The most dangerous sea predator. The most dangerous predators in the world: rating, description and interesting facts
The ocean is home to a huge number of different predators. Some sea predators attack quickly, while others sit in cover for a long time, waiting for their victims.
Every inhabitant of the ocean is eaten by other sea inhabitants; only killer whales and sharks have no enemies.
Sharks
The white shark is most likely the most dangerous predator of the deep sea. People tremble at the very idea of a great white shark.
The white shark has no equal in strength and power among predators in the ocean.
Sharks appeared in the ocean long before man began to dominate the Earth. There are about 400 species of sharks. But the most dangerous shark is the white shark. Individuals of this species can reach 6 meters in length, they weigh about 3 tons and have a powerful toothy mouth. There are about 300 sharp-shaped teeth in the mouth. The teeth on the upper jaw are triangular, and on the lower jaw they are curved. The body shape of the white shark is spindle-shaped, the tail has the shape of a crescent, and the fins are large. White sharks live for about 27 years.
But people are not the target. These predators prefer victims with more serious fat reserves. For example, their favorite treats are sea lions and seals. White sharks do not show too much interest in people, since the human body contains too many tendons and muscles.
Typically, white sharks attack people for two reasons. The first is that a shark associates a person swimming in the water with a sick animal that cannot develop sufficient speed and is easy to catch. The second reason is that surfers floating on a board look like other ocean inhabitants from the water. And since the shark has rather poor eyesight, it can easily make mistakes. To determine whether the prey is edible, the shark bites it, but sometimes sharks tear people to pieces. It is difficult to predict how this predator will behave. When a shark grabs a victim, it shakes its head in all directions, thus snatching pieces from it.
Sea anemone is a predatory animal that more closely resembles a plant. Scientists say sharks are ocean orderlies because they eat dying animals.
Sea anemones
Sea anemone is a predator veiled by beauty. Sea anemones are representatives of cnidarians. Sea anemones have stinging cells that they use as weapons. Sea anemones reach a height of about 1 meter. These creatures lead a sedentary life. They are attached to the bottom by a foot called the sole or basal disc.
The sea anemone has from ten to hundreds of tentacles with special cells - cnidocytes. These cells produce poison, which is a mixture of toxins. Anemones use this poison during hunting and for protection from predators.
The poison contains substances that affect the nervous system of the victim. The prey is paralyzed under the influence of the poison and the predator calmly eats it.
The diet of sea anemones is based on fish and crustaceans. Sea anemone poison is not dangerous for people; it does not lead to death, but can cause quite severe burns.
Killer whales
- predators of the dolphin family, but they are not at all as friendly as dolphins. They are called killer whales. Killer whales attack almost all marine inhabitants: mammals, fish and shellfish. If there is enough food, then killer whales behave quite friendly with other cetaceans, but if there is little food, then killer whales attack their own kind: dolphins and whales.
Killer whales are one of the ocean's formidable hunters. For these predators, the size of the prey is not of great importance; killer whales hunt large animals together. If the victim cannot be killed immediately, the killer whale harasses it by biting off small pieces of it. No one manages to stay alive after a collision with killer whales - not a small fish, not a large whale.
A pod of killer whales acts very harmoniously during the hunt. The predators move in even ranks, like soldiers, and each killer whale has a clearly defined task.
When killer whales lead a sedentary life, they feed mainly on crustaceans and fish. And migrating killer whales prefer large mammals, such as sea lions and seals. Killer whales live up to the name killer whales in the best possible way.
Octopuses
Octopuses are part of the order of cephalopods. These creatures have excellently developed vision, smell, and touch, but they do not hear very well.
Which predatory animals are the largest and most dangerous? Most people probably think of lions and tigers first, but these predators are just babies compared to the really big predators that live on our planet. So, in this post - about the largest and most dangerous predatory animals.
First of all, it is worth dividing predators by habitat. It is clear that marine predators living in water can grow larger than their land-based competitors. But it is impossible to make a clear division here. For example, sharks can attack not only the inhabitants of the sea, but also such completely land animals as deer, horses and bears. On the other hand, many land predators prey on sea creatures. Finally, there are many predators that lead a semi-aquatic lifestyle; they can be found both at sea and on land.
