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Dangerous Marine Animals
The What, Where, Avoiding and Otherwise Dealing with Biological Stinger, Biters, Pokers as a Diver.

Attached Hydrozoans

by Robert Fenner

Not only is it a totally different world “down there”, but diving presents many biological dangers as well. Snorkeling and scuba diving entail risks as we all know; from partial to more decompression sickness and ills of too-fast surfacing, hazards of travel and occasional, though rare rig failure. What is not often fully appreciated is just how common biological hazards there are underwater.

            There are several and diverse organisms to be aware of, and avoid; and for the unfortunate, worthwhile treatments that you can employ should you find yourself stuck, stung or bitten. Herein is my overview of these bio-hazards by group.

Benthic (Attached) Hydrozoans: A Large Cast of “To-Be-Avoideds”

            What do you think of when you hear of "dangers from the deep blue seas", giant squids, sharks, other divers? The actual cause of many, or most discomfort from dealing with marine life is from a group of stinging-celled organisms called hydrozoans; in particular the hydroids and fire-coral. The latter may serve another purpose other than stinging like h, e double hockey sticks when they come in contact with your skin.

“Oh Christmas tree, oh…. Owwww”! Up close and personal many Hydrozoans look like delightful little Christmas trees; but don’t be fooled. Those tiny trees and fans pack a powerful stinging wallop!  Most attached species are a few inches in length maximum; with some being barely visible.

            Though many Hydrozoans look like “corals”; they are closer relatives to jellyfishes; and some are indeed umbrella-like jellies.  Hydrozoans are one of three Classes of the phylum Cnidaria; the “stinging-celled animals”; the others; Class Anthozoa making up the mainly polypoid corals, anemones, sea fans... and Class Scyphozoa comprising the "real" Jellyfishes that live most of their lives as medusas, self-propelled bell-shapes. Most of the Hydrozoans are small, obscure not-so funny to touch Christmas tree sort of affairs; they include such notables as Portuguese Man of War, Fire Corals, and the beautiful delicate Lace Corals (Stylasterines) amongst their ranks. 

We’ll leave off with the jelly Hydrozoans, and deal with them in a later piece. Here we’re going to just deal with attached Hydrozoans.  These are solitary and colonial animal species, sometimes just being a branch per colony, other times being made up of a few individuals coming together as specialized parts (like the "Sail" in the free-living Man 'O War...). The Hydrozoa are differentiated from the other Classes of Cnidarians by their specialized cells, stinging and agglutinant (sticking) structures, as well as life history.

Attached, benthic hydrozoans are comprised of a few types of polyps. The feeding ones bear retractable thin tentacles that in turn house the stinging and sticking inclusions called “nematocysts”.



A close up of Fire Coral in the Bahamas showing the stinging tentacles of the feeding (gastrozooid) polyps. “Nothing feels like fire than to touch a Milleporina in the morning…”


Order Hydroida: These are the Hydroids, the most common Hydrozoans. Most are flexible and "tree-like" in appearance with their stinging cells much like small leaves or ornaments. Most are separate sexes and reproduce sexually, with mature attached colonies releasing small umbrella-like medusae that form at the base of their "branches"... these swim off producing either eggs or sperm, that if joined, metamorphose into a planula larval form that if in turn is fortunate, gets blown by currents to a suitable reef surface and attaches, becoming a new branched colony. 

            There are several species of Hydroids to be found in all areas we dive; and sometimes they occur in great abundance; though often overlooked. Below is a smattering of types you’re likely to encounter.

Aglaophenia cupressina Lamouroux 1812, Feather Hydroid. Indo-Pacific. To about two feet in height. A zooplankton filter feeder. Occurs in whitish and tan varieties. A colony in S. Leyte, P.I.



Gymnangium hians (Busk 1852), Feather Hydroid. Found in areas of good current on underhangs, in caves (pukas). Gray to light brown in color. 2-3 inches in length. Indo-Pacific. Hawaii pic.


