Review of Dytiscidae (Coleoptera) eggs: focus on Japanese species

Dytiscidae are the most diverse aquatic beetle group, but they have been negatively impacted by human population expansion, and their numbers have declined heavily with approximately 40% of all species included on the Japanese Red List. Although egg-related knowledge is scarce and fragmented, understanding eggs and substrates is critical for successful conservation. I collected and reviewed comprehensive information on eggs and egg-laying habits of Japanese Dytiscidae. The goal of this review is to include as broad a range of egg information as was available, with the aim of providing information, ideas on diversity, evolution, and conservation. This review provides information on egg-laying habits and substrate requirements of approximately 29% (40/138) of Japanese species of Dytiscidae, and also includes for the first time egg-laying habit information for five species containing Platynectes and Japanolaccophilus, along with detailed videos and photos. Accumulation of egg information will provide additional aspects to understanding the evolution, niche, and life history of Dytiscidae. This review is the first step towards bridging the knowledge gap regarding Dytiscidae eggs. The study suggested that in some taxa, egg stage may be threatened by declining egg-laying substrates, invasion by exotic species, and global warming. Egg-laying habits and substrate information are essential components of successful in-situ conservation. To effectively conserve the Dytiscidae, we need to understand their eggs more deeply and associate them with conservation strategies.