Crustaceans: Characteristics, Kinds and Odd Deep Sea Ones

Home | Category: Crustaceans (Crabs, Lobsters and Shrimp)


Major decapod lineages. a) Lucifer sp. (southeast Florida) (Dendrobranchiata); b) Stenopus hispidus (Komodo, Indonesia) (Stenopodidea); c) Procaris chacei (Bermuda) (Procarididea); d) Arctides regalis (Maui, Hawaii) (Achelata); e) Cherax quadricarinatus (aquarium specimen) (Astacidea); f) Thor amboinensis complex (Ternate, Indonesia) (Caridea); g) Axiopsis serratifrons (Bali, Indonesia) (Axiidea); h) Stereomastis sculpta (Polychelida); i) Upogebia cf. pusilla (Arcachon Bay, France) (Gebiidea); j) Emerita talpoida (Westerly, Rhode Island) (Anomura); k) Pachygrapsus crassipes (Catalina Island, California) (Brachyura). [Source: Royal Society

Crustaceans are a diverse group that include crabs, lobsters, shrimps, krill, prawns, crayfish, water fleas, copepods, barnacles, a few land creatures such as woodlice and and several other groups of organisms. They are generally water creatures that have tough shells and no backbone and breath through gills. Many are scavengers that feed on detritus. Their shells are made of chitin, the same material that makes up insect shells.

Crustaceans belong to the phylum of arthropods along with insects, centipedes, millipedes and arachnids (including spiders and scorpions). Arthropods account for three fourths of all known animals. All have exoskeletons made of chitin; a body divided into segments and protected by cuticle; jointed legs arranged in pairs; an open circulatory system with organs bathed in a liquid called hemolymph that is pumped around the body by the heart; and a nervous system comprised of paired nerve chords.

The first crustaceans appeared about 500 million years ago when trilobites dominated the seas. Early varieties were similar to trilobites except they had two pair of antennae rather one. Today there are about 35,000 different species of crustacean — four times as many as the total number of bird species. Most are found among rocks and reefs. Some of those found in coral reefs are quite colorful. According to the Census of Marine Life, completed in October 2011, crabs, lobsters and other crustaceans are now believed to be the most common species in the seas of Australia and Japan, whose waters are thought of as the most varied .

Approximately 30,000 species make up the Crustacea this Subphylum. According to Animal Diversity Web: The majority are marine but some are found in fresh water.. Most crustaceans are free-living, but some are sessile ( fixed in one place like a barnacle) ;and a few are even parasitic. Most use their maxillae (bone-like structures in the face) and mandibles to take in food. The walking legs, including specialized chelipeds, may be used to help capture prey. Some crustaceans filter tiny plankton or even bacteria from the water; others are active predators; while still others scavenge nutrients from detritus. The mechanisms by which fertilization and development occur vary greatly. Some crustaceans hatch young that are like miniature adults; others go through a larval stage called a nauplius. [Source: Phil Myers, Animal Diversity Web (ADW) /=]

The main classes of crustaceans are 1) Remipedia; 2) Cephalocarida; 3) Branchiopoda (fairy shrimp, water fleas, etc.); 4) Maxillopoda (ostracods, copepods, barnacles) and 5) Malacostraca (isopods, amphipods, krill, crabs, shrimp, etc.) Many species, including lobsters, crayfish, barnacles, and crabs are important to human economies, some very much so. Others, such as krill, are at the base of extremely important marine food chains. Still others are crucial in recycling nutrients trapped in the bodies of dead organisms. /=\

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal

Crustacean Characteristics

Convergent evolution of carcinized and decarcinized body plans in crustaceans: Phylogenetic evidence demonstrates that carcinized body plans have evolved multiple times (indicated by the colored characteristics on branches). Carcinized clades are: sponge crabs, true crabs, porcelain crabs, hairy stone crabs, and king crabs. Decarcinized clade shown is the frog crabs. from Harvard University

