What Are Sea Urchins?

Sea urchins, or echinoids, are small, seafloor dwelling invertebrates (they have no backbones), that are spiky, often brightly coloured and either globular (e.g. the edible sea urchin), disc-shaped (e.g. sand dollars) or heart-shaped (e.g. heart urchins).  There are over 900 living species, with an evolutionary history that stretches back 450 million years.

 

Above; Fragile pink sea urchins (Strongylocentrotus fragilis) live in communal groups off the west coast  of North America. (Picture courtesy of all-free-download.com)

Sea urchins, comprise the class Echinoidea and are echinoderms, a group which also includes starfish, brittle stars, sea cucumbers and sea lilies.  A familiar feature of echinoderms, particularly the sea urchins, is their spines or sharp projections — a feature which many unwary swimmers are painfully familiar with (Echinodermata is derived from the Greek and Latin for 'prickly skin').

The spines of a sea urchin connect via tubercles to its rigid skeleton (known as a test).  In most species, the test is circular when viewed from above, with the anus on top, and the mouth at the bottom (do sea urchins feed through their bottoms?)

Unlike starfish, sea urchins lack arms.  However, the test of a sea urchin consists of a series of plates which are arranged into pairs of columns, or 'bands', which radiate outward from the top (apex) of the skeleton and have a star-shaped configuration.

Five of the star-shaped 'bands' (termed ambulacra) bear tiny pores.  Through each pore, a tube-like structure extends, each one ending in a circular disc — these are called tube feet, and a sea urchin can move them, by controlling the pressure of water within them.  This system, unique to sea urchins and other echinoderms, allows a sea urchin to move, feed and in some species, respire.

Above: a specimen of Echinus esculentus, with spines removed, showing key features of sea urchins (Courtesy of UCL, Grant Museum of Zoology_specimen S174).

In addition to spines and tube feet, a sea urchin possesses tiny pincer-like structures (pedicellariae).  These help to keep it clean and to avoid parasites (such as marine larvae settling on it).  Many species of sea urchin also bear venom glands, within pedicellariae, which can be used to ward off predators or subdue prey.

In summary, a sea urchin can be thought of as a small rounded ‘box’, consisting of many small plates connected together, which uses its specialised projections (spines, tube feet, pedicellariae) to interact with its environment.

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What Are the Main Types of Sea Urchin?
How and What Do Sea Urchins Eat?
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Where Do Sea Urchins Live?

What are the Main Types of Sea Urchin?

There are two types of sea urchin: regular and irregular urchins.

Regular sea urchins (regular echinoids) are more common.  Regular echinoids are radially symmetrical (a vertical cut from one side of the animal to the other, across the centre, in two or more places will produce two halves that are mirror images of each other).  These sea urchins are hemispherical in shape (crudely resembling the top half of a sphere).  There is no front or back and these urchins can move equally easily in all directions. 

Other key features include: an anus which is in the centre of an urchin’s upper surface; a mouth which is in the centre of its lower surface; and long, pronounced spines.

Regular sea urchins live on the surface of the seabed and graze on seaweed.

Above: a specimen of Echinus esculentus, with spines removed, showing key features of sea urchins as seen on upper surface (Courtesy of UCL, Grant Museum of Zoology_specimen S174).

Above: a specimen of Echinus esculentus, with spines, showing mouth (with Aristotle's Lantern) on lower surface (Courtesy of UCL, Grant Museum of Zoology_specimen S251).



Irregular sea urchins, or irregular echinoids, include the sand dollars and heart urchins.  These have a distinct front and back, because of their bilateral symmetry (only one vertical cut, from front to back, through the centre, will produce two halves which are mirror images of each other).

In sand dollars, the mouth is still on the lower surface, and is either central or nearer the front.  The position of the anus is variable, but is nearer the back end. In heart urchins (e.g.  Echinocardium), the anus is right at the back of the urchin, whereas the mouth is near the front.

Irregular sea urchins have a flattened body, and either live partly or completely buried in soft sediment, such as sand.  Heart urchins live in burrows within the sediment, using secreted mucus to stabilise it.  Specialised tiny beating projections (cilia) on the spines draw in currents of water, containing fresh oxygen, for respiration, and food particles.




