Makito Kobayashi

The goldfish, a member of the Cyprinidae (minnow family) is an important and relevant model for understanding how fish function and reproduce and can be better managed. Here, we review recent studies of this species and its strengths as a model system. First, we review current understanding of how its neuroendocrine system regulates gonadotropin release, and then steroidogenesis and finally gonadal maturation in both males and females. Endocrine determinants of gender and sexual plasticity are described. Effects of the fish androgen, 11-ketotestosterone, on sexual and spawning behavior of males and females are reviewed. We also describe how several hormones (androstenedione, 17,20β-dihydroxy-4-pregen-3-one, prostaglandin F2α) and their metabolites function as “hormonal pheromones” to synchronize male–female spawning behavior and physiology. Insight provided by the goldfish has enhanced understanding of the neurobiology, toxicology, and comparative physiology of other fishes and provided lessons applicable to the culture of commercial carp and the control of invasive species, as well as conservation.

The migration of terrestrial microplastics (MPs) into aquatic environments is considered to have negatively affected aquatic ecosystems. However, quantifying terrestrial pathways is challenging because of the diverse sources of MPs. A poorly understood MP source is shoe outsoles. Therefore, this study determined whether shoes released MPs by abrasion and conducted a series of five experiments to study the potential impacts on aquatic life. The first and second experiments examined the relationship between the distance walked or jogged and the weight loss of shoes. The weight of the shoes decreased significantly as the moving distance increased; however, the values did not differ significantly between walking and jogging. The third experiment tested the size and number of MPs originating from shoe outsole under simulated conditions using sandpaper, which imitated the surface of an asphaltic pavement. The number of MPs increased as the number of contacts with the sandpaper increased. The fourth and fifth experiments examined whether goldfish, Carassius auratus, ingested rubber tips (RTs) derived from the shoe outsoles. When goldfish were given sinking RT or floating RT, they ingested both types of RT with or without feed, suggesting that goldfish recognized RTs as feed. The results of this study suggested that shoes released abrasion debris and particles into the environment as MPs. This study is the first to demonstrate the process of MP release from shoe outsoles and the ingestion of MPs from shoe outsoles by fish.