Evolution of the unique freshwater cave-dwelling tube worm Marifugia cavatica (Annelida: Serpulidae)

Of the approximately 350 described species of serpulid polychaetes, only Marifugia cavatica inhabits fresh water. It is distributed in ground waters of the Dinaric Karst in northeastern Italy, Slovenia, Croatia, and Bosnia and Hercegovina. Five other serpulid species, comprising the genus Ficopomatus, are found in brackish water locations worldwide; otherwise serpulids are all marine organisms. We re-describe M. cavatica and examine the fine structure of its chaetae with SEM as well as summarise its distribution. The morphology of Morifugia provides an ambiguous indication of its phylogenetic relationships, thus DNA sequence data was also used. Phylogenetic analysis of nuclear rDNA 18S and 28S sequences using maximum parsimony, maximum likelihood and Bayesian analyses places Marifugia as a sister group to a clade of brackish-water Ficopomatus species. Osmoconformity and penetration into non-marine waters hence appears to have taken place once in the evolutionary history of Serpulidae. The transition to a subterranean environment may have occurred via ancestral marine shallow water to intertidal or estuarine species (like Ficopomatus) that evolved the necessary physiological mechanisms to withstand low salinity and then penetrated into freshwater caves via surface lakes.