Largest and heaviest animals

The largest animal currently alive is the blue whale. The maximum recorded weight was 190 tonnes for a specimen measuring, whereas longer ones, up to, have been recorded but not weighed. It is estimated that this individual could have a mass of 250 tonnes or more. The longest non-colonial animal is the lion's mane jellyfish.
In 2023, paleontologists estimated that the extinct whale Perucetus, discovered in Peru, may have outweighed the blue whale, with a mass of. However, more recent studies suggest this whale was much smaller than previous estimates, putting its weight at 60 to 113 tonnes. While controversial, estimates for the weight of the sauropod Bruhathkayosaurus suggest it was around 110–170 tons, with the highest estimate being 240 tons, if scaled with Patagotitan, although actual fossil remains no longer exist, and that estimation is based on described dimensions in 1987. In April 2024, Ichthyotitan severnensis was established as a valid shastasaurid taxon and is considered both the largest marine reptile ever discovered and the largest macropredator ever discovered. The Lilstock specimen was estimated to be around whilst the Aust specimen was an even more impressive in length. While no weight estimates have been made as of yet, Ichthyotitan would have easily rivalled or surpassed the blue whale. The upper estimates of weight for these prehistoric animals would have easily rivaled or exceeded the largest rorquals and sauropods.
The African bush elephant is the largest living land animal. A native of various open habitats in sub-Saharan Africa, males weigh about on average. The largest elephant ever recorded was shot in Angola in 1974. It was a male measuring from trunk to tail and lying on its side in a projected line from the highest point of the shoulder, to the base of the forefoot, indicating a standing shoulder height of. This male had a computed weight of 10.4 to 12.25 tonnes.

Heaviest living animals

The heaviest living animals are all whales. There are considerable obstacles to overcome in obtaining accurate weights for them, however, as no scale can accommodate the whole body of a large whale. Whaling factories are usually enlisted to help accomplish the task, but it still remains difficult, and most weighings have been done by parts of flensed whales, leaving much room for error. Mathematical formulas have also been devised to estimate the average weight of various whale species based on body length, but this method is equally prone to inaccuracy.

Rank	Animal	Average mass in tonnes	Maximum mass in tonnes	Average total length in m
1	Blue whale	110	190	24
2	North Pacific right whale	60	120	15.5
3	Southern right whale	58	110	15.25
4	Fin whale	57	120	21
5	Bowhead whale	54.5	120	15
6	North Atlantic right whale	54	110	15
7	Sperm whale	31.25	57	13.25
8	Humpback whale	29	48	13.5
9	Sei whale	22.5	45	14.8
10	Gray whale	19.5	45	13.5

Heaviest terrestrial animals

The heaviest land animals are all mammals. The African elephant is now listed as two species, the African bush elephant and the African forest elephant, as they are now generally considered to be.

Rank	Animal	Average mass in tonnes	Maximum mass in tonnes	Average total length in m
1	African bush elephant	6	10.4	7.5
2	Asian elephant	4.5	7	6.5
3	African forest elephant	2.7	6.0	6.2
4	White rhinoceros	2	4.5	4.4
5	Indian rhinoceros	1.9	4.0	4.2
6	Hippopotamus	1.8	4.5	5.05
7	Javan rhinoceros	1.75	2.3	3.8
8	Black rhinoceros	1.1	2.9	4
9	Giraffe	1.0	2	5.15
10	Gaur	0.95	1.5	3.8

Vertebrates

Mammals (Mammalia)

The blue whale is the largest animal, and therefore the largest mammal, of all time, with the longest known specimen being long and the heaviest weighted specimen being 190 tonnes. The extinct whale species Perucetus colossus was shorter than the blue whale, at but it is estimated to have rivaled or surpassed it in weight, at 85–340 tonnes. At the highest estimates, this would make Perucetus the heaviest known animal in history, although more recent estimates put it at a more moderate 60 to 113 tonnes.
The largest land mammal extant today is the African bush elephant. The largest extinct land mammal known was long considered to be Paraceratherium orgosensis, a rhinoceros relative thought to have stood up to tall, measured over long and may have weighed about 17 tonnes. In 2015, a study suggested that the extinct elephant Palaeoloxodon namadicus may have been the largest land mammal ever, based on a fragmentary femur estimated to belong to an individual with maximum weight of 22 tonnes. This author also suggested that the extinct mastodon "Mammut" borsoni may have rivalled P. namadicus in size.

