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The average depth of the ocean is 3,700 meters (12,100 feet). But the deepest part ever recorded is located in the western part of the Pacific Ocean, in the Mariana Trench, at a depth of around 11,000 meters (36,200 feet).

Bathymetry is the scientific term for measuring the depth of water in oceans, lakes and rivers. Bathymetric maps are similar to land maps in that they show the different underwater landforms in a specific area. Scientists and researchers can use different methods to measure ocean depth.

The different methods are:

1. Sonar

The most common and fastest way of measuring ocean depth uses sound. Ships using technology called sonar, which stands for sound navigation and ranging, can map the topography of the ocean floor. The device sends sound waves to the bottom of the ocean and measures how long it takes for an echo to return. The “echo” is the sound wave reflecting off the seabed and returning to the sonar device.

2. Radar and satellite

Another alternative, though not as fast as sonar, is radar. Similar to sonar, radar requires sending out a type of wave that pings off an object and reflects back. The difference is that radar uses radio waves, a form of electromagnetic wave. But because electromagnetic waves travel slower in water compared with air and become diminished as they travel through water, they are more ideal for atmospheric measurements.

Ocean Depth Measured in the Olden Days

Before the discovery of using sound and radar to measure ocean depth, captains and their crews used a different way to measure the depth of the ocean. Sailors would use a tool called a lead line, which was essentially a lead weight attached to a rope that is marked every 6 feet, a length called a fathom, with a rag or strip of leather. A crew member would then throw the line into the water, and once the lead weight reached the bottom the sailor would measure and record the distance to the ocean floor using the strips on the rope.

The lead line was the most valuable method of measuring depth for navigation and has been used since the fifth century BCE. The tool helped sailors know how deep the water was and if their ship would run aground. The bottom of the lead weight was cupped inward and filled with grease and was used to bring up samples from the ocean floor to help sailors determine if the ocean bed was sand, gravel or mud.

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Ocean floors are mapped using sound equipment and robot submarines. Sonar systems send out high -frequency pulses. The time it takes for the sound pulse to echo back from the ocean floor gives an idea as to the depth of the ocean.

Understanding the depth and shape of the seafloor, called bathymetry, is not only a mapping challenge but it is important if we are to better understand are oceans. This includes understanding ocean circulation, which affects climate, tsunamis, environmental change, underwater geo-hazards, resources, and many other processes affecting the environment, safety, and commerce.

Mapping the seafloor has been occurring since the early 19th century; however, obtaining accurate data has been a challenge until the invention of the sonar. More recent sonars (short fo  Sound Navigation and Ranging) provide far more accurate data, particularly when multibeam echosounder sonars are used.

The Seabed 2030 project is a project attempting to map the seafloor by 2030. Until now, however, only about 20% of the seafloor has been mapped using modern bathymetry methods. In part, the project to map the seafloor will benefit from crowd sourced data obtained from various ocean-going vessels.

However, NOAA is also leading the effort and vessels with sonar equipment are being used to map regions not often travelled by vessels. These vessels are equipped with the latest multibeam sonars that provide hydrographic surveying results that can then build detailed maps with about 0.5 meter resolution.

The mapping efforts are attempting to use different frequencies, from around 12 kHz to closer to 200 kHz, often used in shallower waters. While generally deeper sea levels are easier to map, as sound waves travel and allow a wider region to be surveyed as a ship passes by, shallow areas present challenges, given that multiple passes need to cover less area and interference observed from other sea life and vessels can disrupt data.

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The Pacific is the biggest and the deepest Ocean. It covers a third of Earth's surface and has an area of 180 million km, holding more than 700 million km of water. It is so big that all the continents could fit within its area.

The Pacific Ocean is the largest ocean in the world, making up around 28% of the world’s surface area and almost double that in water surface area. It touches the west coast of North America. It is also home to the deepest trench on earth, the Marianas Trench, where the Challenger Deep is located. It’s 36,037 feet deep, far deeper than Mount Everest is tall. The trench is located between two tectonic plates, accounting for its incredible depth. The ocean’s coast line, along North America and Japan, is prone to earthquakes.

