10. T H E SWIM BLADDER AS A HYDROSTATIC ORGAN
415
(1964) of a swim bladder in a fish caught at about 7000 meters depth
seems to represent the present depth record. Except for typical bottom
fishes most freshwater teleosts possess a swim bladder.
The fact that typical bottom fishes do not possess a swim bladder is
reasonable since they presumably rest on the bottom and thus may
benefit by a slight overweight. The arguments presented to justify the
lack of a bladder in elasmobranchs and in many active pelagic fishes
are less convincing. These arguments often imply that since these fishes
swim all or most of their lives anyway, the additional energy needed
to counteract sinking is insignificant. While the relative energy expenditure for this purpose will be less in active than in passive fishes, it is
hardly less expensive in absolute terms. It is more tempting to suggest
that certain disadvantages are associated with the presence of a bladder
or certain advantages with its absence.
A fish with a certain amount of gas in the bladder is in buoyancy
equilibrium at one depth only. Like a Cartesian diver the slightest vertical displacement will bring it out of equilibrium and the force pushing
upward or downward will increase with the displacement. Volume
changes are checked by an interplay between removal and deposition of
gas. However, these processes are slow as compared to the movements
of the fish. Few fishes are able to refill their bladder at surface levels
in less than a few hours. Thus while a swim bladder saves locomotory
energy for the fish, it inflicts upon it an instability with regard to vertical
position. This may be particularly inconvenient for active fishes which
change depth rapidly, e.g., to chase prey. These difficulties connected
with having a bladder will obviously be less marked the deeper the
fish lives since the volume change is proportional to the relative change
in hydrostatic pressure.
111. THE ARCHITECTURE OF THE SWIM BLADDER
The anatomy of the swim bladder is, from a functional point of view,
very similar in all fishes. It is most instructively displayed in the common
eel, AnguiZZu vulgaris, which will therefore serve as an example throughout this section (Fig. 1 ) .
A. General Organization and Cytology
Embryologically the swim bladder develops as a pouch protruding
from the foregut, and in some fishes the connection between the
415
(1964) of a swim bladder in a fish caught at about 7000 meters depth
seems to represent the present depth record. Except for typical bottom
fishes most freshwater teleosts possess a swim bladder.
The fact that typical bottom fishes do not possess a swim bladder is
reasonable since they presumably rest on the bottom and thus may
benefit by a slight overweight. The arguments presented to justify the
lack of a bladder in elasmobranchs and in many active pelagic fishes
are less convincing. These arguments often imply that since these fishes
swim all or most of their lives anyway, the additional energy needed
to counteract sinking is insignificant. While the relative energy expenditure for this purpose will be less in active than in passive fishes, it is
hardly less expensive in absolute terms. It is more tempting to suggest
that certain disadvantages are associated with the presence of a bladder
or certain advantages with its absence.
A fish with a certain amount of gas in the bladder is in buoyancy
equilibrium at one depth only. Like a Cartesian diver the slightest vertical displacement will bring it out of equilibrium and the force pushing
upward or downward will increase with the displacement. Volume
changes are checked by an interplay between removal and deposition of
gas. However, these processes are slow as compared to the movements
of the fish. Few fishes are able to refill their bladder at surface levels
in less than a few hours. Thus while a swim bladder saves locomotory
energy for the fish, it inflicts upon it an instability with regard to vertical
position. This may be particularly inconvenient for active fishes which
change depth rapidly, e.g., to chase prey. These difficulties connected
with having a bladder will obviously be less marked the deeper the
fish lives since the volume change is proportional to the relative change
in hydrostatic pressure.
111. THE ARCHITECTURE OF THE SWIM BLADDER
The anatomy of the swim bladder is, from a functional point of view,
very similar in all fishes. It is most instructively displayed in the common
eel, AnguiZZu vulgaris, which will therefore serve as an example throughout this section (Fig. 1 ) .
A. General Organization and Cytology
Embryologically the swim bladder develops as a pouch protruding
from the foregut, and in some fishes the connection between the