THE BEHAVIOUR AND PHYSIOLOGY OF HEBRINO AND OTHER CLUPEIDS 321
spawning, for the ovary, at any rate, is mainly composed of proteinaceous yolk.
Immediate changes in density such as those required during
vertical migration, or during movement into water of a different temperature or salinity, may be controlled by the swim bladder or perhaps
by fin movements. The function of the swim bladder may be deduced
from its structure. This has been described by many earlier workers,
but especially in herring and Sprattm sprattus by Maier and Schcuring
(1923) and in Sardinops caerulea arid Enyraulis m d a x by O’Connell
(1966). The clupeids are pliysostomes with an open pneumatic duct
between the swim bladder and the gut. I n clupeids there is also an anal
duct leading to the exterior, but this is not present in Engraulie.
The presence of smooth muscle in the wall of the swim bladder
suggests that the volume may be controlled, but the anal duct does not
seem to be equipped with a sphincter muscle while the pneumatic duct
seems to be designed for peristalsis. There is no well-developed gas
gland. An increase of pressure in the swim bladder, required to compensate for a reduction in volume at greater depths, might be brought
about by secretion of gas from the epithelial lining, or perhaps by
swallowing air at the surface and keeping it under pressure by means
of the muscular wall until a relaxation of the muscles would be neutralized by the increase of hydrostatic pressure at greater depths. O’Connell
suggefited that the pneumatic duct might pump in air (from outside or
perhaps from decomposition products in the gut), while excluding food
particles and parasites by peristaltic action. The anal duct might
collapse if internal pressure was too low and act as a safety valve if too
high. The release of gas from fifih rifling to the Rurface ifl a wc?ll known
phenomenon observed hy finhcrmcrr . Vdwijw1 ( 1 Ijh:$) f i J t 1 r d t h i t l h
specimen of Sprattw spraltus relrmccl gas hi1 hblofi from thc, arisl oparting
after 70 sec when the preRsuro was rodaocd st n rate of 40 mm Hglmin.
Relcafie of gas ~ I H O occurs tlirough tho yrieiimntic ducts where an anal
clrrot is present (Brawn, 1962), but whcther gm might diffuse through
the swim bladder wall is not known.
Jones and Marshall (1963) discussed the high sinking factor
(Density of fish x 100
Density of water
---) of herring (reported by Magnan to be 1034
compared with the range 980-1013 for most other fish) and considered
that the value should be redetermined. This high sinking factor will,
presumably, vary greatly with the fat content m d an average value
would be hard to arrive at. If it is usually as high as this, the fish would
require to make considerable compensation to maintain a level in the
water. Brawn (1962) found a mean sinking factor of 1003 for herring
spawning, for the ovary, at any rate, is mainly composed of proteinaceous yolk.
Immediate changes in density such as those required during
vertical migration, or during movement into water of a different temperature or salinity, may be controlled by the swim bladder or perhaps
by fin movements. The function of the swim bladder may be deduced
from its structure. This has been described by many earlier workers,
but especially in herring and Sprattm sprattus by Maier and Schcuring
(1923) and in Sardinops caerulea arid Enyraulis m d a x by O’Connell
(1966). The clupeids are pliysostomes with an open pneumatic duct
between the swim bladder and the gut. I n clupeids there is also an anal
duct leading to the exterior, but this is not present in Engraulie.
The presence of smooth muscle in the wall of the swim bladder
suggests that the volume may be controlled, but the anal duct does not
seem to be equipped with a sphincter muscle while the pneumatic duct
seems to be designed for peristalsis. There is no well-developed gas
gland. An increase of pressure in the swim bladder, required to compensate for a reduction in volume at greater depths, might be brought
about by secretion of gas from the epithelial lining, or perhaps by
swallowing air at the surface and keeping it under pressure by means
of the muscular wall until a relaxation of the muscles would be neutralized by the increase of hydrostatic pressure at greater depths. O’Connell
suggefited that the pneumatic duct might pump in air (from outside or
perhaps from decomposition products in the gut), while excluding food
particles and parasites by peristaltic action. The anal duct might
collapse if internal pressure was too low and act as a safety valve if too
high. The release of gas from fifih rifling to the Rurface ifl a wc?ll known
phenomenon observed hy finhcrmcrr . Vdwijw1 ( 1 Ijh:$) f i J t 1 r d t h i t l h
specimen of Sprattw spraltus relrmccl gas hi1 hblofi from thc, arisl oparting
after 70 sec when the preRsuro was rodaocd st n rate of 40 mm Hglmin.
Relcafie of gas ~ I H O occurs tlirough tho yrieiimntic ducts where an anal
clrrot is present (Brawn, 1962), but whcther gm might diffuse through
the swim bladder wall is not known.
Jones and Marshall (1963) discussed the high sinking factor
(Density of fish x 100
Density of water
---) of herring (reported by Magnan to be 1034
compared with the range 980-1013 for most other fish) and considered
that the value should be redetermined. This high sinking factor will,
presumably, vary greatly with the fat content m d an average value
would be hard to arrive at. If it is usually as high as this, the fish would
require to make considerable compensation to maintain a level in the
water. Brawn (1962) found a mean sinking factor of 1003 for herring