2.2.3. Mechanical and gametic isolation
The degree to which mechanical and gametic isolation play a role in reproductive isolation between closely related species remain largely unexplored in Daphnia . While it appears that males exhibit little discrimination when approaching heterospecific females, it is uncertain if there are morphological differences in reproductive systems that could prevent successful mating and fertilization. Wuerz, Huebner, and Huebner (2017) described the male reproductive system in D. magna , but no morphological studies provide a contrast of morphological differences in the reproductive systems between species. When there is an excess of males in nature, unmated females are rare (Innes, 1997), and females may copulate with multiple males. While sperm storage appears unlikely (Huebner & Huebner, 2016), research into gamete viability (see Xu et al., 2015) and the process of sperm transmission towards eggs are direly needed. Thus, more studies are required to characterize the morphological differences in the anatomy of the reproductive organs as well as potential mechanisms of gametic isolation after successful heterospecific mating.