Discuss the various adaptations of vertebrates that enable them to live in marine conditions.

Authors Avatar
Discuss the various adaptations of vertebrates that enable them to live in marine conditions.

Vertebrates that live in water are said to be aquatic, and naturally over the course of time they have adapted to these marine conditions. They face a number of problems when it come to living in water, which I shall identify later, but have evolved complex and unique ways of dealing with them. Firstly though there must be a distinction between the types of aquatic vertebrates, although they can obviously be classified according to taxa it is more helpful to distinguish how they have adapted to a marine environment. All fishes are primary swimmers-their ancestors also swam. Other swimming vertebrates are secondary swimmers-their ancestors passed through a terrestrial state, and consequently they have structural and physiological handicaps that have prevented most of them from becoming entirely aquatic again.

It is academic to ask about the survival value of, or reason to evolve, an aquatic lifestyle for primary swimmers. It is a successful way of life, and nature has provided no alternative to most of them. A more interesting point is why did secondary swimmers choose to adopt an aquatic lifestyle again after the change to terrestrial life was so profound that it took about 100 million years to complete. The reverse trend back to water has been "easier" and has occurred many times and possible reasons for this reversal could be:

. To exploit a wide variety of aquatic foods, such as invertebrates, small fish and in one case vegetation.

2. To escape terrestrial predators. They may also subject themselves to aquatic predators of course, but the process of evolution has in specific instances moved in the direction of greater safety.

3. The oceans and major inland waterways are favourable avenues for dispersal and migration.

For whatever the reasons vertebrates have adapted to a marine lifestyle they must all face the same problems. The main categories of problems are motion, homeostasis, acquisition of food, and reproduction.

Motion

Water is a denser medium than air to live in it is harder to move in, and due to is 3-dimensional nature of water much finer control of body orientation is needed. Therefore, all proficient swimmers and divers must:

Reduce the resistance that water offers to motions of the moving body.

2 Propel themselves in a relatively dense medium.

3 Control vertical position in the water.

4 Maintain orientation and steer the body.

Reduce the resistance that water offers to motions of the moving body, Drag.

The resistance that a medium (here water) offers to the motion of an object is called drag. There are several sources, or kinds, of drag. They are interdependent, but can best be presented one at a time. First is frictional drag. Imagine a smooth, spindle-shaped, rigid object moving underwater. A film of water wets its surface and moves with it, yet a short distance away, the water does not move with the object at all. Between the object and the still water is the thin boundary layer where successive layers of water slide past one another; those nearest the object move nearly as fast as it does, and those more and more distant move slower and slower. The shearing forces thus produced tend to slow the moving object and are a source of drag. The boundary layer gets thicker toward the posterior end of the moving object. If a smooth spindle-shaped object moves slowly through the water, successive layers or lamina of the boundary layer slip past one another without any eddies. Flow is said to be laminar.

The next form of drag is pressure drag. This is created as the object moves through the water and suction is caused as the water flows back into the gap the object has displaced, disrupting the boundary layer (backfill).

The final form of drag is wave drag. This occurs of the object is moving close to or on the surface. Energy is extracted from the moving object to create the waves and is lost as drag.

Drag increases with body size, but so does the output of the animal's power plant, and these factors nearly cancel one another. The consequence of some rather complicated physiological considerations seems to be that moderately large swimmers have some advantage. The fastest swimmers are large fishes and small whales. There remain the important variables of body shape and the nature of the surface of the body.

Reduction of Drag by Adaptations of Body Form, Streamlining.

Pressure drag is low when the body is long and slender, like that of a snake or eel, because there is then little displacement and backfill. Frictional drag is minimal, however, when the body is short and plump, because surface area is then minimal. The best compromise is a spindle that is circular in cross section and thickest near the centre of its length where its diameter is one-fourth to one-fifth of its length. The bodies of tunas, swordfishes, and dolphins closely approach this shape. Absence of a functional neck (primary swimmers, cetaceans, sirenians), symmetry of the head, shape of thorax and body musculature, and the distribution of fat and blubber all may contribute to streamlining.

Projections from the basic spindle usually cause turbulence and eddies, and increase drag. Accordingly, expert swimmers reduce or eliminate all major projections not needed for propulsion and steering. Swimmers other than mammals have no external ears or external genitalia in their ancestry. Aquatic mammals secondarily lose their external ears and move the testes back into the abdomen. Nipples or teats and the penis may be withdrawn within the body contour when not functioning. Fast primary swimmers have no limb segments between their fins and bodies. Fast secondary swimmers have very short proximal limb segments to again bring the feet or flippers close to the body. The humerus of cetaceans may be only about as long as it is wide; the femur of pinnipeds may be less than twice as long as wide; the femur of diving birds is short and most of the leg musculature is contained within the contour of the body. Cetaceans and sirenians have reduced the pelvic appendages to internal vestiges. For example a 19m whale would have a 41cm pelvis and a 4cm thighbone. Some other swimmers position the hind limbs in such a way that they do not protrude but instead extend the contour of the spindle shaped body. The knee joints of pinnipeds, beavers, and many diving birds are constructed to allow the necessary reorientation of the limb. Salamanders, crocodilians, and aquatic lizards hold their limbs against the body as they swim with tail and trunk. Lateral fins and flippers that propel the body, on the other hand, must protrude and present a flat surface to the water. Bony fishes also can reduce the area of their fins by folding. Dorsal and anal fins (including the "sail" of the sailfish) may be retracted into grooves on the body surface during fast swimming. Web-footed tetrapods flex their limbs and adduct and curl their toes on the recovery stroke.
Join now!


These considerations of the drag on a rigid object make it evident that fast-swimming vertebrates require adaptations of body form and of the nature of the surface of the body. However, since swimmers are not rigid, drag can also be reduced by certain behavioural adaptations.

Reduction of Drag by Adaptations of Body Surface and Behaviour.

It is advantageous for most swimmers to achieve laminar flow over as much of the body as possible. To maximize laminar flow, fishes evolve small smooth scales, or none at all, and become covered with slime. The small scale like feathers ...

This is a preview of the whole essay