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.