50+ Sea Animals That Start With M: Photos, Facts & Fun

The ocean is home to an incredible diversity of life—and today we’re exploring all the sea animals that start with M. From mysterious mollusks to massive marine mammals, this A-to-Z dive (well, M-to-M!) reveals fascinating creatures from every corner of the sea.

We’ve organized this list by species type—mammals, fish, invertebrates, and more—so you can easily navigate the deep blue. Whether you’re a student, nature buff, or just curious, this article has you covered with bite-sized facts and conservation insights. Get ready to meet over 50 mesmerizing sea animals that start with M!

Sea Animals That Start With M

Manatee (Trichechus manatus)

Manatees are large, slow-moving marine mammals often found in shallow coastal waters and rivers. They have paddle-shaped tails, wrinkled skin, and a docile temperament. Native to the Gulf of Mexico, Caribbean, and parts of the Atlantic, they graze on sea grasses and aquatic plants. Manatees can weigh over 500 kg and measure up to 4 meters long. They surface for air every few minutes but can hold their breath for 20 minutes when resting. They are currently classified as “Vulnerable” due to habitat loss and boat collisions.
Fun Fact: Manatees are also called “sea cows” because of their peaceful grazing habits.

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Mantis Shrimp (Stomatopoda spp.)

Mantis shrimp are colorful marine crustaceans known for their powerful, lightning-fast claws. These animals live in burrows in tropical and subtropical reefs and can strike prey with the speed of a bullet. Despite their name, they are not true shrimp. Some species can see polarized light and more color channels than humans. Their strikes can break glass aquariums and are used to hunt crabs, mollusks, and fish.
Fun Fact: Mantis shrimp have the fastest punch in the animal kingdom.

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Moray Eel (Muraenidae family)

Moray eels are long, serpentine fish that dwell in crevices of coral reefs and rocky shores. They have muscular bodies, sharp teeth, and often display their jaws as a warning. These nocturnal predators feed on fish, octopus, and crustaceans. Some species have a second set of jaws (pharyngeal jaws) that help pull prey into their throat. Moray eels are widely distributed in tropical and subtropical seas.
Fun Fact: Moray eels have a second jaw hidden in their throat—like something out of science fiction.

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Moon Jellyfish (Aurelia aurita)

The moon jellyfish is a translucent, saucer-shaped jelly with four horseshoe-shaped organs visible through its bell. Found in coastal oceans around the world, they drift with the currents and pulse gently to move. They feed on plankton and small fish using stinging tentacles. Though they can sting, moon jellies are generally harmless to humans. Their populations can boom when conditions are right, forming large blooms.
Fun Fact: Moon jellies glow in the dark thanks to bioluminescence.

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Mola Mola (Ocean Sunfish) (Mola mola)

The mola mola is the world’s heaviest bony fish, known for its odd, flattened body and huge dorsal fins. It inhabits temperate and tropical oceans, often seen basking near the surface. Mola mola feed primarily on jellyfish, though they eat various soft-bodied organisms. They can weigh over 1,000 kg and are often mistaken for sharks due to their fin appearance. Despite their bulk, they swim gracefully with their fins flapping like wings.
Fun Fact: A single mola mola can carry over 40 million eggs.

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Mussel (Mytilidae family)

Mussels are bivalve mollusks that attach themselves to rocks, pilings, and other surfaces in intertidal zones. They have dark, oblong shells and feed by filtering plankton from seawater. Mussels play a crucial ecological role in cleaning water and providing habitat for other species. They can form large colonies and are a food source for birds, sea stars, and humans.
Fun Fact: Mussels can filter up to 25 liters of water per day.

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Marlin (Istiophoridae family)

Marlin are large, fast-swimming predatory fish known for their spear-like upper jaw and elongated bodies. Found in both the Atlantic and Pacific oceans, marlins thrive in deep offshore waters and are prized in sport fishing for their strength and speed. They feed on fish like mackerel and squid, often using their bill to stun prey. These fish can leap high out of the water when hooked. While several species exist, all marlins are top predators in their marine ecosystems.
Fun Fact: Marlins can swim up to 80 km/h, making them one of the fastest fish in the sea.

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Marine Iguana (Amblyrhynchus cristatus)

The marine iguana is a unique species of lizard found only on the Galápagos Islands. It is the only lizard in the world that forages in the ocean, feeding on algae growing on underwater rocks. Marine iguanas have flattened tails for swimming, blunt snouts for grazing, and dark coloration to absorb heat. They often bask on lava rocks to warm up after cold dives. These reptiles are strong swimmers but awkward on land.
Fun Fact: They sneeze out excess salt using special nasal glands.