The ocean was named by the explorer Ferdinand Magellan. He chose “pacific” to mean “peaceful sea.” The Pacific Ocean is home to the Ring of Fire, a chain of 450 volcanoes in a u-shape. They reach from South America, along the coast of Japan, the western United States, all the way down into New Zealand. The Pacific Ocean is the world’s largest body of water.

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An ocean is a huge, continuous body of salt water. Oceanographers identify five major oceans on the basis of geography and the continents that surround them. These are the Pacific, Atlantic, Indian, Arctic and Southern (also known as Antarctic) Oceans. They cover about one-third of Earth's surface and contain about 97 per cent of the world's water.

Pacific Ocean

The Pacific Ocean is the largest ocean covering more than 30% of the Earth. This is close to half of the water on Earth.

It touches the west coast border of the Americas along with east Asia and Australia.

The equator divides the Pacific Ocean into two separate parts – the North Pacific Ocean and South Pacific Ocean.

Pacific means “peaceful” in Latin. It has the deepest trenches with an average depth of 3800m.

Atlantic Ocean

The Atlantic Ocean is situated between the Americas and European/African continents. The Atlantic Ocean is the second-largest and saltiest ocean in the world.

It resembles an S-shape between the Americas, Europe, and Africa. “Atlantic” originated from the Greek god “Atlas” who carried the sky for eternity.

The ocean bottom is composed of mid-Atlantic Ridge. This submarine mountain range extends all the way from Iceland to 58 degrees South latitude. It’s part of the longest mountain range in the world.

Indian Ocean

The Indian Ocean is the third largest ocean surrounding a densely populated region. It contains an additional 20% of the water on Earth’s surface.

It borders India in the North, East Africa, Australia, and the Southern Ocean. Because of the higher water temperature, it has limited marine life.

Since about 800 A.D. the Indian Ocean has played an important role in trading. For centuries, navigators have sailed along major ocean currents for shipment routes.

It is bounded by 4 tectonics plate boundaries and may include an additional plate boundary. It is the geologically youngest of the 5 oceans with spreading ridges at divergent plate boundaries.

Southern Ocean

In 2000, the Southern Ocean is the newest ocean recognized by the International Hydrographic Organization. It borders Antarctica in its entirety.

In terms of size, it’s the fourth-largest at 20,327,000 square kilometers. It extends out to 60 degrees South latitude.

It’s an extreme environment and is the least understood of the 5 oceans. This is because it is unexplored, far from populated areas, and has a severe climate.

Despite the Southern Ocean being unexplored, about 80% of all oceans in the world are unexplored. There’s still a lot of work to do for ocean exploration.

Arctic Ocean

The Arctic Ocean is the world’s smallest and shallowest ocean of all 5 oceans. Further to this, it is the coldest and least salty ocean.

In size, the Arctic Ocean is about the size of Russia. Because it’s located at the North Pole, the Arctic Ocean has polar ice. But over the years, glaciers have melted threatening sea levels to rise.

Despite the IHO recognizing it as the “Arctic Ocean”, some oceanographers still call it the “Arctic Sea”.

The Arctic Ocean is the most diverse in terms of fish species. It has a wide variety of marine species including whales, jellyfish, etc.

But because of its frigid temperatures, it has little plant life. This makes it one of the most fragile ecosystems on the planet.

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The difference lies in their depth, area and variety of marine life. An ocean is deeper and covers a greater area as compared to the sea. On the other hand, a sea has more diverse plant and animal life. There are fewer plants in an ocean as there are large areas where sunlight does not penetrate. The deep-sea creatures found in the darkness of oceans are unique.