Crustaceans have two pairs of antennae, a pair of mandibles and compound eyes usually on stalks. Crustacean bodies usually are made up of head, thorax, and abdomen. Their head and thorax is often covered by a shield or carapace and the front of this extends to form a projection called the rostrum. Crustaceans employ a number of strategies when feeding. Large species capture prey and kill it by crushing, stunning or tearing it apart. Other are filter feeders that use their thorical appendages to set up currents in front of their mouth that draw in water which can be filtered for small particles of food. Yet others use their appendages to scavenge or root through sand, mud, algae and other materials.

Crustaceans are generally cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature) and heterothermic (have a body temperature that fluctuates with the surrounding environment). They have setae and sensilla found all over their body. Sensilla are mechanoreceptors or chemoreceptors. Special chemoreceptors are on the antennae. Well developed receptors provide information about appendage position and movement. Crustaceans also have simple and compound eyes.

Phil Myers wrote in Animal Diversity Web: All crustaceans have two pair of maxillae on their heads, followed by a pair of appendages on each body segment. The segments composing these tagmata (anthropod body) differ among different Classes. The appendages are primitively branched (biramous), and although this condition is modified in many species, adults always have at least some biramous (crustacean-style) appendages. Crustaceans respire via gills. Like other arthropods, all have a hard but flexible exoskeleton. [Source: Phil Myers, Animal Diversity Web (ADW) /=]

Crustaceans have series of paired appendages we call legs which are powered by internal muscles within the exoskeleton. These have been adapted for their particular needs. Many have evolved their front legs into claws and pincers (known to scientists as chelipeds). The middle legs are generally used for paddling or walking.

Crustacean appendages have two branches and have a number of functions, including movements, sensing, respirations and egg-brooding. The first pair, often claws or pincers, are used for defense, handling food and even sexual communication. Thoracic appendages called perepods typically have gills. The basal part of some appendages help in walking while abdominal segment often have paired swimming appendages called pleopods or swimmerets. Crustacean leg muscles are attached to prongs near the points inside the exoskeleton. The joints can only move in one plane. To get around this limitation, joints are often grouped in twos or threes on each limp. They are often close together, each operating in different planes, which allows the limb to move in a variety of directions.

Crustacean Exoskeletons

The chitin exoskeleton of crustaceans is strengthened with calcium carbonate. Because they operate almost as well on land as in water, many kinds of crustaceans emerge from the water at beaches and shores or easilt survive when exposed by low tides.

Because the shells can't expand or grow, crustaceans must periodically shed their shells and grow new ones. Before a crustacean molts its absorbs much of the calcium carbonate from its old shell into its blood. This weakens the old shell and allows it to be shed more easily.

The new shell is secreted in the form of wrinkled skin underneath the old shell which splits open and remains mostly intact, resembling a translucent ghost of its former occupant, as the animal crawls out. The animal grows and swells its body by absorbing water. The skin swells and stretches out the wrinkles and hardens gradually into shell. While the shell is hardening the crustacean is vulnerable to attacks and must hide.

The limbs of most crustaceans grow back if they are lost. A quarter of male crabs in one survey lost their claws in combat. Some species of crustaceans can grow back the first set of lost limbs but not a second set. Shellfish like lobsters, crabs and shrimp turn red when cooked because they accumulate red pigment from eating certain plankton and algae. The pigments bond with proteins in the shell, making them invisible until cooking breaks the bond and reveals the red.

