Range expansion of tropical pyrosomes in the northeast Pacific Ocean

Pyrosomes are colonial pelagic tunicates that have fascinated marine biologists for over a century. Their name comes from the “fiery” bioluminescence that luminous organs produce at night time. Blooms of pyrosomes, identified as Pyrosoma atlanticum (Peron, 1804), have recently appeared in the North Pacific Ocean (Fig. 1), prompting questions about environmental factors that triggered their appearance and persistence over multiple seasons as well as potential ecosystem impacts. Pelagic tunicates, which include salps, dolioloids, and pyrosomes, are Urochordates that spend their whole life cycle in the plankton and feed using fine mucus meshes. Pyrosomes are colonies of zooids that are connected in a chitinous tunic and resemble colonial benthic ascidians (Class Ascidiacea). Genetically identical blastozooids are added to the colony via asexual budding. Pyrosome colonies can reach lengths of several meters, with pyrosomes in the northeastern Pacific reaching up to 80 cm in length (Brodeur et al. 2018). Ciliary beating within the zooids achieves both suspension feeding and locomotion (Alldredge and Madin 1982). Each zooid contains luminous organs that may be used to communicate with zooids further away within the colony in response to mechanical or light stimuli (Bowlby et al. 1990). Pyrosomes remain one of the least-studied planktonic grazers, in spite of their widespread distribution and ecological significance. Like other pelagic tunicates, pyrosomes are known to form high density blooms reaching tens of individuals per cubic meter, with swarms of P. atlanticum removing >50% of phytoplankton standing stock in the 0–10 m layer (Drits et al. 1992). Most species, including P. atlanticum,