The largest sea predators
The record holder among marine predators and generally the largest predator in the world is the sperm whale. Sperm whales are huge marine mammals from the order Cetaceans. Modern sperm whales reach 20 m in length and weigh up to 50 tons.
The sperm whale is the largest predator on Earth
Sperm whales live in all areas of the world's oceans and feed mainly on fish and cephalopods. Despite the fact that sperm whales breathe air, they can dive to a depth of 3 km, remaining under water for up to an hour and a half.
How dangerous are sperm whales? The sperm whale is the only predator that can swallow a person whole without even chewing. Nevertheless, sperm whales are not the first to attack people; a scuba diver can swim next to a huge sperm whale without fear. Unfortunately, man himself, as soon as he mastered navigation, began to exterminate marine life, taking up hunting, including sperm whales. And the sperm whales showed the whalers that they were by no means helpless victims. They themselves responded by attacking whaling ships, ramming and even sinking them. Even for modern sea vessels, sperm whales are dangerous.
Another large, smart and effective marine predator from the cetacean order is killer whale. Killer whales are not dangerous to people and do not attack them, but many marine inhabitants are not given a single chance.
Killer whales reach a length of 10 m and can weigh up to 8 tons. They live throughout the world's oceans and mainly hunt fish and seals. Killer whales often hunt in packs; they surround and drive victims, pressing them to the shore or surface of the water. You can understand how dangerous killer whales are from the fact that they even attack large whales and sharks.
The most dangerous and largest predatory fish is, of course, White shark. Great white sharks reach a length of 6 m and a weight of about 2 tons. The white shark is a dangerous and aggressive predator; it often attacks everything that moves, testing buoys, boards and other floating objects. Dozens of swimmers and surfers have been attacked by white sharks.
Over hundreds of millions of years of evolution, these dangerous predators have developed many unique adaptations. For example, sharks have a unique sense of smell, sensing the smell of blood from kilometers away, sensing the slightest temperature changes and even electromagnetic fields. Sharks are not at risk of caries - in addition to the fact that their teeth (of which there are about 300) are very strong, they grow and are renewed throughout their lives.
The largest semi-aquatic predators
There are many animals that can stay for a long time both on land and in the sea. Among them there are also large predators, the largest of which are southern elephant seals. The southern elephant seal lives in the seas of the southern hemisphere, mainly in Antarctica.
Southern elephant seals reach a length of 6 m and weigh up to 5 tons. They hunt mainly for marine life, feeding on fish and squid. Despite their size, these predators are usually not dangerous to people.
Another thing - saltwater crocodiles. The saltwater crocodile, also known as the saltwater crocodile, is the world's largest species of crocodile and a very dangerous and aggressive predator.
These crocodiles can reach a length of 7 m and weigh up to 2 tons. They are able to spend a lot of time at sea, swimming thousands of kilometers. Saltwater crocodiles hunt both land and sea animals, without being very picky. They even attack sharks and elephants.
How dangerous saltwater crocodiles are can be judged by an episode that occurred in February 1945. At this time, the British were trying to capture a Japanese base on an island off the coast of Burma. But to defend the island, the Japanese deployed a detachment of 1215 selected soldiers. Then the English suggested luring the Japanese detachment into the mangrove swamps where saltwater crocodiles lived. The plan worked brilliantly - the crocodiles attacked the Japanese who carelessly entered the swamp, and almost the entire detachment was soon exterminated. Only 20 soldiers managed to escape.
The largest land predators
Of the predators living on land, the largest are bears. The biggest of all bears - polar bear, living in the Arctic.
Polar bears reach a length of 3 m and a weight of up to 1000 kg. These predators mainly hunt seals and fish. Polar bears pose a moderate danger to people, although they are not usually the first to attack.
The largest species of brown bears is kodiak- lives in Alaska and is almost as big as a polar bear.
These bears are omnivores, feeding on both plant and animal foods, especially preferring fish that are caught in rivers during the spawning period.
