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The Euphotic Zone

Plants and Animals

Characteristics of The Euphotic Zone
Plants and Animals
Density Independent and Dependent Factors
Parasitism, Mutualism, Coevolution
Trophic Levels
Human Interaction

On this page we will discover what plants and animals live in The Euphotic Zone.


      Over 1 million species of plants and animals have been discovered in the oceans, and scientists say there may be as many as 9 million species we haven't found yet. One reason the ocean is very important is because of all the algae. If it weren't for marine algae we would not be able to breathe! Through photosynthesis, marine plants and algae provide much of the worlds oxygen supply and take in huge amounts of carbon dioxide. This absorption of carbon dioxide may be a useful tool in reducing the severity of climate change. One type of marine algae is kelp. Kelp is important because it provides shelter and food for a lot of sea creatures. Kelp is also used by humans for many products, including toothpaste and ice cream. Kelp also serves as a buffer by absorbing energy from waves before the waves hit the shoreline, protecting many of the sandy beaches along the California coast. Another important marine plant is phytoplankton. These are tiny plants that serve as food to many of the ocean creatures from the smallest of fish to large whales. Some scientists estimate that phytoplankton provide the earth with almost half of its oxygen! Marine plants live in the euphotic zone of the ocean because they need energy from the sun for photosynthesis.

1. Plankton - drifters, capable of only insignificant free-swimmming. More than 99% of  all marine life is planktonic.

a) Phytoplankton - drifting plants

b) Zooplankton - drifting animals

c) Often further subdivided by size

1) Nannoplankton-<60microns = Coccolithophores & small diatoms

2) Microplankton-larger but need scope = Majority of plankton in sea

3) Macroplankton->1mm = krill, jellyfish, sargassum weed

d) Often further subdivided based on the portion of life cycle for which it is

planktonic (Holoplanktonic=whole life)

2. Nekton - free-swimming organisms - larger and more visible marine organisms, which people tend to think of as dominating the seas.  This is not the case, however, as nekton 

a. Most of the nekton are the mid- and deep-water fish seldom seen by man

b. Marine mammals like whales and dolphins are also in this category

                  Mode of nutrition

A. Autotrophic - capable of making their own food (i.e., mostly plants) that is they get their organic carbon  from inorganic sources such as HCO3-.

1. Photosynthetic autotrophs-plants in photic zone

2. Chemosynthetic autotrophs-bacteria around deep sea vents that use oxidation of H2S to sulfate as an energy source to drive their Corg synthesis

B. Heterotrophic - incapable of making own food (i.e., all animals) that is they get their organic carbon from organic (i.e., premade) sources.


      The Earth's oceans are home to most of the planet’s biodiversity. Here we can find mollusks, fish, whales, crustaceans, bacteria, fungi, sea anemones and many other animals. Animals have to deal with unique living situations in all zones of the ocean. The ocean is a salty place that is often cold. Many animals have special adaptations to handle this difficult environment. Most marine mammals have blubber to survive in the cold water, but sea otters are unique because they don’t have blubber. Instead, they have fur more dense than any other mammal, with up to one million hairs per square inch. Most people have ten times less than that on their heads! The ocean can also support very large life forms. The blue whale is the biggest animal on earth. It can be over 100 feet long. Blue whales are so large that a small person could crawl through their main arteries, and 20 people could stand on their tongue!

More detail on the more complex organisms

A. All the pelagic organisms below belong to the Phylum Chordata         - animals with gill slits and a cartilagi­nous skeletal rod at some stage of their development and the  Subphylum Vertebrata - spinal column of vertebrae,  red blood & 2 pairs of appendages

B. Fish dominate the nekton.  They are found at all depths  and in all the oceans but their distribution patterns are determined directly or indirectly by their dependency on the oceans' primary producers. Concentrated (50% of them) in  upwelling areas, shallow coastal areas and estuaries, which only represent about 2% of the ocean.

1. Wide variety of shapes related to their environment and behavior.

2. Schooling is common amoung certain types.

3. Divided on basis of whether they have a skeleton of cartilage or of bone or whether they are jawless.

4. Most primitive fish - (Class Agnatha) = lamprey, hagfish

5. Primitive cartilaginous  - (Class Condrichthyes)

a. Sharks - cartilaginous skeleton and no gill covers

b. Predate mammals - first appeared 450 mya.

c. Extremely well-adapted to their environment

1) Good eyesight, excellent senses of smell, hearing, electrical impulses (good navigators) & chemistry.

d. Many must swim perpetually to maintain position in water column and get oxygen in through their gills.

e. Carnivores, planktivores and scavengers

f. Skates and rays are flattened, shark-type fish that have adopted a bottom-dwelling life-style.

