Taxonomic study of the brown algal genus <i>Chorda</i> (Chordaceae, Laminariales) with description of the new species <i>Chorda borealis</i> from Alaska and northern Canada

<i>Chorda borealis</i> sp. nov. is newly described from Alaska and northern Canada. <i>Chorda borealis</i> resembles <i>C. filum</i> and <i>C. asiatica</i> in gross morphology, but has a thinner cortical layer than those two taxa and is genetically distinct in the mitochondrial <i>cox</i>1 and <i>cox</i>3, plastid <i>atp</i>B, <i>psb</i>A and <i>rbc</i>L, 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 <i>C. borealis</i>, 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 <i>Chorda</i> species from the Pacific to the Atlantic could have occurred along the coasts of the north-western Arctic Sea.