Halocordyle disticha, the Christmas Tree Hydroid. Branches alternately arranged on single stalks in colonies. Bearing prominent white polyps at ends like Xmas ornaments. To three and a half inches in height. Cozumel pic.


Order Hydrocorallina: These are colonial polypoid hydrozoans that secrete calcium carbonate skeletons, though they are not true corals... The Order comprises two suborders, the Fire Coral, Milleporina and

Suborder Stylasterina: Characterized by having a thick layer of tissue overlying their skeletons. Their specialized feeding and defensive polyps are imbedded within star-shaped openings in their calcareous skeletons.

Stylaster roseus. Here in St. Lucia.


Fire Coral; Millepora: the Poster Child Hydrozoan

            Found worldwide in shallow tropical seas, Millepora is that fawn-colored furry-fuzzy stuff that divers love to hate.  These stony-coral look-a-likes incorporate single celled algae to manufacture at-times massive colonies of reef-building limestone.

Suborder Milleporina, are the stinging fire corals. Unlike the stylasterines, their skeleton is only covered by a thin epidermal layer; and their defensive polyps arise from separate openings that encircle the gastrozooids (feeding polyps). Millepora is the single genus. As you study and observe corals and coral-like animals like the hydrozoans, you'll gain an appreciation for the term polymorphic or "many shapes"; describing the several physical forms a "species" can/does take dependent on nutrient and other growing conditions; sometimes heavily branched and delicate in appearance, other times more blade, fan-shaped and massive. The number of varieties of Millepora are in dispute; systematists J.E.N. Veron states that there are at least 48 nominal species; an unknown number of true species. More to the point for our discussion is the question of "how to tell when you're looking at a fire coral?" period. There's always the touch test; ouch. Most of the time, the colonies are green or yellow-brown (due to endosymbiotic zooxanthellae) fading to whitish at the tips, and "soft", "hairy" and rounded in appearance. On very close inspection, the arrangement of almost microscopic stinging and eating polyps can be seen (image).            

Millepora around the world! Some examples that you’re likely all-too familiar with. A dichotomously branched stand in N. Sulawesi, Indonesia; boulder-like encrusting growth in Wakatobi, S. Sulawesi; fan-shaped unifacial colonies in S. Leyte, P.I.; an upright fan in Egypt’s Red Sea;  and a poly-morphic stand and one burning through a Gorgonian sea fan in Key Largo, FLA.


Avoiding Being Stung:

            With careful placement and cover of most of your body, the only thing a diver has to be watchful of is where they place their uncovered hands. Note that even cloth gloved hands can and do pick up Hydrozoan stinging structures from touching long-immersed mooring ropes, buoys and such; and that it is VERY important to rinse these after and between dives… and NOT touch your eyelids and other thin skin areas to avoid latent stings.

Too Late: Stung; What To Do

            A likely enough source of information: Alert Diver online: http://www.alertdiver.com/Marine_Envenomations_Jellyfish_and_Hydroid_Stings

To paraphrase and add: The single best treatment is to as quickly as possible wash the affected area with household vinegar (which is 5% acetic acid). The type isn’t important, but best to rinse the area rather than rub, or apply ice or freshwater… as they cause even more stinging as nematocysts further fire into your skin. For many folks, application of hydrocortisone cream/ointment brings relief. If you can get to your doctor, they may prescribe a higher dose or even orally administered steroids.

            Some folks are more susceptible to these sorts of stings. Rarely a diver will experience continuing sensation of burning, shortness of breath, fever, nausea, vomiting… IF you experience these, or show no signs of improvement with time, a visit to a medical facility is advised.


            What; where are the really dangerous animals in the seas; the sharky sharks; the humongous biting morays; and stinging stingrays?! These will be covered in later pieces, but for now, let’s focus on the much more likely, more commonly encountered dangerous marines. 

           Yes; attached Hydrozoans are beautiful, fragile and more than slightly to-highly toxic to the touch! Watch your hands underwater... and don't touch your mucus membranes without washing your hands after diving. Ouch! 


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