Sea spaghetti is a long, noodle-like brown seaweed made from Himanthalia elongata, a wrack of the north-eastern Atlantic harvested off the coasts of Ireland, Brittany, Galicia and Wales. Its slender, olive-brown straps look and cook remarkably like pasta, with a tender-crisp bite and a mild, green, faintly bean-like flavour that has made it a favourite of European sea-vegetable cooks.