Let me introduce you to one of our favorite marine creatures: the mantis shrimp. Why are they a favorite? They're cute (sort of), they're very smart (for a crustacean), and they're fun to watch. Unfortunately, they're also difficult to photograph -- more on this a little later!
The individual in the photos on this page is a Shortnose Mantis Shrimp (Odontodactylus brevirostris), from Hawaii. I know that the critter looks pretty big in these photos, but it's not. These are macro images. The mantis shrimp in these pictures was only about two inches (5 cm) long. You can click on any of the photos to see a larger view.
Despite the name, mantis shrimps are not true shrimps. I guess I don't need to explain that they are not mantises, either -- although they apparently acquired their name because they resemble praying mantises somewhat. Like crabs and lobsters and true shrimps, mantis shrimps are Crustaceans.
All mantis shrimps belong to the order Stomatopoda. Stomatopods have a number of features that set them apart from other crustaceans. Among the most notable -- and noticeable -- are their raptorial arms, and eyes that are highly evolved.
Mantis Shrimp Eyes
I've always wondered what the undersea world looks like through the mantis shrimp's eyes, because their eyes are like no other eyes. Take a look at the photos, and notice that the eyes are set at the end of stalks. Each eye is independently movable -- and they seem to move constantly, giving the critters a hyper-alert look. Each eye is trinocular, that is, each has three separate perceptual regions. This would be sort of like having built-in trifocal lenses, except that instead perceiving things at three different levels of magnification, each eye region is specialized to perceive a different kind of visual information -- but all at once.
Mantis shrimp eyes are capable of hyperspectral vision. In other words, not only can they see the visible light spectrum, like we can, but they also can see spectra of light that we cannot, including infrared and ultraviolet. Very recently it was discovered that mantis shrimps have the ability to perceive circular polarized light, too.
Raptorial Appendages
The mantis shrimp's raptorial appendages actually are legs that have evolved into claw-like arms, specialized for killing prey. There are two types: some species have thin, barbed raptorial arms that can spear prey; others have club-like raptorial appendages that smash their prey. (The mantis shrimp species in the photos is a 'smasher'.) When mantis shrimps are at rest or walking about, they keep those killer appendages folded up like closed jack-knives against their bodies, just as the creature in these photos is doing. When they go after prey, those appendages unfold at an incredible speed and they spear or whack the prey.
The spearers tend to choose soft-bodied prey like worms and little fish. The spearers are ambush predators. That is, they sit quietly and concealed until prey comes along and then they attack. In contrast, the smashers usually pursue their prey -- and that is one of the mantis shrimp behaviors we love to watch, when we get the chance.
The smashers use their raptorial arms not just to kill prey, but also to break it apart to eat. The species in the photo eats things like small crabs, and gastropod snails that live in shells. When the mantis shrimp sees one of those and goes after it, first he punches it silly, then he uses his raptorial arms to crack open the shell so that he can pick out the meat with his little forward legs, which are also specialized for that purpose.
The Star of the Puako 'Petting Zoo'
The mantis shrimp in the photos on this page lives off the coast of Puako, Hawaii in an area where we have made countless dives. When you dive in the same area again and again and again over a period of years, you become aware of things that a visiting diver probably never would notice in the course of just a few dives. One thing you learn is where all the different types of creatures live. Except for pelagics that roam the open sea, most marine creatures have a relatively small range. Once you spot where they live, you can usually count on seeing them in the same general area any time you go there. Once you know where to look, there they will be.
Puako has a wonderful fringing reef that parallels the shoreline for a couple of miles. The coral reef area is beautiful, and very accessible, but there are areas beyond the reef that are just as fascinating, if not quite so pretty. Where the seaward edge of the reef ends there is a steep slope. The top of the slope has many rocks and is still covered with quite a lot of coral, but deeper on the slope where less sunlight penetrates there is little live coral. There are more rocks, plus lots and lots of coral rubble -- lumps of dead coral washed down the slope from the reef over time. At the base of the rubble slope the terrain levels out into a sandy plain.
