All About the Giant Isopod: A Deep-Sea Pill Bug

Leave a comment / / Updated on: 11th November 2023

NameGiant IsopodDietScavenger
Scientific nameBathynomusWeight2.5 pounds (1.13 kilograms)
Pronunciationjai-uhnt eye-suh-pod.Length8–50 centimeters (3.1–19.7 inches)
ClassificationMalacostraca, Isopoda, CirolanidaeLocationWorldwide

The Giant Isopod

Giant isopod
Giant Isopod | LindaZ via iStock

The ocean is home to several strange animals. 

One such intriguing creature that lives more than 500 meters below the surface is the giant isopod. 

If you think this 14-legged crustacean looks like a giant version of the woodlice, that’s because they’re related. 

But the giant isopod has managed to evolve into a gigantic size as a result of extreme pressure in the deep sea environment where it lives. 

Gage Beasley's In-Demand Plush Toys
Gage Beasley’s In-Demand Plush Toys

The nature of their habitat also makes it challenging to study them, so relatively little is known about the giant isopod. 

In this article, we’ll explore some of the facts we know about the giant isopod and discuss its ecological and cultural significance. 

Taxonomy and Classification

Gage Beasley's Isopod Stuffed Plush Toy
Gage Beasley’s Isopod Stuffed Plush Toy

The name “giant isopod” is the common name for members of the Bathynomus genus. 

This includes up to 20 species of isopods found in cold, deep waters all over the world. 

They all belong to the order Isopoda, along with woodlice and all their relatives.

The giant isopod is the largest known member of this order.

Isopods are crustaceans, which means they’re distantly related to shrimp and crabs. 

Shrimp, a family relative of the giant Isopod | Floortje via iStock

These are decapods (arthropods with ten legs, compared to 14 for the isopods).

Isopods are among the oldest living animals today. 

They evolved as early as the Carboniferous Period, about 300 million years ago.

Today, about 10,000 species of isopods are found worldwide, and about 20 of them are in the Bathynomus genus (giant isopods).

Physical Characteristics

A giant isopod underwater
A giant isopod underwater | Yiming Chen via Getty Images

Giant isopods are aptly named because they grow larger than other species of isopods found on land and other habitats. 

The enormous size of members of the Bathynomus genus is a typical example of deep-sea gigantism. 

Deep-sea gigantism is a phenomenon where an animal grows larger than usual to survive in the high-pressure environment of the deep sea. 

Regular isopods are usually about five centimeters (two inches) in length. 

Giant isopods grow considerably bigger than this. 

The largest of them, classified as supergiants, typically between 17 and 50 centimeters (6.7–9.7 inches). 

However, smaller members of the genus are generally between eight and 15 centimeters (3.1–5.9 inches) in length. 

supergiant isopod
A supergiant isopod in the making | Joao Paulo Burini via Getty Images

The biggest species in the genus is B. giganteus

Giant isopods are arthropods, which means their body is protected by a thick exoskeleton and made up of overlapping segments. 

Their body is dorso-ventrally compressed, meaning it is compressed from top to bottom.

The giant isopod’s exoskeleton is made up of calcareous materials. 

The first segment is fused to the head and grows over the giant isopod’s eyes. 

The rest of the posterior segments are fused to form a caudal shield over the giant isopod’s abdomen. 

Like terrestrial isopods, giant isopods can curl up their body to protect their abdomen. 

A frontal view of Bathynomus giganteus
A frontal view of Bathynomus giganteus, showing its large, highly reflective compound eyes | Borgx via Wikipedia CC BY-SA 3.0

Giant isopods have large eyes that tend to glow in the dark. 

This is due to a reflective layer at the back of their eyes known as the tapetum that tends to bounce off light. 

Their head also has two pairs of antennae. 

The first set is bigger, while the second one is smaller. 

Some of the biggest Bathynomus individuals can weigh several pounds, with some reaching weights of over 2.5 pounds (1.13 kilograms). 

The giant isopod has seven pairs of legs, also known as pereopods. 

The underside of Bathynomus giganteus
The underside of Bathynomus giganteus | NOAA via Wikipedia Public Domain

The first pair is used to manipulate food into the isopod’s mouth. 

They have a complex mouth with four sets of jaws modified for cutting and tearing up prey. 

Towards the end of the giant isopod’s body, there’s an abdominal section that consists of five curved pieces. 

Each piece has a flat flap on its end, which aids movement and is also used for respiration.

Giant isopods are typically pinkish or pale lilac.