Crustaceans Can Feel Pain, Study Says

A 2013 study by Professor Bob Elwood and Barry Magee from Queen's School of Biological Sciences published in the Journal of Experimental Biology determined that crabs can feel pain based on the reactions and behavior of common shore crabs to small electrical shocks. Professor Elwood's previous research showed that prawns and hermit crabs respond in a way consistent with pain. This latest study provides further evidence of this. Professor Elwood said: "The experiment was carefully designed to distinguish between pain and a reflex phenomenon known as nociception. The function of pain is to aid future avoidance of the pain source, whereas nociception enables a reflex response that provides immediate protection but no awareness or changes to long-term behaviour. "While nociception is generally accepted to exist in virtually all animals the same is not true of pain. In particular, whether or not crustaceans experience pain remains widely debated."[Source: Queen's University Belfast, Science Daily, January 16, 2013

This latest study showed that shore crabs are willing to trade something of value to them — in this case a dark shelter — to avoid future electric shock. Explaining how the experiment worked, Professor Elwood said: "Crabs value dark hideaways beneath rocks where they can shelter from predators. Exploiting this preference, our study tested whether the crabs experienced pain by seeing if they could learn to give up a valued dark hiding place in order to avoid a mild electric shock.

"Ninety crabs were each introduced individually to a tank with two dark shelters. On selecting their shelter of choice, some of the crabs were exposed to an electric shock. After some rest time, each crab was returned to the tank. Most stuck with what they knew best, returning to the shelter they had chosen first time around, where those that had been shocked on first choice again experienced a shock. When introduced to the tank for the third time, however, the vast majority of shocked crabs now went to the alternative safe shelter. Those not shocked continued to use their preferred shelter. Having experienced two rounds of shocks, the crabs learned to avoid the shelter where they received the shock. They were willing to give up their hideaway in order to avoid the source of their probable pain."

Professor Elwood says that his research highlights the need to investigate how crustaceans used in food industries, such as crabs, prawns and lobsters, are treated. He said: "Billions of crustacean are caught or reared in aquaculture for the food industry. In contrast to mammals, crustaceans are given little or no protection as the presumption is that they cannot experience pain. Our research suggests otherwise. More consideration of the treatment of these animals is needed as a potentially very large problem is being ignored.

Source: Royal Society

Crabs and Lobsters Are Sentient Beings, Review Concludes

A review of 300 studies published in scholars 2021 concluded there is strong evidence that some invertebrates — including crustaceans such as crabs, lobsters, shrimp, prawns, and crayfish, and cephalopods such as octopuses, squids, and cuttlefish — are sentient. The review defined sentience as "the capacity to have feelings, such as feelings of pain, pleasure, hunger, thirst, warmth, joy, comfort and excitement." The British government responded by updating it animal welfare laws with a bill to includes octopuses, crabs, and lobsters.[Source: Kelsey Vlamis, Business Insider, November 22, 2021]

An announcement said the bill "already recognizes all animals with a backbone (vertebrates) as sentient beings. However, unlike some other invertebrates (animals without a backbone), decapod crustaceans and cephalopods have complex central nervous systems, one of the key hallmarks of sentience." The decision followed the findings of a government-commissioned independent review by the London School of Economics and Political Science.

The review found there was "strong evidence" that such animals are sentient, which the review defines as having "the capacity to have feelings, such as feelings of pain, pleasure, hunger, thirst, warmth, joy, comfort and excitement."

“The report also made specific recommendations on animal welfare practices based on its findings, including: 1) Banning the declawing of crabs; 2) Banning the sale of live crabs and lobsters to "untrained, non-expert handlers"; and 3) Banning boiling alive and live dismemberment when a viable alternative exists and when electrical stunning is not done first.


Copepods are flealike creatures and one of the most abundant and diverse animal groups on Earth. One sampling of an area the size of a bathroom (5.4 square meters) deep in the South Atlantic Ocean in the mid 2000s turned up 700 species of copepod, 99 percent of them unknown to science. Many copepods are considered zooplankton and are fed on by a huge number and array of sea creatures, most of who are fed on by larger sea creatures. In this way copepods are one of the foundations of the food chain.


