Lessonia tholiformis sp. nov. (Phaeophyta: Laminariales) from the Chatham Islands, New Zealand

A new species of Lessonia Bory, L. tholiformis C.H. Hay is described from the Chatham Islands of New Zealand. Unlike most other Lessonia species, the holdfast of L. tholiformis is a semi-solid dome or mound formed by overlapping layers of flattened, lobed haptera. In this respect, L. tholiformis is somewhat similar to L. nigrescens, a species growing in the intertidal region of Chile, but it differs from that species in other ways. Juveniles of L. tholiformis have rugose blades. However, the rugosity wanes with age and blades of mature plants are consistently smooth. L. tholiformis is the only species of Lessonia found growing on the Chatham Islands where it is apparently endemic. In many localities it is the dominant kelp forming thick, subtidal forests to a depth of 10 m. L. tholiformis is the fifth species of Lessonia now recorded from the south-west Pacific region - the other species being: L. variegata J. Ag. (New Zealand mainland), L. brevifolia J. Ag. (Auckland, Campbell, Antipodes, and Bounty Islands), L. corrugata Lucas (Tasmania) and L. adamsiae C.H. Hay (Snares Island).