 

Sea	Ocean
Sea is also made up of salty water but is smaller in size compared to oceans.	Ocean covers approximately 70% of the earth’s total surface. Ocean is made up of salty water.
The largest sea is the Meditteranean Sea. The area of Meditteranean sea is approximately 1.14 million square miles.	The largest Ocean is the Pacific Ocean with an area of approximately 60 million square miles. The smallest Ocean is the Arctic Ocean covering an area of 5.4 million square miles.
Seas are usually located close to land.	Oceans are not necessarily located close to land.
Aquatic life exists in abundance in seas as seas are usually located close to the Land.	Oceans have reduced aquatic life as it is far away from the land and deeper than the Seas.
There is a large quantity of marine life in the Seas as sunlight is able to penetrate deeper into the seas, allowing photosynthesis.	The marine life in oceans is usually confined to bacteria, shrimp, microscopic planktons. This is due to lack of penetration of sunlight deep into oceans thus not giving much scope for photosynthesis.
Some of the major seas are Mediterranean Sea, Carribean Sea, South China Sea, Black Sea etc. The deepest sea is Carribean Sea with an approximate depth of 6900 metres.	The Deepest Ocean is Pacific Ocean which is around 10,000 metres deep and the Arctic Ocean is around 5600 metres deep.

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Bonnethead sharks were thought to be solely carnivorous, but according to University of California researchers, they're omnivorous they can eat both animals and plants.

Bonnetheads, one of the smallest members of the hammerhead family, are abundant in the waters of the Americas, where they usually feed on crab, shrimp, snails and bony fish. Its plant of choice is seagrass. Researchers fed five bonnetheads on a three-week diet of seagrass and squid. All the sharks put on weight over the course of the study. Tests on the sharks showed that they successfully digested the seagrass with enzymes that broke down components of the plants. A possible reason for the sharks' omnivorous lifestyle is potentially avoiding conflict with other species such as bull sharks or nurse sharks for food.

It is one of the most radical rebrandings in history: contrary to their bloodthirsty image, some sharks are not irrepressible meat eaters, but are happy to munch on vegetation too. According to US researchers, one of the most common sharks in the world, a relative of the hammerhead which patrols the shores of the Americas, is the first variety of shark to be outed as a bona fide omnivore. The bonnethead shark is abundant in the shallow waters of the eastern Pacific, the Western Atlantic, and the Gulf of Mexico, where they feed on crab, shrimp, snails and bony fish. Though small by shark standards, adult females – the larger of the sexes – can still reach an impressive five feet long. Scientists at the University of California in Irvine, and Florida International University in Miami, decided to investigate the sharks’ dietary habits after reading reports of the fish chomping on seagrass, the flowering marine plant that forms subsea meadows in some coastal waters.

To see whether the sharks are truly flexitarian, the scientists retrieved sea grass from Florida Bay and hauled it back to the lab where they re-planted it. As the seagrass took root, the researchers added sodium bicarbonate powder made with a specific carbon isotope to the water. This was taken up by the seagrass, giving it a distinctive chemical signature. The researchers next caught five bonnethead sharks and brought them back to the lab. Once the fish had settled in, they were fed on a three week diet of the seagrass and squid. All of the fish put on weight over the course of the study. The scientists then ran a series of tests on the sharks. These showed that the fish successfully digested the seagrass with enzymes that broke down components of the plants, such as starch and cellulose. Lacking the kind of teeth best suited for mastication, the fish may rely on strong stomach acids to weaken the plants’ cells so the enzymes can have their digestive effects. In all, more than half of the organic material locked up in the seagrass was digested by the sharks, putting them on a par with young green sea turtles.

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First discovered in the 1960s, Deepstaria enigmatica — named for underwater explorer Jacques Cousteau's submersible Deepstar 4000, and the jellyfish's mysteriousness - has only made rare appearances since.  It is looking like a plastic bag. In November 2017, Deepstaria was spotted off the coast of San Benedicto Island, around 3,195 feet deep down in the Pacific Ocean.

Resembling a plastic bag, it has a large, thin bell and no tentacles. Since most jellyfish use tentacles to catch their prey, Deepstaria has to rely on another method. Since its bell can open up to a metre across, Deepstaria uses it to catch upward-moving prey. closing around the prey like a bag.

Deepstaria enigmatica have been observed during jelly-falls. This happens when a jellyfish carcass falls through the water column to the ocean floor. A Deepstaria enigmatica jelly fall was first observed in the lower portion of the oxygen minimum zone of the ocean. It was observed as shrimp and crabs were scavenging its carcass.

