153706 Thong Weed Himanthalia elongata. Length to 1m. Slender yellowish-brown seaweed. Grows attached to rocks on the lower shore, on rather sheltered coasts. FROND is strap-like are rather narrow (less than 1cm across); grows from centre of tough button-like structure attached to rock. STATUS Widespread and locally in W and N Britain; absent from most of SE.