There is an area near the base of the rubble slope, at a depth of about 100 ft (30 meters), that we named the Petting Zoo. We called it that because it teems with small creatures. Sometimes we would go directly to the Petting Zoo, plop ourselves down in one spot, and spend the entire dive watching all the little creatures go about their business. You can learn a lot about marine creatures' behavior that way.
On one of our visits to the Petting Zoo, we noticed this particular mantis shrimp. Some mantis shrimp species are nocturnal, i.e. they only come out at night. This species works the day shift. We first saw him when he emerged from his burrow and began scurrying about. Mantis shrimps can swim a bit. If you look at the second photo above, you'll notice that this creature has a tail not unlike that of a lobster. They can use their tails to propel themselves through the water for short distances, but their more usual method of locomotion is to run about. Notice I said run. They move quickly -- usually too quickly to get a good macro shot.
Mantis shrimps live in burrows. This one had excavated a double-ended tunnel under the sand. I have no idea what it's like inside the burrow -- we used to joke that he probably had an overstuffed chair and a TV in there for all we knew. We did notice that outside the entrance to the burrow we would sometimes see pieces of broken shell in little piles. A few times we saw the critter in the process of housecleaning -- literally throwing bits of shell out the door.
On successive visits we saw this mantis shrimp stalk and kill prey -- usually a small crab. One time we saw him smack a crab that was bigger than he was, dismember it, and drag the body to the opening of his burrow. The mantis shrimp disappeared inside the burrow for a few minutes, then re-emerged and whacked the shell of the now legless crab a couple of times to crack it open and began to dig out the 'meat' for his feast. We watched him until it was time for us to begin our ascent. He was still working on the crab when we left. When we came back a day or two later we looked for the remains. Sure enough, there was a recognizable piece of the crab's empty shell not too far away from the entrance to the mantis shrimp's burrow.
I mentioned in the first paragraph of this article that mantis shrimps can be difficult to photograph. That's because they seem to be in motion constantly when they are outside their burrows. It's not too difficult to get a shot of one peeking out of its burrow, but capturing an image of the whole animal had eluded me for years.
Then one day at the Puako Petting Zoo, this little mantis shrimp came out of its burrow and just stood there looking at us. And -- Hallelujah! -- not only did I have my camera, it was set up for macro photography. Moving as slowly as I could, not to startle the critter, I lay down on my belly on the sand at the edge of the rubble slope, and sort of inched toward the mantis shrimp. He didn't run away.
I focused carefully on his wonderful eyes and pressed the shutter release, figuring this would probably be my one and only shot. So often in underwater nature photography, the flash of the strobe startles a photo subject and it quickly leaves, so one shot is all you get. But no, not this mantis shrimp. I expected him to high-tail it to his burrow, but instead he actually came toward me a bit, stopped and turned sideways, almost posing. Now practically touching him with the end of the lens port, I took another shot, and a third.
Then he turned the other way and positioned himself as if to say, "How's this? Can you see my tail a little better now?" I shot once more, and then he finally scampered over to his burrow and disappeared inside. I did press the shutter one more time while he was in retreat, but his movements had stirred up the sand by then.
I've often wondered what it was that made that mantis shrimp decide to be such a cooperative photo model that day. I've never again had an opportunity like that.
Friday, August 1, 2008
Meet the Mantis Shrimp
Monday, July 14, 2008
WW #42 - The Right Blue's 1st Birthday Review
Not-Quite-Wordless, Special Tuesday/Wednesday Edition
The Right Blue celebrates its first birthday on July 15, 2008. To mark the occasion, we thought we'd post some of our favorite underwater photos this week and next, as a review. All of these have appeared as illustrations to our articles over the past year, but have not appeared before as Wordless Wednesday images.
We know that some Wordless Wednesday visitors only come to see the images, not the articles, but just in case you'd like to read the stories, too, we're including a link to the article where each photo first appeared. We hope you enjoy these photos -- all of which are Bobbie's original work.