Large predators, of course, sometimes attack people, but they are by no means the most dangerous among animals. Rather, the largest predators themselves need protection from humans today. The most terrible and dangerous animal that you really need to be afraid of actually looks different. Here it is:
Malaria mosquito is only about 6 mm in size and weighs approximately 2 milligrams. But these dangerous insects kill many times more people than all sharks, crocodiles and other large predators combined. WHO estimates that these mosquitoes infect more than 300 million people with malaria every year, and more than a million of them die.
Some denizens of the deep would happily feast on us, but most are only dangerous if you attack them first. You can call this the “accidentally stepped on, poisoned and died” principle. In this case, who should you not step on?
The Portuguese man-of-war is an entire colony of jellyfish that hunts other marine life with the help of long, poisonous tentacles. The base of the “ship” floats on the surface of the water at this time, but it is easy to miss. Every year they poison several thousand people.
Box jellyfish have long been famous as one of the most dangerous creatures off the coast of Australia. Their tentacles, numbering up to 60, reach a length of four meters. The poison of some of their species can paralyze a person with one touch and cause him to choke.
Blue-ringed octopuses are as legendary among mollusks as box jellyfish are among cnidarians. These are the most poisonous creatures in all the oceans of the world, the attack of which leads to paralysis and death.
Great white sharks are much scarier on screen than in reality, but that doesn't make them any less fearsome predators. At least 74 unprovoked attacks on people were recorded, including attacks on fishing boats.
Sea snakes are equipped with a more powerful toxic venom than their land-based relatives - simply because fish are not as sensitive to poison. Their venom, like that of all adders, has a paralyzing effect. Fortunately for people, they use their weapons mainly only when hunting, and if handled with care they do not bite.
Lionfish do not waste time on their spines, generously displaying them throughout their body. They very successfully hunt other fish, even capturing territories that are not necessary for the survival of their species. Due to their toxicity and prevalence, lionfish are a real headache for fishermen.
Crocodiles mainly prefer rivers, but their largest representative, the saltwater crocodile, is not at all averse to swimming in salt water. Males of this species grow up to seven meters in length and two tons in weight. Aggressive specimens often attack people.
Large barracudas are impressive predators, growing up to two meters in length. Their teeth are considered to be among the sharpest and most painful in the entire marine world. Barracudas often follow divers out of pure curiosity, but they rarely attack. True, if this happens, then death is guaranteed.
Original taken from billfish561 in Beautiful, but dangerous inhabitants of the seas and oceans.
There are quite a lot of creatures living in sea and ocean waters, an encounter with which can cause a person trouble in the form of injury or even lead to disability or death.
Here I have tried to describe the most common sea inhabitants that you should be wary of when you encounter them in the water, while relaxing and swimming on the beach of a resort or while diving.
If you ask any person "...Which inhabitant of the seas and oceans is the most dangerous?", then we will almost always hear the answer “... shark..." But is this so? Who is more dangerous, a shark or an outwardly completely harmless shell?
Moray eels
It reaches a length of 3 m and a weight of up to 10 kg, but as a rule, individuals are found about a meter long. The fish has bare skin, without scales. They are found in the Atlantic and Indian Oceans, and are widespread in the Mediterranean and Red Seas. Moray eels live in the bottom layer of water, one might say at the bottom. During the day, moray eels sit in crevices of rocks or corals, sticking their heads out and usually moving them from side to side, looking for passing prey; at night they get out of their shelters to hunt. Moray eels usually feed on fish, but they also attack crustaceans and octopuses, which are caught from ambush.
After processing, moray eel meat can be eaten. It was especially valued by the ancient Romans.
Moray eel is potentially dangerous to humans. A diver who has become a victim of a moray eel attack always somehow provokes this attack - he sticks his hand or foot into the crevice where the moray eel is hiding, or chases it. A moray eel, when attacking a person, inflicts a wound that is similar to the bite mark of a barracuda, but unlike a barracuda, the moray eel does not immediately swim away, but hangs on its victim like a bulldog. She can grab the arm with a bulldog death grip, from which the diver cannot free himself, and then he can die.
