Echinoidea - Sea urchins

Echinoidea - Sea urchins


Sea urchins are well known animals and some species are so widespread in the coasts of the Mediterranean Sea that bathing is difficult.














Echinoidea with regular shape







































Echinoidea with irregular shape






















Common name

: sea urchins


Within the classroom Echinoidea of the great phylum of Echinodermata we find the sea ​​urchins, a large group of exclusively benthic animals (i.e. that live in contact with the seabed or in any case attached to a solid substrate) and deuterostomes (the mouth was not formed during the cell differentiation process but subsequently).

The class includes about 950 marine species found in all habitats of the world up to 5000 m of depth. Of these, 26 species are found in the Mediterranean Sea of ​​which 11 are regular and 15 irregular.


The peculiarity of sea urchins is the fact that their body is formed by a rigid skeleton made up of several plates welded together that take on a rigid consistency and provided with more or less showy quills depending on the species.

Sea urchins (like all echinoderms) are exclusively marine animals and are not very tolerant of variations in salinity due to their anatomy.

Among the echinoderms the forms are different depending on the species in fact they are divided into two large groups: echinoidea with a regular-shaped body and echinoidea with an irregular-shaped body.

The species with the REGULAR SHAPE body are the classic sea urchins that we all know where the body assumes a globose or sub-conical shape and are covered with spines formed by calcium carbonate in the form of calcite mixed with organic substance, more or less long and distributed fairly evenly throughout the body depending on the individual species.

They are animals that at the adult stage have pentameral symmetry (the body is divided into five regions arranged around a central disc) and the dimensions vary from a few millimeters up to 30 cm.


The body is divided into two hemispheres:


1) oral hemisphere that it is the part facing the substrate in the center of which is the mouth which is recognized by the presence of the 5 teeth of the Aristotle's lantern which is the structure responsible for chewing surrounded by a membranous area called the peristomal membrane;

2) aboral hemisphere where the anal region is located (called periproctus) and is the part facing upwards formed by a ring with 10 plates and there is a structure called madreporite or madreporic plate through which the liquid of the aquifer system is in connection with the external.

Among the quills there are the pedicellarias which are very mobile appendages with different functions and of different types: pedicellarias that end with a sucker and are responsible for moving or holding the objects with which some species of sea urchins cover their body; pedicellaria in the form of pliers used for grasping; pedicellaria ending in a small hollow thorn that can inject poison into the flesh of an attacker (eg Asthenosoma varium).

The movement takes place through the aquifer system which is formed by vessels very similar to the blood vessels that circulate throughout the body of the animal inside which water drawn from the external environment flows; the water pumped into this system causes a variation in the turgor of the pedicels (see in the photo "skeleton of sea urchins" the double series of small holes in the ambulacral plates where each one carries a pedicel in perfect pentaradiate symmetry) which can thus move. Very particular is the locomotion as it happens in the members of the order Cidaroida where there are very developed and few numerous quills thanks to which they move as if they were on stilts even if it is not easy to see them as they live from 50 to 200 m deep.

Gas exchanges take place thanks to the ambulacral pedicels through which oxygen enters the body.


The reproductive system is formed by five gonads (which are the edible part, the one that is normally appreciated by gourmets) joined together by filaments and when they are ripe they appear voluminous and of a more or less intense orange color and extend from the aboral part where they communicate with the outside almost as far as Aristotle's lantern.

All the external appendages of the sea urchin (thorns, pedicels, pedicels) if lost regenerate very quickly as well as the shell wounds are repaired with the reformation of the calcareous skeleton.

The best known species are the Arbacia lixula (order Arbacioida - family Arbaciidae) known as "black hedgehog" or "male hedgehog" and the Paracentrotus lividus (order Echinoida - family Echinidae) known as "purple hedgehog" or "female hedgehog" which is the species that we are normally used to eat in the Mediterranean.

Arbacia lixula - common name: black hedgehog or male hedgehog

Paracentranthus lividus - common name: female hedgehog or purple hedgehog

Strongylocentrotus droebachiensis (Note 1)

In America the species are more widespread: Strongylocentrotus franciscanus, S. purpuratus is S. droebachiensis (order Echinoida - family Strongylocentrotidae) which are also exported to Japan for use in the famous sushi.

