What is the Biggest fish ever?

In this post, we will meet the Biggest fish ever. We will also learn about the biology of the biggest fish that ever lived on earth and understand a little of its ecology. 

Biggest fish ever

Leedsichthys problematicus was a giant pachycormid, an extinct group of bony fish that lived in the oceans of the Jurassic period 165 million years ago. Although fragments of fossils have been found, there is no record of a fossil fish with a complete spinal column, so its exact size is uncertain. Current estimates show that this fish would be 21 metres long and weigh about 50 tons, thus becoming the largest fish that ever lived.


The Leedsichthys is an actinopterygian bony fish that lived in the late Jurassic period, from 165 to 152 million years ago – Callovian age of Middle Jurassic to Titonian age of Upper Jurassic – in seas from Europe to South America. Recent estimates put it at around 16 metres in length and 45 tons for the largest individuals, making it the largest bony fish are known to exist. A member of the pachycormid family (Pachycormidae), the Leedsichthys is part of a lineage of large filter feeder fish from the Mesozoic, being the oldest filter sea animal to reach this size.

However, several parts of its skeleton including the front of the skull and the vertebrae were not ossified but made of cartilage, a material that decomposes more easily, so the skeleton of Leedsichthys is not entirely known. Its head was probably large and elongated and had bumps on top. 

The jaws had no teeth 

The Leedsichthys fed mainly on zooplankton, filtering the water that passed from the mouth to the gills and retaining small living beings in the 40,000 tracks (a kind of bristle-shaped teeth) attached to its gill arches (structures bones that support the gills). Occasionally, it may also have sucked up patches of marine sediment in search of hidden invertebrates.


It had two large and long pectoral fins, in addition to a dorsal fin and a small anal fin. The tail fin, quite large, was symmetrical and had a rigid and vertical structure. Compared with data from modern teleost fish, scientists found that it could swim, while still maintaining oxygenation of the body’s tissues, at a speed of up to 17.8 km/h. Predators large enough to attack it were Pliosaurus and Liopleurodon.


Leedsichthys fed by filtration, meaning they ate tons of small aquatic beings, called plankton, sucking them up and releasing the water they did not need. This method requires a slow metabolism rate, as the filter feeders are not continually active in search of prey. This also allowed the Leedsichthys to grow as much as today’s large filter feeders.

First discoveries

The first fossil records of Leedsichthys were made in the 19th century (the first dates from 1857, in France), especially important those made by the British Alfred Nicholson Leeds, to whom the animal’s name refers. 

The species was named in 1889, by Arthur Smith Woodward, as Leedsichthys problematicus(“problematic” in Latin), because of the difficulty in identifying and reconstructing the animal – on several occasions’ bones were incorrectly interpreted as stegosaurus spines. Fossils have been found in England, France, Germany, and Chile. Although more than 70 individuals have already been identified, their remains are partial and fragmentary. Much of it consists of bony fin rays (lepidotrichia) and gill arches.

Pachycormiform Order

This order was found in Mesozoic deposits in Eurasia, the Americas, and Antarctica. It is an extinct order of ray-finned marine fish. They have been characterized by having serrated pectoral fins. Although, more recent studies have shown that fin shape diversity in this group was high. Pachycormiformes are joined by: 

–       A compound bone (rostrodermethmoid) forming the anterodorsal edge of the mouth;

–       A reduced coronoid process of the mouth; 

–       Mandible; 

–       Absence of supraorbital associated with a dermosphenophotic defining the dorsal margin of the orbit;

–       Two large suborbital bones similar to plates posterior to the infraorbital;

–       Long and slender pectoral fins; 

–       Asymmetrical branching of the pectoral fin lepidotrichia; 

–       Considerable overlap of the hypural by caudal-fin rays (hyprostegia);

–       Presence of uroneural ossifications distinct from the endoskeleton of the tail fin.

Scientific classification

Kingdom: Animalia

Phylum: Chordata

Superclass: Osteichthyes

Class: Actinopterygii

Order: † Pachycormiformes

Family: † Pachycormidae

Genus: † Leedsichthys

Species: † Leedsichthys problematicus

Pre-Cambrian era

The earliest forms of life developed during the earliest period in Earth history known as the Pre-Cambrian Era, or Pre-Cambrian Period. It began with the formation of the Earth crust about 4 billion years ago and ended approximately 542 million years ago. Scientists do not know exactly when the first life forms appeared, but the oldest fossil ever found is more than 3 billion years old.

