NEKTONNEKTON
There are a variety of different ways to classify the organisms living in
the sea. One of these methods is by grouping the organisms by the type of
marine lifestyle they exhibit.The three major groups in this classification
scheme are plankton, the drifters, benthic, the bottom dwellers and nekton,
the swimmers.
The classification of something as being a swimmer is very broad and thus
there is a huge variety of different types of creatures that have adapted
to living as swimmers.
Within the phylum Chordata there are three major groupings of nekton members.
There are marine
reptiles, marine
mammals and of course fish. Marine reptiles include
turtles, snakes, crocodiles and iguanas. Marine mammals include cetaceans,
pinnipeds, fissipeds (otters) and sirenians (manatees). Fish which are the
largest group of necton can be broken down by class into Agnatha , the jaw
less fish with cartilaginous skeletons, who are also the most primitive
group of living fish, Chondrichthyes, the sharks and rays, who also have
cartilaginous skeletons and Osteichthyes, the bony fish. There are also
a couple of marine invertebrates that have adapted to the nektonic lifestyle.
These include marine arthropods, like shrimp and mollusks, like the octopus
and squid.
All of these creatures face many of the same problems in the marine environment
and have come up with a assortment of adaptations to deal with them.
- Fish have developed many different fin designs to propel them through the water. Scales on their also help reduce drag. Click here to see a diagram of a fish.
- Cetaceans' have lost their hind limbs over time and now use their tail, which is an extension of their back bone, to push them through the water.
- Pinnipeds' have adapted limbs that act as flippers to maneuver through the water
- Both squid and octopi use hydraulic jet propulsion to move
- All of these animals have adapted to the water with a hydrodynamic shape to allow them to move through the dense medium.They all also have a high muscle to body weight ratio. The vertebrates have strong skeletons for the muscles to attach to. Click here to view a diagram of how a fishes body shape affects its effiency to move.
- Most bony fish use gas bladders to control their buoyancy. Sharks and other fish that lack a swim bladder, like tuna, use their pectoral fins like wings to keep them up.
- Marine mammals also use their flippers as wings. Most marine mammals, other than the otters, have blubber to help them float. Its main job, however, is for insulation.
- Fish have gills that, through diffusion, pull oxygen from water into the blood and put carbon dioxide from the blood into the water. The blood and water flow in opposite directions so that there is a faster exchange of gases.
- Marine mammals and reptiles must hold their breath to dive and resurface to breath. The diving capabilities of many marine mammals are extraordinary. The elephant seal, for example, can dive to a depth of 1000 meters for 1:15. In order to make such incredible dives marine mammals have the ability to slow their heart rate (bradycardia), cut off circulation of blood to their extremities (peripheral vasco constriction) and have to think to breath (conscious breathing).
- Fish have many adaptations to help them survive the multitude of predators. Many smaller fish travel in schools which act to confuse would be predators. These schooling fish along with most others have an organ called the lateral line. This "sixth sense" detects low-frequency vibrations and allows the fish to react to nearby movements. Many fish , such as the kelp bass, have passive cryptic coloration that acts to hide the fish. Another common trick for hiding is counter shading.
PREDITORS
- There are two main groupings of preditors, the ambush preditors and the persuit preditors. Ambush preditors include fish like halibut and rock fish. Persuit preditors include fish like salmon and tuna. Go and look at my coho salmon page.