Copepods (meaning "oar-feet") are found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters); some are benthic (living on the ocean floor); a number of species have parasitic phases. Some live on land in swamps, under leaf fall in wet forests, bogs, springs, puddles and water-filled recesses of plants such as bromeliads and pitcher plants. Many live underground in marine and freshwater caves. Copepods are sometimes used as biodiversity indicators. [Source: Wikipedia +]

Copepods are related to shrimp and crabs. They are an important planktonic food source for most of the world's fish species. This high predation pressure has led copepods to evolve extremely fast escape responses when a predator is sensed, and can jump with high speed over a few millimeter. Reaction times to hydrodynamic disturbances of less than 4 milliseconds (thousandths of a second) and escape speeds of over 500 body lengths per second have been measured using 3D high speed digital holographic cinematography (up to 2000 frames per second), [Source: Project Seahorse]

About 13,000 species of copepods are known. As with other crustaceans, copepods go through a larval stage. For copepods, the egg hatches into a nauplius form, with a head and a tail but no true thorax or abdomen. The larva molts several times until it resembles the adult and then, after more molts, achieves adult development. The nauplius form is so different from the adult form that it was once thought to be a separate species. +

Copepods vary considerably, but are typically 1 to 2 millimeters (1∕32 to 3∕32 inches) long, with a teardrop-shaped body and large antennae. Like other crustaceans, they have an armoured exoskeleton, but they are so small that in most species, this thin armour and the entire body is almost totally transparent. Because of their small size, copepods have no need of any heart or circulatory system (one Calanoida has a heart, but no blood vessels), and most also lack gills. Instead, they absorb oxygen directly into their bodies. Most free-living copepods feed directly on phytoplankton, catching cells individually. A single copepod can consume up to 373,000 phytoplankton per day. +

Amphipods — Among the World’s Deepest Living Animals

An amphipod

Elizabeth Kolbert wrote in The New Yorker: “Small crustaceans known as amphipods have been collected from the very bottom of the Mariana Trench, almost thirty-six thousand feet down, where the pressure is so great that the animals’ shells, in theory at least, should dissolve. A team of Japanese scientists reported that one deep-dwelling amphipod, Hirondellea gigas, protects its shell by coating it in an aluminum-based gel, produced from metal that it extracts from seafloor mud. [Source: Elizabeth Kolbert, The New Yorker, June 14, 2021]

Amphipods make up an order of crustaceans with no carapace and, for the most part, with laterally compressed bodies. Ranging in size from 1 to 340 millimeters (0.039 to 13 inches) and resembling shrimp, they are mostly scavengers, who eat detritus on the ocean floor. More than 9,900 amphipod species have been described so far but there is probably a lot more. They are mostly marine animals but there are hundreds of freshwater ones too. Sandhoppers such as Talitrus saltator are examples of ones that live on land. [Source: Wikipedia]

The largest recorded living amphipods — Alicella gigantea — were 28 centimeters (11 inches) long, and photographed at a depth of 5,300 meters (17,400 feet) in the Pacific Ocean. Pieces of Alicella gigantea. retrieved from the stomach of a black-footed albatross were estimated to from creatures 34 centimeters (13 inches) in length. Scientists observed more specimens of A. gigantea in the 10,047-meter- (32,963-foot) -deep Kermadec Trench near Samoa estimated to be 34.9 centimeters long. Some that were 27.8 centimeters long were collected..

Amphipods have been spotted in the Challenger Deep, the deepest known point in the ocean. Located in the western Pacific Ocean, at the southern end of the Mariana Trench, near the Mariana Islands, it is 10,902–10,929 meters (35,768–35,856 feet) deep. The University of Aberdeen’s Oceanlab videoed ones in the Mariana Trench at at a depth of 10,545 meters (34,597 feet) in December 2014. The Schmidt Ocean Institute’s SOI Rock Grabber Lander observed them around 10,668 meters (35,000 feet) in Sirena Deep, the world’s third deepest spot, also in the Mariana Trench south of Guam.

