Our oceans are a peculiar and fascinating world. Many things live in this mysterious world and not just Fish. From the single-celled organism, plankton, to the largest animal on Earth, the blue whale, these creatures keep our nature balanced and help us understand the Earth more. While there are still many marine living organisms undiscovered to this day, let’s look at some of these wonderful creatures. Get ready to pick your favourite! 

But before that, let’s get to know how we classify animals and ultimately, classify ocean creatures. 

Living Organisms Classification

Taxonomy is a very important scientific study that classifies biological organisms based on shared characteristics. The scientist who was behind the current system that is used to classify living organisms is called Carl Linnaeus. He called it the Linnaean System. Organisms are grouped into taxa (a unit of biological classification). These taxa are then given a rank and ultimately create a taxonomic hierarchy. The following ranks are used to classify living organisms:

• Domain
• Kingdom
• Phylum
• Class
• Order
• Family 
• Genus
• Species

Did you know… The singular form of taxa is taxon.

Now, let’s get more specific and talk about each taxon in detail.

Domain

Domain is the highest rank in the taxonomic hierarchy and Species is the lowest. Domain has further three categories, and they are:

• Archaea (archaebacteria): These are single-celled organisms and live in extreme environments such as extremely hot areas or soils with high levels of acid. 
• Bacteria (eubacteria): This domain is commonly known as true bacteria. This bacteria is found in almost every environment and it is a more common domain to find than Archaea. They cause diseases in humans, plants, and animals, but they also work as antibiotic producers and digesters in our stomachs. 
• Eukarya (eukaryotes): This domain is different from Archaea and Bacteria because they have a nucleus in their cells. A nucleus is the part of the cell that contains the chromosomes. The Eukarya are further divided into four kingdoms which are: Protista, Fungi, Plantae, and Animalia. 

Kingdom

Kingdom is the second highest rank in the taxonomic hierarchy. We are going to focus on the four kingdoms of the Eukarya domain. 

• Protista: a unicellular eukaryotic organism that is usually found in environments with moisture and water. Examples of protists include slime moulds, protozoa, and algae. 
• Fungi: they can be unicellular or multicellular. They do not produce their own food and obtain their nutrients through absorption. Some fungi are beneficial to us and provide us with many types of medication such as antibiotics and penicillin. Examples of fungi include mushrooms, yeasts, and moulds. 
• Plantae: they are complex and multicellular organisms that are autotrophic, which means that they produce their own food via photosynthesis. Examples of Plantae include trees, ferns, and flowering plants.

Animalia: they are complex and multicellular organisms that don’t produce their own food. All species of the Animalia kingdom have some type of skeletal support and specialized cells. Their cells do not have walls and are organized into tissues. All species of this kingdom produce sexually. Examples of Animalia include humans, elephants, sponges, vertebrates, worms, and insects.

Phylum

Within the animal kingdom, to which we belong, animals are divided into 36 recognized phyla (the plural of ‘phylum’). Only nine of these phyla contain the vast majority of known species. The following table makes it easier to describe each of the nine phyla.

Class

Class is the fourth rank of classification. Let’s take a look at the Chordata phylum, for example. It has three subphyla: Vertebrata, Urochordata, and Cephalochordata. There are many classes in the subphylum Vertebrata, including Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia. 

Order

Order is the fifth rank of classification. Let’s take a look at some of the orders of Chondrichthyes, Osteichthyes, Reptilia, and Mammalia. We will look at one order from each class.

Family

Family is the sixth rank of classification. The families in the previous table are as follows:

Genus

Genus is the seventh rank of classification. It contains animals that are closely related and share similar features. This rank contains one or two species. For example, both blacktip sharks and bull sharks belong to the Carcharhinus genus. The killer whale, or orca, belongs to the Orcinus genus.

Species

Species is the last rank of classification. Each species within the genus is named after its individual and unique characteristics. If two individuals of the appropriate sexes can breed successfully, they are a species. The scientific name of an animal consists of two Latin words. The first one is the genus and the second is the species. For example, the genus of humans would be Homo and the species is sapiens, so the scientific name would be Homo sapiens. Another example would be the blacktip shark; its scientific name is Carcharhinus limbatus, while the scientific name of the bull shark is Carcharhinus leucas.

Ocean Fish

There are more than 30,000 species of fish in our oceans. We are going to explore together some of these unique-looking fish. We will mainly focus on bony fish, or Osteichthyes, as we learned. 