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Moray Cod (Muraenolepis marmoratus)

Not to be confused with moray eels, the moray cod is a bottom-dwelling fish found in cold southern ocean waters near Antarctica. It has a mottled brownish body and is adapted to live at extreme depths. Moray cod feed on smaller fish and invertebrates and are not commercially fished due to their remote habitat. These rare fish are poorly understood due to the difficulty of deep-sea research.
Fun Fact: The moray cod lives at depths over 1,000 meters.

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Macaroni Penguin (Eudyptes chrysolophus)

The macaroni penguin is a striking seabird with a bright yellow crest that resembles overcooked noodles—hence the name. Found in the sub-Antarctic and Antarctic Peninsula, these penguins breed in large colonies and forage at sea. They dive for krill, squid, and fish, sometimes going over 200 feet deep. Macaroni penguins are excellent swimmers, though clumsy on land. Populations have declined due to changing ocean temperatures and food availability.
Fun Fact: They are named after the flamboyant 18th-century fashion trend “macaroni.”

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Megamouth Shark (Megachasma pelagios)

The megamouth shark is a rare and elusive deep-sea shark known for its disproportionately large mouth. It was only discovered in 1976 and has been seen very few times since. Megamouths feed on plankton and jellyfish, filtering them through gill rakers as they swim slowly. Unlike most sharks, they lack a sleek, fast-moving profile and instead drift along the deep ocean layers. Their soft body and rubbery skin help them survive at great depths.
Fun Fact: Fewer than 100 specimens have ever been recorded.

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Masked Booby (Sula dactylatra)

The masked booby is a large seabird recognizable by its white body, black wings, and distinctive black facial mask. Found across tropical and subtropical oceans, they nest on remote islands and cliffs. These birds dive from great heights to catch fish and squid with precision. Masked boobies live in colonies and often use the same nesting sites for generations. They are monogamous and both parents care for the young.
Fun Fact: They can dive from over 30 meters (100 feet) to catch prey underwater.

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Mudskipper (Oxudercinae subfamily)

Mudskippers are amphibious fish that live in mangroves, estuaries, and muddy flats in the Indo-Pacific. These fascinating fish can breathe through their skin and mouth lining and spend much of their time out of water. They use their pectoral fins to “walk” on land and are known for their territorial displays. Mudskippers feed on insects, algae, and small invertebrates. Their ability to survive both in and out of water makes them evolutionary marvels.
Fun Fact: Mudskippers can blink—something most fish can’t do.

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Marsh Crab (Sesarma spp.)

Marsh crabs are small crustaceans that inhabit salt marshes, estuaries, and mangroves, especially along the Atlantic and Gulf coasts. They dig burrows in mud and sand and play a vital role in aerating sediment. Marsh crabs feed on decaying plant matter, algae, and detritus. They are important to the ecosystem as recyclers of nutrients. During high tide, they retreat into their burrows to avoid predators.
Fun Fact: Their burrowing activity helps maintain marsh soil health.

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Mottled Seahorse (Hippocampus trimaculatus)

The mottled seahorse is a marine fish known for its horse-like head and curled tail. Found in shallow seagrass beds and mangroves in the Indo-Pacific, they rely on camouflage to avoid predators. Mottled seahorses feed on small crustaceans by sucking them into their tube-like snouts. The males carry fertilized eggs in a brood pouch until hatching, a rare trait in the animal kingdom. Habitat destruction has impacted their population in many areas.
Fun Fact: Male mottled seahorses get pregnant and give birth.

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Marine Flatworm (Polycladida order)

Marine flatworms are vibrantly colored invertebrates found in coral reefs and seabeds across tropical oceans. They have soft, flattened bodies and glide along surfaces using cilia. Though they resemble sea slugs, flatworms are a completely separate group and can reproduce both sexually and asexually. They feed on sponges, small crustaceans, or organic matter. Some species are toxic and display bright warning colors.
Fun Fact: Some marine flatworms reproduce by “penis fencing.”

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Marine Toadfish (Porichthys notatus)

Marine toadfish are bottom-dwelling fish with flattened heads, wide mouths, and a grumpy appearance. Found along the Pacific coast of North America, they prefer sandy or muddy seafloors in bays and estuaries. These fish are nocturnal ambush predators that feed on small invertebrates and fish. Some species can produce grunting or humming sounds to attract mates. Though not visually striking, they’re well-adapted to stealth and surprise.
Fun Fact: Male toadfish hum to attract females—sometimes for hours at a time.