The first life forms had only one cell. Among them were the first types of bacteria, algae and protozoa. Living beings with more than one cell began to appear at the end of the Pre-Cambrian. All life forms at that time lived in the oceans of our planet.

Paleozoic era

The next period in Earth history is called the Paleozoic Era or Paleozoic Period. It lasted from approximately 542 million years ago to 251 million years ago. At the beginning of this period, all living things were in the oceans. At its end, there were already plants and animals living on dry land.

Ocean life

At the beginning of the Paleozoic Era, animals were similar to the jellyfish, sponges and worms that live in the oceans today. There were other marine animals, which were arthropods—ancestors of crabs, insects, and spiders. Trilobite is an example of these arthropods. She was an oval marine animal that was 6 to 18 centimetres long. At the end of the Paleozoic Era, Trilobites became extinct.

The first fish appeared about 450 million years ago. They did not look much like fish today. They had hard carapaces and no jaws. It was over time that fish developed jaws, backbones and fins.

The biggest fish today

The biggest extant fish in the world is the whale shark. Do not be fooled by the term whale in this animal name, it is not a mammal! 

This fish is in the group of cartilaginous fish, lives in a marine environment, in tropical and warm temperate waters, and its scientific name is Rhincodon typus.

The whale shark also called the spotted and star dogfish, has a robust body full of spots and stripes, in addition to a characteristic flat, broadhead. This animal can reach up to 20 metres in length and weigh more than 12 tons. The largest whale shark on record weighed an incredible 34 tons.

Despite being a gigantic animal, the whale shark is not a great predator of the oceans. It feeds on small organisms such as small crustaceans that form what is called plankton. To feed, it uses a strategy quite peculiar to such a large animal: suction and filtration.

These animals are ovoviviparous, that is, the development of the embryo occurs inside the mother’s body, but they are nourished through the yolk sac. This type of reproduction is also known as lecithotrophic viviparity.


In this post, we met the Biggest fish ever. We also learnt about the biology of the biggest fish that ever lived on earth and understood a little of its ecology.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): Biggest fish ever

What was the biggest shark in history?

Megalodon lived up to 3 million years ago and its length was estimated to be between 10 and 18 meters long, that is, something like two orcas or a humpback whale. Still smaller than the bony Leedsichthys problematicus that lived in the seas of Europe.

How big is Dunkleosteus?

Dunkleosteus terreli, the name of the powerful predator, was a placoderm, a class of fish that had armoured and articulated plates covering its body. It was up to 11 metres long and could weigh up to 4 tons.

What was the bite force of a Dunkleosteus?

Dunkleosteus terrelli lived more than 360 million years ago. The teeth of the ancient sea monster concentrated the bite force in a small area – the tip of the canine – at a force that would reach 5,625 kilograms per square centimetre.

While dinosaurs ruled the land, who ruled the oceans?

Fish have always been very present in the biological history of our planet. In prehistoric times, gigantic fish with absurd jaw strength dominated the seas and put fear in any other marine predator

Why was Leedsichthys problematicus considered the biggest fish that ever lived?

Current estimates show that this fish would be 21 metres long and weigh about 50 tons, making it the largest fish that ever lived.

When did Leedsichthys live?

It lived in the oceans of the Jurassic period, 165 million years ago.


Friedman, M., Shimada, K., Martin, L. D., Everhart, M. J., Liston, J., Maltese, A., & Triebold, M. (2010). 100-million-year dynasty of giant planktivorous bony fishes in the Mesozoic seas. Science, 327(5968), 990-993.

Martill, D. M. (1988). Leedsichthys problematicus, a giant filter-feeding teleost from the Jurassic of England and France. Neues Jahrbuch für Geologie und Paläontologie-Monatshefte, 670-680.

Liston, J., Newbrey, M., Challands, T., & Adams, C. (2013). Growth, age and size of the Jurassic pachycormid Leedsichthys problematicus (Osteichthyes: Actinopterygii).