Marine biology is the scientific study of
marine life, organisms in the sea.
Given that in biology many phyla, families
and genera have some species that live in
the sea and others that live on land, marine
biology classifies species based on the environment
rather than on taxonomy.
A large proportion of all life on Earth lives
in the ocean.
The exact size of this large proportion is
unknown, since many ocean species are still
to be discovered.
The ocean is a complex three-dimensional world
covering approximately 71% of the Earth's
surface.
The habitats studied in marine biology include
everything from the tiny layers of surface
water in which organisms and abiotic items
may be trapped in surface tension between
the ocean and atmosphere, to the depths of
the oceanic trenches, sometimes 10,000 meters
or more beneath the surface of the ocean.
Specific habitats include coral reefs, kelp
forests, seagrass meadows, the surrounds of
seamounts and thermal vents, tidepools, muddy,
sandy and rocky bottoms, and the open ocean
(pelagic) zone, where solid objects are rare
and the surface of the water is the only visible
boundary.
The organisms studied range from microscopic
phytoplankton and zooplankton to huge cetaceans
(whales) 25–32 meters (82–105 feet) in
length.
Marine ecology is the study of how marine
organisms interact with each other and the
environment.
Marine life is a vast resource, providing
food, medicine, and raw materials, in addition
to helping to support recreation and tourism
all over the world.
At a fundamental level, marine life helps
determine the very nature of our planet.
Marine organisms contribute significantly
to the oxygen cycle, and are involved in the
regulation of the Earth's climate.
Shorelines are in part shaped and protected
by marine life, and some marine organisms
even help create new land.Many species are
economically important to humans, including
both finfish and shellfish.
It is also becoming understood that the well-being
of marine organisms and other organisms are
linked in fundamental ways.
The human body of knowledge regarding the
relationship between life in the sea and important
cycles is rapidly growing, with new discoveries
being made nearly every day.
These cycles include those of matter (such
as the carbon cycle) and of air (such as Earth's
respiration, and movement of energy through
ecosystems including the ocean).
Large areas beneath the ocean surface still
remain effectively unexplored.
== History ==
The study of marine biology dates back to
Aristotle (384–322 BC), who made many observations
of life in the sea around Lesbos, laying the
foundation for many future discoveries.
In 1768, Samuel Gottlieb Gmelin (1744–1774)
published the Historia Fucorum, the first
work dedicated to marine algae and the first
book on marine biology to use the then new
binomial nomenclature of Linnaeus.
It included elaborate illustrations of seaweed
and marine algae on folded leaves.
The British naturalist Edward Forbes (1815–1854)
is generally regarded as the founder of the
science of marine biology.
The pace of oceanographic and marine biology
studies quickly accelerated during the course
of the 19th century.
The observations made in the first studies
of marine biology fueled the age of discovery
and exploration that followed.
During this time, a vast amount of knowledge
was gained about the life that exists in the
oceans of the world.
Many voyages contributed significantly to
this pool of knowledge.
Among the most significant were the voyages
of HMS Beagle where Charles Darwin came up
with his theories of evolution and on the
formation of coral reefs.
Another important expedition was undertaken
by HMS Challenger, where findings were made
of unexpectedly high species diversity among
fauna stimulating much theorizing by population
ecologists on how such varieties of life could
be maintained in what was thought to be such
a hostile environment.
This era was important for the history of
marine biology but naturalists were still
limited in their studies because they lacked
technology that would allow them to adequately
examine species that lived in deep parts of
the oceans.
The creation of marine laboratories was important
because it allowed marine biologists to conduct
research and process their specimens from
expeditions.
The oldest marine laboratory in the world,
Station biologique de Roscoff, was established
in France in 1872.
In the United States, the Scripps Institution
of Oceanography dates back to 1903, while
the prominent Woods Hole Oceanographic Institute
was founded in 1930.
The development of technology such as sound
navigation ranging, scuba diving gear, submersibles
and remotely operated vehicles allowed marine
biologists to discover and explore life in
deep oceans that was once thought to not exist.
== Marine life ==
=== 
Microscopic life ===
As inhabitants of the largest environment
on Earth, microbial marine systems drive changes
in every global system.
Microbes are responsible for virtually all
the photosynthesis that occurs in the ocean,
as well as the cycling of carbon, nitrogen,
phosphorus and other nutrients and trace elements.Microscopic
life undersea is incredibly diverse and still
poorly understood.
For example, the role of viruses in marine
ecosystems is barely being explored even in
the beginning of the 21st century.The role
of phytoplankton is better understood due
to their critical position as the most numerous
primary producers on Earth.
Phytoplankton are categorized into cyanobacteria
(also called blue-green algae/bacteria), various
types of algae (red, green, brown, and yellow-green),
diatoms, dinoflagellates, euglenoids, coccolithophorids,
cryptomonads, chrysophytes, chlorophytes,
prasinophytes, and silicoflagellates.
Zooplankton tend to be somewhat larger, and
not all are microscopic.
