Transoceanic rafting of Bryozoa (Cyclostomata, Cheilostomata, and Ctenostomata) across the North Pacific Ocean on Japanese tsunami marine debris

摘要

Forty-nine species of Western Pacific coastal bryozoans were found on 317 objects (originating from the Great East Japan Earthquake and Tsunami of 2011) that drifted across the North Pacific Ocean and landed in the Hawaiian Islands and North America. The most common species were Scruparia ambigua (d’Orbigny, 1841) and Callaetea sp. Of 36 bryozoans identified to species level, 15 are already known from North America, one of which (Schizoporella japonica Ortmann, 1890) is an earlier introduction from Japan; 18 species are known only from the Western Pacific, one of which (Bugula tsunamiensis McCuller, Carlton and Geller, 2018) is newly described in a companion paper. The 13 additional bryozoans, not taken to species level, are likely derived from the Western Pacific based upon evidence reviewed here; two of these species (Callaetea sp. and Arbocuspis sp.) are undescribed. Seven warm-water species, Metroperiella cf. biformis (Zhang and Liu, 1995), Celleporaria brunnea (Hincks, 1884), Drepanophora cf. gutta Tilbrook, Hayward and Gordon, 2001, Smittoidea spinigera (Liu, 1990), Biflustra grandicella (Canu and Bassler, 1929), Biflustra irregulata (Liu, 1991), and Celleporina cf. globosa Liu, 2001, not known from Japan, may have been acquired by Japanese Tsunami Marine Debris (JTMD) as these objects were carried by ocean currents into more southern waters. Three oceanic bryozoans (Jellyella tuberculata (Bosc, 1802), Jellyella eburnea (Hincks, 1891), and Arbopercula angulata (Levinsen, 1909)) provide insight into the routes that some JTMD items may have taken, and thus the conditions experienced, as they rafted from the Western Pacific to the Central and Eastern Pacific. The cooler-water species J. tuberculata and A. angulata were found primarily on JTMD objects arriving in the Pacific Northwest, whereas J. eburnea was most common on objects landing in the Hawaiian Islands. The most common bryozoan growth forms on these rafted objects were runners (creeping uniserial morphology) and arborescent forms capable of using available surface area provided by other organisms (such as hydroids) on space-limited objects. Species that form flat or mounded encrustations were less frequent, suggesting that they do not fare as well in a potentially space-limited environment.