Scleractinian corals, also known as stony corals — or just hard corals — are the primary reef builders in the oceans. Their polyps secrete calcium carbonate to form a skeleton. A minority of species live as single polyps, but most stony coral species are colonial, and the structures they build ‘grow’ over time. They form a myriad of shapes: mounds, branches, fingers, plates, and encrustations.

In several previous posts I discussed and displayed photos of a number of so-called soft corals, all of which are octocorals, i.e., their polyps have eight tentacles. Stony corals are hexacorals: their polyps have tentacles in multiple of six. Most seem to feed at night, so you are not likely to see the tentacles extended during daylight hours.

The coral in the first photo (top right) forms a rounded hemispherical head that is grooved in a meandering, maze-like pattern. Corals that form these kinds of colonies often are referred to as ‘brain corals’ because the patterns of the ridges and grooves are reminiscent of the sulci of a brain. The one pictured is Diploria labyrinthiformis, a species common throughout the Caribbean Sea.

Below is another example of a ‘brain coral’ — this one from the Red Sea. This species (Platygyra daedalea) forms massive colonies, sometimes more than a meter in diameter. Most brain corals belong to the family Faviidae.

I mentioned above that some hard corals live as single corallites. Members of the family Fungiidae fit that description. Known by the common name Mushroom Corals, these are free-living, i.e., they are not attached to the substrate. The photo below is a macro image of Fungia scutaria, an Indo-Pacific species. This is the most common mushroom coral found in Hawaii, where this one was photographed. The overall shape of the corallite usually is oval, with septae radiating from a central mouth, as shown in the photo. The corallite can attain a width of 15 cm to 18 cm (6 in to 7 in).

While hard corals all have calcareous skeletons by definition, the rest of their anatomy consists mostly of soft parts. Some of those can obscure the skeleton to the point that it is hard to tell, just by looking, that some species are indeed hard corals. A good example is Bubble Coral (Plerogyra sp.), shown in the next photo, below. The stalked corallites of this genus form rounded colonies. The skeleton of the colony is obscured during daylight hours by bubble-shaped vesicles. When this coral feeds, the vesicles deflate, and the tentacles emerge.

By the way, in this macro image you can see clearly that this coral is infested with little pancake-shaped critters. They are Waminoa flatworms. The photo was taken at Bunaken Island, Indonesia where much of the bubble coral seemed to be sporting Waminoa.

Corals of the genus Goniopora seem to feed mostly during daylight hours. The polyps have 24 tentacles arranged in a way that makes them look like flowers. The macro photo below shows the clustered polyps of a Goniopora species from the Red Sea.

A favorite subject with underwater photographers are the colorful members of the family Dendrophylliidae. They grow tubular skeletons topped by a cup-shaped calyx, so they are known by the common names Tube Corals or Cup Corals. Another common name is Cave Coral, a name that refers to this family’s habit of growing on the walls and roofs of underwater caves, and underneath ledges. Some members of the family are colonial, while others are solitary. All of them have gloriously colored tentacles, usually in shades of red, orange, or bright yellow.

I will close with two photos of a colonial Dendrophyllid species from Hawaii, Tubastrea coccinea. Both photos are of the same colony, located inside a small underwater cavelet, but taken at two different times. The first image was captured during daylight hours and the tentacles are retracted into the calyces.

The final photo is a macro image of a polyp from the same colony as above, but this one was taken during a night dive. When their beautifully colored tentacles are extended for feeding, these corals are a sight to behold.

I hope the readers of Photo Synthesis have enjoyed my underwater photos as much as I have enjoyed presenting them during the past month.

Many animals in the sea have evolved colors and forms that allow them to blend in with their surroundings. Some animals use their camouflage to hide from predators — and some predators use camouflage to fool their prey.

It can be difficult to photograph such animals, partly because it’s often hard to find them in the first place. If you look carefully at the photo at right, you will be able to make out the shape of a small purplish slipper lobster (Parribacus antarcticus), right in the center of the photo.

The picture was taken inside an underwater cave in Hawaii, and the lobster was on the ceiling. As I shined my light back and forth, the beam passed over the little lobster several times, but I didn’t spot it. Not until it suddenly scurried across the patch of red encrusting sponge was its presence betrayed.

Many animals that dwell primarily on sandy bottoms do an excellent job of blending in with their surroundings. Usually they are pale, with mottling or light patterns that help them mimic the substrate on which they rest. A good example is the flounder (Bothus mancus) in the next photo. I was preparing to photograph something else, and I didn’t notice the flounder until I very nearly knelt on the poor thing.