It is not poisonous, but since moray eels do not disdain carrion, the wounds are very painful, do not heal for a long time and often become inflamed. Hides among underwater rocks and coral reefs in crevices and caves.
When moray eels begin to feel hungry, they jump out of their shelters like an arrow and grab a victim swimming by. Very gluttonous. Very strong jaws and sharp teeth.
Moray eels are not very attractive in appearance. But they do not attack scuba divers, as some believe; they are not aggressive. Isolated cases occur only when moray eels are in mating season. If a moray eel mistakes a person for a source of food or he invades its territory, then it may still attack.
Barracudas
All barracudas live in tropical and subtropical waters of the World Ocean near the surface. There are 8 species in the Red Sea, including the large barracuda. There are not many species in the Mediterranean Sea - only 4, of which 2 moved there from the Red Sea via the Suez Canal. The so-called “malita”, which has settled in the Mediterranean Sea, provides the bulk of the entire Israeli catch of barracudas. The most ominous feature of barracudas is the powerful lower jaw, which protrudes far beyond the upper jaw. The jaws are equipped with fearsome teeth: a row of small, razor-sharp teeth dot the outside of the jaw, and a row of large, dagger-like teeth inside.
The maximum recorded size of a barracuda is 200 cm, weight is 50 kg, but usually the length of a barracuda does not exceed 1-2 m.
She is aggressive and fast. Barracudas are also called “living torpedoes” because they attack their prey with great speed.
Despite such a formidable name and ferocious appearance, these predators are practically harmless to humans. It should be remembered that all attacks on people occurred in muddy or dark water, where the moving arms or legs of a swimmer were mistaken by the barracuda for swimming fish (This is exactly the situation the author of the blog found himself in in February 2014, when he was on vacation in Egypt, Oriental Bay Resort Marsa Alam 4+* (now called Aurora Oriental Bay Marsa Alam Resort 5*) Marsa Gabel el Rosas Bay . A medium-sized barracuda, 60-70cm, almost bit off the first f alang of the index finger on the right hand. A piece of a finger was hanging on a 5mm piece of skin (dive gloves saved me from complete amputation). At the Marsa Alam clinic, the surgeon put 4 stitches and saved the finger, but the rest was completely ruined ). In Cuba, the reason for attacking a person was shiny objects such as watches, jewelry, knives. It will not be superfluous if the shiny parts of the equipment are painted dark.
The barracuda's sharp teeth can damage the arteries and veins of the limbs; in this case, the bleeding must be stopped immediately, since blood loss can be significant. In the Antilles, barracudas are more feared than sharks.
Jellyfish
Every year, millions of people suffer from “burns” from contact with jellyfish while swimming.
There are no particularly dangerous jellyfish in the waters of the seas washing the Russian shores; the main thing is to prevent these jellyfish from coming into contact with mucous membranes. In the Black Sea, the easiest jellyfish to encounter are Aurelia and Cornerot. They are not very dangerous, and their “burns” are not very strong.
Aurelia "butterflies" (Aurelia aurita)
Cornermouth jellyfish (Rhizostoma pulmo)
Only in the Far Eastern seas does it live enough cross jellyfish, dangerous to humans, the poison of which can even lead to the death of a person. This small jellyfish with a cross pattern on its umbrella causes severe burns at the point of contact with it, and after a while causes other disorders in the human body - difficulty breathing, numbness of the limbs.
Cross Medusa (Gonionemus vertens)
consequences of a cross jellyfish burn
The further south you go, the more dangerous the jellyfish are. In the coastal waters of the Canary Islands, a pirate awaits unwary swimmers - the “Portuguese man-of-war” - a very beautiful jellyfish with a red crest and a multi-colored bubble-sail.
Portuguese man of war (Physalia physalis)
The "Little Man of Portugal" looks so harmless and beautiful at sea... 
And this is what the leg looks like after contact with the "Portuguese man-of-war"....
Many jellyfish live in the coastal waters of Thailand.