Atlantic Bluefin Tuna

First on our list is one of the strongest and fastest fish in the ocean world, the Atlantic Bluefin Tuna.

• It is scientifically known as Thunnus thynnus.
• It belongs to the tuna species and is considered the largest tuna species. They are close relatives to the Pacific bluefin tuna. 
• Atlantic bluefin tuna are born so small that you could see them under a microscope. However, in a span of three or five years, it becomes so gigantic that it reaches 1 metre in length. This makes the Atlantic bluefin tuna one of the largest predators in the ocean. 
• Atlantic bluefin tuna are gigantic and huge. They can weigh more than 250 kilograms. The largest Atlantic bluefin tuna that was recorded in history weighed 900 kilograms and its length reached up to 4.6 metres. 
• When Atlantic bluefin tuna are still young, they feed on zooplankton and other invertebrates, and the bigger they get the bigger their prey also gets. They also get eaten by several predators when they are young. These predators include sharks and killer whales. When Atlantic bluefin tuna reach adulthood, they mainly eat herring and mackerel. 

Atlantic bluefin tuna meat is highly valuable all around the world. It is especially prized in Japan, the home of sushi. Due to the high demand for Atlantic bluefin tuna, its numbers drastically decreased in the Black Sea and the off coast of Brazil, where they formerly were more common.

Atlantic Cod

We have here one of the best-studied fish, the Atlantic cod.

• The scientific name for Atlantic cod is Gadus morhua and they are found in the Atlantic Ocean near North America.
• They are known for their white and flaky flesh.

• Atlantic cod are bony fish with a barbel below their bottom jaw. It is believed that they change colours at different water temperatures. Sometimes their body can be grey-green and other times it appears reddish-brown. Almost their whole body is covered with dark spots except for their stomach. They also have two anal fins and three fins on the upper side of their body.
• Atlantic cod’s length can reach up to 2 metres and weigh over 96 kilograms. They feed on a variety of prey, including other bony fish, American lobster, molluscs, and other invertebrates. Their predators are large sharks, but what is more interesting is that the young Atlantic cod can become food to the adult ones, which makes them cannibalistic.
• Atlantic cod’s meat can be found as the base of many dishes in North America and Europe. But this has made the Atlantic cod population at risk these days. A study in 2012 showed that there were only 100 adult Atlantic cod left in the North sea.
• Atlantic cod usually travel in schools of fish.

Banded Butterflyfish

We now move on to one of the cutest fish you’re going to see, the Banded butterflyfish.

• Banded butterflyfish is one of the 114 species of butterflyfish that are known to us. They are called butterflyfish because their skin is colourful like butterflies. They can be black, orange, yellow, red, and white.
• The scientific name for the Banded butterflyfish is Chaetodon striatus.
• Their body is white with black vertical stripes on it. They have black pelvic fins and their body has the shape of a flattened disk. They are about 16 centimetres in length, which is cutely small!
• Banded butterflyfish feed on coral reefs, invertebrates, and mollusc eggs.
• They can live alone, in pairs, or in small shoals of about 20 individuals for purposes of feeding.
• Large fish are their main threat. Their first instinct when these large fish are present is to flee. In case they can’t do that, they adopt a defensive posture as they lower their head and erect their dorsal spines in a way that tells the predator that they are too spiney to be eaten. 

Did you know… Banded butterflyfish are known to clean other reef fish from parasites such as grunts and parrotfish. 

Blue-banded Goby

We have here another splendid-looking fish! The Blue-banded goby looks exactly like its name.

• It is a red or orange-skinned fish with bright, blue stripes. It definitely is one of the most colourful fish in the ocean. 
• The scientific name for this fish is Lythrypnus dalli.
• It can be found in the eastern parts of the Pacific ocean. Blue-banded gobies live their entire lives hiding amongst reefs and sea urchins. They do not like to stray away from their hiding places because of their fear of being devoured by larger fish.
• They use their teeth to break down and feed on small fish and some crustaceans. 
• Blue-banded gobies have some superpowers! They have the ability to change their gender. It is said that most marine fishes can change their gender, but they can’t change it back to the original one. This is why the Blue-banded goby is special; it can change its gender back and forth.

Bluehead Wrasse 

The Bluehead wrasse is one of the most studied fish.

