What are sea urchins? Members of Class Echinoidea, they’re distant cousins of starfishes that boast imposing defenses and bad reputations. Do sea urchins sting? Not really. It’s more that they impale.
But, if you don’t bother them, these oceanic pincushions won’t bother you. The trick is in not accidentally bothering them.
RATHER THAN APPRECIATING SEA URCHINS, MOST PEOPLE FLEE FROM THEM, CAREFULLY. After all, the likely result of interacting with these oceanic pincushions while doing a close reef crawl or just wading in the shallows is a lot of sharp, lingering pain in a foot or leg or hand.
As good echinoderms, they’re designed with five-part radial symmetry, although that’s hard to discern in the living animals. In most species, it’s all about the spines. Lacking the arms of echinoderms like starfishes and brittlestars, they’re more like globular balls with spines. You can get glimpses of their bodies underneath but it’s the spines that stand out – literally.
DO SEA URCHINS STING? 1): DEFENSIVE ARMAMENTS
So…to get the spine thing out of the way, they’re purely defensive. People refer to being “stung” by them but they don’t really sting the way that jellyfishes do. They impale.
They don’t even impale intentionally. They’re just there in case you want to stick your foot or hand on them, hopefully by accident. They’re shy animals, and nocturnal.
On the reef, they spend their daytime hours half-hidden in nooks and crannies, spines projecting into view, or half-buried in the sand. Some species bore their way into rocks or corals, protecting their bodies but still projecting.
The message: You might meet up with a partially concealed one unwittingly.
DO SEA URCHINS STING? 2): THE SPINES ARE HARD AS HECK TO REMOVE
And once you’ve made contact, you’re stuck. The needle-like spines of a specimen like the often-met Long-Spined Urchin (Diadema antillarum) easily penetrate the skin, and they’re built with little fish-hook-like barbs along the sides that make them difficult to pull out. Plus, they’re brittle and break easily, leaving bits and pieces embedded within the skin. Trying to pull them out with tweezers or a pin is likely to only compound the misery.
Medical sources generally take two lines when it comes to dealing with this. 1) Soak the affected area in water as hot as the victim can tolerate for 30 to 90 minutes. 2) Immerse it in vinegar, with the goal of dissolving embedded pieces or drawing them to the surface. If the initial soak doesn’t do the job, apply vinegar compresses several times a day until they are eliminated.
Untreated, spiny pieces under the skin will dissolve naturally in a couple of weeks. Bits of spine that are deeply embedded in the skin may require surgery to remove them. A big risk is for the area to become infected, so treat with an antibacterial like Neosporin. If it worsens, seek medical care.
And, everyone agrees: Soaking a spine wound with urine is ineffective and is not recommended.
DO SEA URCHINS STING? 3): THE VENOM THING
It should be noted that a relatively small number of true sea urchins do carry venom on their tips or within their spines, but most don’t. With, perhaps, a few exceptions noted below, serious issues from toxins appear low. The literature seems more concerned with the risk of infection from bits left in the skin than from venom.
BUT WHAT ARE SEA URCHINS? 1): “GOOD” URCHINS, SEA & OTHERWISE
To be clear, there are sea urchins and close cousins that don’t represent an impalement threat – among true sea urchins, for example, West Indian Sea Eggs (Tripneustes ventricosus). Their spines are short and blunt, with sort of a Velcro feel to them.
Heart urchins, sea biscuits and sand dollars are urchin variations – with short, felt-like, non-threatening spines. They use them to help bury themselves in the sediment, where they dine on organic matter. Since they live mostly out of sight, animals like sand dollars are best-known by the “petal-embossed” calcium carbonate skeletons that sometimes wash up on beaches.
It doesn’t help that the term “sea urchin” is used loosely, to sometimes talk about all these animals and sometimes only true sea urchins. Naturalists partially skirt the issue by describing true sea urchins as “regular,” reflecting their nearly perfect five-part symmetry, and heart urchins and the like as “irregular,” reflecting a lack of such symmetry.
WHAT ARE SEA URCHINS? 2): ECHINODERMS WITH REAL SPINES
They’re all members of Phylum Echinodermata’s Class Echinoidea, with an estimated 950 species found in waters worldwide, arctic to tropical, deep sea to intertidal. It’s not clear how many are true sea urchins – the numbers are all over the place.
While the term echinoderm means “spiny skin,” it generally refers to the bumpy plates that underlie starfishes’ and feather star bodies. Urchins, on the other hand, not only have the underlying calcium plates in their bodies, they top it off with actual spines.
