Sea Urchin

Sea urchins are interesting animals often offered to the marine aquarium hobby. They can make excellent scavengers, and some species will graze unsightly nuisance algae growths from the reef aquarium. In their natural environment they can be a danger to the unwary holiday maker who might unwittingly step on one. SCUBA divers and snorkellers also become vulnerable to making accidental contact with any part of their body when they fail to notice the menacing spines thrusting out from nooks and crannies, topping rock formations, or massed on the seabed. Urchins often gain a whole range of adjectives added to their recognised name by those who make painful contact with their spines! Urchins are classified as Echinoidea. The Greek word “echinus” literally means “hedgehog” and describes the appearance of many of the short spined species very well. The Class Echinoidea has close to 1000 species altogether, and these include the regular urchin, sea mouse, sand dollar, and other more obscure species.

The spines of urchins are most often quite thin and very sharp, but a few species have comparatively thick spines, and not necessarily sharp. Indeed, some species have distinctly blunt ended spines. Some are long spined while others are short spined. Many of them deliver toxins via the spines, and a few of these toxins can be dangerously powerful. Tube feet, equipped with sucker pad endings, propel them along at sometimes remarkable speeds for such a sluggish-looking animal. Sea urchins have a spherical shape with a pleasing symmetry, displaying a 5-segmented body form, which is usually more clearly seen from dead skeletal specimens. A complex organ which is generally unseen as it is on the underside of the urchin’s body, is the Aristotle’s Lantern. This is basically a grazing device, with teeth which can be extended from the mouth. Some species use it for algae grazing while others can dig into the muddy sea bed to search out food items. What is frequently mistaken to be a single eye-ball gazing up from the centre of the sea urchin’s upper body surface is, in fact, its anus, or faecal sac! One has to say that it really is a very beautiful bottom!

When taking a cursory glance at a long spined urchin, like the common Indo-Pacific species, Diadema setosum, with its dense mass of spines radiating out from the globular central body, there seems nothing better to do than just get away from the creature. But stop and take a closer look from above. There is real beauty tucked away at the bottom of those spines. Look at that central eye-like anus for a start, with different coloured rings surrounding the black central “iris”. Electric blue spotting ornaments each of the five segments of the body (ambulacral areas), visible between the shorter secondary spines and fine hair-like growths called “pedicellaria”. Each segment also carries a brilliant white spot. The spines are not poisonous, but are capable of inflicting seriously painful wounds. This species is very common at source and often reaches the aquarium market. Be sure to only buy a specimen which shows no sign of damage, as this is a tricky creature to handle and pack without it suffering some breakage of its spines. Broken or lost spines do repair quite quickly when the urchin is in its native sea environment. But in the aquarium, following the stressful process of being caught, transported, packed and shipped, there is less chance of recovery for a damaged urchin in a captive situation. A white spined Caribbean species, Diadema antillarum, has a similar form to that of setosum.

Other species from other families may have a very different appearance and behaviour. A totally different type of spine may be seen in examples of Echinometra mathaei. This Indo-Pacific species is more likely to be found as a solitary animal, unlike the long spined black urchins. The spines are very much sturdier too, projecting from the body with a broad base, tapering to a sharp tip. This species also does not carry any venom in its spines. It is able to carve out hiding places in the calcareous rock of the reef, which it does with its jaws and spines.

A decorative species which occasionally comes to the aquarium market is Echinothrix diadema. The spines are prominently banded and quite sturdy at their base, but become very finely pointed at their tip. The globular body colouring is variable. and this urchin is often more colourful and distinctly marked when found at night-time than its normal daytime mode. It is a good scavenger and is one of the more efficient algae grazers. If compatible with the other inhabitants of the reef aquarium this species makes a desirable addition to the collection, but it may be predatory on hard corals!

The most popular and most freely available of the algae grazers is Mespila globulus. This urchin has the added advantage of carrying very short and densely packed spines, leaving an area of its body devoid of spines. There are 10 of these spineless areas which are generally dark bluish in colour. Without very long spines to worry about there is less chance of damage during the collection and shipping of this species. Additionally there is less chance of it damaging other invertebrate life in the reef aquarium, which is something the long spined urchins may do accidentally to anemones and some of the soft corals. They are sometimes difficult to detect on the reef where they frequently move algae and coral debris up onto their body to form an effective camouflage.