• Its head has a beautiful blue colour and the rest of its body’s colour is blue blended with green or yellow. The juvenile’s body, however, is solid yellow.
• The scientific name for the Bluehead wrasse is Thalassoma bifasciatum. 
• The Bluehead wrasse can be found in the Atlantic Ocean, New Mexico, and every Gulf state between these two points.
• It lives in the coral reefs and feeds on small invertebrates. The younger members of a group stay close to their families instead of wandering and exploring, in fear of large predators. 
• The Bluehead wrasse fish uses a unique way of communicating with other members of its group. It produces sounds that can’t be recognized by the human ear and these sounds can be heard from miles and miles away. They use this way of communication to warn each other of an approaching danger so they can flee fast.
• The Bluehead wrasses are very cute fishes that pose no harm to humans. They can be the perfect pet for your aquarium. 

Clown Triggerfish

We now introduce you to one of the most aggressive and colourful fish in the underwater world, the Clown Triggerfish.

• It gets its name from its appearance that almost looks like clown makeup!
• The scientific name of this fish is Balistoides conspicillum.
• It is one of the 40 species of triggerfishes out there. They are called ‘triggerfish’ because of the reaction of their dorsal (back) fin spine when threatened. The first dorsal fin spine becomes erect when facing danger and is locked in place by the second spine, and the first spine cannot be lowered unless the second is pulled back, just like a trigger! That is why they got the name ‘triggerfishes.’ 
• All triggerfishes are known to have very strong jaws and broad teeth, and the Clown triggerfish is no exception. It uses its strong jaw and teeth to feed on hard-shelled invertebrates like sea urchins, crustaceans, and molluscs.
• Not many fish species are known to feed on the Clown triggerfish.
• They can grow to 50 centimetres in length. 
• The Clown triggerfish’s habitat is in the Indian Ocean and the West Pacific Ocean.
• The Clown triggerfish is solitary by nature; it is rarely seen in pairs or in a group. It is only seen in pairs when it is trying to mate.
• They are generally very beautiful creatures with vibrant colours. 

Common Clownfish

We have yet another clown! Common Clownfish is not your typical clown, but it is definitely smaller and cuter. The common Clownfish is perhaps the most iconic and famous ocean fish amongst children because of the Pixar movie ‘Finding Nemo.’ Some children might not believe it, but fish like Nemo actually exist in real life!

• They get their name ‘Clownfish’ from their appearance which looks like clown makeup. They also swim in the water in a very goofy way, making clown-like moves. Let’s get to know some of the most interesting information on clownfish!
• The scientific name for the clownfish is Amphiprion percula. They also go by the name anemonefish.

Where do these goofy clownfish live? And what do they feed on?

•  Well, they live in the Southwest Pacific Ocean, the Red Sea, the Indian Ocean, and the Great Barrier Reef in Australia. 

Did you know… There are currently at least 30 types of clownfish known to us. 

• Deep into the waters, clownfish live among parts of an interesting sea creature called the sea anemone. Anemones are known to be the ‘host’ to clownfish.
• Anemones and clownfish have a very interesting and beneficial relationship. Anemones give clownfish a home that protects them from predators by releasing poison and in return, the clownfish feed on the parasites that endanger the anemone’s life.
• There are more than 100 types of sea anemone, but only 10 are suitable hosts for clownfish. 

Did you know… Clownfish are immune to the poison that anemones release, thanks to the extra layer of mucus on their skin. 

• Clownfish are omnivorous, which means that they feed on both plants and meat. Their diet consists of plankton, molluscs, phytoplankton, small crustaceans, anemone parasites, and algae. 

How do these fascinating fish reproduce?

• Actually, all clownfish that are born are males. When two male fish decide to mate, the biggest one of them changes its sex to female. Once they change their sex, they can’t go back to being male. The male then courts the dominant female until she decides to make a move. When she lays the eggs, the male does most of the ‘egg sitting.’
• Clownfish respect their hierarchy. The female becomes the leader of the school and when she dies, the dominant male becomes a female to take her place. 

Let’s talk a bit about Finding Nemo. Nemo and Marlin (Nemo’s father) in fact belong to a species called false clownfish or false anemonefish (Amphiprion ocellaris). True anemonefish are what we have been talking about so far and they are called Amphiprion percula. Both false and true anemonefish are nearly identical, but they have a slight difference in shape and where they live. 

John Dory 

The John Dory is a famous fish that is targeted by commercial fisheries.