Other very beautiful species that we frequently find in the Mediterranean Sea are: Cidaris cidaris (order Cidaroida - family Cidaridae) of gray - yellowish color that lives on the bottom of the sea and feeds mainly on sponges and gorgons and is known by the common name of "pencil hedgehog";

Cidaris (Note 1)

there Stylocidaris affinis (order Cidaroida - family Cidaridae) which lives on sandy bottoms; the Centrostephanus longispinus (order Diadematoida - family Diadematidae) rare species that lives in sandy and muddy bottoms, which is not easily seen as it lives between 40 and 200 m of depth; Sphaerechinus granularis (order Temnopleuroida - family Toxopneustidae) also very common and is a hedgehog that is also found starting from a few meters deep and up to 100 m, it has a very beautiful and elegant appearance with purple-colored quills with a white tip; Echinus apple tree (order Echinoida - family Echinidae) commonly known as "melon hedgehog" with a fairly spherical case and green quills and lives in the depths between 25 and 1100 m of depth even if it is not very widespread in the Italian seas.

Echinocyamus spp.

In the irregular shapes of the Echinoidea class we find those animals that have adapted to live in the sandy and muddy bottoms. Their shape can be heart-shaped, oval, more or less depressed and the position of both the anus and the oral apparatus can be variously localized (photo on the side). In these forms we speak of secondary bilateral symmetry. The quills are not large in size but are very numerous compared to regular shaped curls.


Hedgehogs are benthic animals that therefore live in contact with the seabed or anchored to a solid substrate. They generally live in rocky bottoms and some species dig real niches in the rocks using the teeth of Aristotle's lantern. The Paracentrotus lividus for example, it is a great digger of rocks as it digs niches in which it burrows and remains there permanently as it often happens that it is no longer able to get out because the opening made when it was in smaller stages, has become so narrow that it does not he is more able to get out of it and depends on the materials that are carried by the current to feed himself.

Sea urchins in general are sedentary animals and the movements take place extremely slowly.

All echinoids are negative phototropic animals, which means that they tend to seek shade and even species such as the Paracentrotus lividus they tend to cover their bodies with pebbles or pieces of shells to protect themselves from the light. Even in tropical areas, most of the species have nocturnal habits and spend the day hidden in crevices or holes in the rock.


Regular hedgehogs that live in rocky substrates are mostly grazing herbivores and their diet is based on algae.

The video below shows how, off the Californian coasts, giant algae grow which are a good source of food for the many sea urchins that attack them and which they greedily feed on ...

The two species A. lixula is P. lividus when they form carpets in the rocks, they completely strip the vegetation of the area from algae. The species of regular sea urchins that live on incoherent bottoms and in deep water are detrivores and consumers of the organic substance that is deposited in the sediment as well as there are irregular urchins that also feed on Diatoms, Foraminifera and other small benthic organisms.


In sea urchins, as in all echinoderms, the sexes are separate and there is no sexual dimorphism as males and females are completely similar to each other. Reproduction occurs without mating as the eggs are laid in the water where they are fertilized by the male seed. In several species, gametogenesis is regulated by the photoperiod so that males and females produce eggs and sperm simultaneously.

From the fertilized egg the planktonic larvae develop whose peculiarity is that they are bilaterally symmetrical which through various metamorphoses arrive at the adult form with pentaradial symmetry.

Generally there is no parental care even if some species are incubators that is to say that they make the eggs grow in the peristome or other species form a kind of tent by compressing the spines around the mouth (Emocidaris nutrix).


The main predators of sea urchins can be both invertebrates and vertebrates. Among the invertebrates we find the sea stars and some Gasteoropods while among the vertebrates the species Balistes vetula (pig fish), some sea birds such as gulls and some mammals such as the sea otter.


The name Echinodermi comes from the Greek echinos "Quill" e dérma "Skin" due to the fact that numerous representatives of this class are provided with quills.