These jelly falls are advantageous to the ocean floor, causing a source of organic enrichment, similar to a whale fall; as carcasses decay away from light and the more oxygen-rich environments of the upper ocean, the animal decays far slower, providing a longer-lasting source of nutrients. The carcasses lead to a restoration of degraded mineral content in the water column. The contribution of these jelly falls is underestimated. The Deepstaria enigmatica fall shows an increase in oxygen availability and organic matter, benefitting the ecosystems found at the bottom of the ocean.

The Deepstaria jellyfish, unlike many jellyfish, lacks tentacles of any kind, which other species of jellyfish commonly use to entrap and consume prey. Instead, Deepstaria trap prey inside their bell, where they are consumed. This method also provides for isopods, who may live inside the jellyfish's bell in a symbiotic relationship

In the 1960s Jacques Cousteau, a French explorer, unexpectedly found the Deepstaria jellyfish in a deep-sea exploration mission. He was exploring the deep sea near Southwest Baker Island in a submarine called the Deepstar 4000, which became the inspiration for the name of this jellyfish. The Deepstaria jellyfish has been found in the Gulf of Mexico, Antarctic, and the Pacific Ocean. In all of these locations, the jellyfish was found 3,000 ft (910 m) below sea level.

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The Tonga Trench in the South Pacific Ocean is the second deepest trench on Earth, only behind the Mariana Trench in the Pacific Ocean. The Tonga's deepest point called the Horizon Deep, is at 10,882  metres below sea level. The Tonga Trench is an 850 miles (1,375 km) long channel. Every point of the trench is not 10 km deep. The average depth is 6000 m or 20000 ft.

The Tonga Trench is situated in the southern part of the Pacific Ocean. The trench lies parallel with the eastern shore of Australia. It is can also be joined with islands of New Zealand with a straight line on a map. The Tonga Trench is followed by the Karmadec Trench in the south. These two forms the active Tonga- Karmadec subduction zone. 

In the Northern Tonga Trench, the convergence between the Indo-Australian and Pacific plate is going on at a fast rate of 9 inch per year. It is too fast for a geological timescale where we have to consider millions of years?

Active tectonic activity around the Tonga trench results in frequent earth quakes around the area. The island of Tonga experience around 350 quakes per year. Often, the large earth quakes generate tsunamis.  The Tonga trench is so deep that the light of sun never reaches to it. As a result, the water inside the deepest parts becomes very cold around 1.1 degree C (34 degrees F).  Due to the abysmal depth of the place the water pressure here is extremely high. Due to the high pressure, no light and lack of oxygen, common sea animals cannot survive here. But there are some special fishes and invertebrates that thrive in this ecological niche.

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While friendly close contact is essential for these acquatic mammals social bonds, sharing space and air can also quickly spread disease.

Three young male dolphins simultaneously break the waters surface to breathe-first exhaling, then inhaling-before slipping back under the waves of the Chesapeake Bay in the U.S. "A perfect sync," said Janet Mann, a dolphin researcher watching from a small skiff.

Synchronised  breathing is something dolphins often do with close pals, such as these males, or that mothers and calves do together, said Mann. It's a way of affirming the relationships that are so important to these highly intelligent and social mammals, like a handshake or a hug among humans. "It says, 'We're together," said Mann, who is based at Georgetown University.

While such close contact is essential to dolphin social bonds, sharing space and air can also quickly spread disease. Mann and other scientists are trying to understand how a highly contagious and lethal disease called cetacean morbillivirus- related to measles in humans and first detected in Virginia and Maryland waters can spread rapidly among dolphins along the Atlantic Coast, as it did from 2013 to 2015.

Thousands of deaths

During that outbreak, more than 1,600 dolphins washed ashore on beaches from New York to Florida, according to the National Oceanic and Atmospheric Administration. Altogether, an estimated 20,000 dolphins died from the virus, and the region's population of coastal dolphins shrank by about 50%.

"It's much like COVID - it's respiratory" in how it spreads, said Mann. "When dolphins breathe together at the surface, they're sharing respiratory droplets just like we do when we're talking or coughing on each other."

She realised that the key to understanding swift virus transmission was tracing dolphin social networks, much as how public health authorities have tracked the COVID-19 pandemic.