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Monk Seal (Monachus spp.)

Monk seals are rare, endangered marine mammals with sleek, torpedo-shaped bodies and expressive eyes. Two species exist: the Mediterranean monk seal and the Hawaiian monk seal. They rest on beaches or in sea caves and hunt in coastal waters for fish, octopus, and crustaceans. Monk seals are shy and elusive, and both species have suffered from habitat encroachment and human disturbance.
Fun Fact: Monk seals were named for their “hooded” appearance and solitary nature.

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Maskray (Neotrygon spp.)

Maskrays are small stingrays known for the striking “mask-like” patterns on their backs. Found in shallow tropical waters across the Indo-Pacific, they rest under sandy or muddy bottoms and use camouflage to hide from predators. They feed on worms, mollusks, and small crustaceans. Unlike larger rays, maskrays are more cryptic and less frequently encountered by divers. Some species are bioluminescent.
Fun Fact: Maskrays can glow in the dark due to natural bioluminescence.

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Minke Whale (Balaenoptera acutorostrata)

The minke whale is one of the smallest baleen whales but still reaches impressive lengths. Found in oceans worldwide, they are sleek and fast swimmers with pointed snouts and a distinctive white patch on each flipper. Minke whales feed on krill and small schooling fish using filter feeding. They are curious and sometimes approach boats. Despite their size, they can be difficult to spot due to their quick surfacing.
Fun Fact: Minke whales can dive for over 20 minutes when hunting.

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Marine Worm (Polychaeta class)

Marine worms encompass a huge variety of segmented worms living in sand, mud, or attached to underwater surfaces. Found in oceans worldwide, these creatures range from tiny bristle worms to spectacular feather duster worms with colorful plumes. They play a key role in the food chain and nutrient cycling. Some species are bioluminescent or venomous.
Fun Fact: Some marine worms can regenerate lost body parts.

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Mangrove Snapper (Lutjanus griseus)

The mangrove snapper, also known as gray snapper, is a popular coastal fish found in the western Atlantic and Gulf of Mexico. It inhabits estuaries, mangrove swamps, reefs, and rocky areas. With a reddish-bronze body and sharp teeth, it preys on crustaceans, small fish, and squid. Juveniles often stay inshore, while adults move to deeper waters. They are targeted in commercial and recreational fisheries.
Fun Fact: Mangrove snappers can change their color slightly to blend in with surroundings.

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Melon-Headed Whale (Peponocephala electra)

The melon-headed whale is a small, social cetacean found in deep tropical and subtropical waters. Despite its name, it’s more closely related to dolphins. It has a rounded head, dark body, and large, sickle-shaped dorsal fin. These whales live in large pods, sometimes over 100 individuals, and are highly vocal. They hunt squid and fish in the open ocean.
Fun Fact: Melon-headed whales often leap and spin while swimming in pods.

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Mottled Grouper (Mycteroperca rubra)

The mottled grouper is a reef-associated fish found in the eastern Atlantic and Mediterranean Sea. It has a stout body with mottled brown and cream coloring for camouflage. Mottled groupers are solitary ambush predators that feed on crustaceans and smaller fish. They are slow-growing and late-maturing, which makes them vulnerable to overfishing.
Fun Fact: Mottled groupers are protogynous hermaphrodites—they start life as females and later become males.

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Macoma Clam (Macoma nasuta)

The Macoma clam, also known as the bent-nose clam, is a soft-shelled bivalve found in mudflats and estuaries along the Pacific coast of North America. It burrows several inches below the surface and uses a siphon to feed on detritus and plankton. These clams are vital to the sediment ecosystem and are prey for birds and fish.
Fun Fact: Macoma clams can rapidly rebury themselves when disturbed.

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Maugean Skate (Zearaja maugeana)

The Maugean skate is a critically endangered species found only in the estuarine waters of Macquarie Harbour in Tasmania. It has a flattened body with wing-like pectoral fins and prefers low-oxygen environments. This elusive skate feeds on crustaceans and worms along the harbor floor. Limited habitat range and environmental changes threaten its survival.
Fun Fact: This skate lives in one of the few brackish water systems that host marine skates.

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Margined Sea Star (Astropecten articulatus)

The margined sea star is a colorful starfish with five pointed arms and a sharp, spiny edge. Found in the western Atlantic from North Carolina to Brazil, it lives in sandy seabeds and shallow coastal areas. It’s an active predator, feeding on mollusks, worms, and detritus. Unlike many sea stars, it can move relatively quickly across the ocean floor. Its bright colors range from orange to purple with pale undersides.
Fun Fact: This sea star buries itself during the day and comes out at night to hunt.

