Facts and Sting Treatment: Stinging Hydroids

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Class Hydrozoa

stinging hydroids, portuguese man of war, fire coral, stinging plant, marine
Stinging hydroids are members of the Hydrozoa class, which also includes the Portuguese man-of-war and fire coral. © istockphoto.com

I am not a doctor! I just like geeking out over diving topics. This article is intended as a source of general information. Do not use it to replace the expertise of a diving doctor. If you are reading this article for first aid advice and you are currently experiencing systemic allergic systems such as respiratory difficulty, a rapid pulse, or generalized swelling, get off the internet and go see a doctor -- now.

There Aren't Any Jellyfish or Coral Heads Nearby, What Just Stung You?

Some divers don't realize that a great deal of marine life lives in the sand and on rocks. Many of these organisms, such as stinging hydroids, have developed systems of feeding and defense that may injure divers. Fern or bush-like stinging hydroids are found throughout the world's oceans, and cause blisters and rashes if touched.

Stinging Hydroids Are Related to Other Aquatic Stingers:

Despite their appearance, stinging hydroids are animals, not plants. They are members of the class Hydrozoa, which includes stinging organisms such as the Portuguese man-of-war and fire coral. Some members of the class live a solitary existence, while others form colonies. Fire coral, stinging hydroids, and the Portuguese man-of-war are all colonial superstructures composed of tiny creatures.

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Stinging Hydroid Identification and Habitat

stinging hydroids vs crinoids
Common stinging hydroids, and a crinoid for comparison. © Wikipedia Commons: The Hantu Blog/Foter/CC-BY-NC. Crinoid image © Wikipedia Commons: berichard.

Stinging hydroid is a generic term typically used by locals to refer to whatever types of hydroid colonies are present in the area. Common names may also include branching hydroid, feather hydroid, bush hydroid, and white ball hydroid (to name a few). Stinging hydroid colonies resemble ferns or feathers. The colonies may be observed as bush-like bundles, branching structures, or standing alone, and have been reported in a variety of colors. Stinging hydroids may anchor to nearly any substrate including rocks, kelp leaves, seafloors and shipwrecks. They are present in temperate, sub-tropical, and tropical waters, and are found at a wide range of depths.

Stinging Hydroids vs Crinoids:

Stinging hydroids might be confused with crinoids, which are feathery echinoderms (the same family as sea stars). Distinguishing between the two may be difficult, although crinoids usually seem to have thicker quills, and do not have branching extensions like many hydroid colonies. Close observation will reveal many individual polyps in a hydroid colony, while criniods have no polyps. A crinoid's plums originate radially from the animal's mouth while stinging hydroids, being colonial, do not have such an organized structure.

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How Do Stinging Hydroids Sting?

Stinging hydroid, nematocysts
Sketch of basic nematocyst function courtesy of NOAA. © Wikipedia Commons: NOAA

Stinging hydroids sting using tiny, barbed, needle-like structures called nematocysts. They are primarily used to inject a toxin into prey, although nematocysts certainly have a defensive application as well. Cells containing nematocysts (cnidocytes) are extremely sensitive, and fire due to gentle contact, pressure waves, and chemical stimulation. This firing reaction is one of the fastest cellular responses in nature. Nematocysts may be released within three milliseconds of contact with the prey. All members of class Hydrozoa sting in a similar fashion.

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Identifying, Treating, and Preventing Stinging Hydroid Injuries

vinegar first aid
The application fo vinegar is the most commonly recommended treatment for stinging hydroid injuries. Natalie L Gibb

Identifying a Stinging Hydroid Injury

Hydroid stings are caused by the same mechanism as fire coral and medusa stings, and although the toxin may be slightly different among species, the identification and first aid is similar.

• Blisters and Rashes:
Hydroid stings cause rashes, burns or blisters, and may look very similar to contact dermatitis from poison oak. The sting is usually described as having a burning sensation. The severity of the reaction varies among divers.
• Immediate and Delayed Reactions Lasting up to 10 Days:
A hydroid sting may be felt immediately upon contact, or may be delayed up to 24 hours. Unfired stinging cells may remain on a diver's skin or clothing, and may fire after the diver has exited the water. Injuries may take 10 days or more to heal.
• Allergic Reactions:
In most cases a stinging hydroid injury is not dangerous. However, divers who experience even the mildest signs of a systemic allergic reaction should seek medical assistance immediately. These signs include an increased heart rate, unusual swelling, and difficulty breathing.
• Infections:
From personal experience, infections seem to be more common with fire coral stings and coral scrapes than stinging hydroid injuries, but monitor blisters and rashes for redness and other signs of infection, and see a doctor if an infection looks likely.

Treating a Stinging Hydroid Injury:

• Minor, Uncomplicated Stings with No Signs of Allergic Reaction:
As with fire coral and jellyfish stings, the recommended first aid for stinging hydroid injuries is washing the area with vinegar. In most cases, vinegar should disable nematocysts and reduce the likelihood of any unfired cells stinging the diver (although one scientific study claimed that vinegar causes nematocysts to fire in a single species of hydroid). The area should be cleansed and disinfected. Soothing ointments such as calamine lotion may be applied to the sting.
• Severe Stings and Allergic Reactions:
In extreme cases, doctors may recommend Benadryl or Cortisone cream. Divers have been prescribed medications for allergic reactions such as Claritin and Benedryl, and in more extreme cases oral steroids and even epinephrine injections have been used to control allergic reactions. Antibiotics may be used to treat infections. As always, it is advisable to consult a doctor before using any medication to treat an aquatic life injury.

Preventing Stinging Hydroid Injuries

Preventing stinging hydroid injuries is easier and more effective than treating them after the fact. Prevent contact with stinging hydroids by staying at least a few feet from underwater structures and refraining from touching the sea floor or rocks. Always use full body exposure protection, such as a dive skin or thin wetsuit, even when it is not required for warmth. Using a full length wetsuit and avoiding contact with underwater organisms will prevent the vast majority of marine life injuries.

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Stinging Hydroids Are Fascinating - Despite Their Potential for Injury

interesting photos of stinging hydroids
Left: Specialized feeding and stinging polyps. Right: A decorator crab camouflages and protects himself with stinging hydroids. Left: © istockphoto.com; Right: © wikipedia commons - Nick Hobgood

Stinging hydroids are fascinating creatures, just not to touch. Here is some quick trivia about stinging hydroids:

• The individual organisms that form a colony often specialize -- some are feeders, some are structural supports, and some spawn during mating. They become dependent upon each other and often cannot survive individually.
• Many aquatic creatures have learned to use stinging hydroids to their advantage. Some animals are impervious to the stings and roost on the branches of the hydroid for protection. Decorator crabs snap off small branches and hook them onto their shells for camouflage and defense.
• A recent study (Feb. 2013) has also found that an isolate from one species of Indo-Pacific hydroid (Aglaophenia cupressina Lamouroux) is an effective antibacterial agent against certain strains of E. Coli. It will be interesting to see what can be done with this discovery.

The Take-Home Message About Stinging Hydroids

Stinging hydroid injuries are completely avoidable - simply do not touch colonies and use a full-length wetsuit anywhere that stinging hydroids could be present. In almost all cases, the recommended first aid is to rinse with area with vinegar, then thoroughly disinfect the area. The presence of tentacles, spines, or barbs indicate the the injury was not caused by a stinging hydroid, and may require additional first aid. In most cases, hydroid stings are not dangerous. However, monitor victims for allergic reactions and infections, and seek medical attention at the first sign that a victim's condition is deteriorating.

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