Above: specimen of Echinocardium, showing its upper surface (Courtesy of UCL, Grant Museum of Zoology_specimen NON2391)

Above: specimen of Echinocardium, showing its lower surface (Courtesy of UCL, Grant Museum of Zoology_specimen NON2391)

(Note that some people constrain the term "sea urchins" to the regular echinoids, but usage here of the term "sea urchins" also includes irregular urchins (sand dollars and heart urchins) — a usage which broadly follows the Natural History Museum Echinoid Directory).

 

Read more:

What Are Sea Urchins? 
How and What Do Sea Urchins Eat?
How Do Sea Urchins Move? 
How Do Sea Urchins Reproduce and Grow?
Where Do Sea Urchins Live?

How Do Sea Urchins Move?

Most sea urchins (particularly regular sea urchins) move using their tube feet.  These are tube-like projections, extending from their body (from the star-shaped ambulacral regions), which end in sucker-like projections.

Tube feet are extensions of a complex set of canals within each sea urchin — a water-vascular system.  Each tube foot can be extended when the urchin pumps fluid into it, allowing the foot to reach out.  The suction cup, at the end of each small foot, sticks to an object, or an adjacent part of the seafloor.  The sea urchin is then pulled towards the object, as the feet contract, when the water pressure within them is decreased.

In regular urchin species, such as Echinus esculentus, the tube feet allows the animal to grip onto rocks, within turbulent marine zones (down to depths of about 50 m), so that it is not detached by waves.  It is able to climb steep rocks and, being radially symmetrical, it is well adapted to moving equally well in all directions.

This method of locomotion — hydraulic powered tube feet — is unique to sea urchins and other echinoderms (which also includes starfish, sea cucumbers, sea lilies and brittle stars).

 

 

Above: a simplified vertical cross-section through a regular sea urchin, showing the tube feet (M = madreporite; GP = genital pore)

 

Irregular sea urchins, such as sand dollars and heart urchins, are adapted to moving in a different environment.  They move using their small, almost fur-like covering of spines, rather than tube feet.  Irregular sea urchins have a distinct front and back end (being bilaterally symmetrical) and they use their lower (oral) spines to move only in a forwards direction.  The urchin, Echinocardium uses it spines in a 'rowing' motion to move forwards within its burrow.

 

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What Are Sea Urchins?
What Are the Main Types of Sea Urchin?
How and What Do Sea Urchins Eat? 
How Do Sea Urchins Reproduce and Grow?
Where Do Sea Urchins Live?

How and What Do Sea Urchins Eat?

In most sea urchins (such as the regular sea urchins), the mouth is on the bottom side of the body, whereas the anus is on the upper surface.  This is an excellent adaptation to living on the seafloor, so that the mouth is in contact with the substrate, enabling it to graze on algae and detritus.  Some species, such as the European Edible Sea Urchin (Echinus esculentus), a North Atlantic species, are omnivorous and feed on both sponges and bryozoa, in addition to seaweed.

In these regular sea urchins, the mouth consists of a complex feeding mechanism (known as Aristotle's lantern) which consists of five strong jaws, each with a single tooth of the mineral calcite.  After food has been grasped by the apparatus, it is then passed up through a complexly coiled gut, and the waste is passed upward through the anus.




Above: a simplified vertical cross-section through a regular sea urchin, showing the digestive organs.  (M = madreporite; GP = genital pore).

 

 

In heart urchins and sand dollars (irregular sea urchins) there is a distinct front and back (due to their bilateral symmetry).  The mouth is still on the lower surface, though it is nearer the front, and the anus is nearer the back end.

Above: specimen of heart urchin (Echinocardium), showing its lower surface, including mouth and anus (Courtesy of UCL, Grant Museum of Zoology_specimen NON2391)

Sand dollars have short spines which move the sand and organic debris over their upper surfaces and into their mouths — allowing the sea urchins to 'sieve' through the substrate surface for food.

Heart urchins, living 10 cm or deeper within burrows, beat tiny hairs (cilia) on their spines to draw in currents of water, containing small particles of food.  These currents also help to wash the faecal waste along a specialised sanitary pipe, at the back of the burrow.

