Coexistence of congeneric native and invasive species: The case of the green algae Codium spp. in northwestern Spain

• We studied the distribution, abundance and reproduction of congeneric Codium species.
• The invasive and native species coexist in all locations and habitats studied.
• Codium fragile was more frequent towards the east of the Cantabrian coast.
• C. fragile matures at a smaller size and in greater proportion than the native species.
• Molecular analyses are necessary for the identification of subspecies of C. fragile.

We examined the patterns of distribution and abundance, and reproductive traits (presence of gametophytes and size at time of reproduction) in the invasive Codium fragile ssp. fragile and the native C. tomentosum and C. vermilara on intertidal habitats of NW Spain at two dates. All three species coexist in the locations and habitats studied, although abundances were low. We found a greater proportion of C. fragile ssp. fragile towards the east of the Cantabrian coast and on upper levels on the shore, where conditions are more stressful. The proportion of thalli bearing gametangia in C. fragile ssp. fragile was greater than in the native species in all habitats. The presence of gametangia was size-dependent for all species, with the invasive species maturing at a smaller size, which combined with the previous features, might confer competitive advantages to this species over the native species. We also demonstrated that molecular analyses are necessary for the correct identification of C. fragile subspecies.