Four new Licmophora species (Licmophorales), with a review of valve characters and exploration of cingulum characters, including a new septum type

Licmophora species are common epiphytes, with new species being discovered in under-sampled areas, and a need for a larger morphological character set for all species. In the rich flora of Guam, there are several distinctive new species, four of which we describe here, with a reconsideration and expansion of their useful taxonomic characters. Licmophora attenuata n. sp. formed large colonies on long mucilage stalks; narrowly cuneate valves, 209–339 µm long, were notable for a large, possibly hollow, spine, and the head-pole rimoportulae set back along the sternum. Licmophora bulbosa n. sp. was solitary or on short, branched mucilage stalks, with broadly cuneate valves, 95–122 µm long, with slightly inflated apices giving it a ‘turkey baster’ shape; valvocopulae bore a novel perforated silica ingrowth along the entire midrib, which we call a Tharngan strip. Licmophora curvata n. sp. attached by mucilage pads, the 87–170 µm long cells were distinctive by their curvature and 21–30 µm broad valves. Licmophora labianatis n. sp. was found in large colonies on long branched stalks, with 311–365 µm long valves that were distinctive for a delicate section mid-way along the valve where vimines were absent. Licmophora cells are heterovalvar and we define the ‘back’ frustule as the one with the basal rimoportula. Frustules are also heteropolar and thus have a left and right side. These terms do not correspond to epivalve/hypovalve and primary/secondary sides of valves and are useful for specifying locations of features. The Licmophora epitheca (i.e., the mature half of the frustule) has a cingulum comprising five copulae (a valvocopula + four pleurae) that differ from each other and between species; we describe their morphology in detail. Copular striae were often denser than valvar striae. Midribs were positioned symmetrically along the midline of the band, or asymmetrically, closer to one margin. Advalvar and abvalvar copular striae consisted of single slits or a series of pores (rimate vs porate) and the type often changed along the band. Midribs on valvocopulae transitioned in distinctive ways from the abvalvar side at the cell apex to the advalvar side. The 2nd and 4th pleurae (closed at the apex of the frustule, like the valvocopula) were sometimes very similar to one another but in other species the 4th pleura was reduced to an apical cap with a very narrow band. The 1st and 3rd pleurae, closed at the base, were usually distinct from one another in the size and shape of the open tips. The expanded set of valve + copula characters will be useful in resolving the taxonomy of the majority of short, straight Licmophora species, but studies of additional species are needed to narrow the list to the most useful copula characters.