Size isn’t everything, and being a small fish in a big pond has its advantages, even when that pond covers 71% of the earth’s surface.
The ocean may be home to the world’s largest animal, the blue whale, but it also provides a home to some of the tiniest species.
But what is the smallest animal in the ocean?
What is the Smallest Animal in the Ocean?
Zooplankton could well be the smallest living organisms in the ocean – at least, that’s what many experts seem to think.
Zooplankton encompasses a variety of species, from single-celled protozoa to minuscule jellyfish, making it a viable contender.
On closer investigation, we managed to track down the tiniest critter living under the umbrella term, zooplankton. The Myxozoa jellyfish is so small it isn’t even measured in millimeters!
Scientists use micrometers to establish the size of this microscopic creature and estimate it to be between 10–300 μm in length. That’s the equivalent of around 0.01 to 0.3 mm.
Having evolved backward, these organisms have gone from being free-swimming, complex creatures to simple parasites.
Not only that, but they’ve also moved from land to sea and back to land again – something few other species have ever achieved.
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13 Small Ocean Animals
To keep you on your toes, we’ll present our 13 small sea animals in reserve order, ending with the minuscule myxozoa jellyfish.
#13 Dwarf Lantern Shark
The Dwarf Lantern shark is the world’s smallest shark, measuring less than 8 inches long. It lives in the cold, deep waters of the Caribbean Sea, rarely ascending above a depth of 1,000 feet.
This tiny shark species was only discovered in 1964 and has since been seen only a handful of times. It subsequently remains something of an enigma. There’s not even enough data available to adequately assess their conservation status.
As its name suggests, the Dwarf Lantern shark can light up due to the photophores located on its belly and fins.
This bioluminescence is thought to camouflage them against the surface, where sunlight streams down from above.
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#12 Star Sucker Pygmy Octopus
Considerably smaller than the Dwarf Lantern Shark, this tiny cephalopod species rarely exceeds one inch in length and weighs just a single gram.
When they hatch, they’re no bigger than a flea and spend the first few months of their lives floating near the ocean’s surface, feeding on zooplankton.
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Preferring the shallow coral reefs of the western Pacific, these little critters, also known as Octopus Wolfi, are solitary creatures with a lifespan of just six months.
The females can lay up to three consecutive clutches of eggs, while most other octopus species only produce one.
An expert hunter and master of disguise, the smallest species of frogfish measures just under an inch long.
Randall’s frogfish is one of the smallest frogfishes and, while difficult to spot, is easy to identify due to the distinct white spots on its body.
Juvenile frogfish are even smaller, sometimes measuring just 3mm long. While they may not be the smallest fish in the ocean, frogfish are both tiny and undeniably cute.
Rather than swimming, they spend most of their time stationary, occasionally using their fins to break into a “slow gallop.”
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Of the 3,000 or so species of nudibranch or sea slug, the smallest is probably the hammerhead doto, or Doto amyra, which measures around 10mm long.
All nudibranchs are carnivorous, and the hammerhead doto is no exception.
The hammerhead doto feeds on a type of marine life known as hydroids and lives along the west coast of America, from Alaska to Baja California in Mexico.
#9 Pygmy Seahorse
There are six different species of pygmy seahorse, of which the smallest measures just 13.8mm from the end of its snout to the tip of its tail.
Pygmy seahorses live on tropical coral reefs and have a complex symbiotic relationship with soft coral species like the Venus Sea Fan.
The brine shrimp that form the main component of the pygmy seahorse’s diet, feed on dinoflagellate algae. In turn, this species of algae uses photosynthesis to create the carbon compounds that nourish the Venus Sea Fan.
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Although the largest species of anglerfish reaches lengths of over three feet, the smallest is just 6.2mm long, making it the smallest fish in the ocean.
The Photocorynus spiniceps occupies tropical and sub-tropical waters in the Atlantic and Pacific Oceans. The female of the species reaches around 42mm long and provides a host for the smaller, parasitic male.
This tiny anglerfish species was only discovered in 2006. Before that, scientists believed the 7.9mm Paedocypris progenetica, or Sumatra larvkarbik, was the smallest fish in the world.
#7 Mysis Shrimp
Mysida or Mysis shrimp are small ocean creatures that measure between 5 and 25mm in length and inhabit both shallow and deep marine environments.
Most species live at the bottom of the sea and live a nocturnal existence that involves burrowing into the seabed by day and hunting for plankton by night.
A few Mysida species are pelagic and spend their time swimming in mid-water ocean environments, while some others while away their days in giant kelp forests.
High in both protein and fat, Mysis shrimp provide an essential food source for many tiny sea creatures, including juvenile pygmy seahorses.
#6 Irukandji jellyfish
This diminutive species of jellyfish has the dubious distinction of the world’s most dangerous sea creature. It may measure just 10 to 20mm in length, but it contains enough venom to kill as many as 60 people in a matter of minutes.
While most jellyfish species drift along on the sea’s currents, the Irukandji, along with other box jellyfish, can propel themselves through the water at speeds of up to four knots, or 7.4kph.