Many Protozoa are zooplankton, including dinoflagellates,
zooflagellates, foraminiferans, and radiolarians.
Some of these (such as dinoflagellates) are
also phytoplankton; the distinction between
plants and animals often breaks down in very
small organisms.
Other zooplankton include cnidarians, ctenophores,
chaetognaths, molluscs, arthropods, urochordates,
and annelids such as polychaetes.
Many larger animals begin their life as zooplankton
before they become large enough to take their
familiar forms.
Two examples are fish larvae and sea stars
(also called starfish).
=== Plants and algae ===
Microscopic algae and plants provide important
habitats for life, sometimes acting as hiding
places for larval forms of larger fish and
foraging places for invertebrates.
Algal life is widespread and very diverse
under the ocean.
Microscopic photosynthetic algae contribute
a larger proportion of the world's photosynthetic
output than all the terrestrial forests combined.
Most of the niche occupied by sub plants on
land is actually occupied by macroscopic algae
in the ocean, such as Sargassum and kelp,
which are commonly known as seaweeds that
create kelp forests.
Plants that survive in the sea are often found
in shallow waters, such as the seagrasses
(examples of which are eelgrass, Zostera,
and turtle grass, Thalassia).
These plants have adapted to the high salinity
of the ocean environment.
The intertidal zone is also a good place to
find plant life in the sea, where mangroves
or cordgrass or beach grass might grow.
=== Invertebrates ===
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 anemones; Ctenophora;
sea worms including the phyla Platyhelminthes,
Nemertea, Annelida, Sipuncula, Echiura, Chaetognatha,
and Phoronida; Mollusca including shellfish,
squid, octopus; Arthropoda including Chelicerata
and Crustacea; Porifera; Bryozoa; Echinodermata
including starfish; and Urochordata including
sea squirts or tunicates.
Invertebrates have no backbone.
There are over a million species.
=== Fungi ===
Over 1500 species of fungi are known from
marine environments.
These are parasitic on marine algae or animals,
or are saprobes on algae, corals, protozoan
cysts, sea grasses, wood and other substrata,
and can also be found in sea foam.
Spores of many species have special appendages
which facilitate attachment to the substratum.
A very diverse range of unusual secondary
metabolites is produced by marine fungi.
=== Vertebrates ===
==== 
Fish ====
A reported 33,400 species of fish, including
bony and cartilaginous fish, had been described
by 2016, more than all other vertebrates combined.
About 60% of fish species live in saltwater.
==== Reptiles ====
Reptiles which inhabit or frequent the sea
include sea turtles, sea snakes, terrapins,
the marine iguana, and the saltwater crocodile.
Most extant marine reptiles, except for some
sea snakes, are oviparous and need to return
to land to lay their eggs.
Thus most species, excepting sea turtles,
spend most of their lives on or near land
rather than in the ocean.
Despite their marine adaptations, most sea
snakes prefer shallow waters nearby land,
around islands, especially waters that are
somewhat sheltered, as well as near estuaries.
Some extinct marine reptiles, such as ichthyosaurs,
evolved to be viviparous and had no requirement
to return to land.
==== Birds ====
Birds adapted to living in the marine environment
are often called seabirds.
Examples include 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.
==== Mammals ====
There are five main types of marine mammals,
namely cetaceans (toothed whales and baleen
whales); sirenians such as manatees; pinnipeds
including seals and the walrus; sea otters;
and the
polar bear.
All are air-breathing, and while some such
as the sperm whale can dive for prolonged
periods, all must return to the surface to
breathe.
== Marine habitats ==
Marine habitats can be divided into coastal
and open ocean habitats.
Coastal habitats are found in the area that
extends from the shoreline to the edge of
the continental shelf.
Most marine life is found in coastal habitats,
even though the shelf area occupies only seven
percent of the total ocean area.
Open ocean habitats are found in the deep
ocean beyond the edge of the continental shelf.
Alternatively, marine habitats can be divided
into pelagic and demersal habitats.
Pelagic habitats are found near the surface
or in the open water column, away from the
bottom of the ocean and affected by ocean
currents, while demersal habitats are near
or on the bottom.
Marine habitats can be modified by their inhabitants.
Some marine organisms, like corals, kelp and
sea grasses, are ecosystem engineers which
reshape the marine environment to the point
where they create further habitat for other
organisms.
=== Intertidal and near shore ===
Intertidal zones, the areas that are close
to the shore, are constantly being exposed
and covered by the ocean's tides.
A huge array of life can be found within this
zone.
Shore habitats span from the upper intertidal
zones to the area where land vegetation takes
prominence.
It can be underwater anywhere from daily to
very infrequently.
Many species here are scavengers, living off
of sea life that is washed up on the shore.
Many land animals also make much use of the
shore and intertidal habitats.
A subgroup of organisms in this habitat bores
and grinds exposed rock through the process
of bioerosion.
=== Estuaries ===
Estuaries are also near shore and influenced
by the tides.
An estuary is a partially enclosed coastal
body of water with one or more rivers or streams
flowing into it and with a free connection
to the open sea.
