Freshwater fish egg dispersal by attaching to waterbirds

Dispersal over geographic barriers plays an essential role in colonization, gene flow, metapopulation dynamics, and invasion (Bowler and Benton 2005). Since dry lands strictly separate freshwater habitats, as expressed by the phrase "islands of water in a sea of dry land" (Faulks, Gilligan, and Beheregaray 2010), dispersal among freshwater waterbodies by themselves is challenging of aquatic organisms. Rumors have existed worldwide that freshwater fish eggs are dispersed by attaching to (ectozoochory) or excretion from (endzoochory) waterbirds (Hirsch et al. 2018). It is well documented that waterbirds disperse aquatic plants, zooplankton, and various aquatic invertebrates, which are co-distributed with fishes (Green et al. 2023). However, there is only two reported cases of empirical evidence of endzoochory in freshwater fishes and no scientific evidence of ectozoochory (Hirsch et al. 2018; Silva et al. 2019; Lovas-Kiss et al. 2020; Green et al. 2023). Here, we show that the southern medaka (Oryzias latipes, hereafter medaka) egg can travel passively by attaching to waterbirds.