Daphnia lumholtzi

Daphnia lumholtzi is a small, invasive water flea that originates in the tropical and subtropical lakes of Africa, Asia, and Australia.^[1][2][3] As an invasive species, Daphnia lumholtzi disrupts aquatic habitats by spreading throughout the warmer waters of lakes and reservoirs.

Description

Daphnia lumholtzi is a small crustacean that is 2–3 mm in length.^[4] It has a large helmet and a long tailspine, usually longer than the length of its body,^[3] that fluctuates in size.^[2][5] Its body structure is arched, extending to a sharp point.^[3] There are roughly 10 prominent spines on the margin of the abdominal shield covering.^[3]

Ecology

Temperature

D. lumholtzi is typically found in the warm, shallow regions^[6] of bodies of water with larger surface areas.^[7] It survives in water temperatures above and around 25 °C (77 °F), and reaches maturity more quickly at higher temperatures. D. lumholtzi spawns at 26–31 °C (79–88 °F), producing large amounts of microscopic offspring. Studies have shown that population density and water surface temperature are positively correlated.^[8] Once favorable temperatures are reached, such as those in the late summer, the previously deposited eggs hatch. It shows a much lower reproductive capacity and lower survival rates in temperatures below 10 °C (50 °F).^[9]

Behavior

Both adult and juvenile D. lumholtzi exhibit a vertical migration pattern, moving upward as the sun sets and downward as the sun rises. This behavior leads to large population densities close to the water surface at night and also occurs in the absence of a suggested predator threat.^[10]

Diet

Daphnia lumhotzi mostly feeds on phytoplankton ranging from 1 to 25 micrometers in size,^[11] but will also eat foods that contain organic detritus, bacteria, and protists which provide an excellent source of nutrients.^[4][11]

Dispersal

Daphnia lumholtzi was originally restricted to the tropical lake and pond regions of southwest Asia, Australia, and most of Africa.^[12] The exact location of geographic origin in the United States has not been identified, but scientists believe the introduction of exotic African fish to lakes most likely caused the distribution.^[12] It was first detected in Missouri and Texas reservoirs in 1991 and has since been found in more than 16 states and over 125 lakes and reservoirs.^[9] Studies have shown that the ability of D. lumholtzi to disperse widely is most likely due to human activity.^[5] Heavy boat traffic on lakes and reservoirs during warmer seasons when D. lumholtzi thrive enable them to expand into other nearby bodies of water.^[13] The long spines and hairs on eggs act as hooks and enable attachment to boats, facilitating dispersal. The presence of D. lumholtzi in smaller ponds is atypical; however it is unlikely that non-human dispersal mechanisms, such as smaller invertebrates moving between bodies of water, have contributed to its widespread distribution.^[5]

Reproduction

D. lumholtzi deposits eggs in lake sediment that can remain dormant for long periods of time. The eggs are characterized by long spines and hairs that act as hooks.^[1] Ephippia are protective shells that cover the egg until favorable conditions occur, such as warmer temperatures or a larger amount of resources. D. lumholtzi is capable of producing 10 times more ephippia than other daphnid species.^[14] In temperatures above the optimal temperature for reproduction, 25°C, the rate of egg development decreases.^[15] In temperatures below 25°C, egg development slows.^[16]

Predators

The main predators of D. lumholtzi are fish and small invertebrate species. Larger fish are almost always successful in their encounters with D. lumholtzi.^[12] Small invertebrate predators are less efficient than large fish in catching D. lumholtzi.

As an Invasive Species

D. lumholtzi exhibits higher survivorship and reproduction in the late summer, under high heat conditions, when compared to other crustaceans living in these conditions. It has been suggested that D. lumholtzi’s more tropical origins may have enabled it to live in these higher temperatures.^[9] This advantage allows them to be a better competitor and ultimately out-compete other species, specifically native zooplankton species, within the same habitat and come out as a successful invader. In accordance with the competitive exclusion principle, no other species can inhabit the same late summer niche as D. lumholtzi; another factor that allows it to have higher survivorship than other Daphnia species and is ultimately a better invader. It is important to note, however, that high survivorship and reproduction are not the only factors that make D. lumholtzi an invasive species.

Competition between D. lumholtzi is increased in habitats that favor the high light intensity of shallow waters. D. lumholtzi showed greater survivorship than other Daphnia species (specifically D. pulex), which made them a stronger competitor for light reception and resources in bodies of water receiving high light intensity. It was found to out-compete other species in areas with high light intensity which in turn contributes to its invasive success.^[17]

D. lumholtzi is capable of producing 10 times more ephippia than other daphnid species,^[14] which can remain dormant until favorable conditions occur. This egg bank gives them an advantage over other species whose eggs cannot withstand desiccation or lower temperatures,^[5] enabling them to produce more offspring that survive longer. The reproductive rate also increases with a higher concentration of food.^[15] Areas exhibiting high food abundance will therefore attract more D. lumholtzi, and in turn result in a higher rate of reproduction.^[17] The greater number of offspring puts pressure on the habitat’s resources and other competitors.

D. lumholtzi is highly plastic,^[9] meaning it has the ability to morphologically adapt to factors within the environment by developing structures that enable it to successfully avoid predation. A long tail spine, large helmet, and additional spines on the abdomen are produced in response to predator kairomones, which are predator hormones, within the water. D. lumholtzi does not produce these protective structures when there are no predators present, and looks morphologically similar to other Daphnia species. When predators are detected, D. lumholtzi responds by producing a tail spine, helmet, abdomen spines for protection; other Daphnia species do not adapt this way to predator threats. With the development of these morphological features, predators have a more difficult time preying on D. lumholtzi. This excess energy the predators put into eating D. lumholtzi lessens predator efficiency, making the predators more likely to choose another prey.^[12] This prey-switching puts an extra strain on other native zooplankton species, reducing predation on D. lumholtzi and allowing it to outperform other competitors.^[8]

Control

Eradication of D. lumholtzi is almost impossible once it has invaded a lake or reservoir. D. lumholtzi is sensitive to various pesticides and manmade chemicals,^[4] but the introduction of chemicals to natural lakes is often harmful to other species. The focus of most control measures is the prevention of initial invasion. For now, scientists recommend simple practices, such as thorough cleaning of boats and avoiding aquarium water dumps, to slow the spread of the species.^[18]

References