Sometimes an animal’s eyes are the one feature that will interrupt the camouflage effect and give it away, however this next image illustrates how even a critter’s eyes can be somewhat camouflaged. This is a crocodile fish (Papilloculiceps longiceps), a bottom-dwelling ambush predator from the Red Sea. Take a good look at its eyes and notice the lappets — the small irregular flaps that partially obscure the eyeballs — a part of its disguise.

Speaking of ambush predators, members of the Scorpionfish family (Scorpaenidae) are camouflage champs among fishes. Below is a Bearded Scorpionfish (Scorpaenopsis barbatus) perched on a reef. Decorated with all kinds of little frills, this fellow can be almost indistinguishable from surrounding plants and soft corals.

Next is another member of the same family, a Devil Scorpionfish (Scorpaenopsis diabolus) from Hawaii. This species hangs out around the rocks and dead coral rubble near the edges of reefs, trying its best to look like just another lump as it waits for unsuspecting prey to pass by. Were it not for its fins, it might go unnoticed by the photographer as well as its prey.

Finally, my favorite scorpionfish species, the Leaf Scorpionfish (Taenianotus triacanthus), which comes in quite an array of colors: nearly black, dark brown, purple, pale pink, white, and scrummy yellow like this one. These laterally compressed fishies perch on the bottom, on ledges, or on coral heads to wait for prey. They sway gently back and forth, just as a leaf might. They often are adorned with algae and scaly crud, which enhances their camouflage. This one, photographed in Hawaii, looks like it is about to molt.

In case you are having trouble figuring out which end is which in the photo above, the head is on the right. (Look for the eye and the pectoral fin). Camouflaged fishes and other creatures may not always be the prettiest animals in the sea, but they are among the most interesting of photo subjects. The only problem is locating them in the photo once you get home!

They look like a cross between a caterpillar and a tricked out centipede. They crawl about with considerable agility, they are voracious feeders, and they certainly know how to defend themselves. Meet the Bearded Fireworm (Hermodice carunculata), a free-moving marine Polychaete worm.

This species is widely distributed from the Caribbean, throughout the warmer waters of the Atlantic Ocean (including Florida and the Bahamas), and all the way to the Mediterranean Sea. All of the images in this post were captured in the Mediterranean, in the waters around Cyprus and Greece.

H. carunculata is a segmented worm, i.e., it is a member of the phylum Annelida. As a Polychaete, one of its distinguishing anatomical features is a set of bristles. In this case, the hair-like bristles, or setae, are arranged in bundles attached to small appendages, called parapodia, at the lateral edge of each segment of the worm.

Although the setae are very fine, they are hollow and contain a venom. The setae also are extremely brittle. If touched they will break off readily and the fragment(s) will lodge in the skin of whatever touched them. The common name Fireworm refers to the burning sensation caused by the venom in the setae.

When the animal is disturbed or threatened, the setae are fluffed out in a defense display, as in the photo below.

In the next photo, below — a sort of quarter-profile view — you can see the how the setae bundles emerge from the critter’s parapodia. The structures above the parapodia are gill filaments. Also visible is the worm’s median antenna, looking a bit like a thin rhinoceros horn.

The species name of H. carunculata refers to the fleshy orange bit on its head, called a caruncle. The caruncle is believed to have a sensory function. The head-on macro shot below presents a more detailed view of the worm’s prostomium, with all of its associated parts. (Don’t be afraid; it looks fierce, but it’s only a photo!)

One final photo will put things back into perspective. In the image below, we see the whole animal as it goes about its business in the muck. This individual was not feeling very threatened, so its setae are semi-retracted.

Speaking of muck, taking photos like these is not a task for the squeamish. Setting up the shots requires that the photographer spend a considerable amount of time lying on her belly in the muck, surrounded by these creepy-crawly critters.

All of the individuals in the above images were between 10 cm and 15 cm (4 in to 6 in) in length. I am told that H. carunculata can grow to a length of 30 cm (12 in) — but for the record, I have never seen one that long, nor would I care to!

The pretty creatures pictured here look like anemones, but they are not true anemones. They are Cerianthids, commonly referred to as ‘tube anemones’, which are taxonomically quite distinct from true anemones.

Cerianthids and true anemones do belong to the same phylum, Cnidaria, and the same class, Anthozoa, but tube anemones belong to the subclass Ceriantipatharia, a taxon that also includes the so-called ‘black corals’ (Antipatharia).