Sea urchins are widely used in the laboratory as, thanks to the large quantity of eggs they produce and the ease with which it is possible to fertilize them even in the laboratory, they are the best animals for the study of fertilization methods and experimental embryology.

The gonads of these animals are much appreciated by gourmets all over the world whether eaten as they are with bread or used to season spaghetti (both dishes are typical of Italian cuisine) and the best time of year to enjoy them is the winter (January - February), the period in which they have the maximum flavor.

In many areas of Italy and the world more generally, their collection outside specific periods of the year is prohibited (for example in Sardinia - Italy - the collection and therefore its consumption is possible only between the months of November and April) in order not to compromise their reproduction.


(1) Original photograph courtesy of NOAA

Sea urchin

Sea urchins (/ ˈ ɜːr tʃ ɪ n z /), are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 meters (16,000 ft 2,700 fathoms). [1] Their hard shells (tests) are round and spiny, usually from 3 to 10 cm (1 to 4 in) across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, triggerfish, and humans.

Like other echinoderms, urchins have fivefold symmetry as adults, but their pluteus larvae have bilateral (mirror) symmetry, indicating that they belong to the Bilateria, the large group of animal phyla that includes chordates, arthropods, annelids and molluscs. They are widely distributed across all the oceans, all climates from tropical to polar, and inhabit marine benthic (sea bed) habitats from rocky shores to hadal zone depths. Echinoids have a rich fossil record dating back to the Ordovician, some 450 million years ago. Their closest relatives among the echinoderms are the sea cucumbers (Holothuroidea) both are deuterostomes, a clade which includes the chordates.

The animals have been studied since the 19th century as model organisms in developmental biology, as their embryos were easy to observe this has continued with studies of their genomes because of their unusual fivefold symmetry and relationship to chordates. Species such as the slate pencil urchin are popular in aquariums, where they are useful for controlling algae. Fossil urchins have been used as protective amulets.

Sea urchins: Do you eat the male or the female?

One of the most common mistakes that we naturalists are horrified by, in addition to the classic confusion between octopus and octopus, is the famous culinary diatribe about sea urchins. The convinced fisherman proclaims "only the female hedgehog is eaten". It's correct?

The "female" sea urchin

What is commonly called "female hedgehog" or "purple hedgehog" is a echinoderm of the species Paracentrotus lividus (Lamarck, 1816)


  • Kingdom Animalia
  • Phylum Echinodermata
  • Echinoidea class
  • Parechinidae family
  • Kind Paracentrotus
  • Species Paracentrotus lividus

P. lividus distributed on all the coasts of the Mediterranean and part of the Atlantic coasts, it prefers rocky bottoms covered with algae and meadows of Posidonia oceanica. It shows a slightly depressed dermaskeleton, robust and not dense spines, the color varies from reddish-brown to purplish.

It is a sciaphilous species, not very fond of light, especially active at night when it grazes in search of algae or Posidonia oceanica. The sexes are separate but there is no sexual dimorphism, it is therefore impossible to recognize the male individual from the female individual from a simple observation. Of P. lividus the gonads are eaten, orange and large, they are considered a real delicacy typical of some Latin countries such as Italy, Spain and France.

The "male" sea urchin

What in popular tradition is instead called "male hedgehog" is the echinoderma belonging to the species Arbacia lixula (Linnaeus, 1758).


  • Kingdom Animalia
  • Phylum Echinodermata
  • Echinoidea class
  • Arbaciidae family
  • Kind Arbacia
  • Species Arbacia lixula

A. lixula has a spherical body, more compressed than P. lividus, its quills are longer and more numerous, black in color. Also in this case the sexes are separated and not recognizable externally, the reproduction takes place through the release of the gametes in water with subsequent fertilization.

It is clear, from what has been said, that for both species of sea urchins there is both a male and a female sex, not recognizable, however, by a simple observation.


They are bilateral organisms, from larvae, while the adults have an apparent radial symmetry, which is secondary and partial. The embryo is in fact bilateral symmetry and resembles that of the chordates. During the growth there is then a preponderant development of the left portion of the body at the expense of the right.