• The scientific name for the John Dory is Zeus faber. 
• It is also commonly known as St. Peter’s fish.
• They are found on the coasts of Africa, Australia, Japan, New Zealand, South East Asia, and Europe. 
• Its body is highly compressed and plate-like, with a large head and a large mouth.
• Its colour ranges from silver to olive-green, with a blue-black spot surrounded by a yellowish ring on each side of the body. 
• It can grow up to 65 centimetres in length and 3 kilograms in weight. 
• It can live up to 15 years. Females live longer than males. 
• It lives close to the seafloor and can be found to depths of 200 metres, over both soft and hard bottoms. 
• It is a solitary species but gets paired to reproduce. 
• It preys on other fish, crustaceans, molluscs, and cephalopods. 
• Large bony fish and sharks are their predators. 

Did you know… The blue-black spot on John Dory’s body helps confuse its predators.

Nassau Grouper

It may not be the cutest fish around in the big marine world, but it surely is unique and has distinctive features.

• The scientific name for Nassau grouper is Epinephelus striatus.
• It is found on the coral reefs around the Caribbean Sea, the Bahamas, Florida, and the Gulf of Mexico. 
• Nassau grouper can swim very deep, but it prefers staying close to shallow reefs and seagrass. Their juveniles stay closer to the coast near the seagrass that offers them protection. 
• It is a very solitary fish, though they are occasionally observed to form a school with other fish when it is the spawning season. These groups are known as ‘spawning aggregations.’
• Although scientists classify this fish as endangered, it can live up to 29 years. 
• It is reddish brown in colour with vertically striped bars across the head and body. Their tail has a black patch around it. This fish can change colours during mating. The body of the male becomes black on top and white below, while the female body becomes almost solid black. 
• Nassau grouper is one of the biggest fish around! It can weigh up to 29 kilograms.
• One unique feature of this fish is the way they feed on its prey. It swallows the prey whole, without chewing! The lips on this fish are thick and it has a very large mouth that enables it to inhale large fish and whole lobsters. It has slender teeth but they are only for preventing the prey from escaping.
• It can change sex! It can change from a female to a male fish and this happens when they turn from juveniles to adults. 

Did you know… Nassau grouper forms a mutualistic relationship with the wrasse fish where it allows the wrasse to clean its mouth from parasites, food particles, and dead tissues and in return, it provides the wrasse with food and protection.

Queen Angelfish

We have looked at clowns, and now we will introduce you to a queen! Queen angelfish is one of the most beautifully coloured fish in the ocean and it has a very good reason for why it is called a queen. Let’s discover the queen together!

• Its scientific name is Holacanthus ciliaris.
• It is found on the coral reefs of the Caribbean Sea, the Gulf of Mexico, and the northern coast of South America. 
• Its habitat can be shallow as well as deep ocean reef waters. It is found alone or as a pair at depths of 76 metres in the ocean. 
• It is yellow and blue in colour, with a black spot on its head that has blue spots and looks like a crown, hence the name queen angelfish. Its dorsal and anal fins are outlined in blue colour. Its pectoral fins are yellow with a large blue spot at the base. 
• The mouth of the queen angelfish is small and has bristle-like teeth. 
• It can grow up to 45 centimetres in length and 1.6 kilograms in weight. Its body is compressed and disc-like. 
• The queen angelfish is omnivorous and preys on plants and animals. Some of its prey include algae, sponges, soft corals, and invertebrates. The juveniles set up cleaning stations and feed on parasites and scales of other larger fish. 
• Large fish, sharks and barracudas are known to feed on the queen angelfish. 

Red Lionfish 

Although it may not be as big as a lion, the red lionfish can be pretty dangerous.

• The scientific name for this fish is Pterois volitans. 
• It lives on the coral reefs of the Indo-Pacific Ocean and the western Atlantic Ocean. 
• It gets its name from the long colourful fin rays that look like a lion’s mane.
• Like all species of the family Scorpaenidae, it has venomous spines to protect it from predators. 
• Its body is brown or maroon with white bands covering the head and the whole body. It has tentacles above the eyes and below the mouth. It has 13 dorsal spines, 14 fan-like pectoral fins, 11 dorsal soft rays, 3 anal spines, and 7 anal soft rays. It also has cycloid scales. 
• It can grow up to 38 centimetres in length and 1.4 kilograms in weight. 
• Red lionfish are known to be top predators of the coral reefs of the Atlantic Ocean. They prey on small fish, shrimps, crabs, invertebrates, and basically anything that can fit their mouth. They eat them in one go and don’t chew. They are invasive and prey on more than 50 species of economically and ecologically important fish. 
• Some of their predators include groupers, eels, snappers, and sharks. 