One of the visible features that distinguishes Cerianthid tube anemones from true anemones is the morphology of their tentacles. The macro photo below shows that Cerianthids have shorter tentacles in their centers, and longer tentacles around the margin. The color of the shorter tentacles usually is different from that of the longer tentacles, making them look a lot like flowers (at least to me).

Cerianthids dwell inside a rubbery tube (thus the name tube anemone) which is built from mucus secreted by the animal. The tube is embedded in mud or packed sand. When not feeding, or when disturbed, the animal retracts inside its tube for protection.

These creatures can be difficult to photograph for several reasons. Most Cerianthids are relatively small; their crowns of tentacles are perhaps 5 cm (2 in) across, so it’s necessary to get very close to them in order to photograph them. If the photographer accidentally touches one of the tentacles, piff! the critter retracts. And although Cerianthids happily feed in gentle currents, any nearby turbulence — like that created by the photographer as he or she moves about — causes the critter to quickly go into hiding.

These are deep-dwelling creatures — all of the examples in this post were photographed at depths greater than 40 meters (130 ft). They are accustomed to low levels of ambient light at those depths, so Cerianthids do not take kindly to blasts of artificial light from a camera strobe. At best, one or two shots of an individual is all that a photographer can hope for before all that is left to photograph is the tube!

All of the Cerianthid tube anemones pictured in this post were photographed off the west coast of Hawaii’s Big Island. This post was adapted from an article I wrote last year on my blog, The Right Blue, where you can find further details and more images of Cerianthids.

Sea fans are among the most beautiful sights seen by divers. Gorgonian sea fans are Cnidarians that build colonies in branching formations that usually are fan-shaped, thus the common name.

Like the Nephtheid soft corals I wrote about recently here on Photo Synthesis, Gorgonians are octocorals: each polyp has eight pinnate tentacles which it uses to capture nutrients suspended in the water column. They are seen most often on reef crests, or jutting out from drop-offs or steep banks in locations where natural currents will sweep plankton and other organic nutrients across the polyps’ tentacles.

Some sea fan species form colonies in a single plane, while others grow their branches in somewhat of a tangle. They come in quite an array of colors, most of which actually are the result of zooxanthellae that live in the structural tissue of the Gorgonian. Beyond giving them attractive coloration, zooxanthellae also produce nutrients through photosynthesis, which benefit the Gorgonians.

By the way, not all Gorgonians form fan-shaped colonies. Some families in this order form long, slender colonies, commonly known as Sea Whips — but we’ll save those for another time. Today I’d like to show you some close-up and macro photos of some Gorgonian sea fan species from several tropical locations around the world to illustrate some of the variability in the morphology of the colonies. (A brief description follows each photo.)

Above is a sea fan from the genus Melithaea, with its polyps open for feeding. The branches of sea fans in this genus tend to intertwine. This one was photographed at Bunaken Island, Indonesia.

The next sea fan, Acabaria sp., forms a fragile looking net-like colony. There are other species in this genus that form denser structures. This one was photographed in the Red Sea, off the coast of the Sinai Peninsula.

The above macro image shows a section of Subergorgia hicksoni, a species widely distributed throughout the Red Sea. This sea fan species is not so beautifully colored, but the colonies grow to an impressive size, very often measuring more than a meter in width. The fans are relatively flat, or gently curving. This species is dominant on many deep reefs in the Red Sea, whereas Acabaria spp. seem to prefer shallower depths.

This purplish sea fan is Gorgonia ventalina, photographed in the Cayman Islands. It forms a tight mesh, and grows in a single plane. G. ventalina is very common throughout the Caribbean Sea. In fact, the sight of this purple fan is almost a hallmark of Caribbean reefs. It tends to live in on patch reefs in relatively shallow water, so it is seen very frequently by divers and snorkelers.

The final photo, below, shows an entire colony of G. ventalina, photographed at West Caicos, in the Turks and Caicos Islands.

Nudibranchs — marine snails without shells — make wonderful photo subjects for the macro photographer. They are small, colorful, and they move slowly (as snails are wont to do). That last characteristic is particularly welcome. Most fishes are in motion almost constantly, and non-sessile invertebrates tend to scurry hither and thither. It’s nice to find a subject that is not only photogenic, but doesn’t turn tail or flat out disappear before the photographer can focus the camera’s lens!

It’s always interesting to find out and record which critters feed on what. Here are some macro photos of two sponge-eating nudibranch species feeding on their favorites. The colorful nudibranch in the first photo, at right, is Chromodoris quadricolor — also known by its common name Striped Pajama nudibranch. While we cannot see the mouth parts of the creature in this photo, or any obvious feeding damage on the sponge, the species is known to feed on these brightly colored sponges (Negombata sp.).