All echinoderms exhibit an external structure with pentameter symmetry at some stage of life: some undergo further modifications that restore bilateral symmetry.

Echinoderms are coelomates deuterostomes closely related to chordates and hemicordates.

In them, a head is not differentiated and their rounded bodies are organized with a criterion similar to that used to build a wheel. From the body of these animals, many appendages of various invoices branch off. The sea lily looks like a strange plant. It adheres to the bottom of the sea and searches for food by waving its long feathery tentacles that can lock up like the petals of a flower. The sea cucumber usually rests on one side of its elongated body and crawls in the same manner as a worm. Starfish have long pointed arms. There Ophiothrix, as it crawls along the bottom, it waves its snake-like arms.

Most echinoderms are well protected against predators. Their bodies are lined with limestone plates, and although this protective coating may seem stiff, some animals can actually make many movements. Echinoderms have two different defense systems: the quills and the pedicels.

The sea urchin's quills are so long and thin that it ends up looking like a ball bristling with needles. Just as dust collects between the fringes of a carpet, debris of various kinds can settle between these quills. When a fragment or a small animal lands on an echinoderm, the pedicels move it. Only the sea cucumber has limestone plates scattered throughout the dermis that lines its soft body. To catch food, sea cucumbers project tentacles out of their bodies.

The echinoderms have, within the body, only one system of aquifers, which are used for breathing, for capturing prey and for locomotion. The water, entering through a perforated plate (madreporic plate), passes through a complicated system of channels connected with small ambulacral pedicels. Each pedicel widens to form a small sac or ampoule, while at the end it ends with a sucker. As the water is forced in and out of the pedicel, this alternately adheres to the ground when it is swollen with water and detaches from it when it contracts. Each animal has hundreds of ambulatory pedicels, which come out through tiny holes in the dermaskeleton. When the echinoderms move on a sandy bottom, the ambulacral pedicels do not work for the suckers to come into action, it is necessary for the animal to move on a hard bottom, for example rocky. Starfish also use suckers to open mollusc shells. Echinoderms are animals without eyes but equally capable of receiving light signals. The ends of the ambulatory pedicels also represent the site on which the photoreceptor cells are arranged, which cooperate synergistically and function as a whole as a single large compound eye [1].

Almost all of these animals feed on small organisms found on the sea floor. The sea cucumber and the sand dollar swallow large quantities of sand, of which they digest only the particles of organic matter contained in it. The digestive system of echinoderms is simple and consists of a mouth placed on the lower face of the animal's body, while the anal opening is located on the upper face, that is, in a position diametrically opposite to the mouth. The mouth and anus are connected by a digestive tract. Echinoderms breathe by means of all the thinned parts that protrude outward. In sea urchins, respiration also occurs by means of digitiform expansions with thin and hollow walls called gills which are found on the ventral surface where the pedicellarias are also present.

The nervous system is represented by a simple ring of nervous tissue, which surrounds the mouth and divides into nerve cords. A fluid fills the body cavity and supplies the organs. In echinoderms, three different parts of the body can be distinguished - the digestive tract, the body cavity and the outer wall.

The sexes are separated and fertilization takes place in water. The echinoderm larva has bilateral symmetry and its development is very different from that of the lower invertebrate larvae. For this reason the echinoderms are considered quite evolved animals and in them it is already possible to establish kinship relations with the more primitive chordates.


By Judy Follo and Daphne G. Fautin

There are about 940 species of echinoids distributed around the world in marine habitats from the intertidal up to 5000 meters deep. Their fossil record is extensive due to their test (an internal skeleton) and dates back to the Middle Ordovician period.

Echinoids are commonly grouped as regular or irregular, with the greatest differences relating to oral structure, body shape and position of the anus. Regular echinoids are sea urchins generally found on rocky substrates. Irregular ecinoids are sand dollars, which are generally found on sandy or soft soils.