But the real scourge for swimmers is the Australian “sea wasp”. She kills with a light touch of multi-meter tentacles, which, by the way, can wander on their own without losing their murderous qualities. You can pay for getting acquainted with the “sea wasp” with severe “burns” and lacerations at best, and with life at worst. Sea wasp jellyfish have killed more people than sharks. This jellyfish lives in the warm waters of the Indian and Pacific oceans, and is especially numerous off the coast of Northern Australia. The diameter of its umbrella is only 20-25 mm, but the tentacles reach a length of 7-8 m and they contain venom similar in composition to cobra venom, but much stronger. A person who is touched by a “sea wasp” with its tentacles usually dies within 5 minutes.
Australian box jellyfish or "sea wasp"
(Chironex fleckeri)
burn from jellyfish "sea wasp"
Aggressive jellyfish also live in the Mediterranean and other Atlantic waters - the “burns” caused by them are stronger than the “burns” of Black Sea jellyfish, and they cause allergic reactions more often. These include cyanea ("hairy jellyfish"), pelagia ("little lilac sting"), chrysaora ("sea nettle") and some others.
Atlantic cyanide jellyfish (Cyanea capillata)
Pelagia (Noctiluca), known in Europe as "purple sting"
Pacific sea nettle (Chrysaora fuscescens)
Jellyfish "Compass"
(Coronatae)
Compass jellyfish chose the coastal waters of the Mediterranean Sea and one of the oceans - the Atlantic - as their place of residence. They live off the coast of Turkey and the United Kingdom. These are quite large jellyfish, their diameter reaches thirty centimeters. They have twenty-four tentacles, which are arranged in groups of three each. The body color is yellowish-white with a brown tint, and its shape resembles a saucer-bell, which has thirty-two lobes, which are colored brown at the edges.
The upper surface of the bell has sixteen brown V-shaped rays. The lower part of the bell is the location of the mouth opening, surrounded by four tentacles. These jellyfish are poisonous. Their venom is potent and often leads to the formation of wounds that are very painful and take a long time to heal..
Yet the most dangerous jellyfish live in Australia and its adjacent waters. Burns from box jellyfish and Portuguese man-of-war are very serious and often fatal.
Stingrays
Stingrays of the stingray family and electric rays can cause trouble. It should be noted that the stingrays themselves do not attack a person; injury can be caused if you step on him when this fish is hiding at the bottom.
Stingray stingray (Dasyatidae)
Electric Stingray (Torpediniformes)
Stingrays live in almost all seas and oceans. In our (Russian) waters you can find a stingray, or otherwise called a sea cat. It is found both in the Black Sea and in the seas of the Pacific coast. If you step on a stingray buried in the sand or resting at the bottom, it can cause a serious wound to the offender, and, among other things, inject poison into it. He has a barb on his tail, or rather a real sword - up to 20 centimeters in length. Its edges are very sharp, and also jagged, along the blade, on the lower side there is a groove in which dark poison from the poisonous gland on the tail is visible. If you touch a stingray lying at the bottom, it will strike with its tail like a whip; at the same time, it sticks out its spine and can cause a deep chopped wound. A wound from a stingray blow is treated like any other.
The Black Sea is also home to the sea fox stingray Raja clavata - large, up to one and a half meters from the tip of the nose to the tip of the tail, it is harmless to humans - unless, of course, you try to grab it by the tail, covered with long sharp spines. Electric stingrays are not found in the waters of Russian seas.
Sea anemones (anemones)
Sea anemones inhabit almost all the seas of the globe, but, like other coral polyps, they are especially numerous and diverse in warm waters. Most species live in shallow coastal waters, but they are often found at the maximum depths of the World Ocean. Sea anemones Usually hungry sea anemones sit completely calm, with tentacles widely spaced. At the slightest changes occurring in the water, the tentacles begin to oscillate, not only they stretch out towards the prey, but often the entire body of the sea anemone bends. Having grabbed the victim, the tentacles contract and bend towards the mouth.
Sea anemones are well armed. Stinging cells are especially numerous in predatory species. A volley of fired stinging cells kills small organisms and often causes severe burns in larger animals, even humans. They can cause burns, just like some types of jellyfish.