 

 

Above: simplified cross-section through a burrow of Echinocardium. Note the long tube feet above and behind the urchin which are used to construct parts of the burrow. Smaller sticky tube feet, at the front of the urchin, catch food particles and pass them to the mouth.

 

 

Read also:
What Are Sea Urchins?
What Are the Main Types of Sea Urchin? 
How Do Sea Urchins Move? 
How Do Sea Urchins Reproduce and Grow?
Where Do Sea Urchins Live?

Where Do Sea Urchins Live?

Like other echinoderms (which also include starfish, brittle stars, sea cucumbers and sea lilies), sea urchins are marine invertebrates.

They occur in oceans, mainly in shallow coastal waters.  Lacking the ability to osmoregulate (control the balance of salt and water), they rarely venture into brackish waters (where there is a freshwater content).

Most sea urchins (particularly the regular sea urchins) live on harder ocean substrates, clinging onto rocks in turbulent zones using their tube feet.  The irregular sea urchins (including sand dollars and heart urchins) prefer more sandy substrates.  Sand dollars live just beneath the surface, whereas heart urchins live deeper (10 cm or more) within burrows. 

Read more:
What Are Sea Urchins?
What Are the Main Types of Sea Urchin?
How and What Do Sea Urchins Eat?
How Do Sea Urchins Move?
How Do Sea Urchins Reproduce and Grow?

How Do Sea Urchins Reproduce and Grow?

The sexes are separate in sea urchins. At breeding time, normally in the summer, the gonads (containing eggs or sperm) swell considerably within the sea urchins' plated skeletons.

Spawning (releasing of eggs and sperm) takes place on a massive scale, at the same time within members of the same population.  These products are released from the sea urchins via the genital plates, at the top of the organisms.

 

Above: a simplified vertical cross-section through a regular sea urchin, showing the gonads.  (M = madreporite; GP = genital pore)

 

 

External fertilisation then takes place with new sea urchins starting their lives as swimming larvae (echinopluteus). The swimming larvae, unlike most sea urchin adults, are bilaterally symmetrical (one imaginary plane divides them into two equal halves).

These larvae also have arms, unlike all sea urchin adults, which they use to trap plankton for food.  After several weeks, a part of each larva (the rudiment) starts to develop into an adult sea urchin and structures such as the arms are resorbed (dissolved and assimilated).

The newly developed sea urchins — those that have not been eaten by predators — then sink to the seafloor.  Most of these will have no front or back end (being radially symmetrical) and will be able to move equally easily in all directions.

Some species of sea urchin produce eggs which are brooded by females and develop directly into adult sea urchins (there is no planktonic stage).  In these species, the females can be more easily distinguished from the males as they have specialised brood pouches for their eggs.  Species which adopt this life-cycle strategy are usually confined to polar waters.

 

 

Read more:
What Are Sea Urchins?
What Are the Main Types of Sea Urchin?
How and What Do Sea Urchins Eat?
How Do Sea Urchins Move?
Where Do Sea Urchins Live?

“Sea Urchins Feed through Their Bottoms”. True or false?

"Sea urchins feed through their bottoms"... I have heard several times, including a similar remark from the film, Johnny English [DVD](upon hearing this, in a sushi bar, Rowan Atkinson's character then proceeds to spit out the sea urchin dish, in disgust).

This myth stems from the remarkably different anatomy, or body plan, of sea urchins (echinoids).

Sea urchins consist of a somewhat spherical body, covered in spines.  In most species, the mouth is on the lower or 'bottom' surface (known as the 'oral surface') — whereas its ‘true’ bottom or anus is on the opposite side of the organism, on its upper surface (‘aboral surface’).

Thus, sea urchins feed through their ‘bottom’ surface, but this includes their mouth, not their anus. So Rowan Atkinson’s character need not be so disgusted…

This unusual body plan (at least compared to our anatomy) appears to be an adaptation of the sea urchin to living on the ocean floor, so that its mouth is in contact with the substrate, allowing it to feed on algae or detritus.  As the anus is situated on its upper surface, waste does not therefore become mixed up in its food.

Read more about sea urchins...