Sunday, July 13, 2008
The Right Blue's First Birthday
The Right Blue has a birthday coming up this week. Launched on July 15, 2007, The Right Blue is now one year old. To celebrate the blog's first year, we thought it would be fun to present a review of some highlights of the past 12 months, which we will do during the coming week.
By the way, we chose the photo at right for this birthday post because we thought it looked festive -- like a party favor. [Click on the photo to enlarge.]
Bobbie shot this macro photo of Social Feather Dusters (Bispira brunnea) off West Caicos, in the Turks and Caicos Islands. In case you are wondering about these creatures, they are annelid worms. They live inside those calcareous tubes, which they secrete, and they feed by extending those fluffy 'feather duster' parts to catch particles. You can see that some of the worms have retracted their 'feathers' into their tubes. (They're going to miss the birthday party!)
So then, back to the year-end review. Some of our regular readers have been with us since The Right Blue began, but we know we have acquired many new readers along the way, too. Some of those newer readers may not know much of the background about who we are, and how The Right Blue came to be. As a review, here are a few links to articles that will serve to explain some things about us, and about the project that came to be called The Right Blue:
About 'The Right Blue' - July 15, 2007
Prologue: How 'The Right Blue' project came about - July 16, 2007
Prologue: Our training as divers - July 20, 2007
Prologue: The supporting cast - August 1, 2007
Learning underwater photography -- The easy parts - August 5, 2007
Since you've asked: Answers to readers' questions - November 1, 2007
More to follow - including a collection of some of our favorite photos from The Right Blue, which we will post for Wordless Wednesday, this week and next.
Tuesday, May 13, 2008
Friday, May 9, 2008
Bubble Coral Plus
In the previous article, we introduced our readers to Bubble Coral (Plerogyra sinuosa), a type of stony coral recognizable by the inflated bubble-like vesicles that give rise to its common name. Sometimes other little creatures live in or on the Bubble Coral. We refer to that situation as Bubble Coral Plus.
For Wordless Wednesday, we posted a macro photo of Bubble Coral with a tiny shrimp (Vir phillipinensis) that makes its home among the Bubble Coral's vesicles. The first photo on this page shows another instance of Bubble Coral Plus. This time the Bubble Coral is host to an entirely different critter, the Waminoa flatworm.
The first time we saw Waminoa on Bubble Coral we thought it looked like some kind of algae growing on the surface of the vesicles. We found out only recently that the spots actually were tiny critters. Apparently not too much is known about these flatworms, except that they inhabit some soft corals, as well as the soft parts of the Bubble Coral, and once they are established, the reproduce rather quickly.
This second photo is a 1:1 macro shot. If you click on the photo to enlarge it, you'll be able to see that the Waminoa flatworms look like little pancakes clinging to the surface of the Bubble Coral vesicles. Apparently not a whole lot is known about the Waminoa flatworms, but it is thought that they in turn are host to tiny dinoflagellates -- single cell life forms that produce nutrients through photosynthesis, some of which they share with their host.
I took both of the photos on this page in the Celebes Sea at Bunaken Island. The little island is part of an offshore marine reserve near Manado, on the northern finger of Sulawesi Island in Indonesia. Most of the Bubble Coral in this area has a golden or greenish tinge to it, whereas most of the Bubble Coral we saw in the Red Sea was white, sometimes with a pale bluish tinge. We are not certain if the Bubble Coral in the Celebes Sea is the same species as that in the Red Sea, but it belongs to the same genus, Plerogyra.
Someone who commented on the Wordless Wednesday photo of Bubble Coral mentioned that she had been snorkeling for a long time, and had visited many places, but had never seen Bubble Coral. That was not surprising to us, since it is not usually found at snorkeling depths. We have seen Bubble Coral at many dive sites in the Red Sea, and in various places in the waters around Indonesia and Malaysia, but never at depths of less than about 15 meters (50 feet). Thus, it is unlikely that snorkelers would get to see it.