Octopuses
Octopuses (Octopoda) are the most famous representatives of cephalopods. “Typical” octopuses are representatives of the suborder Incirrina, bottom-dwelling animals. But some representatives of this suborder and all species of the second suborder, Cirrina, are pelagic animals that live in the water column, and many of them are found only at great depths.
They live in all tropical and subtropical seas and oceans, from shallow waters to depths of 100-150 m. They prefer rocky coastal zones, looking for caves and crevices in the rocks for habitat. In the waters of the Russian seas they live only in the Pacific region.
The common octopus has the ability to change color to adapt to its environment. This is explained by the presence in his skin of cells with various pigments that, under the influence of impulses from the central nervous system, can stretch or contract depending on the perception of the senses. The usual color is brown. If the octopus is scared, it turns white; if it is angry, it turns red.
When enemies approach (including divers or scuba divers), they flee, hiding in rock crevices and under stones.
The real danger is an octopus bite if handled carelessly. The secretion of the poisonous salivary glands can be injected into the wound. In this case, acute pain and itching is felt in the bite area.
When a common octopus bites, a local inflammatory reaction occurs. Heavy bleeding indicates a slowdown in the blood clotting process. Usually, recovery occurs within two to three days. However, there are cases of severe poisoning in which symptoms of damage to the central nervous system occur. Wounds caused by octopuses are treated in the same way as injections from poisonous fish.
Blue-ringed octopus (Blue-ringed Octopus)
One of the contenders for the title of the most dangerous marine animal for humans is the octopus Octopus maculosus, which is found along the coast of the Australian province of Queensland and near Sydney, found in the Indian Ocean and, sometimes, in the Far East. Although the size of this octopus rarely exceeds 10 cm, it contains enough poison to kill ten people.
Lionfish
Lionfish (Pterois) of the family Scorpaenidae pose a great danger to humans. They are easily recognized by their rich and bright colors, which warns of the effective means of protection these fish have. Even marine predators prefer to leave this fish alone. The fins of this fish look like brightly decorated feathers. Physical contact with such fish can be fatal.
Lionfish
(Pterois)
Despite its name, it cannot fly. The fish got this nickname because of its large pectoral fins, which look a little like wings. Other names for lionfish are zebra fish or lion fish. She got the first due to the wide gray, brown and red stripes located throughout her body, and the second due to her long fins, which make her look like a predatory lion.
The lionfish belongs to the scorpionfish family. The body length reaches 30 cm, and the weight is 1 kg. The coloring is bright, which makes the lionfish noticeable even at great depths. The main decoration of the lionfish is the long ribbons of the dorsal and pectoral fins, which are what resemble a lion's mane. These luxurious fins conceal sharp, poisonous needles, which make the lionfish one of the most dangerous inhabitants of the seas.
The lionfish is widespread in the tropical parts of the Indian and Pacific oceans off the coasts of China, Japan and Australia. It lives mainly among coral reefs. lionfish Since it lives in the surface waters of the reef, it therefore poses a great danger to swimmers, who can step on it and be injured by sharp poisonous needles. The excruciating pain that occurs is accompanied by the formation of a tumor, breathing becomes difficult, and in some cases the injury leads to death.
The fish itself is very voracious and eats all kinds of crustaceans and small fish during the night hunt. The most dangerous include puffer fish, boxfish, sea dragon, urchin fish, ball fish, etc. You only need to remember one rule: the more colorful the fish and the more unusual its shape, the more poisonous it is.
Cube body or box fish (Ostraction cubicus)
hedgehog fish (Diodontidae)
fish ball (Diodontidae)
In the Black Sea there are relatives of the lionfish - the noticeable scorpionfish (Scorpaena notata), it is no more than 15 centimeters in length, and the Black Sea scorpionfish (Scorpaena porcus) - up to half a meter - but such large ones are found deeper, further from the coast. The main difference between the Black Sea scorpionfish is its long, rag-like flaps, supraorbital tentacles. In the noticeable scorpion fish these outgrowths are short.
conspicuous scorpionfish
(Scorpaena notata) 
black sea scorpionfish
(Scorpaena porcus)
The body of these fish is covered with spines and growths, the spines are covered with poisonous mucus. And although the scorpionfish’s venom is not as dangerous as that of the lionfish, it is better not to disturb it.
