#daphniidae

Water Flea (Daphnia sp.), family Daphniidae, order Anomopoda, Poland

Don't Flea from the Water Flea!

Daphnia pulex is the most common species of water flea, and perhaps one of the most abundant animals in the world. They are found in nearly every permanant, stagnant body of fresh water, including lakes, ponds, slow-moving streams, and small puddles. They typically reside near the surface of the water, but will occasionally venture deeper into the column, particularly during the day when they are more vulnerable to predators.

Colonies of D. pulex feed on bacteria, phytoplankton fine detritus, and certain types of algae. They are filter feeders, using their feathery front legs to create small currents in the water which draws in food. Due to their small size, D. pulex is vulnerable to many predators including fish, insect larvae, and other water fleas. Populations often undergo cyclomorphosis, a process in which they change size and shape to make it harder for potential predators to feed on them.

Despite being quite abundant, D. pulex is rarely seen, as individuals are usually between 0.2-3.0 mm long. Males are usually smaller than females, and have longer antennae. Although they are crustaceans, it is difficult to distinguish segments of their body, and different populations may have anywhere from 4-6 pairs of legs. The outer shell is largely transparent, exposing the internal organs.

D. pulex typically utilizes either cyclical parthenogenesis to reproduce. In cyclical parthenogenesis, the cycle starts in the winter with females laying eggs which are fertilized by the males. The eggs can remain dormant for up to twenty years, and can resist freezing and drying. After hatching, the young can live up to 100 days without predation. In this second part of their cycle, which occurs during the summer, the emerging population is entirely female, and the eggs they lay can develop into both males and females. Under ideal conditions, populations may consist entirely of parthenogenic females.

Conservation status: D. pulex has not been evaluated by the IUCN. Due to their large range and population they are largely stable, although overharvesting for the aquarium trade may pose a threat to more isolated populations.

Photos

Paul Herbert

Lance Biechele

Photos by @gemfyre

Another freshwater crustacean from the Dingo Rock gnamma hole (and there's some even tiny Ostracods in the photo, too).

I'm not sure which genus - there are certainly Daphnia species known from gnamma holes - D. jollyi, for example, is a WA endemic, but we get other genera too, such as Simocephalus.

There are at least 150 species in Daphnia, alone. Most of them, like their relatives, can reproduce asexually, with occasional sexual reproduction to produce eggs that can survive extended drought or transport over long distances.