Stem-mammals ([Synapsida])

The Late Triassic Lisowicia bojani, from what is now southern Poland, probably was the largest of all non-mammalian synapsids, at in length, in height and in weight. However, one study suggested a more conservative weight of 4.87 tonnes to 7.02 tonnes for the adult taxon, with an average body mass of 5.88 tonnes. The largest carnivorous synapsid was Anteosaurus at and.

Caseasaurs ([Caseasauria])

The herbivorous Alierasaurus was the largest caseid and the largest amniote to have lived at the time, with an estimated length around. Another huge caseasaur is Cotylorhynchus hancocki, with an estimated length and weight of at least and more than.

Sphenacodontids ([Sphenacodontidae])

The biggest carnivorous synapsid of Early Permian was Dimetrodon, which could reach and. The largest members of the genus Dimetrodon were also the world's first fully terrestrial apex predators.

Tappenosaurids (Tappenosauridae)

The Middle Permian Tappenosaurus was estimated at in length which is comparable in size with the largest dinocephalians.

Therapsids ([Therapsida])

The plant-eating dicynodont Lisowicia bojani is the largest-known of all non-mammalian synapsids, at and. The largest carnivorous therapsid was the aforementioned Anteosaurus from what is now South Africa during Middle Permian epoch. It reached long, and about in weight.

Reptiles (Reptilia)

The largest living reptile, a representative of the order Crocodilia, is the saltwater crocodile of Southern Asia and Australia, with adult males being typically long. The largest confirmed saltwater crocodile on record was long, and weighed about. Unconfirmed reports of much larger crocodiles exist, but examinations of incomplete remains have never suggested a length greater than. Also, a living specimen estimated at and has been accepted by the Guinness Book of World Records. However, due to the difficulty of trapping and measuring a very large living crocodile, the accuracy of these dimensions has yet to be verified. A specimen named Lolong caught alive in the Philippines in 2011 was found to have measured in length.
The Komodo dragon, also known as the "Komodo monitor", is a large species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, Gili Motang, Nusa kode and Padar. A member of the monitor lizard family, it is the largest living species of lizard, growing to a maximum length of more than in rare cases and weighing up to approximately.
Nevertheless, current extant reptiles are still dwarfed by their prehistoric ancestors. The largest non-dinosaurian terrestrial reptile to have ever lived were crocodilomorphs such as Deinosuchus, Sarcosuchus and Purussaurus, each have weight varying between 5–10 metric tons. The largest snake to have ever lived was Titanoboa which could grow up to and weigh up to. Prehistoric marine reptiles from the Mesozoic were even larger. Archelon was the largest testudines, being around long from head to tail and in weight. Mosasaurus hoffmanni was the largest squamate lizard to have ever lived, with a length of and a body mass of However, the largest reptiles were the shastasaurids, specifically Ichthyotitan, which approached lengths rivalling to exceeding those of a blue whale, at around in length.

Largest living reptiles

The following is a list of the largest living reptile species ranked by average weight, which is dominated by the crocodilians. Unlike mammals, [|birds], or fish, the mass of large reptiles is frequently poorly documented and many are subject to conjecture and estimation.