To understand how diseases circulate in social animals such as humans, dolphins, or chimpanzees scientists must scrutinise not only the biology of a virus, but also how vulnerable populations interact, said Jacob Negrey, a researcher who studies animal viruses at Wake Forest School of Medicine. "Contact networks represent a double-edged sword," he said. "Your friends that you need are also the individuals most likely to get you sick."

Dolphins are extremely playful animals and often swim close together, sometimes even touching fins. "We call it holding hands," said Mann.

"The males stay pretty coordinated with each other. The females sync, but not as regularly. They sync mostly with their offspring," adds Mann. That difference in behaviour may help explain why males died in greater numbers during the most recent cetacean morbillivirus outbreak - a hypothesis the researchers are examining.

Depleted populations While Atlantic bottlenose dolphins are not endangered, NOAA considers their coastal populations to be "depleted", meaning "below optimum sustainable population".

Outbreaks of the virus emerge here every 25 years or so. And they strike dolphins and their close marine relatives elsewhere, including some endangered whale species.

University of Hawaii researcher Kristi West called the disease - which causes skin lesions, pneumonia, brain infections and a suppressed immune system - "the most significant threat to dolphins and whales on a worldwide scale".

While viruses naturally occur in the wild, human disruption of marine habitats has made animals more vulnerable. "The disease becomes an even more significant threat when we combine it with other stressors that dolphins and whales throughout the world are facing," said West. AP.

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NEWCASTLE University scientists claim that an enzyme isolated from marine bacterium Bacillus licheniformis cuts through plaque on teeth and cleans hard-to-reach areas. Dr Nicholas Jakubovics of the university's School of Dental Sciences said: "Plaque is made up of bacteria which join together to colonize an area in a bid to push out any potential competitors. Traditional toothpastes work by scrubbing off the plaque containing the bacteria. But that's not always effective which is why people who religiously clean their teeth can still develop cavities." When bacterial cells die, the DNA inside leaks out and creates a biofilm that sticks to teeth, protecting the bacteria from brushing, chemicals or even antibiotics. Bacillus licheniformis, found on the surface of seaweed, releases an enzyme which breaks up the biofilm and strips away harmful bacteria.

Researcher Prof. Burgess said: "The zyme breaks up and removes the bacteria esent in plaque and importantly, prevents build-up of plaque too. If we can contain it with'n toothpaste we would be creating a product which could prevent tooth decay. The enzyme also has huge potential in he ping keep clean medical implants such as artificial hips and speech valves which also suffer from biofilm infection."

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Seamounts are underwater mountains rising over 1000 metres from the ocean floor. They do not reach the water's surface. They are formed by extinct volcanoes whose lava piles suddenly rise to form peaks. They are important manne ecosistems that support a wide variety of oceanic wildlife, from corals to sharks . Seamounts and guyots are most abundant in the North Pacific Ocean, and follow a distinctive evolutionary pattern of eruption, build-up, subsidence and erosion. In recent years, several active seamounts have been observed, for example Loihi in the Hawaiian Islands.

Because of their abundance, seamounts are one of the most common marine ecosystems in the world. Interactions between seamounts and underwater currents, as well as their elevated position in the water, attract plankton, corals, fish, and marine mammals alike. Their aggregational effect has been noted by the commercial fishing industry, and many seamounts support extensive fisheries. There are ongoing concerns on the negative impact of fishing on seamount ecosystems, and well-documented cases of stock decline, for example with the orange roughy (Hoplostethus atlanticus). 95% of ecological damage is done by bottom trawling, which scrapes whole ecosystems off seamounts.

Because of their large numbers, many seamounts remain to be properly studied, and even mapped. Bathymetry and satellite altimetry are two technologies working to close the gap. There have been instances where naval vessels have collided with uncharted seamounts; for example, Muirfield Seamount is named after the ship that struck it in 1973. However, the greatest danger from seamounts are flank collapses; as they get older, extrusions seeping in the seamounts put pressure on their sides, causing landslides that have the potential to generate massive tsunamis.

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An organism's appearance and body colour serve various purposes and they are strongly linked to the environment. In the marine environment, colouration is tied to how light penetrates ocean water and an animal's ability to blend in with its surroundings. A new study has found that fish communities on the Great Barrier Reef in Australia may become less colourful as corals bleach. The researchers studied the reef health, coral types, and resident fish over three decades.

