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[1] [a] Although shoaling fish can relate to each other in a loose way, with each fish swimming and foraging somewhat independently, they are nonetheless aware of the other members of the group as shown by the way they adjust behaviour such as swimming, so as to remain close to the other fish in the group. Shoaling groups can include fish of ...
These fish are adapted for an active life under low light conditions. Most of them are visual predators with large eyes. Some of the deeper water fish have tubular eyes with big lenses and only rod cells that look upwards. These give binocular vision and great sensitivity to small light signals. [37]
Diagrammatic vertical section through the eye of teleost fish. Fish have a refractive index gradient within the lens which compensates for spherical aberration. [4] Unlike humans, most fish adjust focus by moving the lens closer or further from the retina. [5] Teleosts do so by contracting the retractor lentis muscle.
A bait ball, or baitball, occurs when small fish swarm in a tightly packed spherical formation about a common centre. [1] It is a last-ditch defensive measure adopted by small schooling fish when they are threatened by predators. Small schooling fish are eaten by many types of predators, and for this reason they are called bait fish or forage fish.
The vertebrate eye is usually either optimised for underwater vision or air vision, as is the case in the human eye. The visual acuity of the air-optimised eye is severely adversely affected by the difference in refractive index between air and water when immersed in direct contact.
The central nervous system is confined to the lateral line and olfactory systems, the eyes are small and may not function, and gills, kidneys and hearts, and swim bladders are small or missing. [ 48 ] [ 55 ] These are the same features found in fish larvae , which suggests that during their evolution, bathypelagic fish have acquired these ...
[14] [15] [16] Other examples of adaptations are larger fins for more energy-efficient swimming, and a loss of scales and swim bladder. [17] [18] The loss can be complete or only partial, for example resulting in small or incomplete (but still existing) eyes, and eyes can be present in the earliest life stages but degenerated by the adult stage ...
Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains squalene, and their cartilage, which is about half the normal density of bone. [51] Their liver constitutes up to 30% of their total body mass. [35]