Rank	Animal	Average mass	Maximum mass	Average total length
1	Saltwater crocodile	450	2,000	4.5
2	Nile crocodile	410	1,090	4.2
3	Orinoco crocodile	380	1,100	4.1
4	Leatherback sea turtle	364	932	2.0
5	American crocodile	336	1,000	4.0
6	Black caiman	300	1,000	3.9
7	Gharial	250	1,000	4.5
8	American alligator	240	1,000	3.4
9	Mugger crocodile	225	700	3.3
10	False gharial	210	590	4.0
11	Aldabra giant tortoise	205	360	1.4
12	Loggerhead sea turtle	200	545	0.95
13	Green sea turtle	190	395	1.12
14	Slender-snouted crocodile	180	325	3.3
15	Galapagos tortoise	175	417	1.5

Dinosaurs (Dinosauria)

s are now extinct, except for birds, which are a type of theropod.

Sauropods ([Sauropoda])

The largest dinosaurs, and the largest animals to ever live on land, were the plant-eating, long-necked Sauropoda. The tallest and heaviest sauropod known from a complete skeleton is a specimen of an immature Giraffatitan discovered in Tanzania between 1907 and 1912, now mounted in the Museum für Naturkunde of Berlin. It is tall and weighed 23.3–39.5 tonnes. The longest is a long specimen of Diplodocus discovered in Wyoming, and mounted in Pittsburgh's Carnegie Natural History Museum in 1907. A Patagotitan specimen found in Argentina in 2014 is estimated to have been long and tall, with a weight of 69–77 tonnes.
There were larger sauropods, but they are known only from a few bones. The current record-holders include Argentinosaurus, which may have weighed 100 tonnes; Supersaurus which might have reached in length and Sauroposeidon which might have been tall. Some abnormal specimens such as specimen BYU 9024 of the Barosaurus/''Supersaurus genus could reach an astounding 45–50 meters long, with mass varying from the 'modest' 60–66 tons to the more immense 92–120 tons. Two other such sauropods include Bruhathkayosaurus and Maraapunisaurus. Both are known only from fragments that no longer exist. Bruhathkayosaurus might have been between in length and 175–220 tonnes in weight according to some estimates, with recent estimates placing it between 110 and 170 tons. Maraapunisaurus'' might have been approximately 35–40 m long and weighed 80–120 tonnes or more. Each of these two 'super-sauropods' would have easily rivalled the largest blue whale in size.

Table of the largest sauropods

Theropods ([Theropoda])

The largest extant theropod is the common ostrich.
The largest theropod known from a nearly complete skeleton is the most complete Tyrannosaurus rex specimen, nicknamed "Sue", which was discovered in South Dakota in 1990 and now mounted in the Field Museum of Chicago at a total length of. Body mass estimates have reached over 9,500 kg, though other figures, such as Hartman's 2013 estimate of 8,400 kg, have been lower.
Another giant theropod is Spinosaurus aegyptiacus from the mid-Cretaceous of North Africa. Size estimates have been fluctuating far more over the years, with length estimates ranging from 12.6 to 18 m and mass estimates from 7 to 20.9 t. Recent findings favor a length exceeding 15 m and a body mass of 7.5 tons.
Other contenders known from partial skeletons include Giganotosaurus carolinii and Carcharodontosaurus saharicus.

Armored dinosaurs ([Thyreophora])

The largest thyreophorans were Ankylosaurus and Stegosaurus, from the Late Cretaceous and Late Jurassic periods of what is now North America, both measuring up to in length and estimated to weigh up to 6 tonnes.

Ornithopods ([Ornithopoda])

The largest ornithopods were the hadrosaurids Shantungosaurus, a late Cretaceous dinosaur found in the Shandong Peninsula of China, and Magnapaulia from the late Cretaceous of North America. Both species are known from fragmentary remains but are estimated to have reached over in length and were likely the heaviest non-sauropod dinosaurs, estimated at over 23 tonnes.

Ceratopsians ([Ceratopsia])

The largest ceratopsians were Triceratops and its ancestor Eotriceratops from the late Cretaceous of North America. Both estimated to have reached about in length and weighed 12 tonnes.

