Dangerous reef organisms
Dangerous reef organisms hot buttons for dangerous reef-organisms section of BIOLOGY OF CARIBBEAN CORAL REEFS website hot button for irritating chemicals section of BIOLOGY OF CARIBBEAN CORAL REEFS website hot button for bites section of BIOLOGY OF CARIBBEAN CORAL REEFS website hot button for sharp spines/bristles section of BIOLOGY OF CARIBBEAN CORAL REEFS website hot button for poisonous flesh section of BIOLOGY OF CARIBBEAN CORAL REEFS website hot button for stings section of BIOLOGY OF CARIBBEAN CORAL REEFS website

There aren't many Caribbean-reef orgnanisms that will kill you dead, but there are certainly quite a few that can hurt you. CLICK ON an icon to learn about what you should avoid.

NOTE it goes without saying that a good and careful SCUBA diver will be swimming well above the reef and will not be touching anything. For this reason, most dive-boat operators frown on their customers wearing gloves

icon for stings section of BIOLOGY OF CARIBBEAN CORAL REEFS website


Most things that sting are found in Phylum Cnidaria, a group that includes sea anemones, fire corals, and jellyfishes. Fortunately, most cnidarians are harmless to humans but, nonetheless, they are all best left alone.

The stinging device in cnidarians is microscopic and is known as a stinging "cell", or nematocyst. The nematocysts are contained within cells on tentacles, such as on sea anemones or coral polyps, or on special defensive polyps known as dactylozooids, as on fire corals or Portuguese-man-of wars.

NOTE lit. “finger animal”, perhaps referring to the often stubby shapes of such defensive polyps.

seahorse dive leader in BIOLOGY OF CARIBBEAN CORAL REEFS website

"Oh, here's something to avoid...fire coral. It's not really a coral, but who cares when you're rolling in agony. Well, maybe not that bad but, still, pretty bad! Some people who touch it get a rash that last for several days" - Turks & Caicos 2003

NOTE fire corals Millepora spp. are common on Caribbean reefs. If you look closely at the specimen in the second part of the video, you may see a white fuzz covering the branches. These are mostly defensive polyps, bearing the highly toxic stinging cells that cause all the misery. Although you shouldn't be touching anything on the reef, be especially careful around fire corals


photo collage of defensive polyps of a fire coral
Fire coral bears nematocysts on special defensive polyps that when touched can produce a painful sting. Often there is an accompanying rash that in some people may last for days.

NOTE fire corals are not actually corals. Rather, they are in the Class Hydrozoa, while corals are in Class Anthozoa. Along with tiny bottom-dwelling hydroids and their floating medusae, and Portuguese-man-of-wars, the hydrozoans are by far the most common source of stings in the Caribbean



diagrammatic view of how a nematocyst works
Nematocysts are tiny capsules filled with an hygroscopic (i.e., takes up water) protein-gel and most have an internally coiled hollow thread that is capable of rapid eversion. There are many types of nematocysts. Ones with sticky or coiling threads function to attach to the substratum or to entangle a prey. Killing or disabling types of nematocysts have everting threads that penetrate the skin of the prey, sometimes aided by barbs along their lengths. To visualise how this works, imagine blowing out the everted finger of a rubber glove. The everting thread is driven by the expansion of the protein gel, which swells from intake of water, and which is also the toxin. When it can go no further, the tip of the thread ruptures and the toxin is released. The toxin acts by inhibiting nerve function and is also necrotic, causing breakdown of tissues. Note in the drawing on the Left how small is the nematocyst, only a few micrometers in length.

NOTE 1micrometer = 1um = 1/1000th of a millimeter

seahorse dive leader for BIOLOGY OF CARIBBEAN CORAL REEFS website photograph of a

"When you see an anemone like this one, with well-demarcated white bands, keep away. They really can sting! Notice how much damage the anemone has caused to the coral" - Bonaire 2008

NOTE a corkscrew anemone Bartholomea annulata


photograph of corkscrew anemone within boulder coralStings of the corkscrew anemone are not only painful but, on sensitive skin, can produce a raised and persistent rash.  The white bands on the tentacles show the location of batteries of nematocysts.