Tuesday, May 6, 2008
BNSullivan.jpg "Bubble coral with tiny shrimp")
Sunday, May 4, 2008
Bubble Coral
We have displayed a lot of photographs of reef scenes on The Right Blue. Reefs are formed by corals. The stony corals in the order Scleractinia are the reef-building corals that secrete calcareous skeletons for their polyps, which accumulate over time to become the hard framework of reefs.
We have posted quite a few photos of different kinds of soft corals, ranging from 'broccoli coral' and other Nephtheids to sea fans. Soft corals generally are more colorful and showy, and so they often are more attractive subjects for photography, but without hard corals, there would be no coral reefs. Until now, we haven't really focused much on stony corals, so we thought it was time we introduced some of the more interesting ones to readers of The Right Blue.
Some hard coral colonies form massive lumpy structures that look much like rock. Other stony corals form into fingers, pillars, antler-like branches, or even structures that look like plates. The ridged ones commonly referred to as 'brain coral' belong to the stony coral group, too.
One of the more unusual-looking stony corals we know about is Bubble Coral, an organism in the family Caryophyliidae. The first photo on this page shows a large colony of Bubble Coral on a reef in the Red Sea. If you click on the photo to enlarge it, you will see clusters of what look like bubbles -- thus the common name. Those bubbles, or 'vesicles' as they are properly called, are little balloon-like structures with rubbery skin that inflate during the day.
At left is a macro photo of the vesicles. The species in these photos, Plerogyra sinuosa, is quite common in the Red Sea. Usually it is found on steep slopes or on the vertical walls of underwater cliffs at depths of about 15 meters (50 feet) to about 40 meters (130 feet). I shot the first two photos on this page at Ras Mohammed, a well-known reef at the tip of the Sinai Peninsula.
I took the third photo on this page during a night dive off the coast of Tiran Island, which is situated at the mouth of the Gulf of Aqaba. Although we are not 100% certain, we believe that the organism in the photo is Bubble Coral of the same species as the photos above. Unfortunately, we did not get to return to the same spot in daylight to verify that this was indeed Plerogyra sinuosa.
At night, Bubble Coral looks very different than it does during daylight hours. The Bubble Coral's vesicles deflate, exposing its tentacles and its mouth.
In the photo at right you can see what appear to be the partially deflated vesicles, as well as the little tentacles. The tentacles can sting.
By the way, the vesicles of this species usually are whitish, often with striations of grey or very pale blue. They are translucent, and can look slightly opalescent. We have seen Bubble Coral elsewhere with a golden or greenish tinge. Next time we'll show you some more Bubble Coral, this time from Indonesia. The Bubble Coral we saw there had some 'extras' that led us to think of it as Bubble Coral Plus.
Wednesday, April 23, 2008
Monday, April 21, 2008
Crinoids - Also Known As Feather Stars
In the previous post, about our final dives on the wreck of the Zenobia, I mentioned that we saw crinoids inside the shipwreck. As I wrote that, I realized that many readers, and especially non-divers, probably had never seen or even heard of crinoids. They're strange creatures, and I was hard pressed to describe them for that story, so I thought it would be a good idea to show readers of The Right Blue what a crinoid looks like.
The photos on this page show a crinoid known as Klunzinger's Feather Star (Lamprometra klunzingeri), a species found commonly in the Red Sea. This is not the species we saw inside the Zenobia, but these photos should work well to illustrate what a crinoid is like.
Crinoids belong to the same phylum (Echinodermata) as sea stars and urchins. The phylum name means "spiny skinned" and most members of the phylum do have some kind of spiny structures on their outer coverings.
The crinoids have feathery arms, which are jointed. They can (and do) bend every which way. The arms have rows of protrusions, called pinnules, which run the length of the arm, making them resemble feathers. The crinoids catch their food by extending their arms like a fan. Bits of plankton are caught on the pinnules.
These creatures also have a set of appendages, called cirri, that serve as feet. They can move along on the cirri a little bit, but they also use their feathery arms to propel themselves. Sometimes they bend their arms down in a sort of arc, and use them like extra legs to skitter across sand or other flat surfaces. They use their cirri to hold on tightly to whatever they decide to perch upon.