Birds ([Aves])

The largest living bird, a member of the Struthioniformes, is the common ostrich, from the plains of Africa. A large male ostrich can reach a height of and weigh over. A mass of has been cited for the common ostrich but no wild ostriches of this weight have been verified. Eggs laid by the ostrich can weigh and are the largest eggs in the world today.
The largest bird in the fossil record may be the extinct elephant birds of Madagascar, which were related to the kiwis. Aepyornis exceeded in height and, while Vorombe could reach a similar height and a mass of. The last of the elephant birds became extinct about 300 years ago. Of almost exactly the same upper proportions as the largest elephant birds was Dromornis stirtoni of Australia, part of a 26,000-year-old group called mihirungs of the family Dromornithidae. The largest carnivorous bird was Brontornis, an extinct flightless bird from South America which reached a weight of and a height of about. The tallest carnivorous bird was Kelenken, which could reach 3 to 3.2 meters in height and 220 to 250 kilograms. The tallest bird ever was the giant moa, part of the moa family of New Zealand that went extinct around 1500 AD. This particular species of moa stood up to tall, but weighed about half as much as a large elephant bird or mihirung due to its comparatively slender frame.
The heaviest bird ever capable of flight was Argentavis magnificens, the largest member of the now extinct family Teratornithidae, found in Miocene-aged fossil beds of Argentina, with a wingspan up to, a length of up to, a height on the ground of up to and a body weight of at least. Pelagornis sandersi is thought to have had an even larger wingspan of about, but is only about, half the mass of the former.

Heaviest living bird species

The following is a list of the heaviest living bird species based on maximum reported or reliable mass, but average weight is also given for comparison. These species are almost all flightless, which allows for these particular birds to have denser bones and heavier bodies. Flightless birds comprise less than 2% of all living bird species.

Rank	Animal	Binomial name	Average mass	Maximum mass	Average total length	Flighted
1	Common ostrich	Struthio camelus	104	156.8	210	No
2	Somali ostrich	Struthio molybdophanes	90	130	200	No
3	Southern cassowary	Casuarius casuarius	45	85	155	No
4	Northern cassowary	Casuarius unappendiculatus	44	75	149	No
5	Emu	Dromaius novaehollandiae	33	70	153	No
6	Emperor penguin	Aptenodytes forsteri	31.5	46	114	No
7	Greater rhea	Rhea americana	23	40	134	No
8	Domestic turkey/wild turkey	Meleagris gallopavo	13.5	39	100–124.9	Yes
9	Dwarf cassowary	Casuarius bennetti	19.7	34	105	No
10	Lesser rhea	Rhea pennata	19.6	28.6	96	No
11	Mute swan	Cygnus olor	11.87	23	100–130	Yes
12	Great bustard	Otis tarda	10.6	21	115	Yes
13	King penguin	Aptenodytes patagonicus	13.6	20	92	No
14	Kori bustard	Ardeotis kori	11.4	20	150	Yes
15	Trumpeter swan	Cygnus buccinator	11.6	17.2	138–165	Yes
16	Wandering albatross	Diomedea exulans	11.9	16.1	107–135	Yes
17	Whooper swan	Cygnus cygnus	11.4	15.5	140–165	Yes
18	Dalmatian pelican	Pelecanus crispus	11.5	15	183	Yes
19	Andean condor	Vultur gryphus	11.3	14.9	100–130	Yes

Amphibians ([Amphibia])

The largest living amphibian is the South China giant salamander. Formerly considered conspecific with the Chinese giant salamander, the maximum size of this nearly human-sized river-dweller is and almost. Before amniotes became the dominant tetrapods, several giant amphibian proto-tetrapods existed and were certainly the dominant animals in their ecosystems. The largest known was the crocodile-like Prionosuchus, which reached a length of.

