Resumen:
Larval and juvenile air breathing fish may experience nocturnal and/or seasonal aquatic hypoxia.
Yet, whether hypoxia induces respiratory developmental plasticity in larval air breathing fish is
uncertain. This study predicted that larvae of two closely related anabantid fish—the facultative
air breather the Siamese fighting fish (Betta splendens) and the obligate air breathing blue gourami
(Trichopodus trichopterus)—show distinct differences in developmental changes in body, gill,
and labyrinth morphology because of their differences in levels of dependency upon air breathing
and habitat. Larval populations of both species were reared in normoxia or chronic nocturnal
hypoxia from hatching through 35–38 days postfertilization. Gill and labyrinth variables were
measured at the onset of air breathing. Betta splendens reared in normoxia possessed larger,
more developed gills (~3× greater area) than T. trichopterus at comparable stages. Surface area
of the emerging labyrinth, the air breathing organ, was ~ 85% larger in normoxic B. splendens
compared to T. trichopterus. Rearing in mild hypoxia stimulated body growth in B. splendens, but
neither mild nor severe hypoxia affected growth in T. trichopterus. Condition factor, K (~ 1.3 in
B. splendens, 0.7 in T. trichopterus) was unaffected by mild hypoxia in either species, but was
reduced by severe hypoxia to <0.9 only in B. splendens. Severe, but not mild, hypoxia decreased
branchial surface area in B. splendens by ~40%, but neither hypoxia level affected Trichopodus
branchial surface. Mild, but not severe, hypoxia increased labyrinth surface area by 30% in
B. splendens. However, as for branchial surface area, labyrinth surface area was not affected in
Trichopodus. These differential larval responses to hypoxic rearing suggest that different larval
habitats and activity levels are greater factors influencing developmental plasticity than genetic
closeness of the two species.