There are teeny tiny hooks on their ends of the cirri, which help them to grab onto their perch. We have seen these actually puncture a sponge enough to leave a scar. I should also add that, on occasion, we have attempted to move a crinoid from one location to another. It's fairly easy to put a gloved finger next to a crinoid's little feet, and nudge it to perch there. The trouble comes in getting the crinoid to release its hold on that gloved finger again!
Crinoids are nocturnal creatures. They fold themselves up into a ball and hide in crevices in the reef during daylight hours. They usually emerge from their hiding places at dusk, and situate themselves on a favorite perch -- on coral, a large sponge, a sea fan -- wherever they can anchor themselves well. Then they unfold their feathery arms and feed all night, returning again to their hiding places at first light.
Crinoids are sensitive to light. When they are exposed to a bright light, they immediately begin to fold in their arms. For this reason, it is sometimes difficult to photograph them in their full glory, with all their arms completely outstretched. Crinoids are fairly plentiful on many reefs, so they are relatively easy to find during night dives. However what often happens is, we shine our lights around and spot a lovely crinoid, but as soon as it senses the light beam it begins to curl up. So, we switch off our lights and wait. Eventually the crinoid will unfold again, but then the photographer is lucky to get more than one or two shots before the light from the camera's strobe prompts the crinoid to fold into itself again. It takes patience to photograph crinoids.
The two photographs on this page were shot in quick succession. In the first photo, the crinoid's arms are fully extended. (Take note of its little cirri, hanging onto the coral it has chosen as a perch.) In the second photograph, the crinoid already is reacting to the light emitted by the flash during the first shot, so it's beginning to curl up. I photographed this crinoid during a night dive in the Red Sea, off the coast of Egypt's Sinai Peninsula. [Click on the photos to enlarge.]
Wednesday, March 5, 2008
Wednesday, February 27, 2008
Wednesday, February 20, 2008
Saturday, February 2, 2008
Nephtheid Soft Corals - Pale Pastels
In the past several posts, we've been displaying images of soft corals of the Nephtheidae family. To soothe any damage we may have inflicted on our readers' retinas with yesterday's collection of corals with fiery colors, today we're presenting images of Nephtheid corals in pale pastel colors.
By the way, the species of a Nephtheid soft coral cannot be determined merely by looking at the colors. Corals of a particular species in this family may come in a variety of colors.
All of the corals on this page, as well as on yesterday's post, belong to the genus Dendronephthya, but we're not certain of the species. In fact, I have been told by a marine biologist friend who studies these corals, that the only reliable way (short of DNA analysis) to identify a species of Dendronephthya is by examining certain internal structures, called spicules, with a microscope. In other words, it's next to impossible to reliably determine the species in the field.
For our purposes, we don't mind that we can't be sure of the exact species. When it comes to soft corals, we have been more concerned with collecting images of the different color varieties than with precise species identification. We're content just to know that these are Nephtheids.
At first glance, the more brightly colored varieties certainly are impressive, but we think there is something appealing about the pastels, too. The paler colors suggest a certain fragility, perhaps.
The stalks of almost all of these corals are somewhat translucent. But take a look at the macro image at right: The stalk is nearly transparent! (To see even more detail, you can click on any of the photos on this page to enlarge.)
Some of the corals in this family have greenish coloration, making them resemble plants. A common nickname for those varieties is Broccoli Coral. Other Nephtheid soft corals are sometimes referred to as Carnation Coral. The series of images below may give you an idea of why they acquired this common name.
I took all of the photos on this page while diving in the Red Sea, at reefs along the Sinai Peninsula. I hope you enjoy them.
Friday, February 1, 2008
Nephtheid Soft Corals - Fire in the Night
This past week I began a series about soft corals in the Nephtheidae family, arguably some of the most beautifully colored marine life on the planet. I mentioned that these corals come in a wide variety of colors, and that while they sometimes look like plants, they are in fact colonies of small animals -- coral polyps -- that arrange themselves in bundles on a stalk or stem.