Frogs (Anura">Frog">Anura)

The largest member of the largest order of amphibians is the African goliath frog. The maximum size this species is verified to attain is a weight of and a snout-to-vent length of. The largest of the toads is the cane toad. This infamous, often invasive species can grow to maximum mass of and measure a maximum of from snout-to-vent. Rivaling the previous two species, the African bullfrog can range up to a weight of and from snout to vent. Another large frog is the largest frog in North America, the American bullfrog, which can reach weights of up to and snout-to-vent-length of. However, the toad Beelzebufo ampinga, found in fossil from the Cretaceous era in what is now Madagascar, was estimated to grow to long and weigh up to, making it the largest frog ever known. But in more recent studies, animals of this species have been estimated to have grown to at least , which is around the size a modern African bullfrog can reach. The largest tree frog is the Australasian white-lipped tree frog, the females of which can reach a length of from snout to vent and can weigh up to. The family Leptodactylidae, one of the most diverse anuran families, also has some very large members. The largest is the Surinam horned frog, which can reach in length from snout to vent and weigh up to. While not quite as large as Ceratophrys cornuta, Leptodactylus pentadactylus is often heavier; it can reach long and weigh. The largest dendrobatid is the Colombian golden poison frog, which can attain a length of and nearly. Most frogs are classified under the suborder Neobatrachia, although nearly 200 species are part of the suborder Mesobatrachia, or ancient frogs. The largest of these are the little-known Brachytarsophrys or Karin Hills frogs, of South Asia, which can grow to a maximum snout-to-vent length of and a maximum weight of.

Rank	Frog species	Maximum mass	Maximum snout-vent length	Family
1	Goliath frog			Conrauidae
2	Helmeted water toad			Calyptocephalella
3	Lake junin giant frog			Telmatobiidae
4	Blyth's river frog			Dicroglossidae
5	Cane toad			Bufonidae
6	African bullfrog			Pyxicephalidae
7	Mountain chicken frog	1 kg		Leptodactylidae
8	American bullfrog			Ranidae
9	Surinam horned frog			Ceratophryidae
10	Smokey jungle frog			Leptodacylidae

Caecilians ([Gymnophiona])

The largest of the worm-like caecilians is the Colombian Thompson's caecilian, which reaches a length of, a width of about and can weigh up to about.

Salamanders ([Urodela])

Besides the previously mentioned Chinese and South China giant salamanders, the closely related Japanese giant salamander is also sometimes cited as the largest living amphibian, but salamanders of a greater size than and have never been verified for this species. Another giant of the amphibian world is the North American hellbender, which can measure up to. The recently described reticulated siren of the southeastern United States rivals the hellbender in size, although it is more lean in build. The largest of the newts is the Iberian ribbed newt, which can grow up to in length.

Fish

The largest fish of all time was likely Megalodon, estimated to have reached sizes of. The largest living fish, as well as the largest cartilaginous fish and largest living non-cetacean animal, is the whale shark. The largest individual had a length of. The largest extant bony fish is the giant sunfish, with the largest recorded individual having a weight of 2,744 kg

Invertebrate chordates

Tunicates (Tunicata)

The largest tunicate is Synoicum pulmonaria, found at depths of, and are up to in diameter. It is also present in the northwestern Atlantic Ocean, around the coasts of Greenland and Newfoundland, but is less common here than in the east, and occurs only at depths between.
;Entergonas
;Pleurogonas
;Aspiraculates

[Thaliacea]

The largest thaliacean, Pyrosoma atlanticum, is cylindrical and can grow up to long and 4–6 cm wide. The constituent zooids form a rigid tube, which may be pale pink, yellowish, or bluish. One end of the tube is narrower and is closed, while the other is open and has a strong diaphragm. The outer surface or test is gelatinised and dimpled with backward-pointing, blunt processes. The individual zooids are up to long and have a broad, rounded branchial sac with gill slits. Along the side of the branchial sac runs the endostyle, which produces mucus filters. Water is moved through the gill slits into the centre of the cylinder by cilia pulsating rhythmically. Plankton and other food particles are caught in mucus filters in the processes as the colony is propelled through the water. P. atlanticum is bioluminescent and can generate a brilliant blue-green light when stimulated.

