• visual visual

Food Habits

Nymphs of Hexagenia limbata are detritivores. They filter feed bits of organic material and occasionally algae by creating a current in their burrows with their gills. By waving their gills, the water current causes the food particles to move to the mouthparts of the nymphs. Like all adult mayflies, adults do not have functional mouthparts and do not feed during their short lifespan. Instead, their alimentary tract is actually filled with air, which helps with flight.

• Primary Diet
• detritivore detritivore

• Plant Foods
• algae

• Other Foods
• detritus detritus

• Foraging Behavior
• filter-feeding filter-feeding

Predation

Many aquatic animals prey on Hexagenia limbata nymphs, such as dragonfly larvae including Macromia illinoiensis , and many freshwater fish species, including yellow perch , lake whitefish , and smallmouth bass . Though the nymphs burrow deep into the sediment, this behavior does not seem to prevent them from being preyed upon by dragonfly larvae. Many bird species, such as tree swallows and other aerial predators, such as adult dragonflies, can prey on the adult mayfly swarms. These swarms, as well as synchronous emergence of adults from immature stages in large groups is one defense against predators, as it decreases the risk of predation on any one individual.

Ecosystem Roles

Populations of Hexagenia limbata in Lake Erie have received considerable research attention, as this species disappeared from the region for over 30 years due to pollution and subsequent eutrophication and hypoxia episodes. The disappearance of this and other Hexagenia species and the state of the lake caused people to reevaluate dumping phosphorous and other nutrients. After this was curbed and the lake was restored to habitable conditions, Hexagenia limbata reappeared in the 1980s and is once again very prevalent in the region. As a source of prey for many fish, birds, and insects in the area, its disappearance could have had a significant impact on the other fauna of the ecosystem. Also in Lake Erie, Hexagenia limbata populations are dealing with the effects of invasive zebra mussels ( Dreissena polymorpha ) and quagga mussels ( Dreissena bugensis ) that have spread throughout and now dominate the Great Lakes. Numbers of Hexagenia limbata nymphs are lower in areas where the mussels have a high density, due to the buildup of mussel shells on the sediment, which prevents construction or maintenance on the burrows in which the nymphs live. However, the mussels do produce feces that the nymphs might feed on, which could be beneficial.

Nymphs are significant agents of bioturbation in their benthic habitat. Through the currents they create with their gills, as well as their burrow building, nymphs likely cause some sediment bound chemicals such as cadmium to stay in the water, instead of sinking into the sediment. This may cause a slower recovery of contaminated areas. Their respiration activities may also play a role in oxygen depletion in the sediment-water interface area in which they live. Hexagenia limbata is often found in the same habitat as Hexagenia rigida , another burrowing mayfly, particularly in Lake Erie and the eastern range of its habitat, they are often studied together. Whichever of the two is dominant often changes from season to season and the two species can often be found co-dominating a habitat. Hexagenia limbata has been the dominant species in the last few years, which may be due to larger body size, allowing for better dispersal, as well as higher fecundity. Crepidostomum cooperi , a trematode that matures in sunfish of family Centrarchidae , uses nymphs of Hexagenia limbata as a second intermediate host.

Economic Importance for Humans: Positive

Nymphs of Hexagenia limbata can be used as bioindicators of the sediment toxicity of the lakes and streams they inhabit. They can also be obtained commercially for testing sediment samples and are the recommended species to use for sediment testing. Mass emergence of Hexagenia limbata from the water, molting from nymph to subimago, can be a significant event for fisherman. Flies are often designed after these mayflies, as many fish prey on this species. Fisherman have also learned to mimic the way subimagos sit on the water before taking off for shore, enticing fish to bite their line. Fishing during a "Hex hatch" can be very successful.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

Swarms of Hexagenia limbata and other Hexagenia species can be huge and are often a nuisance to communities. Swarms were a problem particularly in the 1950s in the Lake Erie region, and as the populations have re-established in that region, are again causing problems. Since mayflies are attracted to lights, swarms often collect at street lights, causing power outages when swarms collect at illuminated power transformers. The dead bodies of mayflies can also quickly pile up.

Conservation Status

Hexagenia limbata has no special conservation status, though as demonstrated by the populations in Lake Erie, these mayflies are susceptible to human pollution and invasive species, and efforts to prevent these ecosystem changes should continue.

Other Comments

In July 1999, a swarm of mayflies made up of Hexagenia limbata and Hexagenia rigida on the shore of Lake Erie was so large that it was visible on Doppler radar. The swarm was thought to be 3 to 6 km wide, 16 to 25 km long, and 125 to 250 m in height.

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

Angela Miner (author), Animal Diversity Web Staff, Leila Siciliano Martina (editor), Texas State University.

References

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