I have a big problem with these corals: There are so many wonderful colors and varieties that I can't stop taking pictures of them!
The colors range from pastel pale, to richly saturated, to just plain knock-your-eyes-out. Today, we'll show you a few that we call "Fire in the Night" -- brilliant reds, oranges and golds.
I took all of the photos on this page during night dives in the Red Sea. These Nephtheid varieties feed mostly at night, so that is when they are plumped up and looking their best, with their feathery tentacles extended like flower petals. Click on any of the photos to enlarge and see more detail.
The first photo on this page is a a soft coral that had attached itself to the underside of a ledge. This bright red and yellow color combination is, to me, the prototypical "Fire in the Night" color. In case you are wondering, that is not an official name; it's just our name for this color variety.
Next is a macro shot of that same coral. You can see that it is the tentacles of the polyps that are bright red. The stalk itself is relatively pale. Those bright yellow bits on the surface of the stalk are sclerites -- hard structures that help it to hold its shape when it is plumped up. They serve a purpose similar to battens in a sail. They also pose a lighting problem to photographers, since they tend to be more reflective than the rest of the coral.
This next variety is a little less fiery than the one above, but is definitely bright and rich. We call this "Golden Glow." This time it's the stalk and sclerites that are golden, while the tentacles on the polyps are almost white.
A similar variety is the one we call "Hot Peach." It has a yellowish stalk, and bright yellow sclerites, but the flower-like polyps are a rosy pink. The combination of the yellows and pinks make it look quite "peachy," we think.
We'll wind up today with a macro shot of the "Hot Peach" color variety. According to my dive log, this image was captured just after dusk. The photo clearly shows that the tentacles were not fully retracted, but not yet fully extended, either.
These corals are not exactly easy to photograph. First of all, they don't need bright sunlight to prosper, so many of them live in deep water, or under ledges and overhangs, or inside cavelets. So, first the photographer has to find them.
Secondly, since these corals are bushy and branchy, they have many planes, and it's difficult to choose a point of focus. Also, since some parts -- especially those calcareous sclerites -- are more reflective than other parts, lighting them evenly can be a challenge.
Shots like the ones on this page were taken at night. Often there is virtually no ambient light underwater at night. So, the photographer (and her helpful dive companion) must locate the subjects to photograph by shining their handheld searchlights this way and that. It helps a lot if the team already has done at least one thorough survey of the area during daylight!
Next, the photographer composes the shot in light provided by a submersible flashlight. Underwater strobes only light a relatively small area -- so the photographer really has to get in close. Meanwhile we have to mind our buoyancy, depth, elapsed time, and our air supply, among other technicalities. But somehow it all comes together, at least some of the time.
Next time we'll show you some paler specimens of Nephtheid soft corals from our photo collection.
Wednesday, January 30, 2008
Tuesday, January 29, 2008
Broccoli Coral
This is the second in a series of posts intended to showcase the many colors and shapes of soft corals in the taxonomic family Nephtheidae.
In the first post I mentioned that these corals appear in a countless array of colors. Most grow as tree-like structures having a stem, or stalk, with many branches. Near the ends of the branches are clusters of polyps which open like tiny flowers when they are feeding. When the polyps are closed, they look like little beads.
In fact, when the polyps are retracted, these corals -- especially the greenish colored ones -- resemble broccoli plants, and "Broccoli Coral" is one of the common nicknames given to this type of coral. One look at the photo here, and you'll understand why. (Click on the photo to enlarge.)
Although this kind of coral can resemble broccoli in appearance, it definitely is not a plant. It is a colony of animals -- the coral polyps. The polyps arrange themselves in bundles at the ends of the rubbery stalks. Each polyp has exactly eight short, feathery tentacles. In order to feed, the polyps open and close their little tentacles, grabbing tiny nutrient particles that are suspended in the water.