The individual in the first image was photographed in the Red Sea at Tiran Island.

In the image above, we see a Mediterranean nudibranch species, Peltodoris atromaculata, known by the common name Dotted Sea Slug. The nudibranch is on a brown sponge (Petrosia ficiformis), which is thought to be its primary food source. Both the sponge and the nudibranch species are found throughout the Mediterranean, including the Adriatic and Aegean Seas.

The final image, above, is much more convincing as a feeding record. Here again we have P. atromaculata, but this time the feeding scars on the sponge, P. ficiformis, are very obvious. It is not uncommon to see several of these nudibranchs feasting on a single sponge at the same time. They can leave the sponge quite scarred.

Each of the two photos of P. atromaculata is of a different individual. Both were photographed off the coast of Greece, near Cape Sounion.

Note: Although the nudibranchs in the second and third photo appear larger than the C. quadricolor in the first photo, in real life they all are about the same size: approximately 5 cm (2 in) long. The second and third photos were enlarged.

Many animals in the ocean seek shelter from predators by living on or among other animals. Among fishes, members of the Damselfish family (Pomacentridae) often seek protection this way. Some of these relationships also are commensal or even symbiotic.

One of the most well known symbiotic relationships in the marine world is that between anemones and fishes commonly known as ‘clownfish’ or ‘anemonefish’. Clownfish form a subfamily, Amphiprioninae, of the Dameselfish family. Each of the twenty-some species in this subfamily lives symbiotically with one or more anemone species.

Both the fishes and the anemones are believed to benefit from the relationship. The fishes are protected from predators by the stinging nematocysts of the anemeones’ tentacles; the anemones feed in part on waste matter from the fish.

In the photo at the top right of this page, an Indo-Pacific clownfish species, Amphiprion ocellaris, hides among the tentacles of an anemone (Heteractis sp) where it makes its home. The photo was taken at Bunaken Island, Indonesia.

In some cases, several species live together in the same host anemone. In the macro photo above, a juvenile Two-bar Anemonefish (Amphiprion bicinctus) — the only anemonefish species known from the Red Sea — shares its living space with some Anemone Shrimp (Periclimenes longicarpus), a commensal species. I photographed this particular anemone (Stoichactis sp.) at a reef in the Straits of Tiran. This one anemone was host to a number of these shrimp, plus an adult pair of A. bicinctus, and several juveniles. A. bicinctus also shares its anemone home at times with another Red Sea damselfish species, Dascyllus trimaculatus.

I was photographing Cerianthid ‘tube anemones’ in Hawaii one day when I noticed a very tiny fish that appeared to be sheltering among the tentacles of the individual in my viewfinder! The fish was so tiny that I could not identify it then with any confidence. Only after the photos were enlarged could I see that it was a barely post-larval Hawaiian Dascylllus (Dascyllus albisella), a damselfish species endemic to Hawaii. The image above is one of those photos. At that time I had never before seen this relationship, but I have since seen this pairing a few more times. I do not know if the relationship is commensal, or if the little fishies were merely sheltering among the Cerianthid’s tentacles until they grew large enough to migrate to the reef.

Many other species in the damselfish family find shelter not in anemones, but in branching hard corals, especially Acroporidae. It is not uncommon to see a group of these fishes hovering above a formation of ‘antler coral’. From a distance they can look like a colorful shimmering mass above the coral colony. If a predator (or a diver) approaches too quickly or too closely, the fishes drop down instantly and in unison into the branches of the coral, where they hide until the threat has passed. (The precision of this choreography is impressive.) The image above, photographed in the Red Sea, shows a number of fluorescent green fish (Chromis caerulea) snuggled in among the branches of a stand of Acropora sp.

I have seen this behavior in many species of damselfish. Interestingly, these fishes seem to practice a sort of species segregation. Only one species at a time will inhabit a given coral colony. Once a group of conspecifics has established itself at a coral colony, no other species of damselfish will be tolerated there.

One of the many marine animals that works the night shift is the crinoid species pictured here: Lampometra klunzingeri, a member of the Mariametridae family. During daylight hours, these crinoids hide in crevices in the reef. Shortly before sunset, like clockwork, they emerge from their hiding places and laboriously crawl up onto the corals in search of a perch for feeding.

In the Red Sea, where these photos were taken, these crinoids are seen frequently among the branches of Millepora dichotoma, a calcareous reef-building Cnidarian commonly known as ‘fire coral’. At dusk, these crinoids crawl onto the branches and grasp the surface with their cirri, as you see in the first photo on this page. Then they unfold their segmented arms, spreading them widely to capture plankton. The whole process is quite a wonderful sight to witness.

