9. AIR BREATHING IN FISHES
363
Among air-breathing fishes, thosc occupying brackish and marine
habitats are rare because of the well-stirred turbulent conditions of these
waters. However, several members of marine gobiid and blennid families
show structural adaptations for air breathing ( Schottle, 1931; Oglialoro,
1947). Estuarine waters may show large spatial, diurnal, and seasonal
fluctuations in oxygen availability (Todd and Ebeling, 1966). More
exceptionally, fishes like Periopthalmus have developed air-breathing
habits in support of behavioral acts such as feeding and escape reactions.
11. NATURE OF THE STRUCTURAL
ADAPTATIONS FOR AIR BREATHING
Structural adaptations for air breathing in fishes are extremely diverse.
The extent of modifications from structures typical of an aquatic breather
often reflects the relative role of air breathing in overall gas exchange.
Some species rely predominantly on water breathing with gills and only
supplement gas exchange with air breathing when adverse respiratory
qualities in the water make aquatic breathing insufficient or too costly
for extraction of the required oxygen. Others are obligate air breathers
and succumb if denied access to air for a short time (Rauther, 1910;
Johansen, 1968).
All air-breathing fishes have a hollow compartment as a receptacle
or holding space for air. The air chamber may have evolved as a modification of an already existing structure or it may be a neomorphic
structure. It is richly supplied with blood vessels, and fine capillaries
or lacunar spaces invest the membranes bordering the air space. The
interface between blood and air is specialized to reduce the diffusion
distance and other diffusion barriers between air and blood. Frequently,
various forms of structural changes such as foldings, papillations, or
arborizations of the gas exchange membranes have resulted in a considerable surface expansion that enhances the diffusion exchange. Such
structural specialization is more prominent in species utilizing air breathing as a major means of gas exchange. A concurrent trend is for the
primary aquatic gas exchange organs, the gills, to degenerate progressively to few, coarse, and sparsely perfused gill filaments as air
breathing assumes a more important role (Carter and Beadle, 1931b;
Dubale, 1951). In some species, such as the electric eel, Electroplzorus
electricus, the gills have become vestigial (Evans, 1929; Johansen et al.,
1968). Air-breathing fishes also show structural specializations for rhythmic ventilation of the air-breathing organs, while the muscle and skeletal
363
Among air-breathing fishes, thosc occupying brackish and marine
habitats are rare because of the well-stirred turbulent conditions of these
waters. However, several members of marine gobiid and blennid families
show structural adaptations for air breathing ( Schottle, 1931; Oglialoro,
1947). Estuarine waters may show large spatial, diurnal, and seasonal
fluctuations in oxygen availability (Todd and Ebeling, 1966). More
exceptionally, fishes like Periopthalmus have developed air-breathing
habits in support of behavioral acts such as feeding and escape reactions.
11. NATURE OF THE STRUCTURAL
ADAPTATIONS FOR AIR BREATHING
Structural adaptations for air breathing in fishes are extremely diverse.
The extent of modifications from structures typical of an aquatic breather
often reflects the relative role of air breathing in overall gas exchange.
Some species rely predominantly on water breathing with gills and only
supplement gas exchange with air breathing when adverse respiratory
qualities in the water make aquatic breathing insufficient or too costly
for extraction of the required oxygen. Others are obligate air breathers
and succumb if denied access to air for a short time (Rauther, 1910;
Johansen, 1968).
All air-breathing fishes have a hollow compartment as a receptacle
or holding space for air. The air chamber may have evolved as a modification of an already existing structure or it may be a neomorphic
structure. It is richly supplied with blood vessels, and fine capillaries
or lacunar spaces invest the membranes bordering the air space. The
interface between blood and air is specialized to reduce the diffusion
distance and other diffusion barriers between air and blood. Frequently,
various forms of structural changes such as foldings, papillations, or
arborizations of the gas exchange membranes have resulted in a considerable surface expansion that enhances the diffusion exchange. Such
structural specialization is more prominent in species utilizing air breathing as a major means of gas exchange. A concurrent trend is for the
primary aquatic gas exchange organs, the gills, to degenerate progressively to few, coarse, and sparsely perfused gill filaments as air
breathing assumes a more important role (Carter and Beadle, 1931b;
Dubale, 1951). In some species, such as the electric eel, Electroplzorus
electricus, the gills have become vestigial (Evans, 1929; Johansen et al.,
1968). Air-breathing fishes also show structural specializations for rhythmic ventilation of the air-breathing organs, while the muscle and skeletal