Estuaries form a transition zone between freshwater
river environments and saltwater maritime
environments.
They are subject both to marine influences—such
as tides, waves, and the influx of saline
water—and to riverine influences—such
as flows of fresh water and sediment.
The shifting flows of both sea water and fresh
water provide high levels of nutrients both
in the water column and in sediment, making
estuaries among the most productive natural
habitats in the world.
=== Reefs ===
Reefs comprise some of the densest and most
diverse habitats in the world.
The best-known types of reefs are tropical
coral reefs which exist in most tropical waters;
however, reefs can also exist in cold water.
Reefs are built up by corals and other calcium-depositing
animals, usually on top of a rocky outcrop
on the ocean floor.
Reefs can also grow on other surfaces, which
has made it possible to create artificial
reefs.
Coral reefs also support a huge community
of life, including the corals themselves,
their symbiotic zooxanthellae, tropical fish
and many other organisms.
Much attention in marine biology is focused
on coral reefs and the El Niño weather phenomenon.
In 1998, coral reefs experienced the most
severe mass bleaching events on record, when
vast expanses of reefs across the world died
because sea surface temperatures rose well
above normal.
Some reefs are recovering, but scientists
say that between 50% and 70% of the world's
coral reefs are now endangered and predict
that global warming could exacerbate this
trend.
=== Open ocean ===
The open ocean is relatively unproductive
because of a lack of nutrients, yet because
it is so vast, in total it produces the most
primary productivity.
The open ocean is separated into different
zones, and the different zones each have different
ecologies.
Zones which vary according to their depth
include the epipelagic, mesopelagic, bathypelagic,
abyssopelagic, and hadopelagic zones.
Zones which vary by the amount of light they
receive include the photic and aphotic zones.
Much of the aphotic zone's energy is supplied
by the open ocean in the form of detritus.
=== Deep sea and trenches ===
The deepest recorded oceanic trench measured
to date is the Mariana Trench, near the Philippines,
in the Pacific Ocean at 10,924 m (35,840 ft).
At such depths, water pressure is extreme
and there is no sunlight, but some life still
exists.
A white flatfish, a shrimp and a jellyfish
were seen by the American crew of the bathyscaphe
Trieste when it dove to the bottom in 1960.
In general, the deep sea is considered to
start at the aphotic zone, the point where
sunlight loses its power of transference through
the water.
Many life forms that live at these depths
have the ability to create their own light
known as bio-luminescence.
Marine life also flourishes around seamounts
that rise from the depths, where fish and
other sea life congregate to spawn and feed.
Hydrothermal vents along the mid-ocean ridge
spreading centers act as oases, as do their
opposites, cold seeps.
Such places support unique biomes and many
new microbes and other lifeforms have been
discovered at these locations .
== 
Subfields ==
The marine ecosystem is large, and thus there
are many sub-fields of marine biology.
Most involve studying specializations of particular
animal groups, such as phycology, invertebrate
zoology and ichthyology.
Other subfields study the physical effects
of continual immersion in sea water and the
ocean in general, adaptation to a salty environment,
and the effects of changing various oceanic
properties on marine life.
A subfield of marine biology studies the relationships
between oceans and ocean life, and global
warming and environmental issues (such as
carbon dioxide displacement).
Recent marine biotechnology has focused largely
on marine biomolecules, especially proteins,
that may have uses in medicine or engineering.
Marine environments are the home to many exotic
biological materials that may inspire biomimetic
materials.
=== Related fields ===
Marine biology is a branch of biology.
It is closely linked to oceanography and may
be regarded as a sub-field of marine science.
It also encompasses many ideas from ecology.
Fisheries science and marine conservation
can be considered partial offshoots of marine
biology (as well as environmental studies).
Marine Chemistry, Physical oceanography and
Atmospheric sciences are closely related to
this field.
== Distribution factors ==
An active research topic in marine biology
is to discover and map the life cycles of
various species and where they spend their
time.
Technologies that aid in this discovery include
pop-up satellite archival tags, acoustic tags,
and a variety of other data loggers.
Marine biologists study how the ocean currents,
tides and many other oceanic factors affect
ocean life forms, including their growth,
distribution and well-being.
This has only recently become technically
feasible with advances in GPS and newer underwater
visual devices.Most ocean life breeds in specific
places, nests or not in others, spends time
as juveniles in still others, and in maturity
in yet others.
Scientists know little about where many species
spend different parts of their life cycles
especially in the infant and juvenile years.
For example, it is still largely unknown where
juvenile sea turtles and some year-1 sharks
travel.
Recent advances in underwater tracking devices
are illuminating what we know about marine
organisms that live at great Ocean depths.
The information that pop-up satellite archival
tags give aids in certain time of the year
fishing closures and development of a marine
protected area.
This data is important to both scientists
and fishermen because they are discovering
that by restricting commercial fishing in
one small area they can have a large impact
in maintaining a healthy fish population in
a much larger area.
== See also ==
== Lists