The Great Barrier Reef, the largest coral reef system on Earth, has been subject to frequent bleaching events due to global warming in the past few decades. Coral bleaching events profoundly change the make-up of the ecosystem. Researchers say the loss of corals is a likely factor in the disappearance of brightly-coloured fish in the GBR.

The study looked at the diversity of colours found in communities of reef fishes and related that to the types of habitats where those fishes live.

 Fish communities in healthy parts of the reef with plentiful complex corals were compared to other areas where bleached corals dominate in the wake of major disturbances, such as heatwaves. The researchers found that as the cover of structurally complex corals increases on a reef, the range of colours present on fishes living and around them also increases. The diversity of fish colours declines in areas covered in dead, damaged corals. The bleached and dead corals probably provide less protection from predators for brightly-coloured fishes.

The researchers also found that the eye-catching yellow and green fishes, such as the lemon damselfish and green coral goby, have declined by about two-thirds, since the first recorded mass bleaching event of the reefs around Orpheus Island in 1998.

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The black piranha has the most powerful bite of all carnivorous fishes, living or extinct. Even at their small body sizes, new research indicates that piranhas will attack and bite chunks of bony fins and flesh from prey many times larger than themselves. Piranhas have a reputation as the most  ferocious fish in the world, With razor sharp teeth, they hunt in packs, stripping a body of its flesh. The first bite-force measurements taken from wild specimens of this largest species of piranha in the Amazon, shows that this creature can bite with a force more than 30 times greater than its weight.  The black piranha has jaw muscles of an 'extraordinary' size and a highly modified jaw-closing lever. In fact, the muscle complex makes more than two percent of the black piranha's total body mass. This allows the fish to exert bite force equivalent to 30 times its bodyweight. The measured bite force of the black piranha, at 320 newton (N), was nearly three times greater than that exerted by an American alligator of comparative size, said the study. The Black Piranha, has outsized jaw muscles allowing it to exert bite force equivalent to 30 times its bodyweight, a feat unmatched in the natural world.

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Tyrannoneustes  lythrodectikos or 'tyrant swimmer', a marine crocodile that looked part shark and part sinister dolphin, has been identified as one of the world's oldest-known "super predators", meaning carnivores that can feed on prey that's as large or larger than themselves.

"It is the oldest-known metriorhynchid macrophage- an animal that was adapted to feeding on large-bodied prey," says Mark Young of the University of Edinburgh's School of Biological Sciences. The term 'metriorhynchid' refers to a group of marine crocodiles that were superficially similar to living dolphins. "During the Middle Callovian Age 165 million years ago, much of Europe was covered by a shallow sea, creating a chain of large to small islands. Tyrannoneustes lived in this shallow sea, along with numerous other marine reptiles."

Tyrannoneustes was first discovered in the late 1800s in the Oxford Clay Formation, which is a Jurassic soil sediment found by England and originally thought to have been a close relative called Dakosaurus. It was locked away in the storage system in the Hunterian Museum in Glasgow, and remained hidden there collecting dust and untouched for over a century. It was finally taken out and re-examined in early 2013 (January) by Mark Young and some of his colleagues and found to be a whole new, largest, and oldest species of marine crocodile. 

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Despite their name, starfish are not fish. They lack a backbone, fins and gills found in fish. They are marine animals. They do not have gills, scales, or fins like fish do. Because they are not classified as fish, scientists prefer to call starfish "sea stars." If you thought a starfish has only five arms, think again. There are some species with as many as 50 arms. They have light-sensitive eyespots on the tips of their arms which help them in finding food. They can regenerate lost limbs and swallow large prey using their unusual stomachs. This means that some starfish can even regenerate an entirely new sea star from just one arm and a portion of the star's central disc. This won't happen too quickly, though; it takes about a year for an arm to grow back. There are about 2,000 species of sea stars.2? Some live in the intertidal zone, while others live in the deep water of the ocean. While many species live in tropical areas, sea stars can also be found in cold areas—even the polar regions. Starfish (or sea stars) are beautiful marine animals found in a variety of colors, shapes, and sizes.

Credit : ThoughtCo.

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