There are some pretty big creatures in the seven seas.

But what about the other end of the scale? There are some truly remarkable tiny critters at the other end of the spectrum. On this list, readers can learn about ten of the most interesting small ocean animals.


Size: 35mm

Location: Antarctic ocean

Krill to scale in terms of size
A krill is so small it can fit onto the end of your finger

Credit: Sophie Webb/NOAA (CC0 1.0)

This term has almost become encompassing for various species. But broadly speaking krill are a collection of small crustaceans. The word roughly translated means “fry” which is the word for a baby fish. Although they are not members of the fish family.

Perhaps the most interesting thing about krill is that they are so numerous. They form a large part of the diet of the aforementioned Blue Whale which as you may be aware is the largest animal to have ever lived.

Pea Crab

Size: 20mm

Location: Atlantic and Pacific coastlines

Pea Crab
This minuscule crustacean is a type of crab only millimeters in size

Credit: Jessica87nz (CC-BY-SA-3.0)

Pea crabs are a tiny species of crab. They are very widespread.

Like most crab species they have pincers, although there are no occurrences of this causing danger to humans.

Karenia Brevis

Size: At their smallest 20mm

Location: Gulf of Mexico

Karenia Brevis
Karenia Brevis are single-celled organisms

Credit: FWC Fish and Wildlife Research Institute (CC BY-NC-ND 2.0)

This tiny single-celled organism has a big legacy. It is the organism that causes the infamous Florida Red Tides. While it isn’t directly an issue to human beings rapid blooms can wreak havoc on marine ecosystems making this little organism a big problem. They are clever organisms that use their twin flagella in order to propel themselves and can travel great distances.

Irukandji Jellyfish

Size: 10mm

Location: Australia

Irukandji Jellyfish
The Irukandji Jellyfish is the smallest jellyfish in the world

These might be considered giants compared to some of the creatures on this list, but they are still pretty small measuring in at roughly half a centimeter. They’re small but they pack a punch. They have a potent toxin that can be fatal in rare cases, perhaps a surprise given its tiny size but even if it does not kill you directly it can lead to Irukandji syndrome one of the effects of this is that it creates a feeling of certain doom and triggers anxiety attacks and nausea. All of which can be pretty bad if you are in the sea. There are thought to be a wealth of unattributed deaths that might have been caused by this tiny creature.

Brine Shrimp

Size: 15mm

Location: Worldwide in saltwater and marshes

Brine Shrimp
Brine shrimp, or ‘sea monkeys’ are only millimeters in size

Credit: Hans Hillewaert (CC-BY-SA-4.0)

You may know brine shrimp by their commercial name as they are sold in their dormant form and dubbed sea monkeys. They do hold the distinction of being one of only a few animals that have the ability to survive without a metabolism. Or, the sum of all the chemical processes in an organism.

Mysida or Mysis shrimp

Size: 5mm

Location: freshwater lakes of North America

The Mysis shrimp is so small its almost naked to the human eye

Credit: Perhols (CC-BY-SA-3.0)

While slightly bigger than some of the creatures on this list (they start at about half a centimeter) they still sit proudly on the list. Their diet consists of another creature on this list as they are often fed brine shrimp. For fans of “Sea Monkeys”, this might make Mysis shrimp enemy number one.


Size: 1mm

Location: Worldwide in saltwater

The Copepods or ‘Copepodkils’ is merely 1mm in size

Credit: Uwe Kils (CC-BY-SA-3.0)

Again, these are a favorite of the reef-keeping community. Often a colony of these tiny creatures will develop independently in a reef setup. They are vast and tiny. They are a major food source and are grazed on from everything from clownfish to the wonderfully vibrant Mandarin Goby. They are also the main food source for seahorses who need an almost constant supply of tiny critters to survive. Some dwell on the ocean floor and on rocks. Others swim in the currents.


Size: At their smallest 12 μm

Location: Worldwide

Chlamydophrys are so small its hard to see one from another

These tiny aquatic organisms are so small they are hard to distinguish from one another. It is a eukaryotic species and feeds on organic matter.


Size: At their smallest, 0.5 μm

Location: Worldwide

Algae is incredibly small
Algae may be incredibly small but it has a massive impact on our environment

Algae may be considered by some to be a plant. Interestingly algae are responsible for the majority of the earth’s oxygen-rich atmosphere. Far more than trees (which are often attributed the moniker of oxygen kings due to their photosynthesis.) The truth is that algae are not truly a plant or an animal. It is difficult to classify at all. However, there are a vast array of types and it survives and thrives in fresh water and saltwater alike.


Size: At their smallest 0.2 μm

Location: Worldwide (in fresh and saltwater)

Plankton are some of the smallest marine creatures in the ocean

Credit: Christian Sardet/CNRS/Tara expeditions (CC-BY-SA-4.0)

This is a diverse range of creatures. And believe it or not, they live in the air as well. They are unable to move and instead rely on the current of the water (or air) to provide them with movement. They consist of bacteria, algae, and protozoa. Their unusual and interesting appearance makes them interesting to study.


What is μm?

An μm is a micrometer. On the metric scale of length it moves from meters, down to centimeters, then millimeters, and then micrometers.

Is it true that people eat live Pea Crabs?

Often they do, either unwittingly or by choice. Pea Crabs act in the way you might expect a parasite to act. They can often be found in the shells of commonly eaten seafood such as oysters and mussels.

Is the Irukandji jellyfish something to worry about?

If you don’t live in Australia or nearby you should not worry about it too much. However, if you worry about being attacked by sea creatures these are the ones to fear. They kill more people per year than sea snakes, sharks and stingrays combined.