The close-up photo above shows L. klunzingeri with its arms spread for feeding. You can see how this array could be a fairly efficient plankton sieve, but the next image is even more convincing.

The image above is a macro photo of a section of a single arm of L. klunzingeri. Clearly visible are the rows of fine hairlike structures on each pinnule. Once we have a look at an image like this, it is easier to understand how it is possible for the crinoid to capture enough food to sustain itself. All but the most microscopic bits that might be suspended in the water that passes over this crinoid’s arms will be caught.

I mentioned above that this crinoid species hides in the day and comes out at dusk. Clearly, the relative presence or absence of ambient light is an important regulator of this animal’s behavior. In fact, like many other nocturnal animals, this crinoid species is very sensitive to light. This photosensitivity can make it difficult to photograph. When exposed to bright light, the animal rapidly retracts its arms, curling them inward.

For the photographer, this means it is necessary to set up a shot of the animal with its arms extended in next to no ambient light! In order not to prematurely disturb the animal when photographing this species, I use a light with a narrow beam, like that of a penlight, to locate a likely subject. One of my camera strobes has a modeling light — also a very narrow beam — which I use to quickly aim and focus the camera lens. As soon as the strobes flash once, the crinoid will begin to curl in its arms. As a result, I have many more photos of these crinoids that look like the final one, below, than of the arms completely spread out.

Note: All of the photos on this page are the same species, Lamprometra klunzingeri, but each image is of a different individual. All of the photos were taken during night dives in the Red Sea, off the coast of the Sinai Peninsula.

Crinoids, a class of marine animals in the phylum Echinodermata, are pretty creatures. The photo at right shows a crinoid perched on a Malaysian reef with its fluffy arms extended for feeding. Looking at the photo, it’s easy to see how they acquired their common name, Feather Stars.

This is how divers usually see crinoids, and this is how they are most often photographed. As pretty as crinoids are — and they come in a vast array of colors — photographs of the whole animal don’t reveal much about the animal’s structure or behavior. Whole-animal photos of creatures like this should be just a starting point. A photo study of an animal, or class of animals, should include close-up and macro photos as well.

Today I present three close-up photos of crinoids that show something of their structure. Tomorrow I will continue with some macro photos that reveal even more about crinoids.

In the photo above, you can see the arrangement of the animal’s feathery, many-jointed arms around a central disc. The animal’s mouth is in the disc, which is covered by many cirri. Crinoids are filter feeders. They capture tiny bits of organic matter suspended in the water that passes over the pinnules on their arms. The food is then transported inward, down the length of the arms to the mouth.

This second close-up photo is an aboral view of the same individual. (It can be difficult to photograph this view, but this very accommodating individual happened to choose a protruding strand of wire coral as its perch, so I was able to swim around behind it to take the photo!) Here we have a good view of the form and function of the cirri on the underside of this crinoid. These cirri are used for mobility: the crinoid can skitter along using the cirri like little feet. Then once the animal finds a suitable perch, it uses its cirri to hold itself in place. In this case, the cirri are wrapped around the wire coral in a tight grasp.

As filter feeders, crinoids optimally position themselves in places where currents — tidal and otherwise — pass over them. The cirri of crinoids have tiny hooks at the ends, which help them to hold on and stay in place during feeding. While this may be functional for the crinoids, should they decide to perch on a soft surface, those hooks on the cirri can do damage. In this final photo (a different individual and probably a different species than the previous two photos), a crinoid has decided to perch on a sponge and has dug into the sponge’s flesh with its cirri. As you can see, the sponge has been damaged (although it will likely heal over in time).

About the photos: The crinoid in the image at the top of the page was photographed at Pulau Sipadan, Malaysia. The three close-up images were taken at Manado Tua, off the northeastern tip of Sulawesi Island, Indonesia.

Shown above: Thalassoma klunzingeri, from the Red Sea, about 15 cm (six inches) long. This one was photographed near Sharm el Sheikh, Egypt.

Shown above: Halichoeres hortulanus, about 23 cm (nine inches) long. Also from the Red Sea, this one was photographed at Ras Mohammed, at the tip of the Sinai Peninsula.

Shown above: Thalassoma pavo, a Mediterranean species, about 12 cm (about five inches) long. This one was photographed near Cape Greco on the southeastern coast of Cyprus.

Shown above: Coris julis, a smaller Mediterranean species, about 7.5 cm (three inches) long. This one also was photographed near Cape Greco, Cyprus.