Fish Behavior 1 by unknow

Author:unknow
Language: eng
Format: epub
Publisher: John Wiley & Sons, Incorporated
Published: 2020-04-03T00:00:00+00:00

2.2.6.2. Differences in buoyancy

The gas pressure in the gas bladder of many teleosts, such as cyprinids, enables them to change their buoyancy during swimming movements, whether they are diving or swimming to the surface. The development of this hydrostatic organ conditions the way of life, and the pelagic forms that evolve in open water often have a more developed gaseous bladder than the demersal* and benthic* forms, which enables them to have wide variations in buoyancy. A single case of sexual dimorphism has been reported in a small Asian cyprinid of the Psilorhynchus genus, with the gas bladder of males five times larger in diameter and 98 times more voluminous; this is probably related to the respective volumes of the ovaries and testicles, with the former clearly the more developed.

Elasmobranchs, which do not have this anatomo-physiological regulatory device, maintain a certain hydrostatic balance thanks to their hepatic lipids, which results in sharks, rays and others actively swimming in marine waters. A select few freshwater species such as the bull shark, Carcharhinus leucas, and the sawfish, Pristis pristis, undergo a strong modification of their buoyancy in correlation with a decrease in water density; their buoyancy then becomes negative, being multiplied by a factor of 2–3, which should justify a considerable hepatic increase of ×4 to ×8 which does not occur. The increased expenditure of energy when swimming compensates for this difference in buoyancy, which is a serious handicap to the occupation of freshwater environments by the vast majority of elasmobranchs. Fresh water is an environment not easily frequented by sharks.

Bibliography: Biol.Lett., 2014, 10: 20140348 & DOI:10.1098/rsbl.2014.0348, J.Exp.Biol., 2015, 218: 1099-1110 & DOI:10.1242/jeb.114868

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