How to distinguish male Daphnia from female Daphnia

In many Daphnia species, females can reproduce through a process called parthenogenesis, in which a female daphnid will produce a clutch of viable eggs without the need for fertilization. These asexual eggs, which are genetically identical to the mother and to each other, are carried in a brood chamber under the mother's carapace.  Since this process does not require fertilization, a culture of parthenogenetic daphnids will contain only females. Often, however, parthenogenetic species will alternate between asexual and sexual reproduction seasonally (cyclic parthenogenesis).

Figure 1. Female Daphnia with empty 

brood chamber

Figure 2. Female Daphnia with 

parthenogenetic eggs in the brood 

chamber

In wild populations that we've sampled, males are rarely found during periods of peak population growth. However, we have captured wild males at the end of the summer season, when the population begins to dwindle. Environmental cues such as the presence of certain predators, deteriorating water quality or dwindling food supply may induce the female population to switch reproductive modes. When female daphnids are exposed to environmental stresses like these, they will still produce parthenogenetic eggs, but some of these eggs will be male, and usually a high proportion of the clutch will be males. These males hatch, mature and mate with with other females in the population. The fertilized eggs that result from mating are packaged into a protective structure called an ephippium or resting egg, which preserves them until the appropriate environmental cues induce them to hatch the following season.

Figure 3. Female Daphnia lumholtzi with ephippium in the 

brood chamber (a), and ephippia released from Daphnia (b)

In daphnid cultures, male production usually indicates a stress response, perhaps due to the culture temperature fluctuating or to poor water quality. If you see males in your cultures, and you don't want to take down or re-start the culture, you should remove them and any females that are carrying ephippia, and change the culture water.

With a bit of practice, you can learn to identify the males visually while they swim. Males viewed from the top appear to have wider "shoulders" than the females. Looking at the male from the ventral aspect shows why: the top third of the carapace is opened wider ventrally in male daphnia. So as you watch the male swimming, he appears to be swimming around with his carapace opened. The female carapace opening is narrower along the ventral aspect, so she appears more streamlined. Adult females are also larger than adult males.

The differences in physical appearance appear to be related to reproduction, as the male has two pairs of modified appendages that he uses for grasping during mating: one pair is a set antennules near the rostrum. Just posterior to these are the first set of phyllopods, or limbs, each of which has an elongated seta (stiff bristle) at the tip. While you can't see these modified appendages at lower magnifications, you can learn to visually distinguish the male from the female by his more squat, triangular shape and ventrally opened carapace.

These features are easily distinguishable microscopically. The rostrum is much reduced relative to the female, giving them a flat-faced appearance, just below which the modified antennules can be seen. Just below these, the opened carapace and grasping setae are clearly evident. In contrast, the female has a distinct rostrum, lacks modified grasping appendages, and a narrower ventral margin.

Figure 4. Comparison of the rostrum region of female and male daphnids

Figure 5. Scanning electron micrograph images of male Daphnia lumholtzi. Images by Melissa Pompilius

All media by Melissa Pompilius
Middle Tennessee State University
Molecular Biosciences PhD Program
mp4n@mtmail.mtsu.edu