This may surprise some of you out there, but I hear a lot of talk about sea urchins. Especially from divers and snorkelers who’ve had the good/bad luck of encountering these gorgeous beasts. The good luck is in observing some of the strangest of all the Earth’s marine animals. The bad luck is in getting a bit too close to certain kinds. I would like to highlight the latter concept a little bit, and perhaps clear up some of the misinformation out there. Or simply add some information that isn’t all that easy to come by, even though urchins are among the most common and conspicuous animals you can see on a dive or in a tidepool. Doing this during the Expedition is easy because, well, the diversity of the urchins here is pretty amazing, and they’re right out there off the place where we are staying in the case of most species, especially the ones who can cause some damage.
To recap, sea urchins (Echinoidea) are in the phylum Echinodermata (“echino” = spiny; “derm” = skin). The urchins are related to things like starfish, brittlestars, sea cucumbers, and sea lilies. The most familiar of the sea urchins are globose things, adorned with spines distributed over the body. The shape of the urchin is maintained by a skeleton of tightly sutured columns of plates made of a type of limestone (a.k.a. calcium carbonate) secreted as a biological form of calcium carbonate called “stereom”, making a structure not unlike your skull in that it is covered with skin. In urchins, this skin, or epithelium, even forms a thin layer over all the external appendages, including the spines. The columns of plates are arranged in a radiating pattern based on five. Why five is a subject for another day, perhaps. Nevertheless, this is a powerfully unique way to identify an echinoderm. Look for that 5-part radiating symmetry. The mouth of a sea urchin is on the bottom, and the anus is on the top. Because the skeleton is technically internal to the epithelium (and not external like the shell of a snail or clam), it gets a special name, the “test”. There are lots of puns I could make here, but I’ll largely refrain and instead only indicate that terminology tests us all at times. But new terms add precision, and that’s what science is all about. So test it is.
Back to stinging. It always startles and pleases me to learn just how many divers have learned the genus name of a sea urchin: Diadema. Necessity is the mother of learning, I guess, because the memory of the name is almost always linked to a negative encounter with this black-spined urchin that decorates so much of the world’s coral reefs. I actually really like this animal, but this is not a unanimously held reaction. However, with a little caution and awareness of where your body parts are while drifting over a mass of Diadema, you can enjoy the encounter quite unscathed. I’ve been collecting and observing these guys for many years, and I’ve only been hit once in a way that really mattered. It served to instill respect that I hold close to this very day. Most of the black urchins with the really long spines belong to the genus Diadema. There are two common species of Diadema in the Philippines’ reefs. Here they are:
The one on the left, D. setosum, is arguably the most common species ’round these parts, and is easily distinguished from the one on the right when seen alive and under natural light by the amazingly bright red ring in the little raised area on the top of the body (called the anal sac). In both species, there are beautiful patterns of iridescence on the top of the test, and this makes urchin-watching just that much more special.
When hidey-holes under coral rubble are available, D. setosum tends to be solitary and able to defend against attacks (usually by triggerfish if anecdotal reports hold) by directing spines outwards. However, out in the open they become more gregarious, gathering in small gangs of nervous pincushions, with the spines just touching to maintain their own respectful distances, spines constantly waving a little, particularly towards changes in light and pressure waves from passing animals such as fish or divers. The effect can be dramatic, with just the right hint of menace.
Look, but don’t touch. The long spines are very, very sharp, and come to a point so fine that it’s hard to see precisely where they end in a watery medium where distances can be deceptive to start with. The spines can penetrate flesh so easily and quickly that once you feel it, it’s way too late. You’ve been perforated. More than likely, the spine (or spines, if you are really unfortunate) will break off in the skin. You can try to pull them out, but the delicate and easily broken structure of the spine (which is hollow) makes that difficult. Most people know about the barbs, but what they don’t realize is that the barbs don’t point backwards toward the animal, but towards the tip of the spine. So the damage is done going in. The other common misconception is that these spines are poisonous. It only feels that way. Most of the post-encounter discomfort comes when tissues inside the hollow spine start to decompose and attract bacteria.
And remember that epithelium I mentioned? It also can break down and cause infection in the wound. Best way to deal with that is to immerse the afflicted body part in vinegar. This does three things. It makes the urchin tissues inert to bacterial feasting, kills the bacteria themselves, and dissolves the spine skeleton, which is also made of the calcium carbonate stereom described above for the plates in the test. Being limestony, this material fizzes and dissolves readily in any acid such as vinegar. Vinegar adds to the hurtin’ at first, but trust me, it helps and greatly reduces future damage that can be caused by leaving the spine in there whole. If you don’t have vinegar, you can also roll the skin around the spine or tweeze the spine in the wound until it crushes up into smaller pieces for your natural immune system defenses to deal with.