Corkscrew anemone Bartholomea annulata
in a boulder coral Montastrea sp. 0.6X

treatment for nematocyst stings

2. IRRIGATE WITH MEAT-TENDERISER to deactivate the toxin
3. APPLY COLD PACKS to reduce inflammation
4. SEEK MEDICAL HELP if there are any symptoms of allergy such as excessive and persistent swelling, or shock

TAKE CARE when handling portions of attached tentacles to avoid contacting undischarged nematocysts

seahorse dive leader for diversity section of BIOLOGY OF CARIBBEAN CORAL REEFS website photograph of tentacles of Portuguese-man-of-war Physalia taken from a video These are the fishing tentacles of a large-sized Portuguese-man-of-war Physalia sp. collected from the beach and placed in an aquarium tank. The tentacles are mostly fishing ones, and are constantly being pulled up and let down. The "bumps" along their lengths are just the foldings of the tentacles as their lengths are compacted, although nematocyst batteries may also be visible - Port Aransas, Texas 2003

photograph of Portuguese-man-of-war on a beach in the Gulf of Mexico
The Portuguese-man-of-war is a colonial hydrozoan with features similar to those of fire corals. The colony comprises several types of polyps on tentacles buoyed up by a float (bell). The float itself is a type of polyp and, in fact, is the first one to grow from the larva. The most potent nematocysts are borne on long fishing tentacles. These extend many meters downwards to catch small fishes and crustaceans that blunder into them. The fishing tentacles on a large specimen may be 30m in length. How does the animal bring captured prey up to its mouths on the feeding polyps? The tentacles can contract about 70-fold, enabling prey to be brought close to the belll.

Species of Physalia are also known as "blue-bottles", for their colour. After storms they may be stranded on the beach by the hundreds. The nematocysts can remain active for days after photograph of Portuguese-Man-of-War courtesy Cindy Young, Vancouverstranding, as long as they are moist, so watch where you walk with bare feet.


Physalia physalis
stranded on a beach at Port Aransas, Texas 0.5X


Juvenile Portuguese-man-of-war Physalia physalis showing
several feeding polyps (these are the darkest blue-colored
ones, some curved), and several lighter-blue fishing tentacles,
including 2 longish ones. The fishing tentacles bear many
specialised polyps known as dactylozoids, used in both food
capture and defense. These polyps have nematocysts that are
highly toxic 1X. Photograph courtesy Cindy Young, Vancouver.


photograph of Portuguese-man-of-war Physalia with prey fishesThe high potency of the nematocysts enables the capture and eating of large prey such as fishes. Physalia stings may be life-threatening to humans, especially to individuals with allergy to the protein toxin in the nematocysts. Any contact with the tentacles should be followed by a visit to a doctor or hospital.




Physalia sp. with a pair of small prey fishes.
If not stranded on the beach, the fishing
tentacles would contract and pull the fishes
up to the bell region for ingestion 0.5X


drawing of sting-apparatus of a cone shell
Another potentially deadly stinger is the cone shell, Conus. These snails capture worms, fishes, and other snails using a hollow spear-like device modified from part of the ancestral feeding apparatus, the radula. When thrust into the flesh of a prey by a muscular mouth structure known as the proboscis (see drawing on Right), the spear releases toxins that operate to inhibit nerve-impulse transmission. The prey is quickly paralysed.

NOTE a tooth-bearing device present in all molluscs save bivalves, that is extended from the mouth to scrape up food materials. Depending upon the food habits of the snail in question, algae can be rasped and bivalve shells drilled with the photograph of Caribbean cone shell Conusradula

West-Indian alphabet-cone Conus spurius 2X

treatment for cone-shell stings

1. ELEVATE THE LIMB and APPLY COLD PACKS to slow movement of the toxin.

One good feature of Caribbean cone shells, if you can call it that, is that they are not nearly as toxic as related Indo-Pacific species, whose toxins can readily kill...