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Marine Life Encyclopedia

Corals and Other Invertebrates

Bluebottle

Physalia utriculus

Distribution

Indo-Pacific waters

ECOSYSTEM/HABITAT

Coastal to open ocean (pelagic)

FEEDING HABITS

Foraging predator

TAXONOMY

Class Hydrozoa (hydrozoans), Order Siphonophora (siphonophores), Family Physaliidae (men o’ war)

The bluebottle, or Indo-Pacific Man o’ War, is not a jellyfish but a siphonophore, which is a colony of tiny, specialized polyps working together as colonies. The bluebottle is easily recognized by its blue, gas-filled sac (pneumatophore) that floats on the water’s surface. During summer in the Southern Hemisphere, strong winds carry bluebottles to the shores of Australia, where thousands of bluebottle stings are reported each year.

The bluebottle is comprised of four different colonies of polyps that depend on each other to survive. The species is named after one of its polyps, the gas-filled sac, often referred to as “the float,” which resembles a blue bottle floating in the ocean. The float moves depending on the wind and supports the other three types of polyps that are responsible for catching prey, digesting food, and reproduction. The float can be 0.8 to 6 inches (2 to 15 cm) long, while the feeding tentacles reach lengths of 30 feet (10 m).

Bluebottles are foraging predators that feed mostly on larval fish and small crustaceans and mollusks. Their predatory tentacles are equipped with stinging cells that are used to paralyze and capture prey. Once caught, the prey is transferred to the mouths of digestive tentacles that expand to more than 0.8 inches (2 cm) wide to ingest the prey. Bluebottles use their reproduction tentacles to produce their own eggs and sperm that make larva. The larva then divides itself many times until a colony is formed.

Few species eat the bluebottle because its translucent blue body is difficult to see against the water, but some predators that are immune to their stinging cells (e.g., nudibranchs and snails) are known to feed on this and other siphonophores. Bluebottles are similar to the Portuguese Man o’ War (Physalia physalis) in appearance and behavior, but are smaller and less venomous. And unlike the Portuguese Man o’ War, bluebottle stings have yet to cause any human fatalities. However, a bluebottle sting still causes pain and swelling, and tentacles should be removed carefully by beachgoers using tweezers. The species is not valuable or fished commercially, but strong winds and currents can carry massive swarms of bluebottles to the coasts where they sometimes wash up on beaches.

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