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Moon Snail (Naticidae family)

Moon snails are predatory marine snails with large, round shells and a powerful foot used for burrowing into sand. Found in shallow coastal waters worldwide, they prey on clams by drilling through shells with an acid-secreting tongue called a radula. They are slow-moving but highly effective hunters. Their shell has a smooth, glossy surface with pale colors.
Fun Fact: Moon snails drill perfect circles into clam shells to feed.

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Mangrove Whipray (Urogymnus granulatus)

The mangrove whipray is a large stingray found in tropical Indo-Pacific waters, often near mangrove forests and lagoons. It has a rounded body with a long, thin tail and can reach impressive sizes. This species prefers shallow, muddy areas where it feeds on benthic invertebrates and small fish. Its coloration allows it to blend in with sediment.
Fun Fact: It can bury itself completely in mud for stealth and protection.

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Marine Isopod (Cymothoidae family)

Marine isopods are crustaceans with segmented, armored bodies resembling pill bugs. Found at all ocean depths, from intertidal zones to deep trenches, some are scavengers while others are parasitic. Certain species attach to fish and feed on blood or tissue. Despite their creepy look, many isopods are harmless and essential decomposers in marine ecosystems.
Fun Fact: Some parasitic isopods replace the tongues of fish with their own body!

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Marine Catfish (Ariidae family)

Marine catfish are found in coastal and estuarine waters around the world, especially in the Americas and Southeast Asia. They have whisker-like barbels around their mouths and armored heads. These bottom-feeding fish consume crustaceans, small fish, and detritus. Uniquely, males often carry eggs in their mouths until they hatch. Some species are known to sting with venomous spines.
Fun Fact: Male marine catfish carry eggs in their mouths for weeks.

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Mussurana Eel (Pseudomugilidae sp. – commonly misnamed)

While “mussurana eel” isn’t an officially recognized species in marine biology, the name sometimes pops up in informal lists as a misidentification of elongated marine fish or eels with similar features. Mussurana is actually the name of a land-dwelling snake genus, not a true sea creature. However, for the sake of completeness, we’ll note that this often-confused term may refer to slender, eel-like fish found in coral reef ecosystems.
Fun Fact: The name “mussurana eel” is often mistakenly used for totally unrelated creatures.

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Marine Plankton (Various species)

Though plankton isn’t a single animal, marine zooplankton includes countless tiny sea animals like copepods, krill, and larvae. These creatures drift with ocean currents and form the base of the marine food web. Found from surface waters to the deep sea, plankton feeds on microscopic algae and is, in turn, consumed by whales, fish, and seabirds. Despite their size, they play a giant ecological role.
Fun Fact: Plankton produce over 50% of the world’s oxygen.

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Mud Crab (Scylla serrata)

The mud crab is a large, clawed crustacean found in mangroves, estuaries, and tidal flats across the Indo-Pacific. Its hard shell and massive claws make it a popular seafood item. Mud crabs are omnivorous and opportunistic, feeding on fish, mollusks, and detritus. They’re strong diggers and hide in burrows at low tide. Some species can even tolerate slightly fresh water.
Fun Fact: Mud crabs can regenerate lost claws within a few molts.

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FAQs About Sea Animals That Start With M

How many different types of sea animals start with M?

There are over 50 unique sea animals that start with the letter M, ranging from massive whales to tiny invertebrates.

Are all the animals listed here marine species?

Yes, each animal featured is either fully marine or spends most of its life in saltwater environments like oceans, seas, or estuaries.

Which “M” sea animal is the largest?

The mola mola (ocean sunfish) and minke whale are among the largest “M” marine animals, both weighing hundreds to thousands of kilograms.

Do any of these animals live in freshwater too?

A few, like mudskippers and marine catfish, can tolerate brackish or slightly fresh water but are primarily marine or coastal.

Why are so many marine animals endangered?

Threats like habitat loss, pollution, climate change, and overfishing have placed many sea animals, including monk seals and Maugean skates, at risk.

Conclusion

From mysterious mollusks to majestic mammals, the sea animals that start with M showcase the incredible variety of life beneath the waves. Each creature plays a unique role in the ocean’s delicate web, whether it’s a massive minke whale or a microscopic plankton.

Learning about these animals helps us appreciate the beauty and importance of our oceans—and hopefully, inspires greater conservation. The next time you hear about a manatee or a mantis shrimp, you’ll know they’re just part of the magnificent “M” marine lineup.