Doliolida ([Doliolida])

The largest doliolida is Doliolida The doliolid body is small, typically 1–2 cm long, and barrel-shaped; it features two wide siphons, one at the front and the other at the back end, and eight or nine circular muscle strands reminiscent of barrel bands. Like all tunicates, they are filter feeders. They are free-floating; the same forced flow of water through their bodies with which they gather plankton is used for propulsionnot unlike a tiny ramjet engine. Doliolids are capable of quick movement. They have a complicated lifecycle consisting of sexual and asexual generations. They are nearly exclusively tropical animals, although a few species are found as far north as northern California.

Salps ([Salpida])

The largest salp is Cyclosalpa bakeri s) long. There are openings at the anterior and posterior ends of the cylinder which can be opened or closed as needed. The bodies have seven transverse bands of muscle interspersed by white, translucent patches. A stolon grows from near the endostyle. The stolon is a ribbon-like organ on which a batch of aggregate forms of the animal are produced by budding. The aggregate is the second, colonial form of the salp and is also gelatinous, transparent and flabby. It takes the shape of a radial whorl of individuals up to about in diameter. It is formed of approximately 12 zooids linked side by side in a shape that resembles a crown. are largest thetyses: Thetys vagina Individuals can reach up to long.

Larvaceans ([Larvacea])

The largest larvacean is Appendicularia in body length.

Cephalochordates ([Leptocardii])

The largest lancelet is the European lancelet "primitive fish". It can grow up to long.

Invertebrate non-chordates

Echinoderms (Echinodermata)

The largest species of echinoderm in terms of bulk is probably the starfish species Thromidia gigas, of the class Asteroidea, which reaches a weight of over, but it might be beaten by some giant sea cucumbers such as Thelenota anax. However, at a maximum span of, Thromidia gigas is quite a bit shorter than some other echinoderms. The longest echinoderm known is the conspicuous sea cucumber Synapta maculata, with a slender body that can extend up to. In comparison, the biggest sea star is the brisingid sea star Midgardia xandaros, reaching a span of, despite being quite slender. Evasterias echinosoma is another giant echinoderm and can measure up to across and weigh.

Crinoids ([Crinoidea])

Sea urchins and allies ([Echinoidea])

Sea cucumbers ([Holothuroidea])

Brittle stars ([Ophiuroidea])

Sea stars ([Asteroidea])

Flatworms (Platyhelminthes)

;Monogenean flatworms
;Flukes
;Tapeworms

Arrow Worms (Chaetognatha)

The largest arrow worms is Parasagitta setosa rows to a maximum length of, its tail being up to a quarter of this length.

Segmented worms (Annelida)

The largest of the segmented worms is the African giant earthworm. Although it averages about in length, this huge worm can reach a length of as much as and can weigh over. Only the giant Gippsland earthworm, Megascolides australis, and a few giant polychaetes, including the notorious Eunice aphroditois, reach nearly comparable sizes, reaching, respectively.

Ribbon worms (Nemertea)

The largest nemertean is the bootlace worm, Lineus longissimus. A specimen found washed ashore on a beach in St. Andrews, Scotland in 1864 was recorded at a length of.

Mollusks (Mollusca)

Both the largest mollusks and the largest of all invertebrates are the largest squids. The colossal squid is projected to be the largest invertebrate. Current estimates put its maximum size at long and, based on analysis of smaller specimens. In 2007, authorities in New Zealand announced the capture of the largest known colossal squid specimen. It was initially thought to be and. It was later measured at long and in weight. The mantle was long when measured.
The giant squid was previously thought to be the largest squid, and while it is less massive and has a smaller mantle than the colossal squid, it may exceed the colossal squid in overall length including tentacles. One giant squid specimen that washed ashore in 1878 in Newfoundland reportedly measured in total length, head and body length, in circumference at the thickest part of mantle, and weighed about. This specimen is still often cited as the largest invertebrate that has ever been examined. However, no animals approaching this size have been scientifically documented and, according to giant squid expert Steve O'Shea, such lengths were likely achieved by greatly stretching the two tentacles like elastic bands.