According to the Schmidt Ocean Institute: Amphipods thrive below 8000 meters, perhaps because there are no fish eating them. There are scavenger species that eat organic material from above, and predatory species that eat the scavengers. Why do these organisms thrive where fish dare not go? At the moment, we can’t say. If pressure is the key, perhaps these organisms have been evolving longer than fish and have more pressure-resistant proteins. There are hints. Scientists in Japan reported that a hadal Mariana amphipod has an enzyme that digests wood! Moreover, this enzyme (unlike many fish proteins) works better under pressure than it does at low pressure.

Might amphipods also have better piezolytes (compounds that stabilize proteins against high undersea pressures)? In amphipods collected from various depths on other expeditions, we found that the deepest species have high TMAO like snailfish (the deepest-living fish). But they are also high in other interesting molecules including glycerophosphorylcholine (GPC) and scyllo-inositol. Both are protective agents. GPC occurs at high levels in mammalian kidneys, where it protects your proteins from harmful effects of waste products. Scyllo-inositol is being tested to restore mis-folded brain proteins associated with Alzheimer’s Disease. Do these also protect proteins at high pressure?[Source: Schmidt Ocean Institute]

Cystisoma — the Amphipod with an Invisible Body

Cystisoma make up a genus of amphipod notable for its nearly completely transparent body, adapted for life in low light waters. Emily Underwood wrote in Smithsonian magazine; “There are few places to hide in the open ocean. Even in the “twilight zone” — the depths where sunlight gradually fades away — a mere silhouette can mean the difference between being a meal and finding one. But Cystisoma, a little-studied crustacean cousin of the sand flea, has a dazzling way to elude predators: It’s as clear as glass. Now researchers at Duke University and the Smithsonian have discovered how the solid creature manages to be so transparent — a finding that could lead to our very own invisibility cloak. [Source: Emily Underwood, Smithsonian magazine, January-February 2017]

“Cystisoma belong to a suborder of marine crustaceans called hyperiid amphipods, which live in every ocean, from just below the surface to right near the floor. The insect-like animals are masters of disguise and evolved dramatically different camouflage depending on the depth. Below 3,000 feet, where sunlight ends, the species are red or black. Transparent species such as Cystisoma tend to live between 30 and 1,000 feet, where the light is increasingly dim.

“To get to the bottom of Cystisoma’s disappearing act, Duke marine biologist Laura Bagge and Karen Osborn, a Smithsonian zoologist, went hunting off the coasts of Mexico, California, Florida and Rhode Island. Trawling with nets and searching with deep-sea-diving robots, they captured specimens of the roach-like critters, which are about the size of a human hand. In the lab, the scientists studied small bits of the animal’s shell under an electron microscope. The analysis revealed minute spheres all over the shell, as well as “tiny, hair-like, nipply-looking things” growing out of it, says Bagge. When the researchers used a computer to study how such microscopic structures affect light, they found the coating canceled out 99.9 percent of the light reflections, much as the egg-crate foam walls in a recording studio absorb sound. Moths’ eyes have a similar anti-glare coating, but this is the first time scientists have seen it used for camouflage.

“The curious spheres look like bacteria living on the shell surface, but they’re smaller than any bacteria we know of, says Bagge; the team is using DNA analysis to be sure. The finding could be useful either way. Engineers might be able to design similar structures to increase the transparency of glass and the absorption of solar panels, or even aid a kind of invisibility cloak that similarly distorts light. For her own research, Bagge wants to find out how being almost invisible affects the crustaceans’ social lives: “How does one clear animal find another to mate with?”