Their dark coloration serves as a camouflage that keeps them hidden in the deep sea environment they call home. 

Habitat and Distribution

Giant isopods are bottom-dwelling crustaceans found in all three of the major oceans: the Atlantic, Pacific, and Indian oceans. 

Species in the Bathynomus genus are found in various regions of the Atlantic Ocean, including the North and South Atlantic, but not in the East Atlantic. 

They have been recorded off the coasts of North America, South America, and West Africa.

Some species are also known to inhabit the Indian Ocean, with records of their presence off the coasts of India and Sri Lanka.

Giant isopods live in the Pacific Ocean, including regions near Japan, Australia, and New Zealand. 

However, no giant isopod species have been identified from the East Pacific. 

Deep sea ocean floor
Deep sea ocean floor | stock_colors via iStock

Although there are no specific records of giant isopods in the other oceans, some species likely exist in deep-sea environments in these oceans. 

Giant isopods typically live in cold waters, usually less than 20 °C (68 °F).

They’re mostly found in deep-sea environments, from the sublittoral zone (depth of about 170 meters or 560 feet) to the bathyal zone (more than 2,000 meters or 6,560 feet). 

These regions of the ocean are characterized by high pressure and temperature. 

Behavior and Social Structure

Giant isopod with its creepy crawlies
Giant isopod with its creepy crawlies | Joao Paulo Burini via Getty Images

Giant isopods are primarily scavengers. 

They rely on a diet of dead and decaying marine organisms that sink to the ocean floor. 

Their complex mouths, characterized by powerful mandibles, are adapted to tearing into tough flesh. 

Giant isopods also use their long antenna to detect the scent of decaying organic matter in the water.

This guides them to potential food sources. 

Giant isopods are active creatures but are more active at night than during the day, which means they’re nocturnal. 

During the day, they often find shelter in crevices or burrows to avoid potential predators and conserve energy. 

Giant isopod hunting at night
Giant isopod hunting at night | Darren Lynch via Getty Images

They emerge at night to scavenge.

Members of the Bathynomus genus are not territorial in the conventional sense but are also not social. 

They do not actively defend the specific areas where they forage, but there’s no evidence of complex intraspecific interactions. 

Instead, they all search for food individually, wherever it may be found.

Their solitary lifestyle is well-suited to the deep-sea environment, where resources are scarce and widely dispersed.

Although they spend most of their time walking slowly on the seafloor and scavenging for food, giant isopods are free-swimming arthropods. 

This means they’re capable of swimming and gliding through the water. 

Since some of them are found at lower depths of up to 170 meters, it is likely that they exhibit some level of vertical migration in the water column. 

Diet and Feeding

barren ocean floor
An underwater shot of the sandy and barren bottom of the Atlantic Ocean | Snorre Hatlestad Roberg via iStock

The deep sea environment where the giant isopod lives is mostly barren. 

However, freshly dead or decaying organic matter may sink into these deep-sea environments and be consumed by scavenging bottom-dwellers like the giant isopod. 

Their primary food source is the carcasses of marine animals that sink to the ocean floor. 

This includes a wide range of organisms, from small fish and squid to larger marine mammals such as whales. 

They may also feed on other forms of detritus, including decaying plant matter, dead plankton, and other organic debris.

Giant isopod walking under the sea
Giant isopod walking under the sea | DigiPub via Getty Images

The long antennae and smaller antennules of the giant isopods are well-developed and effective for detecting decaying matter in the water. 

They also have robust mandibles capable of tearing through tough flesh and breaking down the carrion they encounter. 

These mandibles are well-suited for processing the hard and fibrous tissues of various kinds of deceased marine animals.

Giant isopods may also prey on living, slow-moving animals that stray into their ecosystem, such as sea cucumbers, nematodes, sponges, and radiolarians.  

Given the opportunity, they may also prey on live fish, but this rarely occurs.

Reproduction and Life Cycle

Giant insopod in an aquarium for scientific study
Giant insopod in an aquarium for scientific study | Mikeac1 via Wildkratts Fandom

Due to the accessibility of the typical Bathynomus habitats in the wild, detailed observation of their reproductive behavior is limited. 

However, we can infer some aspects of their reproductive behavior based on the observation of other related isopod species.

Mating in giant isopods typically takes place during winter and spring. 

Resources are most abundant during this period, compared to winter when there is a food shortage. 

Bathynomus giganteus likely engages in some form of mating rituals, which may involve chemical and tactile cues. 

Males and females may use their specialized sensory antennae and antennules to locate each other. 