Like all echinoderms, ecinoids are pentaradially symmetrical, have a hydro-vascular system and have an internal skeleton consisting of calcitic ossicles (plaques). A distinguishing feature of ecinoids is that the ossicles interlock (overlap) and are fused in a globular or discoid test its flattened or concave oral side faces the substrate and the aboral side is arched in most species. The mouth, in the peristomal membrane, contains a powerful masticatory apparatus called Aristotle's lantern. The lantern is made up of five jaws and is capable of extending through the mouth of some hedgehogs. The mouth leads to the intestine and anus, which is found in the center of the aboral surface in regular echinoids. The anus is posterior or on the oral surface of the irregular echinoids.

Spines and tubular feet that surround the peristome function in locomotion, burrowing and collecting food. Generally, hedgehogs have longer spines and sand dollars have shorter spines which give them a fuzzy look. The tubular feet are part of the water vascular system characteristic of all echinoderms. The pincers located between the spines are called pedicellariae. Some types of pedicellariae and specialized spines of hedgehogs contain poison used for self-defense.

In normal curls, the ossicles, or plates, of the test are arranged in ten longitudinally oriented columns. Two adjacent columns each form one of the five ambulacral series. These are the plates through which the tube feet extend. On the aboral side, the tube feet function in breathing and sensation. The series of ambulacral plates are evident in the clean sand dollar test: limited to the aboral side, they are arranged in a petaloid pattern. At the aboral end of the interambulacral series of regular curls are the five (sometimes four) genital plates, through which the gonopori is opened. One of the genital plates serves as a sieve plate, or madreporite, for the water vascular system. Together the madreporite, the anus and the gonoporae make up the periproct.

Most echinoids have five obvious gonads arranged interambulacrally. The sexes are separate. In some species, gametogenesis is regulated by the photoperiod so that spawning of most or all members of a population occurs at the same time. Some hedgehog females hatch their young externally, under the protection of their spines or tubular feet. In species with indirect development, an echinopluteus larva is produced. This larva is bilaterally symmetrical and undergoes metamorphosis to reach the pentaradial symmetry of the adult.

Echinoids graze on virtually anything they encounter, plants or animals. This includes algae, bryozoans, and dead animals.

Members of this class are food for crabs, starfish, fish, birds, otters, and other mammals. Probably the most important contribution of these animals to scientific knowledge is their embryological development. Researchers study the development of deuterostomes using sea urchin eggs, due to the clear radial cleavage during the development of a zygote. Echinoids of economic importance to the United States are red ( Strongylocentrotus franciscanus ), purple (S. purpuratus ) and green ( S. droebachiensis ) Sea urchins. These hedgehogs are harvested for their eggs and exported to Japan. Bottarga, known as uni, is used in sushi.


Brusca, R.C. and G.J. Abrupt. 1990. Chapter 22: Phylum Echinodermata. Invertebrates. Sinauer Associates, Inc., Sunderland, Massachusetts.

Hyman, L.H. 1955. Invertebrates: Echinodermata: The Coelomate Bilateria. Volume IV. McGraw-Hill Book Company, Inc. New York and other cities.

Kozloff, E.N. 1990. Chapter 21: Phylum Echinodermata. Invertebrates. Saunders College Publishing. Philadelphia and other cities.

Price, R.J. and P.D. Tom 1995. Sea urchins. Publication of the Sea Grant Extension program.


Judy Follo (author), Daphne G. Fautin (author).

General informations

Hedgehogs live on the bottom of the seas and are sedentary animals and their movements take place extremely slowly. They present a rough and thorny outer coating called dermaskeleton, made up of calcareous platelets joined together and contained in the thickness of the skin. The spines are articulated on the platelets and through small holes, the ambulacral pedicels are astro-flexed, tubes that end with suckers and allow the hedgehog to move. Breathing is essentially cutaneous, but there are also small gills near the mouth, called "Aristotle's Lantern" consists of 5 teeth. Some species, on the other hand, do not have teeth.

