LAUSR

ABSTRACT Chorda borealis sp. nov. is newly described from Alaska and northern Canada. Chorda borealis resembles C. filum and C. asiatica in gross morphology, but has a thinner cortical layer than those two taxa and is genetically distinct in the mitochondrial cox1 and cox3, plastid atpB, psbA and rbcL, and nuclear rDNA ITS1-5.8S-ITS2 region DNA sequences. The erect thalli are simple, cord-shaped, medium to light brown, and up to 90 cm in length and 2 mm in diameter. The mid-thallus cortex is composed of 2–4 cells and is 110–230 µm thick. When mature, unilocular zoidangia develop among unicellular clavate paraphyses and measure 30–48 µm long and 8–13 µm in diameter, containing presumably 16 zooids, each with an eyespot. The distributional pattern of C. borealis, which is distributed from the northern Pacific Ocean to the Labrador Sea and connected to the north-western Atlantic through the Arctic, suggests that the dispersal of Chorda species from the Pacific to the Atlantic could have occurred along the coasts of the north-western Arctic Sea.