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SUMMARY Conspecific populations with different life‐cycle seasonality, particularly of their reproductive season, have been reported for many seaweed species. However, the number of genetic analyses of such seasonal populations is limited. Herein, based on the mitochondrial cytochrome oxidase subunit 3 gene (cox3) haplotype, we evaluated the genetic diversity of winter (January–March) and spring (April–June) fertile populations of the brown alga Sargassum horneri in Sado Island, Japan. Obviously, dominant cox3 haplotypes were different between seasons (even at a single site), and the N ST values indicated the presence of a strong genetic differentiation between the two seasonal populations. AMOVA analyses confirmed that most of the genetic diversity was between seasons rather than between regions. Herein, the possibility of allochronic isolation is discussed. Considering the phylogeny of the mitochondrial cox3 DNA sequence, autumn or winter seasonal populations such as those in this investigation have evolved independently at least twice in the species. The present study shows that a shift in breeding season could be expected to conserve the genetic diversity of S. horneri in a certain region.

Hydaticus pacificus conspersus Régimbart, 1899 (Coleoptera: Dytiscidae), is currently considered “endangered,” according to the national Red List of Japan. Herein, the developmental period of the immature stages of H. pacificus conspersus during laboratory rearing is described in detail for the first time. We found that this species deposits eggs not only on aquatic plants but also on the surface of driftwood. The observed durations of developmental stages were as follows: egg period – 3 days; first instar – 1–2 days (1.7 ± 0.5); second instar – 1–4 days (2.4 ± 0.6); third instar – 4–7 days (5.0 ± 0.7 days); and construction of a pupal chamber to emergence: 8–13 days (9.6 ± 0.8). Based on these observed results, H. pacificus conspersus has one of the shortest larval developmental periods among the Dytiscidae species. The short larval period and the ability to lay eggs on substrates other than plants may be an adaption of the species to increase reproductive success in unstable environments.