Lectotypification of Lithophyllum arcticum (Corallinales, Rhodophyta) and a study of its relationships within the Melobesioideae

Three separate collections of the type material of Kjellman's Lithophyllum arcticum are re-examined and a lectotype is selected. It is confirmed that the thallus is unattached, at least 4.5 cm in diameter, composed of up to eight superimposed more or less discoidal lamellae, provided with concentric striations on the surface. Individual lamellae are usually 100 to 200 mum thick (reaching I mm), developing dorsally from the main thallus and expanding centrifugally. The internal organization is dorsiventral with a polystromatic hypothallium, giving rise to an ascending perithallium with small subepithallial initials and rectangular (in TS) epithallial cells. It is found that patches of coaxial-like growth occur sporadically in the hypothallium and the perithallium, and that ventral lamellae may grow back-to-back. Somatic cells exhibit both large and narrow cell fusions. Pore plates of the raised multiporate conceptacles are slightly sunken to flattened and perforated by 16 to 31 pores. Pore canals are conical (narrowing towards the top) and are bordered by filaments composed of both undifferentiated and slender-elongate cells. Old conceptacles are overgrown by vegetative filaments and empty chambers are embedded in the perithallium. Collectively these features indicate that L. arcticum belongs to the subfamily Melobesioideae. The development of an unattached-superimposed thallus, patches of coaxial growth, short subepithallial initials and specialized pore cells suggest a position either in Mesophyllum, or in an amended Leptophytum to include even species with coaxial patches and unattached-superimposed habit (characters presently segregating Leptophytum from Synarthrophyton). The holotype of Lithophyllum zonatum from East-Finnmark, previously considered to be related to L. arcticum, is re-examined and shown to belong to a different species. A previous Arctic record of Mesophyllum lichenoides from Spitsbergen is abolished, and thus the disjunct distribution of L. arcticum in relation to Mesophyllum suggests a position in the Synarthrophyton-Leptophytum complex which shows a bipolar to temperate distribution.