#5 Red Tide Dinoflagellate
This microscopic photosynthetic organism is a type of marine bacteria also known as Karenia Brevis.
Measuring between 20 and 40mm, it’s one of the ocean’s biggest killers. Creating toxic red tides, dinoflagellate has been known to kill as much as 1,700 tons of marine life in a few weeks.
Karenia Brevis is an adaptable marine organism that gets its nutrients from sunlight and through the consumption of other single-celled forms of marine life, including microscopic mircomonas species.
Capable of traveling great distances, this species of dinoflagellate has been found along the coast of North Carolina and as far south as Mexico.
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#4 Pea Crab
In its larval form, the tiny pea crab scrambles inside an oyster shell where it grows to maturity, living off food stolen from the oyster.
This behavior is consistent amongst all kleptoparasites, and while it won’t kill the host, it can be detrimental to its overall health, even if it doesn’t affect its flavor!
Female pea crabs measure between 8 and 12mm, while the male pea crab rarely grows beyond 7mm long. Benefiting from the protection of the oyster shell, they remain soft-shelled throughout their lives.
Apparently sweet, salty and chefs and diners across North America enjoy just a little crunchy, pea crabs.
The term zooplankton covers a variety of small ocean animals, ranging from microscopic protozoans to the 3000kg Sunfish or Mola mola. Scientists classify zooplankton as any marine organism incapable of swimming against the current.
The smallest plankton, known as microzooplankton, measure less than 2mm long and feed on the tiniest plant or phytoplankton cells in the ocean.
As small as they are, zooplankton play a big part in all marine ecosystems, providing nutrients for all sorts of sea creatures, from tiny corals to enormous fish species like the whale shark.
A type of tiny crustacean, copepods measure just 0.11mm long. Making up a significant proportion of the ocean’s zooplankton population, scientists consider copepods to be amongst “the most abundant metazoans on earth.”
Copepods aren’t restricted to marine environments, with many species living in freshwater systems.
Despite their numbers, recent research indicates a worrying decline in ocean-dwelling copepod populations, which scientists believe is related to “higher temperatures and greater predator abundance.”
#1 Myxozoa Jellyfish
There are an estimated 30,000 different species of Myxozoa jellyfish, none of which exceed 0.3mm in length.
Capable of living in both fresh and marine environments, Myxozoans are microscopic parasites that are, for the most part, so simplistic they lack the genes necessary for cell-to-cell communication, coordination, and even aerobic respiration.
Myxozoa jellyfish have devolved rather than evolved and, in doing so, have abandoned their digestive systems, mouths, and “ability to survive outside a host.”
They still hold onto one complex structure, known as a nematocyst, which resembles the stinging cells of its larger jellyfish cousins.
While completely dependent on their hosts for their survival, Myxozoa jellyfish are nevertheless capable of altering those hosts for their own benefit.
As an evolutionary biologist, Paulyn Cartwright observes, the Myxozoa jellyfish is “so weird” that it “absolutely redefines what we think of as animal.”
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Discovering this much about the smallest ocean creatures should have piqued your interest and left you wanting more.
At the Dutch Shark Society, we’re always happy to oblige our readers, so we’ve come up with some answers to the questions we suspect are hovering on the tip of your tongues.
What’s the Smallest Fish in the Ocean?
Photocorynus spiniceps, a type of anglerfish, is our smallest fish.
The males, which are parasitic and attach to the much larger females, only reach a length of 6.2 millimeters long, which is less than a quarter of an inch!
What is the Smallest Animal in the Ocean?
The myxozoa jellyfish could be the smallest animal in the ocean, but it’s a tough call to make. All types of microzooplankton, or picoplankton, have a chance of winning the title of the smallest ocean animal.
Regardless of which tiny sea creatures are the smallest of them all, they all play a vital role in the ecosystem.
Being infinitesimally small, they can obtain nutrients that few others can access and convert them into food that other, larger species need to survive. Without these small ocean animals, we wouldn’t have the biggest ones either.
What is the Smallest Mammal in the Ocean?
The smallest mammal in the ocean is a giant compared to the smallest sea creature.
Measuring around 140cm long, the sea otter is the world’s smallest sea-dwelling mammal.
Also known as the marine otter, this rare animal lives on the pacific coast of South America, where an estimated 3,000 individuals remain.
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From the diminutive pygmy sea horse to the weird yet rather wonderful Myxozoa Jellyfish, the ocean contains as wide a variety of small sea creatures as it does giant ones.
Although difficult to spot when swimming, snorkeling, or diving, these tiny life forms are well worth looking out for.
You may not want to come face to face with a deadly Irukandji jellyfish, but interacting with a Star Sucker pygmy octopus or juvenile frogfish would be as memorable an experience as swimming with whale sharks.
After all, size isn’t everything, even when your home is the biggest pond on earth.
Nicky is a British adventurer and animal lover who spends her time exploring the natural world and writing about her experiences. Whether on horseback, underwater, running, hiking or just standing with a fishing rod in hand, she embraces everything her adopted home of South Africa has to offer.