Rediscovery and reinstatement of the New Caledonian endemic filmy fern Hymenophyllum pumilio Rosenst.

The filmy fern Hymenophyllum pumilio was described in 1911 from a single collection made during 1910 from New Caledonia. It was not recorded again until its rediscovery during an expedition to New Caledonia in 2012. In the interim, it was synonymised in 2007 with Hymenophyllum minimum, which had been regarded as endemic to New Zealand. After comparing the new collections with H. minimum from New Zealand, we reinstate H. pumilio as a distinct species. The most notable morphological differences are that H. pumilio lacks secondary pinnae and the spines on the outer surfaces of the indusial flaps that are found in H. minimum. There are also substantive genetic differences, with 37 substitutions across an alignment length of 2104 base pairs of rbcL and trnL-trnF chloroplast DNA sequences. Hymenophyllum pumilio is a small and easily overlooked species that remains poorly documented. It is symptomatic of the need for more taxonomic attention on New Caledonia's ferns.