Daphnia lumholtzi

Daphnia lumholtzi is typically found in the warm, shallow regions[6] of bodies of water with larger surface areas.[7] While most species of Daphnia see high mortality at temperatures greater than 25 °C,[8] D. lumholtzi individuals can survive and reproduce at temperatures up to 30 °C,[8][9][10] with a thermal optimum occurring at 29 °C.[9] Studies have shown that population density and water surface temperature are positively correlated.[11] Once favorable temperatures are reached, such as those in the late summer, the previously deposited eggs hatch. The lower temperature range of D. lumholtzi extends to 5 °C, where some individuals are able to survive,[8][9][10] with survival increasing significantly at 10 °C[8][10] and reproduction beginning to occur at 15 °C.[8][10] This significantly broad temperature range in which D. lumholtzi can survive has led to it being labeled a eurythermal species.[12]

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]

Daphnia lumhotzi mostly feeds on phytoplankton ranging from 1 to 25 micrometers in size,[13] but will also eat foods that contain organic detritus, bacteria, and protists which provide an excellent source of nutrients.[4][13]

Daphnia lumholtzi was originally restricted to the tropical lake and pond regions of southwest Asia, Australia, and most of Africa.[14] 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.[14] 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.[15] 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.[16] 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]

Daphnia 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. These ephippia are able to survive in environmental conditions, including oxygen, salinity, and temperature ranges, that are inhospitable to adult daphnids, although exact ranges for D. lumholtzi ephippia have not been found.[5] This ephippia stage is an example of diapause, a state of suspended animation an organism can enter in order to survive a harsh environment.[17] D. lumholtzi is capable of producing 10 times more ephippia than other daphnid species.[18] In temperatures above the optimal temperature for reproduction, 25 °C, the rate of egg development decreases.[19] In temperatures below 25 °C, egg development slows.[20]

The main predators of D. lumholtzi are fish and small invertebrate species. Larger fish are almost always successful in their encounters with D. lumholtzi.[14] Small invertebrate predators are less efficient than large fish in catching D. lumholtzi.

Daphnia lumholtzi individuals prefer areas with high levels of dissolved oxygen and avoid areas where oxygen levels are low. Population surveys have found robust D. lumholtzi populations in water with oxygen saturation levels ranging from 65-163%, while no populations were found in water with saturation levels ranging from 7-50%.[21] The mechanism D. lumholtzi uses for respiration is very similar to that used by other species of Daphnia, with gas exchange occurring through gills that are fed oxygenated water by appendages on the thorax.[22]

As with most other members of the order Cladocera, D. lumholtzi lives in freshwater and is hyperosmotic to its environment.[23] D. lumholtzi is generally found in habitats with a salinity near 0 grams per liter, but can withstand slightly saline water, up to 1.5 grams per liter, for short amounts of time.[23] This ability to survive short bouts of salinity has likely contributed to D. lumholtzi’s ability to invade North America.[23]