Giant Isopods of the Ocean Floor

Isopods make up an order of crustaceans that includes wood lice, pill bugs and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Females brood their young in a pouch under their thorax. There are over 10,000 identified species of isopod worldwide, with around half of them living on the ocean, mostly on the seabed. The fossil record of isopods dates back to the Carboniferous period (in the Pennsylvanian epoch), at least 300 million years ago. At that they lived in shallow seas. The name Isopoda is derived from the Greek roots iso- ( "equal") and -pod ("foot"). The largest isopod is in the genus Bathynomus. Some large species are fished commercially and eaten in Mexico, Japan and Hawaii. [Source: Wikipedia]

giant isopod

Giant isopods live in the deep sea, beyond the reach of daylight. They belong to the genus Bathynomus and were first discovered in 1879. They have 14 legs and can reach lengths of more than 30 centimeters (one foot). They spend the majority of their time on the seabed, waiting for food to fall from higher in the water column. They are thought to be abundant in cold, deep waters of the Atlantic, Pacific and Indian Oceans. Otherwise they are little studied and not much is known about them. [Source: Emily Osterloff, Natural History Museum]

According to Miranda Lowe, Principal Curator of Crustacea at the Natural History Museum, saidL When ocean animals die and begin to decay, parts that aren't consumed nearer the surface begin to fall towards the seabed. It can look white and fluffy, like snowflakes. But due to the scarcity of food, animals often have to be patient and wait for long periods to get what they need. This could mean waiting for years. Their metabolism is incredibly slow - a giant isopod kept in captivity in Japan reportedly survived for five years without eating.

On why they are so bigm she said: In the deep ocean, animals need to carry more oxygen, so their bodies can become bigger and have longer legs. Another possible factor in increased body size is that the deeper an animal lives, the less predators there are. This would mean that animals can safely grow to larger sizes. 'They don't have many natural predators. They've got a really hard outside shell, like other crustaceans, and there's not much meat in the animal - there's less meat than in a crab. So there aren't a lot of other animals that are going to want to eat it for anything substantial.'

Not only does the deep offer minimal food, there is also a lack of light. The isopods have sensory adaptations to help them navigate in the dark. “Bathynomus has very long antennae which are about half the length of its body. They're a large sensory organ so they can feel their way around. Giant isopods also have very large eyes in comparison to their body too. The isopods also have little hooked claws at the ends of their legs. These make the animal more stable on the ocean floor.

Giant Isopod Lives Five Years Without Food.

In February 2014, a giant isopod was found dead at Toba Aquarium in Japan after going five years without eating. This has been reportedly as longest period of time an animal has gone without food. Scientists do not know why it refused to eat. Keepers at the aquarium even play with its food and pretended to eat to try to encourage the crustacean to dig in. [Source: Donna Sawyer, Daily Mail, February 17, 2014]

giant isopod

After the creature died, the Daily Mail reported: The giant male isopod, called No. 1 to distinguish it from the nine other giant isopods kept at the Japanese aquarium, had not eaten since January 2009, when it ate a whole horse mackerel. No. 1's keeper Takeya Moritaki has been dumbfounded by the crustacean's hunger strike, claiming the deep-sea creature would at times pretend to eat by moving its mouth and front legs around the food to appease aquarium staff, however it never actually swallowed.

Mr Moritaki, was preparing the mackerel for No. 1's six-monthly feed, however the giant isopod sat motionless as the bait was lowered in to the tank, according to Rocket News. Realising something was wrong, No. 1 was lifted out of the tank and declared dead. At that point, the creature had starved itself for five years and 43 days. It has been widely reported as the longest time any animal under observation has gone without food.

“The death of the giant isopod, has been confirmed by the aquarium and a dissection was performed, however no cause of death was found. Research is underway on the remains of No.1 to find out why it refused to eat. Giant isopod No. 1 was taken to Toba Aquarium from the Gulf of Mexico in September 2007, measuring 29 centimeters and weighing one kilogram.


