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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.
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.
The early ontogeny of Triportheus albus and T. angulatus, two fish species of Triportheidae, is described using morphological, meristic, and morphometric characters. These species are exploited by subsistence fisheries and have potential as an alternative source of fish, given the decline in the natural stocks of other commercially important fish species in the Amazon. The specimens were collected in the open water limnetic zone, under of the macrophyte stands, and in subsurface areas near sandbars in the Amazon basin. Intra and interspecific morphometric analyzes were performed to evaluate growth models between species. The combination of color pattern, body morphology, morphometric proportions and myomeres number distinguishes the species from each other and from other congeners. Some morphometric relationships related to head as snout length and eye diameter as well as length from the snout to the origins of anal and length from the snout to the origins of pelvic, related with standard length were different between the two species of Triportheus, reflecting different growth models between them. An identification key for larvae and juveniles of some species of Triportheus from the Eastern Amazon is presented.
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