A global review of pyrosomes: Shedding light on the ocean's elusive gelatinous “fire-bodies”

Pyrosomes are colonial tunicates that form gelatinous tubes and occasionally produce bioluminescent swarms. The rapid “bloom-bust” dynamics of pyrosomes have the potential to outcompete other zooplankton, restructure marine food webs, enhance carbon export, and interfere with human activities. Pyrosomes have been recorded for at least two centuries, yet much remains unknown about their physiology, bloom mechanisms, and ecosystem impacts. Recent bloom reports prompt the question of whether frequencies or densities are increasing, and whether potential population changes are caused by ocean change. This review clarifies current information on pyrosome biology and ecology and explores apparent contradictions in habitat preferences and diel vertical migration (DVM). We posit that the dominant species, Pyrosoma atlanticum, may preferentially inhabit waters below 18°C and alter its DVM activity to maintain overall body temperature near this threshold. Expanding our knowledge of pyrosomes is needed to facilitate their inclusion in ecosystem models and forecasts of future population distributions.