Among the dangerous Black Sea fish, the sea dragon (Trachinus draco) should be noted. An elongated, snake-like, bottom-dwelling fish with an angular large head. Like other bottom-dwelling predators, the dragon has bulging eyes on the top of its head and a huge, greedy mouth.
sea dragon
(Trachinus draco)
The consequences of a poisonous injection from a dragon are much more serious than in the case of scorpionfish, but not fatal.
Wounds from scorpionfish or dragon thorns cause burning pain, the area around the injections turns red and swells, then general malaise, fever, and your rest is interrupted for a day or two. If you have suffered from ruff thorns, consult a doctor. Wounds should be treated like regular scratches.
The “stone fish” or wart (Synanceia verrucosa) also belongs to the order of scorpion fish - no less, and in some cases more dangerous than lionfish. 
"stone fish" or wart
(Synanceia verrucosa) 
Sea urchins
Often in shallow waters there is a risk of stepping on a sea urchin.
Sea urchins are one of the most common and very dangerous inhabitants of coral reefs. The hedgehog's body, the size of an apple, is studded with 30-centimeter needles sticking out in all directions, similar to knitting needles. They are very mobile, sensitive and react instantly to irritation.
If a shadow suddenly falls on a hedgehog, it immediately points its needles towards danger and puts them together, several at a time, into a sharp, hard peak. Even gloves and wetsuits do not guarantee complete protection from the formidable peaks of the sea urchin. The needles are so sharp and fragile that, having penetrated deep into the skin, they immediately break off and it is extremely difficult to remove them from the wound. In addition to spines, hedgehogs are armed with small grasping organs - pedicillariae, scattered at the base of the spines.
The venom of sea urchins is not dangerous, but it causes burning pain at the injection site, shortness of breath, rapid heartbeat, and temporary paralysis. And soon redness and swelling appear, sometimes there is loss of sensitivity and secondary infection. The wound must be cleaned of needles, disinfected, and to neutralize the poison, hold the damaged part of the body in very hot water for 30-90 minutes or apply a pressure bandage.
After meeting a black “long-spined” sea urchin, black dots may remain on the skin - this is a trace of pigment, it is harmless, but it can make it difficult to find the needles stuck in you. After first aid, consult a doctor.
Shells (molluscs)
Often on the reef among the corals there are wavy valves of bright blue color.
tridacna clam
(Tridacna gigas)
According to some reports, divers sometimes get caught between its doors, as if in a trap, which leads to their death. The danger of tridacna, however, is greatly exaggerated. These clams live in shallow reef areas in clear tropical waters, so they are easy to spot due to their large size, brightly colored mantle, and ability to spray water at low tide. A diver caught in a shell can easily free himself by inserting a knife between the valves and cutting the two muscles that compress the valves.
Poisonous Clam Cone
(Conidae)
Do not touch beautiful shells (especially large ones). Here it is worth remembering one rule: all mollusks that have a long, thin and pointed ovipositor are poisonous. These are representatives of the conus genus of the gastropod class, having a brightly colored conical shell. Its length in most species does not exceed 15-20 cm. The cone injects with a needle-sharp spike that protrudes from the narrow end of the shell. Inside the thorn there is a duct of the poisonous gland, through which very strong poison is injected into the wound. 
Various species of the genus cone are common on coastal shallows and coral reefs of warm seas.
At the moment of the injection, a sharp pain is felt. At the site where the spike was inserted, a reddish dot is visible against the background of pale skin.
The local inflammatory reaction is insignificant. A feeling of acute pain or burning appears, and numbness of the affected limb may occur. In severe cases, there is difficulty speaking, flaccid paralysis quickly develops, and knee reflexes disappear. Death may occur within a few hours.
In case of mild poisoning, all symptoms disappear within 24 hours.
First aid consists of removing thorn fragments from the skin. The affected area is wiped with alcohol. The affected limb is immobilized. The patient is taken to the medical center in a supine position.