Once they form a mating pair, the male typically transfers sperm to the female through specialized structures called “spermatophores.” 

These structures contain sperm, and the male attaches them to the female’s body, where they fertilize the eggs.

isopods mating
Two giant isopods mating | Jeffrey0409 via Nature Rules Fandom

After fertilization, the female carries the eggs in a specialized egg pouch known as a marsupium until they hatch.

The pouch is formed by overlapping plates known as oostegites. 

The giant isopod’s pouch may contain between 20 and 30 eggs. 

The eggs of the giant isopod are considered the largest of all marine invertebrates, with a diameter of about 13 millimeters (0.51 inches). 

Brooding females do not move around or feed. 

Instead, they bury themselves in the sediments to insulate the eggs and protect them from predators.

The gestation period may vary from one species to the other, but it’s typically about two to four weeks on average. 

The hatchlings emerge from the eggs as fully-formed miniature versions of the adults. 

They’re typically about six centimeters (3.4 inches) long by the time they emerge from the eggs. 

A mature giant isopod walking under the ocean
A mature giant isopod walking under the ocean | Clip image via oceanexplorergov

The only difference between juveniles and adult forms of giant isopods is the absence of the last pair of walking legs in the juveniles. 

The hatchlings are known as manca.

Like other arthropods, giant isopod juveniles molt multiple times as they develop into adults. 

The number of molts depends on environmental factors and the individual’s age, but it can be as many as ten or more molts before reaching full adult size. 

Giant isopods have a slow metabolism. 

This allows them to survive on the relatively infrequent food falls that occur in their habitat from time to time.

Ecological Role and Interactions

Giant isopod on the table
Giant isopod on the table | Dornicke via Wikimedia Commons

As scavengers, Bathynomus species play a critical role in recycling nutrients and organic matter in the deep sea environment. 

Giant isopods consume dead and decaying marine animals that sink to the ocean floor, 

Without scavengers like this, the excessive accumulation of organic debris will disrupt the balance of the marine ecosystem. 

Bathynomus is a potential prey item for larger predators in the deep-sea environment. 

deep-sea octopus
A potential predator of the giant isopod, a deep-sea octopus | Volodymyr Ivanenko via iStock

Some of their potential predators include deep-sea fish and cephalopods. 

By feeding on carcasses, Bathynomus giganteus indirectly transfers energy from deceased marine organisms to these predators that occupy higher levels in the food web. 

The large size and tough exoskeleton of the giant isopod can provide some level of protection for them. 

They’re also capable of curling up into a ball when attacked, which can help protect their soft underside. 

However, giant isopods are not immune to predation from larger animals in their habitat. 

Conservation Status and Threats

The giant isopod (Bathynomus) is not currently listed on the IUCN Red List. 

This means that its conservation status is not yet fully assessed due to the lack of sufficient data on its population and abundance. 

However, they do face several threats, chief of which are habitat loss, climate change, and pollution. 

oil rig at sea
Oil rig at sea | Vladimirovic via iStock

Giant isopods live in the deep sea, which is increasingly affected by human activities such as bottom trawling and oil and gas drilling. 

These activities can destroy the seafloor habitat that giant isopods rely on for food and shelter.

In recent years, there’s been a growing trend of the ocean becoming warmer. 

Giant isopods are adapted to cold, oxygen-rich water and will have a hard time adjusting to the warm, acidified seawater caused by climate change.

Giant isopods are also vulnerable to pollution, such as plastic debris, which can be toxic when ingested. 

deep sea pollution
Massive plastic pollution of the ocean bottom | Andriy Nekrasov via iStock

Current conservation efforts for most deep-sea animals focus generally on protecting the general ecosystem instead of individual species like the giant isopods. 

Some initiatives include the establishment of protected areas and international agreements to regulate activities in the deep sea. 

Various countries are also introducing measures aimed at regulating deep-sea fisheries to minimize instances of deep-sea creatures getting caught as bycatch and other indirect activities that may affect the deep-sea ecosystems. 

Unique Adaptations and Survival Strategies

Deep sea creatures like the Bathynomus need to be hardy and adaptable to survive the extremely cold and nutrient-deficient ecosystem where they live. 

The giant isopod has some of these adaptations that allow it to thrive in the abyssal depth where it lives. 

The massive size of this isopod is a type of adaptation known as deep-sea gigantism. 

hard exoskeleton of giant isopod
The hard exoskeleton of the giant isopod | 3D Image via Sketchfab

Animals with this adaptation tend to grow bigger in response to the extreme pressure in their environment, which can be up to 100 times higher than the pressure at the surface.  

