Are similar worms different? A comparative tracer study on bioturbation in the three sibling species Marenzelleria arctia, M. viridis, and M. neglecta from the Baltic Sea

The invasive spionid polychaetes of the genus Marenzelleria spp., consisting of the three sibling species M. neglecta, M. viridis, and M. arctia, has been found in the Baltic Sea since the 1980s. Because of difficulties in species identification, little is known about species-dependent sediment reworking and solute transport. The closely related species are apparently similar in feeding and sediment-dwelling behavior, but size and burrowing depth indicate differences in bioturbation and the biogeochemical consequences thereof. To investigate these potential differences, a tracer experiment with artificial particles (luminophores) and solute tracer (bromide) was conducted. Polychaetes were identified to species level using a molecular genetic key. Modeled results show that all three species display markedly low particle reworking rates with biodiffusion coefficients (Db) of 1.76 cm(2) yr(-1) (M. viridis) and 0.07 cm(2) yr(-1) (M. neglecta) at an abundance of 1273 individuals m(-2) and 0.44 cm(2) yr(-1) (M. arctia) at twice that abundance. Nonlocal transport coefficients are negligible in all cases. Solute transport by M. neglecta and M. viridis are more similar to one another than to M. arctia, whose solute transport mode is much more diffusive in character (10.9-fold enhanced diffusivity with nonlocal irrigation coefficients [alpha] of 55.3 yr(-1)) than that of the two other species, which affect tracer distributions in the sediment predominantly through a nonlocal, advective transport mode (alpha of 108.9 yr(-1) M. viridis and 130.9 yr(-1) M. neglecta). Thus, a functional grouping of the sibling species in terms of bioirrigation is not recommended.