Many Nephtheid coral colonies contract during daylight hours, making them look a bit shrunken and withered. They expand to feed at night, plumping themselves out and extending all of their little tentacles like bouquets of flowers at the tips of their stalks. For this reason, many of the best photos of these corals are taken during night dives.
I took the photo on this page in the Red Sea, during daylight hours. You can see that the polyps are tightly shut. In tomorrow's "Wordless Wednesday" post, you'll see some macro photos of this same variety of soft coral.
Wednesday, January 23, 2008
Wednesday, December 19, 2007
Sunday, November 11, 2007
Fire coral: Another view
After the previous post about fire coral was published, Jerry asked, "Why didn't you include a picture that shows what the whole thing looks like, instead of just some macros? You know, so that divers and snorkelers will know what fire coral looks like before they get too close."
I had to admit, I was remiss in not including such a shot. The photo at right is Jerry's choice to show fire coral as divers and snorkelers are likely to see it. (Click on the photo to enlarge.)
The fire coral species shown here is Millepora dichotoma. Like several other fire corals of the same genus, it tends to have whitish tips while the main structure of the colony has some color. Those of you who have been following this blog for some time could probably guess that this photo was taken in the Red Sea as soon as you saw those bright orange fishies. I've mentioned several times that these little fish, called Scalefin Anthias (Pseudanthias squamipinnis), are ubiquitous in the Red Sea.
We'd also like to note that the background color of this photo is pretty close to The Right Blue!
Friday, November 9, 2007
Fire Coral: Look, but do not touch
When is a coral not a coral? When it is a fire coral.
Fire corals (Family: Milleporidae) look like corals, and that's how they came to be called corals, although some biological characteristics related to their life cycle set them apart distinctly from true corals.
Fire corals are reef builders just like true corals. They secrete a calcareous skeleton. They take on a variety of different shapes ranging from lacy branches (like the example in the first photo on this page), to plate-like structures, to crusts that can cover other things growing on the reef, such as sea fans.
Fire corals and true corals do belong to the same phylum (Cnidaria). A characteristic that all Cnidarians have in common are nematocysts.
Nematocysts are tiny structures on the surface tissue of the organism that are used to capture food, and to defend against predators. Nematocysts actually are very cool structures -- one of nature's engineering marvels. They consist of a fine tube, inside of which is a little coiled thread with microscopic barbs on the end. When something touches the end of the nematocyst it fires the barbed thread into whatever has touched it, just like a little harpoon fired from a harpoon gun.
The second photo on this page is an enlarged 1:1 macro shot of fire coral. The little hair-like structures on the surface (called dactylozooids) are armed with the nematocysts. Touching those little hair-like structures triggers the nematocysts to fire. Click on that photo to see an even larger, more magnified version.
The nematocysts of fire corals carry a toxin that is intended to paralyze the minute bits of plankton that are the fire coral's prey. The toxin also causes pain to predators. Divers who touch or even accidentally brush against fire coral, experience a painful sting that burns like all get out -- thus the name fire coral.
Marine biologists who study these kinds of organisms have discovered something that is useful to know. The nematocysts of fire coral (and their first cousins, the hydroids) are de-activated by acids. Thus, it's a very good idea for divers to include in their kits a small container of vinegar when they visit places where they may possibly encounter hydroids or fire coral. A little squirt of vinegar immediately stops the nematocysts that may still be stuck to the skin from firing.
Conversely, fresh water and soapy solutions actually aggravate the nematocysts. The last thing you want to do if you are stung by hydroids or fire coral is to rinse the skin with fresh water. It actually prompts any remaining nematocysts to fire.
If you are ever stung by hydroids or fire coral, you won't forget it. Usually a welt or rash arises immediately and stays for a week or more, burning again every time you take a shower or bath.
For the record, the fire coral in the first image on this page, is Millepora alcicornis, a Caribbean species. It was photographed in the Cayman Islands. The species in the second photo is Millepora dichotoma, photographed in the Red Sea near Sharm El-Sheikh, Egypt.
