Abstract: Rare species may exhibit some characteristics, such as a narrow distribution area and a specialization of habitat utilization. These characteristics may provide crucial information about the underlying mechanisms of species endangerment. Bunops scutfrons, a rare cladocera, has received more and more attention due to its rarity. Understanding the causes of its rarity could advance ecological theory. This species has only been recorded in four samples in China, with limited taxonomic descriptions. Its key biological and ecological characteristics remain largely unknown. Live specimens of B. scutfrons were firstly collected from the Dianchi Lake Basin during a systematic clado-ceran survey in 2022. Through successful propagation, sufficient individuals were available, allowing for an investigation into population structure, reproductive capacity, and population dynamics of this species. I carried out observations in 2022 and 2023. B. scutfrons was a small cladoceran (body length: 0.30-1.15 mm), with sexual maturation at a small size (0.52 mm). Notably, no male individuals were detected in the population. In 2022, the fecundity of the outdoor population (summer eggs) was (2.2±1.4) eggs·ind^-1 (n=334, max=8 eggs·ind^-1), which increased to (4.8±2.8) eggs·ind^-1(n=94, max=13 eggs·ind^-1) in 2023. During the reproduction of winter eggs, two eggs were produced per ephippium. In life table experiment, individuals were fed with mixed algal solution (Chlorella spp.). The maximum number of reproductive instars in the first (F₁) and second (F₂) generation was 8 and 7, respectively. The total reproductive rate was 30.3 eggs·ind^-1 and 9.4 eggs·ind^-1, respectively, with maximum lifespans of 21 and 16 days, and intrinsic growth rates of 0.37 and 0.36. When cultured in a mud slurry medium, the maximum reproductive instars for both the F₁ and F₂ generations was 8, with a total reproductive rate of 40.3 and 36.1 eggs·ind^-1, respectively. The lifespan extended to 28 and 27 days, with intrinsic growth rates of 0.31 and 0.30, respectively. Under suitable conditions, the population of B. scutfrons grew rapidly, with an instantaneous growth rate of 0.39 per day. By the 10th day, population size increased 50-fold, reaching a relative density of 510 ind·L^-1, and the population entered a plateau phase. In the natural environment, however, the relative abundance remained extremely low. In 2022, the population disappeared during the dry season, while in 2023, it vanished completely from the observation area during the flood season. These findings suggested that B. scutfrons exhibits a relatively long lifespan, high reproductive capacity, and strong potential for population growth under favorable conditions. Compared to many common species, B. scutfrons does not show significant disadvantages in its fundamental biological or population ecological traits. Its rarity in natural environments is likely due to its susceptibility to environmental pressures rather than intrinsic biological limitations.

Key words: Bunops scutifrons, population structure, fecundity, population dynamics, rare species