Marine life helps determine the very nature of our planet. Marine organisms produce 80% of the oxygen we breathe and probably help regulate the earth's climate. Shorelines are in part shaped and protected by marine life, and some marine organisms even help create new land.
Our oceans and seas are teeming with life from fish to plankton. Microscopic life undersea is incredibly diverse and still poorly understood.
As on land, invertebrates make up a huge portion of all life in the sea. Invertebrate sea life includes Cnidaria such as jellyfish and sea anemone; Ctenophora; sea worms including phyla Plathyhelminthes, Nemertea, Annelida, Sipuncula, Echiura, and the Phoronida; Mollusca including shellfish, squid, octopus; Crustaceans; Porifera including sponges, Bryozoa, Echinodermata including starfish; and Urochordata - sea squirts or tunicates.
Fish
Fish have evolved very different biological functions from other large organisms. Fish anatomy includes a two-chambered heart, operculum, secretory cells that produce mucous, swim bladder, scales, fins, lips and eyes. Fish breathe by extracting oxygen from water through their gills. Fins propel and stabilize the fish in the water.
Well known fish include: sardines, anchovy, tuna, clownfish (also known as anemonefish), and bottom fish which include halibut and ling cod. Predators include sharks and barracuda.
Reptiles
Reptiles which inhabit or frequent the sea include sea turtles, marine iguanas, sea snakes, and saltwater crocodiles. Since all extant marine reptiles are oviparous and need to return to land to lay their eggs, most reptile species live on or near land, rather than in the ocean. Some extinct marine reptiles, such as ichthyosaurs, evolved to be viviparous and had no requirement to return to land.
Sea Birds
Seabirds are species of Birds adapted to living in the marine environment, examples including albatross, penguins, gannets, and auks. Although they spend most of their lives in the ocean, species such as gulls can often be found thousands of miles inland.
Marine Mammals
A marine mammal is a mammal that is primarily ocean-dwelling or depends on the ocean for its food. Mammals originally evolved on land, but later marine mammals evolved to live back in the ocean.
There are five groups of marine mammals:
Order Sirenia: the manatee, dugong.
Order Carnivora, family Ursidae: the polar bear
Order Carnivora, superfamily Pinnipedia: the seal, sea lion, and walrus
Order Carnivora, family Mustelidae: the Sea Otter and Marine Otter
Order Cetacea: the whale, dolphin, and porpoise
Since different groups of marine mammals originate from different ancestors, this is a case of convergent evolution.
Since mammals originally evolved on land, their spines are optimized for running, allowing for up-and-down but only little sideways motion. Therefore, marine mammals typically swim by moving their spine up and down. By contrast, fish normally swim by moving their spine sideways. For this reason, fish mostly have vertical caudal (tail) fins, while marine mammals have horizontal caudal fins.
Some of the primary differences between marine mammals and other marine life are:
- Marine mammals breathe air, while most other marine animals extract oxygen from water.
- Marine mammals have hair. Cetaceans have little or no hair, usually a very few bristles retained around the head or mouth. All members of the Carnivora have a coat of fur or hair, but it is far thicker and more important for thermoregulation in Sea Otters and Polar Bears than in seals or sea lions. Thick layers of fur contribute to drag while swimming, and slow down a swimming mammal, giving it a disadvantage in speed.
- Marine mammals have thick layers of blubber used to insulate their bodies and prevent heat loss. Sea Otters and Polar Bears are exceptions, relying more on fur and behavior to stave off hypothermia.
- Marine mammals give live birth. Most marine mammals only give birth to one calf or pup at a time, and are never able to birth twins or larger litters.
- Marine mammals feed off milk as young. Maternal care is extremely important to the survival of offspring that need to develop a thick insulating layer of blubber. The milk from the mammary glands of marine mammals often exceeds 40-50% fat content to support the development of blubber in the young.
- Marine mammals maintain a high internal body temperature. Unlike most other marine life, marine mammals carefully maintain a core temperature much higher than their environment. Blubber, thick coats of fur, bubbles of air between skin and water, countercurrent exchange, and behaviors such as hauling out, are all adaptations that aid marine mammals in retention of body heat.
- The polar bear spends a large portion of its time in a marine environment, albeit a frozen one. When it does swim in the open sea it is extremely proficient and has been shown to cover 74 km in a day. For these reasons, some scientists regard it as a marine mammal.
Find out more about marine animal life here: Marine at Animal Corner