Variety and edibility

The Echinoidea class includes about 950 marine species widespread in all habitats of the world up to 5000 m of depth. Of these, 26 species are found in the Mediterranean Sea of ​​which 11 are regular and 15 irregular. THE "regular" curls they are the most common, are widespread everywhere and are equipped with quills: the Paracentrotus is of a variable color from brown to purple and is edible.
THE "irregular" curls they have an oval body, they live buried in the debris or in the mud, their skeleton shows on the upper part a star figure that represents the transformed locomotor system. They have very short and fragile quills that form a kind of furry coat around the animal. The best known species are:

Arbacia lixula

Known as "black hedgehog" or "male hedgehog".

It is so named because of the erroneous belief that it is the male specimen of Paracentrotus lividus is a sea urchin of the Arbaciidae family. A. lixula has the body slightly compressed along the vertical axis and can reach 6 cm in diameter. Its entire surface is covered with numerous sharp quills of glossy black color and on the ventral side is the very wide oral opening.

Paracentrotus lividus

Known as "purple hedgehog" or "female hedgehog".

It is the species that we are normally used to eat in the Mediterranean. It is called "female hedgehog" due to the erroneous belief that it is the female specimen of Arbacia lixula, it is a sea urchin of the Echinidae family. He has a depressed body and quills violet color but sometimes also olive green or reddish brown depending on the concentration of pigment that the quills contain. It is about two distinct species, respectively Arbacia lixula, the inedible one and Paracentrotus lividus, that of gastronomic interest of which the part we consume is constituted by the gonads, the reproductive system, of both sexes. When they are ripe they appear voluminous and of a more or less intense orange color.
In America the most common species are: Strongylocentrotus franciscanus, S. purpuratus, Strongylocentrotus franciscanus, S. droebachiensis, also exported to Japan to be used in the famous sushi.

How to take them

They are detached with any knife, inserted under the hedgehog and then pulled. Once collected, they are placed in a retina.

When is it forbidden to collect hedgehogs?

In many areas their collection outside specific periods of the year is prohibited (for example in Sardinia the collection and therefore its consumption is only possible between the months of November and April) in order not to compromise their reproduction.

If we prick ourselves with the spines of the hedgehog

The sea urchin's spines can sting, provoking pain and burning. The quills are fragile and very often break inside the wound, they do not inject poison, but the sting is painful and becomes easily infected. We must proceed with one thorough disinfection of the wound and to removal of quills with sterilized tweezers, making sure that the plug or part of it is completely removed from the skin. They can be applied repeatedly vinegar compresses, which dissolve any fragments of thorn. Some species are poisonous and their quills can cause symptoms such as swelling, local paralysis, in extreme cases even death.

On the market

You can find them in the markets or by fishmongers, already clean and therefore cheaper or to clean, otherwise you can buy the sea urchin pulp packaged in glass jars or aluminum cans whose conservation is guaranteed by the pasteurization process.

The word Echinoderms derives from the Greek: "echinos = sting" and "dérma = skin" and means "skin with thorns".

Idioms in dialect

In Bari dialect: "Go a scazze le rizze c'u cule" = "Go and squash the sea urchins with your butt" that is: shut up that you are saying nonsense.

In other languages

  • In English they are called: sea urchins
  • In French: oursins
  • In Spanish: erizos de mar
  • In German: seeigel


  • In Croatia hedgehogs are not eaten. Asking for explanations about it, the answer is that trivially, they are not to their liking! (photo below).

  • Sea urchins are a lot used in the laboratory grazie alla grande quantità di uova che producono e alla facilità con cui è possibile fecondarle, sono i migliori animali per lo studio delle modalità di fecondazione e della embriologia sperimentale.
  • Di recente si è scoperto che i ricci di mare possono raggiungere l'età di 200 anni senza mostrare segni evidenti di invecchiamento e con capacità riproduttiva non inferiore a quella degli esemplari giovani.

Idee originali

I loro gusci, privati degli aculei (basta lasciarli a bagno nella candeggina) si presentano di vari colori e possono essere utilizzati a scopo ornamentale o per esempio come delle originali palle per l'albero di Natale, come ci viene suggerito dal blog o ancora farli colorare con le tempere dai bambini e usarli poi come simpatici e colorati portapenne o soprammobili.

Video: Japanese Street Food - GIANT SEA URCHIN Uni Sashimi Japan Seafood