What is the difference between the effects of the density and the viscosity of water from the perspective of an aquatic vertebrate?

Question

What will be an ideal response?

Answer 1

Density and viscosity have overlapping effects on some activities of vertebrates and act independently in other situations.
Both the velocity of an animal's movement through water and the ability of an aquatic organism to ventilate its respiratory surfaces are retarded by the density and viscosity of water. The high density of water means that an aquatic animal must work to force the water aside, and its high viscosity means that water flows slowly compared to the flow of a less-dense fluid such as air.
The density of water allows aquatic animals to achieve neutral buoyancy so they can maintain their vertical position in the water column without needing to swim up or down. The viscosity of water plays essentially no role in this behavior.
The high density of water renders gravity irrelevant to aquatic animals. Its high viscosity is the main physical property that makes streamlining important for aquatic vertebrates.
Because aquatic vertebrates are essentially neutrally buoyant, their skeletons lack the weight-bearing characteristics seen in the skeletons of terrestrial vertebrates.
Free of the need to support a massive body, aquatic vertebrates can evolve very large body sizes. The blue whale, the largest aquatic vertebrate, reaches a length of 30 m and weighs 180,000 kg. A few of the huge dinosaurs of the Mesozoic era were longer than a whale, but they weighed less. Argentinosaurus is the largest dinosaur currently known; it is estimated to have been about 34 m from nose to tail tip, but it weighed only 91,000 kg.