Codium fragile in Norway: subspecies identity and morphology

Abstract

The green alga Codium fragile consists of 10 subspecies, of which subspecies fragile is a well-known invasive seaweed. Morphological work carried out in the 1950s suggested that there were three subspecies along the Norwegian coast: subsp. fragile, subsp. atlanticum and subsp. scandinavicum. However, more recent molecular data have shown the existence of only two subspecies and that these are frequently misidentified. The aims of the present study were therefore to verify which subspecies occur in Norway using the rpl16-rps3 chloroplast marker, to ascertain their likely time of arrival and to compare their morphology to their genetic identity. DNA sequences were obtained for 60 thalli from 18 sites along the coast (57–69° N) and 10 herbarium specimens (1902–1950). The sequences indicated that both subsp. fragile and subsp. atlanticum occur at present and have been in Norway since at least 1932 and 1948, respectively. The subspecies co-occurred at one site, but in general, subsp. atlanticum appears to have a narrower distribution than subsp. fragile, both geographically and in terms of habitat. Importantly, mucron length, other utricle features, or habitat were not always sufficiently reliable to give an accurate subspecies identification, demonstrating the necessity of DNA sequencing for the identification of these subspecies.