For very colourful species of urchin it is necessary to look to the venomous creatures. They have sufficient protection from their venom-packed spines to not need the added security of inconspicuous colouring. One species which is quite common in the Red Sea is Asthenosoma varium. This same species from Indo-Pacific areas is somewhat different in appearance. Another variable species, Tripneustes gratilla, is also common in the Red Sea. There are a number of other animals which will sometimes be found living on toxic urchins in a commensal state, but there are two particularly spectacular ones worth mentioning because they would make remarkable additions to a suitably appointed reef aquarium where toxic urchins are successfully maintained. Zebra crabs are extraordinary by any standards, and have an ability to install themselves well down into the spines, which are loaded with a powerful toxin. Although quite differently coloured to the host toxic urchins, which are usually Asthenosoma ijimai or varium, the crab is marked in such a way as to make it very difficult to detect on the urchin. The other creature is a very pretty shrimp, Coleman’s shrimp. Being dark red and yellow this colourful shrimp shares some of the toxic urchin’s livery, yet when it is out on top of the spines it is very obvious. But when it takes shelter lower down and well into the spiny cover, it virtually disappears. Both of these animals share the valuable protection afforded by the toxicity of the host urchin, but the author has never seen both being hosted by a single urchin. Unlike anemones which often host both crabs and shrimps along with one or more species of symbiotic anemone fish, urchin’s commensal creatures seem to be more territorial. The author once accidentally brushed one of these urchins during a night diving expedition, only lightly scratching the side of one hand, but the instant pain was severe.

Astropyga radiata is another toxic urchin, and is more likely to be seen as an aquarium specimen than the much larger and more potent species discussed above . When juvenile it may have very elongated spines, and occasionally carry these into maturity, but the shorter spined form is more suited to the process of bringing this urchin to market. A. radiata may also carry commensal shrimps, but they are quite minute to suit the size of the host animal. The shrimps may be seen occasionally hopping off the urchin to forage for food, but keeping very close to its protector, ready to climb aboard again if danger threatens. This species of urchin is one of the prettiest around! It is blessed with a warm orange/red body, cream coloured spines banded with rusty-red, and a superb array of royal blue spots radiating across the body in ten rows, extending from the black-centred orange faecal sac down to the bottom of the spherical body.

One of the most spectacular urchins is also potentially deadly dangerous. This is the Flower Urchin, Toxopneustes pileolus. When its pedicellaria are fully opened this urchin looks for all the world like a bunch of pale pink flowers with a strong red centre. But this animal is too dangerous to be handled for the aquarium trade, and has been responsible for fatalities where accidental contact with it has been suffered. One researcher in the US who was working with the species and was well aware of its potency, but managed to get himself injured by a specimen. Although not being punctured by a large number of spines he was critically ill for some time and was lucky to have survived to tell the tale. When the author first found a specimen of this urchin it was liberally covered with broken coral rubble at a very shallow site in the Lembeh Straits of Sulawesi. In order to get a good picture the rubble was carefully picked off by hand, unbeknown to him at that time how toxic this urchin could be! The fact that this species often camouflages itself quite thoroughly with debris makes it a major hazard for potential accidental contact .

Some urchins are so removed from the more recognisable and familiar form they may easily be mistaken for some other creature. One such species is Prionocidaris verticillata. At first sight this strange animal could be taken for an unusual species of anemone. It spreads its thick spines out in a similar fashion and has fleshy tubercles which tend to mask the true hard spiny nature of an urchin. The spines have a knobbly appearance and have quite blunt endings. This is an urchin with a very ancient history, having fossilised relatives dated at some 300 million years of age! It is, however, a rarity on the reef and is unlikely, therefore, to be collected for the aquarium trade.

Another very different species of urchin which does come onto the aquarium market from time to time is the Slate Pencil Urchin, Heterocentrotus mammillatus. When seen tucked away in some crevice during daylight hours on the reef, one wonders how on earth it manages to get in and out of such a confined space when its body is encumbered with such a massive array of thick, heavy spines? But it is able to manage this perfectly well, having such obligingly mobile “slate pencils” to manipulate. There is a lot of variation of colour between specimens of this urchin, and a lovely red form is the most desirable from an ornamental point of view, but it is rarely available. A related and much darker coloured species sometimes shows up in aquarium shops too. This is H. trigonarius which has longer and more slender spines. Incidentally, this urchin is not named after slate pencils, but is so-called because its “spines” were indeed the very things which were slate pencils in former times.