1) Planktivores, carnivores

6. TRUE FISH (Class Osteichthyes) - bony skeletons, scales, gill covers, swim bladder

a. Commercial Species of Bony Fish - sardine, anchovy, menhaden, herring (planktivores) and mackerel, pompano, tuna & swordfish (predatory) and flounder, halibut, turbot, sole (demersal = bottom fish) and perch and snapper (along seafloor in the shallower, nearshore areas)     

b. Most in surface layers (<200m) and are streamlined, active, predatory, and capable of high-speed, long-distance travel.

c. Deep-sea species of bony fish-not well known (>200m)

1) Predators have highly specialized equipment for catching their prey (lures).

2) Usually large mouths and hinged jaws so they can take advantage of the rare, large meals on which they subsist.

3) Many have specialized light organs to confuse predators or attract prey

C. Class Reptilia - sea snakes & turtles                     

1. Reptiles are few and far between

a. A few marine lizards (Galapagos)

b. Sea Snakes - about 50 kinds - extremely poisonous, only in Pacific and Indian oceans -  coastal waters    

1) Some of their venom is 50X more potent than cobra 

c. Turtles - live in ocean but nest on land, 1 of 4 types=herbivorous, while the other 3 are carnivorous.

D. Class Mammalia - Seals, manatees, whales, dolphins

1. Include:

a. Coast-dwelling herbivores ( Sirenians (sea cows) – dugongs.manatees

b. Coastal carnivores (spend most of lives in waters and come ashore to breed

1) Sea otter

2) Pinnipeds (Sea lions, seals, and walruses)

c. Pelagic Forms  (Cetaceans = whales, porpoises, and dolphins)

2. Because mammals evolved from reptiles on land some 200 million years ago and no marine forms are known earlier than 60 mya, it is believed that all marine forms evolved from some ancient land-dwellers.

3. Coast-dwelling herbivores ( Sirenians (sea cows) – dugongs & manatees

a. Much like elephants and live as long (60-70 years)

b. Found south of North Carolina on the east coast of the USA and along the Gulf Coast

c. Harmless, docile, slow-swimming grazers

4. Coastal carnivores (spend most of lives in waters and come ashore to breed

a. Sea otter

b. Pinnipeds (Sea lions, seals, and walruses)

E. Pelagic Mammals  (Cetaceans = whales, porpoises, and dolphins )

1. Two types  =  Toothed whales (Odontoceti, one blow hole)  and Baleen (Mysticeti, two blow holes)

a. Cigar-shaped bodies

b. Nearly hairless

c. Insulated with thick blubber

d. Forelimbs are modified into flippers that move only at shoulder

e. Hind limbs are vestigial (not attached to rest of skeleton)

f. Skull highly modified

1) One blow hole for toothed

2) Two blow holes for baleen

g. Propel themselves by vertical movements of horizontal fluke (Tail)

2. Toothed whales (Odontoceti) feed on fish and squid although the Killer Whale is know to feed on a variety of larger animals, including other whales.

a. Killer Whales, Sperm whales, porpoises and dolphins

3. Baleen (Mysticeti) whales feed primarily by filtering krill, copepods and small fish at depths ranging from surface down to an including the sediment of shallow ocean basins.

4. Modifications ot increase swimming speed

a. Cetaceans' muscles are not vastly more powerful than other mammals

b. Modifications reduce frictional drag

5. Modifications to allow deep diving

a. Humans can free dive to a maximum of 100 m and stay for 6 minutes.

b. Sperm whale dives deeper than 2200 m (1.5 miles)

c. Elephant seal can stay down for up to 2 hours.

d. Can alternate between periods of normal breathing and cessation of breathing.

1) Cetaceans have exceptionally large concentrations of capillaries that exchange gasses and hold air in lungs longer so can extract much more of the oxygen (90%) than terrestrial mammals (20%)

2) Have much greater blood volume per unit of body mass  so can store more oxygen

3) Swimming muscles can function without oxygen during dive

4) Other organs not necessary during dive can be shut off from circulatory system (digestive tract, kidneys, etc.) so can reduce heart rate by 20-50%.

6. Baleen whales probably evolved from toothed whales about 30 mya

a. Instead of teeth, they have baleen, plate of horny material, that hang down from the upper jaw to serve as a sieve.

b. Blue, finback, humpback, right, gray, etc.

7. Some whales migrate seasonally many thousands of miles

a. California Gray-Arctic Ocean to Baja, California

b. Humpback-As far north as the Gulf of Alaska and as far south as the Mariana and Hawaiian Islands