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What Are the Smallest Animals in the Ocean?

Come meet some of the teeniest, tiniest critters in the sea

Marine Wildlife_Pygmy Seahorse_Tanya Lister
Pygmy Seahorse, Tulamben, Indonesia © Tanya Lister

It’s finally springtime for us up in the northern hemisphere and the animal kingdom seems to be waking up after a looong winter’s nap, especially those chirping birds outside my bedroom window! The earth is softening, tiny buds are popping up on all the trees and little critters everywhere are “twitterpated,” from deer and rabbits to the tiniest frogs and worms.

As with most things, this got me wondering about the ocean and its critters. What are our favorite ocean animals up to in springtime? What are the biggest animals in the ocean? And what are the tiniest animals in the ocean? Dive in with me…

Zooplankton

Zooplankton may very well be the smallest type of animal in the ocean and encompass a variety of species, from single-celled protozoa to some jellyfish and crustaceans like krill. Not to be confused with phytoplankton (which are plants), these little critters are typically less than one inch in length and some are so small you need a microscope to observe them. But don’t mistake their size for their importance. Zooplankton may be at the bottom of the food chain but they are critical in supporting both marine and freshwater food webs.

2048px-Zooplanktonsm
© Matt Wilson/Jay Clark, NOAA NMFS AFSC

Irukandji jellyfish

Found in tropical regions including the Caribbean and the Indo Pacific, the Irukandji jellyfish are quite possibly the smallest, and most venomous, jellyfish in the world. At just 0.06 cubic inches, the Irukandji are unique for more than just their size. While most sea jellies have stingers (or nematocysts) only on their tentacles, the Irukandji have stingers on their bell as well. Their venom has been described as “100 times as potent as that of a cobra and 1,000 times stronger than a tarantula’s.” Yikes! Reactions to their venom have been so severe that these tiny jellies have an entire syndrome named after them.

Sexy shrimp

That’s right, the sexy shrimp—also known as squat shrimp or Thor amboinensis—comes in around 0.5 inches. These itty-bitty flirtatious shrimp get their name from the cute little dance they do with their bodies. A popular species for home aquariums, sexy shrimp are most commonly found in the Atlantic and Indo-Pacific and live in corals and anemones.

Pygmy Seahorse

The pygmy seahorse is both itty-bitty and a master of disguise! Ranging from 0.5 to one inch long, there are several types of pygmy seahorses and they all blend in well with seagrasses and soft corals. They are so good at camouflage, in fact, that the first little one was discovered only when its coral home was being observed in a lab!

Pygmy Seahorse
© Inusuke via Canva.com

Nudibranchs

Shockingly enough, these lovely sea slugs don’t really belong on this list! Some of our favorite nudibranchs can actually grow to be up to 12 inches long. However, the smallest only grow to about ¼ inch, so we’ll keep them here as an honorable mention. Did you know there are more than 2,000 species of nudibranch? And that’s just those we know about today.

Beautiful blue dragon nudibranch
© S. Rohrlach

Dwarf Lanternshark

While they’re not the smallest animal in the ocean either, the world’s smallest shark is the dwarf lanternshark and can literally fit in your hand, reaching a maximum of 7.9 inches. Like other lantern sharks, this little buddy has light-generating organs called photophores along its stomach to attract prey and hide from predators. A native to the Atlantic and Caribbean Sea, this member of the dogfish shark family feeds on krill, our other tiny planktonic crustacean friends.

What are your favorite microscopic ocean critters?

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