Aplacophorans (Aplacophora)

Chitons (Polyplacophora)

Bivalves (Bivalvia)

Gastropods ([Gastropoda])

Cephalopods ([Cephalopoda])

While generally much smaller than the giant Architeuthis and Mesonychoteuthis, the largest of the octopuses, the giant Pacific octopus, can grow to be very large. The largest confirmed weight of a giant octopus is, with a arm span and a head-to-tentacle-tip length of. Specimens have been reported up to but are unverified. A weight of 10–50kg is a much more common size.

Tusk Shell ([Scaphopoda])

The largest tusk shell Fissidentalium metivieri class range in length

Roundworms (Nematoda)

The largest roundworm, Placentonema gigantissima, is a parasite found in the placentas of sperm whales which can reach up to in length.

Velvet worms (Onychophora)

The largest velvet worm known is Solórzano's velvet worm. An adult female was recorded to have a body length of 22 cm.

Water bears (Tardigrada)

The largest tardigrade is Echiniscoides sigismundi, which is less than long. Typical tardigrades are about long. Only the larger tardigrades are visible to the naked eye, but since they're also transparent, magnification is needed to see them.

Arthropods (Arthropoda)

The largest arthropod known to have existed is the eurypterid Jaekelopterus, reaching up to in body length, followed by the millipede relative Arthropleura at around in length.
Among living arthropods, the Japanese spider crab is the largest in overall size, the record specimen, caught in 1921, had an extended arm span of and weighed about. The heaviest is the American lobster, the largest verified specimen, caught in 1977 off of Nova Scotia weighed and its body length was. The largest land arthropod and the largest land invertebrate is the coconut crab, up to long and weighing up to on average. Its legs may span.

Arachnids ([Arachnida])

Spiders ([Araneae])

Scorpions ([Scorpiones])

Pseudoscorpions ([Pseudoscorpiones])

Thecostracans (Thecostraca)

The largest known thecostracan Facetotecta y-larvae are Hansenocaris cristalabri, about 0.5 millimeters long.

Barnacles and allies ([Cirripedia])

The largest barnacle is Balanus nubilus, reaching a diameter of and a height of up to, and containing the largest known muscle fibres.

Crustaceans ([Crustacea])

The largest crustacean is the Tasmanian giant crab, with a weight of and a carapace width of up to. It is the only species in the genus Pseudocarcinus. Males reach more than twice the size of females. At a length of up to, Lysiosquillina maculata is the largest mantis shrimp in the world. Tasmanian giant freshwater crayfish in weight and over long have been known in the past, but now, even individuals over are rare. The species is only found in Tasmanian rivers flowing north into the Bass Strait below above sea level, and is listed as an endangered species on the IUCN Red List.

Branchiopods ([Branchiopoda])

and allies ([Maxillopoda])

Ostracods ([Ostracoda])

Amphipods, isopods, and allies ([Peracarida])

Remipedes ([Remipedia])

Horseshoe crabs (Xiphosura)

The four modern horseshoe crabs are of roughly the same sizes, with females measuring up to in length and in weight.

Sea spiders (Pycnogonida)

The largest of the sea spiders is the deep-sea species Colossendeis colossea, attaining a leg span of nearly.

Trilobites (Trilobita)

Some of these extinct marine arthropods exceeded in length. A nearly complete specimen of Isotelus rex from Manitoba attained a length over, and an Ogyginus forteyi from Portugal was almost as long. Fragments of trilobites suggest even larger record sizes. An isolated pygidium of Hungioides bohemicus implies that the full animal was long.