The Sea Mouse Maretia planulata, is mostly buried in the sand during the daytime and only comes out to forage for food at night. It is found in open sandy or silty areas, and will bury when disturbed at an amazing rate. It can also be quite a fast mover when it is travelling around the substrate during its busy food gathering excursions. The body shape is not pronouncedly globular like that of its close relatives discussed above. It tends to have distinct head and tail ends to its body, which is approximately tear-drop shaped and relatively flattened The colour can vary from an off-white through to dark brown. If the Sea Mouse becomes available to the reef aquarium keeper there seems to be little to recommend its purchase, as it would be rarely seen, and doubtful that it would serve any utilitarian purpose to earn its keep.

These amazing animals have their place in the aquarium hobby, although urchins are certainly not everyone’s choice. If aquarium keepers are not having to be careful of injecting themselves with venom, they will probably be watching out for the sharp spines of other non-venomous species. Those that graze algae undoubtedly have a place in a reef aquarium plagued by persistent algae problems, an is in need of some natural form of control. The commensal possibilities would certainly make a very interesting additional feature in a suitably appointed reef aquarium.

This is the menacing array of spines of the Long Spined Black Urchin, largely concealing the globular body below, with secondary spines, hair-like pedicellaria, and intriguing patterning of electric-blue and five brilliant white spots identifying each of the body’s five segments. (Diadema setosum).

This single specimen of the Caribbean white spined species of urchin, Diadema antillarum, clearly shows the typical form of a long spined species as it travels across the sand at night in search of food

Echinometra marthaei is an urchin which has very sturdy spines radiating out from its typically globe- shaped body. The base of each spine has a white “collar” encircling it.

This urchin has distinctively marked banded spines, and often reveals a large faecal sac with fine white spotting. The base colouring is quite variable. The specimen shown has a maroon body and the lighter areas of the spines show a pleasing pink. (Echinothrix calamaris)

Most of the short spined urchins manipulate marine rubble up onto their spines for camouflage. Algae and coral stone is seen attached to this Mespilia globulus, which is often sold for its active algae browsing activity.

The colourful Red Sea toxic sea urchin, Asthenosoma varium grows quite large and needs to be accommodated in an adequately proportioned aquarium. The ball- tipped pedicellaria are a prominent feature. The powerful toxin is carried in these ball-shaped glands.

The Indo-Pacific toxic sea urchin, also Asthenosoma varium but looking quite different to the Red Sea form, frequently carries commensal animals with it. The Zebra Crab, Zebrida adamsii, is one such “passenger” which is able to firmly cling onto its host, nestling down into the spines where it is difficult to detect.

Coleman’s Shrimp (Periclimenes colemani) is one of the prettiest marine shrimps, and almost invariably in pairs when found on the toxic urchin Asthenosoma varium . The large female has come up to the surface and is quite easily seen there, but the smaller male has stayed down among the dense thicket of fearsome white tipped spines where it is almost invisible. Note the elaborate form of the spines.

Astropyga radiata is a toxic urchin which does become available to the aquarium trade from time to time. It is a very pretty species with 10 lines of blue spots which demarcate each of the five double rows of ambulacral plates, typical of the urchins.

This toxic urchin is too dangerous to be collected for the aquarium hobby. Its venom is potentially lethal. At the very centre the flower-like pedicellaria are almost fully open, while the surrounding ones are still closed up. The effect is very much like a pretty bunch of flowers when the whole array is fully open. But shorter, sinister venom-bearing spines lie just beneath the ornamental canopy!

At a glance this obscure species of urchin, Prionocidaris verticillata, looks more like some strange anemone. The erroneous impression is aided by the tubercular growths radiating from the animal’s body, and the irregular rugged form of the actual spines.

The Sea Mouse is a very busy type of urchin which emerges from its daytime rest beneath the sand, to scurry along on its tube feet, searching out food from the substrate. The five segments of the double ambulacral plated body is clearly seen in this species.

Slate Pencil urchins can be quite colourful, with “spines” that are thick and blunt, and quite different to the wicked spiky array normally associated with sea urchins. These were collected and used for writing on slates in bygone days. The species shown here is Heterocentrotus mammillatus.


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