Barrel Jelly

Overview

The Barrel Jellyfish (Rhizostoma pulmo) is a large, robust scyphozoan frequently observed in the Mediterranean, Black Sea, and Northeast Atlantic. Often appearing in significant numbers during late summer and early autumn, these jellyfish are known for their rhythmic, lung-like pulsing motion and tendency to form large seasonal blooms.

Appearance

This species features a distinctive, highly arched, bell-shaped umbrella that can reach up to one meter in diameter. The bell is typically white or translucent with subtle blue, pink, or yellow hues and is bordered by a characteristic violet-blue fringe. Unlike many other jellyfish, it lacks marginal tentacles; instead, it possesses eight thick, fused oral arms beneath the bell that resemble a cauliflower-like structure.

Biology & behavior

Rhizostoma pulmo is a pelagic drifter that relies on passive movement and bell pulsations to navigate. While they possess nematocysts (stinging cells) for defense and prey capture, they are considered only mildly venomous and pose no serious threat to humans, typically causing only minor skin irritation. These jellyfish serve as a vital food source for various marine life, most notably the leatherback turtle. Young fish are sometimes observed sheltering beneath the bell or among the oral arms, a relationship that may provide protection for the fish.

Feeding

As a non-macrophagous feeder, this species relies entirely on micro- and mesozooplankton. It captures prey using its specialized oral arms, which are equipped with mucus secretions and numerous small mouth openings to trap and ingest planktonic particles.