The first observation of sexual reproductive behaviour of a tardigrade was reported in 1895, and much knowledge on this topic has accumulated since then. Up to the 2000s, these observations were described in detailed reports including text, sketches and photographs. Then, innovative video recordings were made in 2016 of the reproductive behaviours of tardigrades that oviposit their eggs into exuviae, allowing the clear visualization of the phenomenon. Recently, we recorded tardigrade species laying their eggs freely in the environment. Thus, knowledge of tardigrade reproduction has been increasing. We therefore summarize the known sexual reproductive behaviours of tardigrades and present some unresolved questions herein.
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