Does larval ability to modulate body buoyancy explain successful colonization of freshwater environments by diadromous gobies?

Salinity is an environmental factor that strongly characterizes the habitat use patterns of aquatic organisms. However, knowledge is biased toward the effect of differences in osmotic pressure among salinity habitats; how ambient specific gravity (SG) differences determine species distribution is scarcely understood. Diadromous fish, which migrate between marine and freshwater habitats, may encounter this SG problem when they are unexpectedly landlocked in or colonize freshwater areas with low environmental SG. This is particularly serious for planktonic larval fish, which must maintain neutral buoyancy for foraging and passive locomotion, although their swimbladders are generally underdeveloped. Then, we hypothesized that the SG problem limits the establishment of freshwater resident populations in marine-originated diadromous fishes. To test this hypothesis, the SG modulation ability of newly hatched larvae was compared among three closely related diadromous goby species in Gymnogobius, one of which has freshwater resident populations. The aquarium experimental results did not support that only the species deriving freshwater residents can maintain neutral buoyancy even in freshwater conditions; that is, all three species could modulate their body SG almost equally to those of both fresh and sea waters. This suggests that the ability to maintain neutral buoyancy in freshwater had been pre-adaptively acquired prior to larval freshwater colonization. On the other hand, it is highly noteworthy that the early larvae of the target group maintained neutral buoyancy in various SG environments using swimbladders, which is the first such evidence in teleosts.