Echinoids routinely occupy habitats as diverse as rocky shores, coral reefs, sandy and muddy seabottoms and seagrass beds.
Species like the Pacific Purple Urchin (Strongylocentrotus purpuratus) and Green Urchin (Strongylocentrotus droebachiensis) may be as familiar for their presence in tide pools and shallow ledges along the U.S. Pacific or Atlantic coasts as in deeper habitats.
In the Caribbean, Longspined Sea Urchins (Diadema antillarum) and Magnificent Urchins (Astropyga magnifica) are common sights. Indo-Pacific equivalents: Black Longspine (Diadema setosum) and Radiant Sea urchins (Astropyga radiata).
WHAT ARE SEA URCHINS? 3): ECHINOID BODY ANATOMY
Echinoid bodies are supported by rows of tightly interlocking calcium carbonate plates, creating a rigid capsular “test,” or skeleton – much less flexible than those of starfishes, brittlestars and featherstars. In sea urchins’ case, their bodies are round, even globular. In true sea urchins, mouths are on the underside, anuses on the upper surface. in heart urchins, they’re at opposite ends.
Sea urchins’ bodies can be as small as 1 in/2.5 cm or as large as 6 in/15 cm or more. In the Caribbean, the bodies of Slatepencil Urchins (Eucidaris tribuloides) measure about 2 in/5 cm, West Indian Sea Eggs (Tripneustes ventricosus) about 5 in/13 cm, Magnificent Urchins about 6 in/15 cm. Spines are something else.
While the West Indian Sea Egg sports short spines, in the half-inch range and the Purple Urchin (Arbacia punctulata) 3 -4 in/8-10 cm, the Long-Spined Urchin carries spines up to 8 in/20 cm in length.
WHAT ARE SEA URCHINS? 4): ECHINOID REPRODUCTION & LONGEVITY
Sexually, Echinoids are dioecious – that is, males and females (Note: in the ocean, which is which is hard to tell, although presumably not to other echinoids). They reproduce through broadcast spawning, launching their gametes into the current for external fertilization, with larvae then carried wherever. Depending on the species, it may take two to five years to develop into adults.
Once they are adults, they typically can count on lifespans of 20 to 30 years. But some species are extremely long-lived. Scientists have identified specimens of the Red Sea Urchin (Mesocentrotus franciscanus) that they say are as old as 200 years. But 30 years may be more typical.
DO SEA URCHINS STING? 4): SPINES – AND PEDICELLARIAE
The familiar long-spined urchins found pretty much worldwide are generally black with some white highlights, but overall, sea urchin spines come in an array of hues – reds, blues, purples, greens, oranges, all whites.
Depending on the species, the calcium carbonate spines may be solid or hollow. They’re attached with ball-and-socket joints, and movable. In true sea urchins, long defensive spines are able to react to contact by all pointing in the direction of the touch. In heart urchins and sand dollars, they’re a pitiful defense but they’re excellent at passing sand over the body, enabling the animal to burrow into the sediment.
If the spines aren’t venomous, structures between the spines can be. These are pedicellariae, shorter, moveable stalked structures often described as claw-like appendages with moveable jaws. Pedicellariae functions are not entirely understood but they may play a role in keeping the urchin’s body free of algae and encrusting organisms. In some species of echinoderms, they appear to play a role in trapping food.
Some 80 species carry toxins. Whether they represent serious threats depends on what source you read. It would appear that the incidence of serious complication is low. But, in the Indo-Pacific, the Flower Urchin (Toxopneustes pileolus), with prominent pedicellariae, is considered particularly venomous.
LOCOMOTION
Urchins lack brains, eyes and legs but as good echinoderms, they do have hundreds of little tube feet with which to ambulate.
Like all echinoderms, they rely on water vascular systems that use hydraulic pressure to control five rows of paired tube feet across their bodies. The tubes end in suckers that let them glue themselves to whatever substrate they’re on, or to creep along as the slurping up food.
Some sources suggest that they creep slowly. Once, on a night dive under a pier, I watched a Variegated Urchin (Lytechinus variegatus) move and he was zipping along, like the classic old Little Old Man comedy routine. Watching a sea urchin tiptoe across a sandy bottom is either hilarious, ridiculous or charming.
“ARISTOTLE’S LANTERNS”
To begin with, sea urchins have five arrow-shaped teeth, each held by a separate jaw in the center of their undersides. Researchers have found that, at least in the Pink Sea Urchin (Strongylocentrotus fragilis), the teeth automatically sharpen themselves as they are used. Urchins’ five part mouths are called “Aristotle’s lanterns, since the Greek philosopher first described it in ancient times.
