Bythotrephes longimanus

Bythotrephes longimanus, commonly referred to as the spiny water flea, are predatory planktonic crustaceans[1]. These microscopic organisms are native to Eurasia, and are considered invasive to the Great Lakes region of North America. Spiny water fleas are observed in abundance between the late-summer and fall months[2].

Taxonomic Classification

Kingdom: Animalia

Subkingdom: Bilateria

Infrakingdom: Protostomia

Superphylum: Ecdysozoa

Phylum: Arthropoda

Subphylum: Crustacea

Class: Branchiopoda

Order: Diplostraca

Suborder: Cladocera

Infraorder: Onychopoda

Family: Cercopagididae

Genus: Bythotrephes

Species: Bythotrephes longimanus

Appearance-Defining Characteristics

Bythotrephes longimanus under magnification.

Bythotrephes longimanus zoomed in under microscope.

Bythotrephes longimanus (spiny waterfleas) are extremely small, growing between 0.25 - 0.6 inches long at maturity. The females typically grow larger than the males, however. These organisms are classified as zooplankton, appearing opaque in color. In regard to their size, spiny water fleas have large and powerful mandibles used to consume their prey[4]. They are primarily identified, though, by their long and spiny tail which accounts for about 70% of its total length[4]. Adult spiny water fleas will averagely have 4 pairs of barbs along their tails, while juveniles only have one pair. These barbs, or spikes, are utilized as defense against predators. The spiny water flea has one large eyespot, either black or red in coloration.

Habitats and Range

Bythotrephes longimanus (spiny waterfleas) originated across regions of Northern Europe and Asia[3]. Native to Lake Ladoga, near the Baltic Sea in Russia, these microscopic organisms can be found among the zooplankton within bodies of freshwater [4].

Even though this species did not originally live in North America, it accidentally became introduced into the United States through the Great Lakes by the discharging of cargo ballast water which was found to be contaminated, leading to some of the first discoveries of spiny water fleas found in Lake Ontario in the year 1982[3]. By 1987, they were found to have spread out beyond Lake Ontario into Lake Superior[3]. They are frequently found on fishing line in clusters that looks like gelatin with a wet cotton-type of texture[3].

Environmental/Biological Conditions

This organism is found primarily close to where other zooplankton reside, in upper water columns within both small and large temperate lake water bodies, and they have the ability to withstand brackish water type environments[2]. The frequency of Bythotrephes and their population density is primarily based upon the water temperatures and the salt concentration found in these small or large temperate lakes, due to Bythotrephes organisms only having the ability to live in environmental conditions where both temperate lake water temperatures are between 4°C and 30°C and the salinity level is between 0.04 to 8.0 parts per thousand (ppt)[2]. However, the ideal environmental conditions in which they tend to thrive are when lake temperatures are between 10°C and 24°C, and salinity levels of 0.04 to 0.4 ppt[2].

Diet

This organism is a predatory type of zooplankton[3]. During the daytime hours, they move into water bodies that have a greater depth in order to be able to hide effectively from other predators[3]. They eat other types of zooplankton, and during the night they go to shallow water to hunt down their food[3]. One of the specific types of organisms they eat are Daphnia, a genus of smaller zooplankton[4]. In addition to eating other types of zooplankton, it also competes with other larger organisms, such as fish, for food[4].

Reproduction

Throughout the spring and summer months, spiny waterfleas go through a reproductive process in which they reproduce via cloning[4]. When the lake temperatures become colder or there is less food (such as in the autumn months), they will reproduce through the process of sexual reproduction to produce embryos, in which they are able to generate tough eggs that are unsusceptible and impervious to both drying out and freezing[4]. After these eggs are generated by the females, the females will carry both their eggs and newly-born offspring on their backs[4].

Ecological Effects and Consequences

The most important types of zooplankton are known by the genus name Daphnia[4]. The Daphnia genus makes up approximately 100 freshwater species, and these very small organisms are very essential and important when it comes to overall lake health[4]. Some of the critical ecological contributions that Daphnia provide include serving as a big food source for fish, and consuming floating algae[4]. They graze and consume on the surface algae through their constant leg movement, which creates very tiny currents which cause the algae to be moved across the surface of the water, pulling itself into the Daphnia's tiny maws[4]. Through their grazing and consumption of surface algae, they are capable of maintaining healthy, non-excessive levels of algae in the lake water body, which in turn means Daphnia is helping to keep the water body ecosystem healthy and balanced[4].

However, due to the spiny waterflea, (their strong and invasive predator), the population densities of these essential Daphnia zooplankton have been greatly declined and destroyed[4]. This destruction caused by the invasive spiny water flea species is making the overall Daphnia population decrease in Lake Superior, in addition to almost every other water body within the region and vicinity of the Great Lakes[4]. Additionally, recent studies have discovered that in various lake water bodies located in Minnesota, the overall populations of native plankton have decreased by up to 60% due to the spiny waterflea[4]. This significant amount of decline of plankton has also led to impacts of other aquatic species, such as yellow perch, walleye, and other native game fish species[4]. Fish species that reside in lakes heavily invaded and impacted by the spiny waterflea have a tendency to grow and develop at a slower rate throughout the first year of their lives especially because young fish eat plankton, and this also makes them more susceptible to being consumed by predators[4]. Fish cannot simply just eat spiny water flea plankton because the presence of its long, barbed and spiny tail easily gets stuck the fishs' throats[4].

References

[1] ITIS - Report: Bythotrephes longimanus. (n.d.). . https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=684624#null.

[2] USGS Spiny Waterflea (Bythotrephes longimanus) Species Profile. (n.d.). . https://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=162#:~:text=Ecology%3A%20Bythotrephes%20longimanus%20is%20found,in%20late%20summer%20and%20autumn.

[3] Spiny waterflea (Bythotrephes longimanus). (n.d.). . https://www.dnr.state.mn.us/invasives/aquaticanimals/spinywaterflea/index.html.

[4] This ferocious water flea is mauling the Great Lakes. 2020, December 2. . https://www.nationalgeographic.com/environment/article/invasive-water-fleas-decimating-plankton-in-great-lakes.