The waters surrounding the Hawaiian Islands are home to a varied ecosystem, including several species of jellyfish. Certain types of cnidarians present a risk to ocean users due to their stinging cells, known as nematocysts. Understanding the appearance and behavior of these creatures helps swimmers and beachgoers enjoy the Pacific safely. The jellyfish encountered range from common, mildly stinging species to those requiring immediate attention upon contact.
Common Visitors to Hawaiian Shores
The most frequently seen cnidarian is the Moon Jellyfish, scientifically known as Aurelia aurita. This jellyfish is recognizable by its translucent, saucer-shaped bell, which can grow up to 16 inches in diameter. A distinct identifying feature is the four horseshoe-shaped gonads visible through the top of the bell. Moon Jellies possess mild stinging cells that pose little threat to humans, making them less hazardous encounters in quiet bays and harbors.
The Moon Jellyfish feeds on minute prey like micro-plankton and small crustaceans, which explains its relatively mild venom. They are commonly found floating nearshore or within protected areas like harbors, often carried by gentle currents. These species represent the majority of jellyfish populations worldwide and are often encountered without incident.
Identifying the Highly Venomous Box Jellyfish
The Hawaiian Box Jellyfish (Alatina alata) demands greater caution. This species is named for its cube-shaped bell, which is mostly colorless and transparent, making it difficult to spot. The bell is small, often hand-sized, with four thin, trailing tentacles that can extend significantly below the body.
The danger stems from its potent venom, injected through microscopic stinging cells lining the tentacles. While the Hawaiian species is not considered deadly like its Australian cousin, Chironex fleckeri, its sting is extremely painful and can cause severe systemic reactions. These animals are active predators, capable of directed swimming toward a light source, and are considered the fastest swimmers among their relatives.
The appearance of Alatina alata nearshore is highly predictable. They typically arrive in large numbers on south-facing shores, such as Waikiki and Hanauma Bay, approximately 8 to 12 days after the full moon. This monthly aggregation is tied to the lunar cycle, cueing the mature animals to swim shoreward to spawn. Their presence is concentrated on the leeward sides of the islands, with warning signs often posted at popular beaches during this three-day period.
The Portuguese Man-of-War: A Related Hazard
The Portuguese Man-of-War (Physalia physalis or the Pacific variety Physalia utriculus) is often mistaken for a jellyfish but is biologically classified as a siphonophore. This means it is a colony of four specialized individual organisms, called zooids, that work together.
One of these zooids forms the most recognizable feature: a gas-filled, blue or violet bladder that floats on the ocean surface, acting as a sail. This float, or pneumatophore, can rise up to six inches above the waterline and gives the creature its common name. Beneath the float, long, venomous tentacles trail in the water, sometimes reaching lengths of 30 feet or more, delivering a powerful sting.
The Man-of-War is generally found far out at sea, but strong onshore winds and currents frequently blow them onto Hawaiian beaches, especially those on the windward (east-facing) side.
Essential Sting Response
Responding quickly and correctly to a sting is necessary to minimize the effects of the venom. For stings from the Hawaiian Box Jellyfish, the recommended first aid is a two-step process focusing on neutralizing remaining stinging cells and treating the injected venom.
The initial step involves rinsing the affected area with vinegar for at least 30 seconds to prevent unfired nematocysts from injecting more toxin. After the vinegar rinse, any remaining tentacles should be carefully removed using tweezers, not scraped with a card, as scraping can trigger a massive discharge of venom.
The second step involves immersing the stung area in hot water, ideally between 104 and 113 degrees Fahrenheit, for 20 to 45 minutes to denature the venom and reduce pain. Applying ice is not recommended for box jellyfish stings, as research indicates it can enhance the venom’s activity. Seek medical attention immediately if the sting covers a large area or if the victim experiences severe symptoms like difficulty breathing or nausea.