Barnacles (balanus glandula) are sticky crustaceans. They clamp on to rocks with a cement that is so strong scientists are studying it to make strong-water-resilient glues and other commercial uses. They are filter feeders who prefer strong currents to bring lots of nutrients passing their way. They are often found on wharfs, docks and ships. People stranded at sea have survived by eating fish attracted to barnacles that grew on their life rafts or boats in the middle of the sea. [Source: NOAA]

Of the more than 1,400 species of barnacles found in the world’s waterways, the most common ones are called acorn barnacles. As anyone who’s ever maintained a vessel knows, removing barnacles requires some elbow grease (or a pressure washer). That's why some boaters call them by their slang name: "crusty foulers."

How do barnacles stick to the undersides of vessels, to other sea life, to each other, and to pretty much anything they come in contact with? They secrete a fast-curing cement that is among the most powerful natural glues known, with a tensile strength of 5,000 pounds per square inch and an adhesive strength of 22-60 pounds per square inch. The glue is so strong that researchers are trying to figure out how it can be used commercially.

Barnacles like places with lots of activity, like underwater volcanos and intertidal zones, where they reside on sturdy objects like rocks, pilings, and buoys. Moving objects like boat and ship hulls and whales are particularly vulnerable to the pesky critters. Large barnacle colonies cause ships to drag and burn more fuel, leading to significant economic and environmental costs. The U.S. Navy estimates that heavy barnacle growth on ships increases weight and drag by as much as 60 percent, resulting in as much as a 40 percent increase in fuel consumption!

Barnacles feed through feather-like appendages called cirri. As the cirri rapidly extend and retract through the opening at the top of the barnacle, they comb the water for microscopic organisms. They quickly withdraw into their protective shells if they sense a potential threat. Barnacles secrete hard calcium plates that completely encase them. A white cone made up of six calcium plates forms a circle around the crustacean. Four more plates form a "door" that the barnacle can open or close, depending on the tide. When the tide goes out, the barnacle closes up shop to conserve moisture. As the tide comes in, a muscle opens the door so the feathery cirri can sift for food.

Sea Spiders

Sea spiders are not crustaceans but I didn’t know where else to put them so I put them here. Also known as Pantopoda or pycnogonids, they are marine arthropods of the class Pycnogonida found especially in the Mediterranean and Caribbean Seas, as well as the Arctic Ocean and near Antarctica. There are over 1300 known species, ranging in size from 1 to 10 millimeters (0.039 to 0.39 inches) to over 90 centimeters (35 inches) in some deep water species. Those found in relatively shallow depths tend to be small while those in Antarctic waters can grow quite large. Some small ones are so small some to their organs are on the their legs. [Source: Wikipedia]

sea spider

Although "sea spiders" are not true spiders, or even arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well known arthropod groups, such as insects or crustaceans. However this is in dispute, as genetic evidence suggests they may even be an ancient sister group to all other living arthropods. Most sea spiders survive by sucking juices of coral, sponges and sea anemones.

Reporting from northern California, Mel White wrote in National Geographic: “I'm rock-hopping above Horseshoe Cove with Jackie Sones, research coordinator at the Bodega Marine Reserve. "This," she says, holding up a pale orange creature about the size of her fingernail, "is a pycnogonid, commonly called a sea spider." Through a hand lens, it does in fact resemble a spider, although one with a touch of the Michelin Man, body and legs ribbed and puffy. "It uses its proboscis to puncture sea anemones and suck out fluids," Sones says. But this minute predator has a nurturing side. Sones upends the pycnogonid to reveal a cluster of tiny spheres like whitish caviar. "The males care for the developing young," she says. "They gather the eggs from different females and hold them with specially modified legs." Pycnogonids fascinate biologists because they're one of very few types of animals in which only males care for the young. [Source: Mel White, National Geographic, June 2011]

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated April 2023

This site contains copyrighted material the use of which has not always been authorized by the copyright owner. Such material is made available in an effort to advance understanding of country or topic discussed in the article. This constitutes 'fair use' of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without profit. If you wish to use copyrighted material from this site for purposes of your own that go beyond 'fair use', you must obtain permission from the copyright owner. If you are the copyright owner and would like this content removed from, please contact me.