Dispersal and gene flow in free-living marine nematodes

Marine nematodes are amongst the most abundant and diverse Metazoa in marine sediments [1,2]. Estimates of species diversity, including terrestrial and parasitic species, vary widely and range from 105[3] to 108[4]. This huge taxonomic diversity encompasses a wide variety of feeding strategies and life history characteristics, but has at the same time hampered ecological studies because species identification is difficult. Consequently, ecological studies on free-living nematodes typically pool species into functional groups based on different feeding strategies [5], tail shape [6], body size [7], life history [8], or a combination of several of these parameters [9]. Next to these ecological studies, considerable work has been done over the last decades to provide an evolutionary framework for the phylum Nematoda [10,11], with a special focus on terrestrial [12], marine [13,14] or parasitic nematodes [15]. These studies show that convergent evolution is a frequent phenomenon for nematode morphology, feeding strategy and habitat. In contrast, there are only few studies on the importance of micro-evolutionary processes (gene flow, genetic drift and selection) for nematode evolution, even for parasitic nematodes [16]. Defining the scales of connectivity among marine populations and identifying the barriers to dispersal and gene flow are however crucial to understand the ecological and evolutionary properties of populations and the dynamics and persistence of populations under environmental changes.Gene flow describes the exchange of genetic information between populations through migration, whereas dispersal is defined as the movement of individuals from one genetic population to another [17]. Consequently, from a population genetics perspective and for species where individuals rather than eggs or propagules are the mechanism for dispersal, dispersal and gene flow are synonyms [18]. Both terms are used throughout this review. For the marine environment, barriers to ge