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Respiration

D. 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%.^[1] 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^[2].

Response to salinity

As with most other members of the order Cladocera, D. lumholtzi lives in freshwater and is hyperosmotic to its environment.^[3] 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.^[3] This ability to survive short bouts of salinity has likely contributed to D. lumholtzi’s ability to invade North America.^[3]

Temperature maintenance

What generally differs D. lumholtzi from most North American daphnid species is its ability to tolerate and thrive in relatively high temperatures. While most species of Daphnia see high mortality at temperatures greater than 25° C,^[4] D. lumholtzi individuals can survive and reproduce at temperatures up to 30° C,^[5][4][6] with a thermal optimum occurring at 29° C.^[5] This ability to survive at higher temperatures in comparison to other daphnids may be a result of genetic changes in D. lumholtzi that resulted in enzymes that are better able to function in the higher temperatures of its native tropical habitats.^[7] The lower temperature range of D. lumholtzi extends to 5°C, where some individuals are able to survive,^[5][4][6] with survival increasing significantly at 10° C^[4][6] and reproduction beginning to occur at 15° C.^[4][6] This significantly broad temperature range in which D. lumholtzi can survive has led to it being labeled a eurythermal species.^[7]

Ephippia stress response

D. lumholtzi, as with most daphnid species, produces eggs called ephippia when exposed to high environmental stress.^[8] These ephippia are able to survive temperature, oxygen, and salinity levels significantly lower and higher than those hospitable to adult daphnids, although exact ranges for D. lumholtzi ephippia have not been found.^[8] This ephippia stage is an example of diapause, a state of suspended animation an organism can enter in order to survive a harsh environment.^[9]           

References

JacobMwiki (talk) 00:28, 29 April 2016 (UTC)