Experts also think a lack of large predators in their ecosystem may have contributed to the large size of the giant isopods. 

The exoskeleton and internal structures of these marine arthropods are also designed to withstand extreme pressure in their environments. 

Another vital environmental adaptation is the slow metabolism of this giant isopod, which is perfectly suited to the deep-sea environment where they live.

Food is scarce and unpredictable in this part of the ocean. 

Giant isopod with its well-developed eyes
Giant isopod with its well-developed eyes | Image via Pinterest

The slow metabolism of the giant isopod helps them conserve energy so they can endure long periods without food. 

They also have a range of adaptations to find food that falls to the bottom of the ocean. 

These include well-developed, light-sensitive eyes and antennae that can detect decaying organic matter in the water. 

The long antennae and smaller antennules are highly sensitive and can detect tactile and chemical cues in the water.

The dark coloration of the giant isopod also provides some degree of camouflage in the deep-sea environment. 

This helps them blend into the dark substrate of the ocean floor, making them less conspicuous to potential predators and prey.

Cultural Significance and Human Interactions

Giant isopod in an aquarium
Giant isopod in an aquarium | NR1&GLAFan2004 via FictionRulezForever Fandom

The giant isopod is the largest isopod living in the world’s oceans. 

However, it is not well-known to the general public and has limited cultural significance because of its remote location in deep-sea habitat. 

Due to its size and distinctive appearance, this deep-sea scavenger has fascinated scientists and enthusiasts curious about deep-sea creatures. 

Bathynomus species are of interest to marine biologists, oceanographers, and researchers studying deep-sea ecosystems.

Research into these species focuses on their unique adaptations, behavior, and, most importantly, their role in recycling nutrients in the ecosystem. 

Giant isopods are not a target of commercial fisheries, mainly because they hold little commercial value and also live beyond the reach of traditional fishing rigs. 

public aquarium
Mother and daughters looking at the fish in a big public aquarium | damircudic via iStock

However, they have been known to attack and destroy fish caught in fishing trawls due to their feeding activities. 

Some public aquariums and marine science centers may keep giant isopods in special exhibits dedicated to deep-sea life. 

These exhibits serve an educational purpose, introducing visitors to the incredible diversity of marine life in the deep sea.

People have been known to consume giant isopods as food by preparing them for East Asian cuisines such as ramen.

The meat is said to taste like crab or lobster but with a firmer texture. 

However, despite their large size, there’s very little “meat” to be harvested from the body of a giant isopod, which discourages people from consuming them. 

Future Prospects and Research

giant isopod specimen
Specimens of Bathynomus doederleinii (front) and Bathynomus kensleyi (behind), in the collections of the National Museum of Marine Biology and Aquarium in Pingtung, Taiwan | Tiouraren via Wikipedia CC BY-SA 4.0

Giant isopods were discovered for the first time in 1879. 

Since then, there has been considerable interest in understanding these deep-sea arthropods and how they live. 

Unfortunately, very little is known about them despite their intriguing looks and immense size. 

There’s sufficient information about their taxonomy and relationship with other arthropods. 

But giant isopods are yet to be extensively studied in their natural habitat because of the challenging nature of deep-sea ecosystems. 

deep sea vehicle
Underwater robot explores the deep sea | S_Bachstroem via iStock

More recent technological advancement, such as the invention of remotely operated deep-sea exploration vehicles, is making it easier for scientists to explore and document these deep-sea ecosystems more comprehensively. 

Ongoing expeditions will help identify new species within the genus and unravel various aspects of their behavior and ecological interactions. 

Understanding the population, distribution, and ecological factors that affect these deep-sea species can also inform conservation efforts and help identify vulnerable areas that may require protection in the future. 


The giant isopod is a massive arthropod found in deep-sea ecosystems all over the world. 

They’re related to woodlice but several times bigger. 

Giant isopods have been found in depths of up to 2,500 meters below the surface. 

They’re deep-sea scavengers that feed mainly on the carcasses of marine animals and other debris that fall to the bottom of the ocean. 

Although we do not have accurate data on their population, there’s evidence that human activities such as deep-sea fishing and plastic pollution adversely affect deep-sea creatures like giant isopods. 

This can be detrimental to the balance of the deep-sea ecosystem because these deep-sea arthropods play a major role in getting rid of debris and cleaning up the ocean. 

Consequently, we need to invest more effort in learning more about deep sea scavengers like the giant isopod in order to better understand them and put measures in place to protect them. 


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