Corals
Coral, both living and dead, can cause painful cuts (be careful when walking on coral islands). And the so-called “fire” corals are armed with poisonous needles that dig into the human body in case of physical contact with them.
The basis of the coral is made up of polyps - marine invertebrate animals measuring 1-1.5 millimeters or slightly larger (depending on the species).
As soon as it is born, the baby polyp begins to build a cell house in which it spends its entire life. Micro-houses of polyps are grouped into colonies from which a coral reef ultimately appears.
When hungry, the polyp sticks out tentacles with many stinging cells from its “house”. The smallest animals that make up the plankton encounter the tentacles of the polyp, which paralyzes the victim and sends it into the mouth. Despite their microscopic size, the stinging cells of polyps have a very complex structure. Inside the cell there is a capsule filled with poison. The outer end of the capsule is concave and looks like a thin spirally twisted tube called a stinging filament. This tube, covered with tiny spines directed backwards, resembles a miniature harpoon. When touched, the stinging thread straightens, the “harpoon” pierces the victim’s body, and the poison passing through it paralyzes the prey.
Poisoned coral harpoons can also injure humans. Dangerous ones include, for example, fire coral. Its colonies in the form of “trees” made of thin plates have chosen the shallow waters of tropical seas.
The most dangerous stinging corals from the genus Millepora are so beautiful that scuba divers cannot resist the temptation to break off a piece as a souvenir. This can be done without “burns” and cuts only with canvas or leather gloves.
Fire coral (Millepora dichotoma)
When talking about such passive animals as coral polyps, it is worth mentioning another interesting type of marine animal - sponges. Typically, sponges are not classified as dangerous marine inhabitants, however, in the waters of the Caribbean there are some species that can cause severe skin irritation to a swimmer upon contact with them. It is believed that the pain can be relieved with a weak solution of vinegar, but the unpleasant consequences from contact with the sponge can last several days. These primitive animals belong to the genus Fibula and are often called touch-me-not sponges.
Sea snakes (Hydrophidae)
Little is known about sea snakes. This is strange, since they live in all the seas of the Pacific and Indian oceans and are not among the rare inhabitants of the deep sea. Maybe it's because people just don't want to deal with them.
And there are serious reasons for this. After all, sea snakes are dangerous and unpredictable.
There are about 48 species of sea snakes. This family once left land and completely switched to an aquatic lifestyle. Because of this, sea snakes have acquired some features in the structure of the body, and in appearance they are somewhat different from their terrestrial counterparts. The body is flattened laterally, the tail is in the form of a flat ribbon (in flat-tailed representatives) or slightly elongated (in swallowtails). The nostrils are located not on the sides, but on the top, so it is more convenient for them to breathe, sticking the tip of the muzzle out of the water. The lung stretches throughout the body, but these snakes absorb up to a third of all oxygen from the water with the help of the skin, which is densely permeated with blood capillaries. A sea snake can stay underwater for more than an hour. 
The venom of the sea snake is dangerous to humans. Their venom is dominated by an enzyme that paralyzes the nervous system. When attacking, the snake quickly strikes with two short teeth, slightly bent back. The bite is practically painless, there is no swelling or hemorrhage.
But after some time, weakness appears, coordination is impaired, and convulsions begin. Death occurs from paralysis of the lungs within a few hours.
The great toxicity of the venom of these snakes is a direct result of their aquatic habitat: to prevent the prey from escaping, it must be immediately paralyzed. True, the venom of sea snakes is not as dangerous as the venom of snakes that live with us on land. When a flattail bites, 1 mg of poison is released, and when a swallowtail bites, 16 mg is released. So, a person has a chance to survive. Out of 10 people bitten by sea snakes, 7 people remain alive, of course, if they receive medical assistance in time.
True, there is no guarantee that you will be among the last.
Among other dangerous aquatic animals, especially dangerous freshwater inhabitants should be mentioned - crocodiles that live in the tropics and subtropics, piranha fish that live in the Amazon River basin, freshwater electric stingrays, as well as fish whose meat or some organs are poisonous and can cause acute poisoning.
If you are interested in more detailed information about dangerous species of jellyfish and corals, you can find it at http://medusy.ru/