Green Barrel Sea Squirt

Overview

The Green Barrel Sea Squirt is a colonial tunicate frequently observed on tropical Indo-Pacific reefs. These organisms form distinct, urn-shaped colonies that attach firmly to hard substrates like coral or rock, often appearing as small, wobbly bulbs.

Appearance

Colonies typically reach up to 10 cm in diameter and exhibit a soft, flexible consistency. Their body wall, or tunic, is perforated by numerous tiny inhalant pores, while a single large atrial siphon at the top serves as the primary exit for filtered water. The colony's coloration—ranging from white and brown to vibrant green—is influenced by the presence of symbiotic cyanobacteria within its tissues. White, spherical calcareous spicules embedded in the tunic provide structural support and often appear more concentrated around the siphon openings.

Biology & behavior

This species maintains a unique symbiotic relationship with Prochloron cyanobacteria, which reside in its tissues and provide sugars through photosynthesis. To protect these symbionts from intense tropical sunlight, the sea squirt produces specialized molecules called mycosporines that act as a natural sunscreen by filtering out harmful ultraviolet radiation. While sessile, the colony can exhibit a form of "mobility" by slowly dividing and shifting its position across coral branches to optimize its exposure to light. When disturbed, the colony may exude a thick, sticky mucus.