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Adaptations To Life In The Sea

The types of locomotion marine animals have adopted.

There are several main types of locomotion adopted by marine animals. Perhaps the simplest is cytoplasm streaming. Here the animal projects its cytoplasm forward into pseudopodia then flows into this projection. Amoeba move in this way. Secondly some animals move by means of ciliary propulsion e.g. protozoa or marine worms. Thirdly we have propulsion by changes in body shape e.g. certain jellyfish, squid and starfish. Fourthly there is propulsion by undulations of the body e.g. many kinds of fish. Fifthly we should consider propulsion by means of appendages, fins, tails and limbs as seen in more complex marine animals. These are the principle types of locomotion adopted by marine animals. In addition vertebrate animals have a number of interesting adaptations that help them in locomotion. Briefly these are adaptations in shape, muscle types, fin shapes, tail shapes, swim bladders and internal organs. A fusiform shape e.g. tuna helps in streamlining. Fast pelagic fish have muscles adapted for fast swimming e.g. tuna, marlin , mackerel and swordfish. Tail shapes in sharks are adaptations helping with sudden surging movements necessary in their locomotion. Many fish have swim bladders which helps with their buoyancy and some species of shark have large oily livers which help with buoyancy and indirectly with locomotion.

Methods of feeding in marine animals.

Broadly speaking there are several main methods of feeding in marine animals. The simplest form of feeding is possibly by animals engulfing food and digesting in food vacuoles within the cell. This is known as phagocytosis. Then there is the method of feeding by digestion in a gastrovascular cavity e.g. sea anemone. Many organisms are filter or suspension feeders e.g bivalve molluscs, tubeworms and krill. This method is Used at all scales in the marine environment as some whales filter feed on zooplankton. Deposit feeders sift out nutrients from the seabed e.g. sea cucumber. Some organisms are grazers for example gastropod molluscs use rasping teeth called radula. Finally there are a whole range of carnivores from the dog whelk who feeds using a drill mechanism to whales, sharks and dolphins and ambush predators such as the angler fish using bioluminescent lures. These are the main methods of feeding in marine animals but there are many variations within each category.

The sensory systems in marine animals.

What is the dominant sense in the sea ? We should consider at least six major sensory systems in marine animals. The dominant sense in the sea is hearing so we should begin with this. Many invertebrates detect sound by cilia. Fish detect by sensory hairs in the otolith organ in the inner ear. Lateral line systems also enable fish to detect sound vibrations, movements of prey and fish in schools and changes in ocean currents. When we consider vision there are also a variety of systems in use. There are relatively simple systems such as eyespots e.g. polychete worms to the spherical lens systems of fish which allow them to have light perception beyond the capabilities of man. Vision is essential for both prey and predators. The humpback whale also has a complex eye and by moving their eyes backward to the flatter part of the lens they can see clearly above water and under water they use the front part of their eyes which are more spherical and covered with mucus in a protective layer. Next we should consider the sense of orientation in marine animals. Several species can detect the pull of gravity with organs known as statocysts. In vertebrates the semicircular canal in the ear performs this function. Next we come to chemoreception covering the senses of taste and smell. The sense of smell (olfaction) is extremely well developed in sharks and salmon. Electroreception is another sense used by sharks and some other predatory fish who posses electrosensory organs. In sharks these are known as ampulla of Lorenzini. Finally there is the sense of magnetoreception and magnetite crystals have been found in salmon that may enable them to navigate over long distances. Much research remains to be done in this area it seems.

The secondarily adapted features of marine animals (cetaceans).

Lets us consider the main secondarily adapted features of cetaceans and in particular the humpback whale. These secondary adaptations are as a result of the ancestors of whales and dolphins returning to the sea about 50Ma. As they had not developed gills etc over the last 45 Ma these animals have made many secondary adaptations to enable them to cope with the marine environment. If we consider the humpback whale we could broadly divide these secondary adaptations into external and internal. Externally the humpback whale has lost its hind legs, tail flukes have developed massively, dorsal fins have developed for stability, the forearm has evolved into a flipper with fused elbow, necks have become short and fused, genitals do not protrude to interfere with streamlining, ears and body hair have disappeared and eyes have become modified with 120 degree vision. Internally the spinal vertebrae have become massive to deal with the huge muscles and tail, bones within the rib cage have become lighter, improved hearing has been developed with wax filled tubes and echolocation. The baleen whales have developed huge throat groves for improved feeding. Blubber has evolved for advanced thermoregulation in the cold seas and other temperature controls developed such as the rete mirabile. In addition more efficient kidneys help with water conservation, unihemispherical sleeping helps guard against predation, muscular blowholes have developed to seal off the entry of seawater, voluntary breathing systems developed and advanced diving techniques. In these ways the cetaceans have developed an amazing array of secondary adaptations to life in the sea.

Dr Simon Harding

www.biblon.com

www.chronos.es

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