These animals are mostly herbivores. Moving out into the open during the nighttime hours, they feed mostly on algae, up to and including kelp where kelp forests exist. But they’ll also feast on decaying organic matter like dead fishes and on sponges, sea cucumbers, brittlestars, worms and mussels.
They use their formidable jaws to scrape or tear the stuff from rocks or corals. Or, they dredge up algae, detritus and other treats from the sediment. Sea urchins actually make good vacuum cleaners and contribute to keep algae growth on the reefs in check.
On the other hand, too many sea urchins can wreak havoc on a habitat. During population explosions, they can strip a kelp forest very quickly, upset the balance of diversity on a reef by devouring its algae, or erode the coral itself with their complex jaws.
PREDATORS & FELLOW TRAVELERS
Fortunately, despite their substantial defenses, they are vulnerable to predators as varied as wolf eels, triggerfishes, starfishes, lobsters and crabs. And sea otters, major sea urchin gourmands.
How to take a sea urchin? Go in low. A triggerfish’s observed technique: Hover sideways close to the sand, blow strong puffs of water at the urchin’s base, knock it over, grab the unprotected body, munch.
Additionally, a significant predator is the human race. The popular, creamy sushi ingredient Uni is usually referred to sea urchins’ roe, or eggs. Actually, it’s created from their gonads, which produce the roe. Uni’s flavor is often described as light, sweet and somewhat briny – and an acquired taste.
If some underwater animals seek out sea urchins for lunch, others make use of them for protection. A tiny fish called the urchin clingfish (Diademichthys lineatus) makes its home among the forests of long spined urchins. Juveniles dine on the pedicellariae, adults on the tube feet.
WHAT ARE SEA URCHINS? 4): THE HEART URCHINS’/SAND DOLLARS’ DIFFERENCE
Echinoids all, heart urchins, sea biscuits and sand dollars are “irregular” urchins because they lack the near-perfect symmetry of true urchins. For example, in contrast to the mouth underneath, anus on upper side true urchin design, heart urchins come with more of a dome shape with the mouth at the front end, the anus at the rear. They lack the true urchins’ Aristotle’s lantern mouth.
Sand dollars and sea biscuits do have Aristotle’s lantern mouths. Flattened, disc-shaped sand dollars are most familiar from their calcium carbonate shells embossed with the five-petal design, but in fact all three display the pattern. Their dense coverings of short spines assist them in burrowing in the sediment, where they feed on organic matter.
COLD WATER FINDS
SEEING STARS: OTHER ECHINODERMS ON POSEIDON’S WEB – EXPLAIN THE REEF!
- Stomach-Everting, Arm-Regenerating Starfish Explained
- Brittlestars: Well-Armed, Laid-Back and Full of Tricks
- Sea Cucumbers – Superheroes of the Seas
- All Arms, All the Time: Feather Star Facts
PRINCIPAL SOURCES: Marine Biology, Peter Castro, Michael Huber; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Coral Reef Animals of the Indo-Pacific, Terrence Gosliner, David Behrens, Gary Williams; Reef Creature Identification, Florida, Caribbean, Bahamas/Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; The Coral Reef at Night , Joseph Levine, Jeffrey Rotman; Marine Life of the North Atlantic, Andrew Martinez; Sea Urchin, Tree of Life Web Project; The Echinoidea, University of California Museum of Paleontology – Berkeley; In situ Wear Study Reveals Role of Microstructure on Self-Sharpening Mechanism in Sea Urchin Teeth, Matter; Sea Urchin Facts: Animals Of The Oceans, World Atlas.com; What is the Difference Between Sea urchins and Sand Dollars? Natural History Curiosities; Sea Urchins, Animal Network; Green sea urchin Facts, Thought.com; Pacific Purple Sea Urchin, Oceana; Some Sea Urchins are Venomous But Usually Not Very Dangerous, liveabout.com; Sea Urchin Injuries to the Hand: A Case Report and Review of the Literature, Iowa Orthopedic Journal; Sea Urchin Stings, Merk Manual, Professional Version; Sea Urchin Stings and Puncture Wounds, eMedicine Health; How to recognize and treat a sea urchin sting, Medical News Today; Sea Urchin, Encyclopedia Britannica; Sea urchin, Tripneustes ventricosus, Pedicellaria, et.al., Wikipedia.com.