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Why are thaliaceans adapted for life in open water?

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Bright blue sea squirt Rhopalaea crassa with yellow-edged openings grows on a coral reef among various corals in the Philippines. — tunicate Blue spot sea squirt (*Clavelina moluccensis*), a colonial tunicate.

Thaliaceans, which include pyrosomes, dolioloids, and salps, are pelagic animals adapted for life in open water through specific structural and functional modifications of their ascidian ancestors.1

Their primary adaptation for open-water mobility is a specialized form of jet propulsion. While many other tunicates have poorly developed musculature, thaliaceans have evolved modified muscles that allow them to constrict their atrial cavity and eject water. In dolioloids and salps, this exhalant current creates a powerful jet stream that propels the animal forward through the water column. Additionally, these organisms exhibit specialized reproductive strategies, including complex alternations between sexual and asexual phases, which support their survival and proliferation in pelagic environments.231