Myriapods ([Myriapoda])

Centipedes ([Chilopoda])

Millipedes ([Diplopoda])

Symphylans ([Symphyla])

Insects ([Insecta])

s, a class of Arthropoda, are easily the most numerous class of organisms, with over one million identified species, and probably many undescribed species. The heaviest insect is almost certainly a species of beetle, which incidentally is the most species-rich order of organisms. Although heavyweight giant wētās are known, the elephant beetles of Central and South America, and, the Titan beetle of the neotropical rainforest or the Goliath beetles, and, of Africa's rainforest are thought to reach a higher weight. The most frequently crowned are the Goliath beetles, the top known size of which is at least and. The elephant beetles and titan beetle can reach greater lengths than the Goliath, at up to, respectively, but this is in part thanks to their rather large horns. The Goliath beetle's wingspan can range up to.
Some moths and butterflies have much larger areas than the heaviest beetles, but weigh a fraction as much.
The longest insects are the stick insects, see below.
Representatives of the extinct dragonfly-like order Meganisoptera, such as the Carboniferous Meganeura monyi of what is now France and the Permian Meganeuropsis permiana of what is now North America, are the largest insect species known to have existed. These creatures had a wingspan of some and a mass of over, making them about the size of a crow.

Cockroaches and termites ([Blattodea])

Beetles ([Coleoptera])

Earwigs ([Dermaptera])

True flies ([Diptera])

Mayflies ([Ephemeroptera])

True bugs ([Hemiptera])

Ants and allies ([Hymenoptera])

Moths and allies ([Lepidoptera])

; Mantises
; Scorpionflies
; Alderflies and allies
; Net-winged insects
; Dragonflies and damselflies
; Grasshoppers and allies
;
;Stoneflies
;Caddisflies
;Lice
;Booklice
;Fleas
;Thrips
;Angel insects
;Silverfish and allies

Cnidarians (Cnidaria)

The lion's mane jellyfish is the largest cnidarian species, of the class Scyphozoa. The largest known specimen of this giant, found washed up on the shore of Massachusetts Bay in 1870, had a bell diameter of, a weight of. The tentacles of this specimen were as long as and were projected to have a tentacular spread of about making it one of the longest extant animals.
;Corals and sea anemones
The largest individual species are the sea-anemones of the genus Discoma, which can attain a mouth disc diameter of. Longer, but much less massive overall, are the anemones of the genus Ceriantharia, at up to tall.
Communities of coral can be truly massive, a single colony of the genus Porites can be over, even though the individual polyps are quite small. In 2024, the largest ever was found off Malaulalo island in the Solomon Islands, a clonal colony of Pavona clavus over 32 by 34 meters — the size of two basketball courts.
;Hydrozoans
The colonial siphonophore Praya dubia can attain lengths of. The Portuguese man o' war's tentacles can attain a length of up to. On 6 April 2020 the Schmidt Ocean Institute announced the discovery of a giant Apolemia siphonophore in submarine canyons near Ningaloo Coast, measuring diameter with a ring approximately long, claiming it was possibly the largest siphonophore ever recorded. Stygiomedusa, commonly known as the giant phantom jelly, is the only species in the monotypic genus of deep sea jellyfish, Stygiomedusa. It is in the Ulmaridae family. With only around 110 sightings in 110 years, it is rarely seen, but is believed to be widespread throughout the world, with the exception of the Arctic Ocean.
;Box jellyfishes
The largest box jellyfish Chironex fleckeri is the largest of the cubozoans, many of which may carry toxic venom. Its bell usually reaches about in diameter but can grow up to. Each of the four corners of the bell trails a cluster of 15 tentacles. The pale blue bell has faint markings; viewed from certain angles, it bears a somewhat eerie resemblance to a human head or skull. Since it is virtually transparent, the creature is nearly impossible to see in its habitat, posing significant danger to swimmers.

Sponges (Porifera)

The largest known species of sea sponge is the giant barrel sponge, Xestospongia muta. These massively built sponges can reach in height and can be about the same thickness at the thickest part of the "body". Some of these creatures have been estimated to be over 2,400 years of age.

Calcareous sponges ([Calcarea])

The largest known of these small, inconspicuous sponges is probably the species Pericharax heteroraphis, attaining a height of. Most calcareous sponges do not exceed tall.

Hexactinellid sponges ([Hexactinellida])

A relatively common species, Rhabdocalyptus dawsoni, can reach a height of once they are of a very old age. This is the maximum size recorded for a hexactinellid sponge.