Sharks and Rays

This group of animals may be top predators in the oceans, but they are actually very important to keep our ecosystem balanced. Sharks and rays belong to the Chondrichthyes class, along with skates and chimaeras. There are over 1000 species within this class.

The species within this class share some unique features. Their skeleton is made of cartilage instead of bones; cartilage is the flexible substance that forms our ears and nose. Only their teeth, and sometimes their vertebrae, contain calcium. They have the ability to replace their teeth with new sets. They have a pair of very powerful jaws that helps the catch and chew on their prey. Their skin is covered with placoid scales that are spiny and toothlike projections. They also have paired fins, a two-chambered heart, and a pair of nostrils. Some of the species possess electric organs or poison that they use to sting and defend themselves or for predation. 

They have a complex digestive system, though not as complex as ours. It comprises a mouth, pharynx, stomach, intestines, and cloaca. The cloaca in females is part of the reproductive system. Adult males have claspers on their pelvic fins which they use to transfer sperms to the cloaca of the female. They are also poikilotherms, which means that they cannot regulate their internal Body temperature. Here are some examples of Chondrichthyes:

• Sharks: the blacktip shark and the bull shark.
• Rays: the blue-spotted ribbon-tail ray and the cownose ray.
• Skates: little skate and small deepwater skate.
• Chimaeras: the whitespot ghost shark and the whitefin chimaera. 

We will focus in this section on sharks and rays. 

Blacktip Shark

The blacktip shark may not be the deadliest species of sharks out there, but it is definitely dangerous. Let’s discover more about this creature. 

• The scientific name for this shark is Carcharhinus limbatus. 
• The blacktip shark is found worldwide. It is found in shallow coastal waters. 
• They can live for at least 12 years, sometimes even longer. 
• They have black-tipped fins and this is where they get their name from. They have a vigorous body that is grey to brown above and white below, with a pointed snout. The teeth of their upper jaw are broad with narrow cusps, while the teeth of the lower jaw are narrow. They have five pairs of gill snits. 
• Adult males can grow to 1.8 metres in length, while females grow to 1.82 metres. The largest recorded blacktip shark was a 2.47 m female. 
• A baby blacktip shark is referred to as a pup, and some people find blacktip pups very cute!
• Blacktip sharks are not picky eaters. They attack schools of small fish. They eat a wide variety of fish such as herring, sardines, anchovies, sea catfish, and some smaller sharks like sharp-nose and smoothhound sharks. They can also consume rays, skates, cephalopods, and crustaceans. 
• Blacktip sharks aren’t particularly aggressive towards humans, but being near them while they are swimming at full speed can be quite dangerous. 
• There are no natural predators for these sharks. However, their pups can be eaten by larger sharks of other species. 

Bull Shark

A simple look at the name will tell you that this may be one of the most aggressive shark species, and you are absolutely right. We will learn some interesting facts about the infamous bull shark. 

• The scientific name for this shark is Carcharhinus leucas.
• This shark can live in both saltwater and freshwater. It mostly lives in freshwater. 
• It is known to be the shark species that comes in contact with humans the most because it lives in coastal areas. That’s why it is most likely to get attacked by a bull shark than any other species. 
•  It has a big plump body with a long pectoral fin and blunt snout. The body has a grey coat on top and white below. 
• Similar to the blacktip shark, the female bull shark is bigger than the male. Adult males can grow to 2.1 metres in length, while adult females can grow to 3.3 metres. Adults can weigh between 91 to 227 kilograms. Pups can weigh around 3 kilograms. 
• Bull sharks prey on small sharks, bony fish, and sometimes other bull sharks, which makes them cannibalistic. They occasionally eat sea turtles, crustaceans, birds, and dolphins. 

Did you know… The bull shark has very special glands and kidneys that help it maintain salt in its body while it is roaming freshwater.

Blue-spotted Ribbon-tail Ray

We now reached one of the most unique-looking fish in the marine world. The blue-spotted ribbon-tail ray does not look anything like its savage cousins, sharks, but it has some deadly features.