Although the long spines of Diadema are not venomous, there are toxin-bearing spines on all members of the family Diadematidae, to which Diadema belongs. In Diadema, these are relatively short, very sharp (yes, even sharper than the long spines), and almost never reached by an errant hand or foot or whatever, because you hit those long “guard spines” first. That generally keeps you from reaching the stinging spine layer, unless you are really unfortunate and set up for a trip to the hospital because you put all your weight down on a Diadema. Each of these shorter spines has a slight swelling at the tip where gland cells in the epithelium make and accumulate a toxin that causes a real, honest-to-goodness sting.
Although these gland-bearing spines are hard to reach (or even see) in Diadema, they are really prominent in another diadematid genus, Echinothrix:
Again, we have two species common in the Philippines. E. calamaris has lighter spines, with beige or brown on the test, a nicely speckled anal cone (not a phrase you will see everyday), and very obvious and exposed, light brown spines tipped with poison glands, as in the close-up below (red arrow).
Note that the long spines of this close relative of Diadema are not sharp. In fact, they are hollow with thin walls, like a straw with the end closed off. In the juveniles, these spines are so large relative to the test that the urchin looks like it’s carrying little, narrow vases sticking out from its test. Weird.
The other Echinothrix, which is very black with very nice, blue iridescent patches on the test, also has these shorter, poison-bearing spines. You can see this iridescence and the poison glands (red arrow again) well when you get close up.
Then there is the fire urchin, Asthenosoma varium. This is an urchin whose characteristics are so unusual I just have to tell you about it. It’s also common in coral reefs just about everywhere. Nature has found a special way to tell us “do not touch this animal”. The bright colors might be inviting, but when you see that in nature, it usually hints at something dangerous. This lovely photo is courtesy of Terry Gosliner, who has the same respect for this relatively large and powerful stinger as I do:
The test of this urchin, which can exceed 20 cm across, is flexible because the plates that make it up are separated by connective tissues that allow the plates to hinge against each other. The urchin keeps its shape by gently inflating itself with sea water, but if you poke it (use something other than a body part, please), it yields a bit like a crunchy balloon. The fire urchin uses this feature to get into crevices, and possibly also to economize on the metabolically expensive calcium carbonate that other urchins use to make up their stiffer tests. This isn’t so much a factor in coral reefs, where dissolved calcium carbonate is relatively abundant. But this urchin species has as its closest relatives a bunch of equally bizarre forms that live in the deepest parts of the ocean. Evolutionary studies show that the ancestors of the fire urchin live in the abyss where calcium carbonate is harder to come by and to shape into urchin skeletal parts, selecting for species with thin, flexible tests in which calcium carbonate is used sparingly. And guess what? Those deep-sea relatives of A. varium compensate for the relative lack of an armored test by having the worst stinging capability of any urchin that I know. I got hit by one in my left middle finger while doing deep-sea work in the Bahamas — just a tiny pinprick of one spine — and my left arm was useless for several hours.
Here is a close up of the spines on a fire urchin. Most of that blobby, balloon-like tissue on the spines is filled with toxin. Never pick up a fire urchin. There is some evidence to suggest that you could go into shock if enough spines zap you at the same time.
Finally, I would like to mention one more stinging urchin with a difference. This one hurts a lot, but not because of the spines. The so-called “flower urchin”, Toxopneustes pileolus, is another one very pretty to look at, but deserving of respect:
The flowers are not spines, but a special structure unique to urchins called pedicellariae (for you sticklers — pun intended — out there, the pedicellariae on starfish are an independent evolutionary invention and only superficially similar to urchin pedicellariae). Pedicellariae (singular: pedicellaria — not “pedicellarium”) are ice-tong-like pincers mounted on the ends of stalks interspersed all over the test among the spines. All urchins have pedicellariae, but they are usually small and inconspicuous, and too small to do any damage unless you are a tiny barnacle larva trying to find a nice home to stick to on the top of a sea urchin. This is the usual type of thing that pedicellariae are used to defend against. In Toxopneustes, the spines are very short and not very sharp. This urchin protects itself from larger animals with the grossly enlarged pedicellariae instead. Although they look like flowers, inside the pink fleshy bit that makes the “bloom” are three tongs that meet together at their points when the “flower” closes, tearing a hole in the transgressor’s flesh and injecting a toxin into the wound:
You can tell when the animal is all worked up and in the urchin equivalent of “DEFCON 1″ when the whitish spines lie down to expose the open jaws of the pedicellariae. Put your hand on that and you’ll get a powerful dose of toxins from several pedicellariae at once. A complicating factor is that this urchin, like many other species, likes to cover itself with bits of coral rubble, sometimes making it hard to see in the shadows. Still, a beautiful animal and always interesting to see in its native habitat. It’s a real favorite of underwater photographers with an interest in abstract art.
So that’s my primer on stinging urchins. I wanted to call this blog, “Oh test, where is thy sting?”, but I wasn’t sure if that might have been a bit unforgivable — or even obscure. Heck, I don’t know my Shakespearean sonnets or Corinthians either.