Surface lebensspuren produced by amphipods and isopods (crustaceans) from the Isonzo delta tidal flat, Italy

Substrate properties, such as grain size, water content, shear strength, and content of organic mucus, influence the life activity of benthic organisms and their trace-fossil record. This study deals with actualistic experiments using small crustaceans (amphipods and isopods) moving mainly over plaster of Paris surfaces in various stages of hardening. Several consistencies, such as semi-fluid, very soft, soft, soft-stiff, stiff, and very stiff are distinguished. The morphology of surface lebensspuren shows a broad variety that directly depends on stiffness of the substrate and the capability of the organisms to cope with it. Semi-fluid substrates hinder the organisms in their motility— they move by plowing, whereby sediment flows back behind the animal and refills the furrow, leaving an indistinct line on the surface. With increasing stiffness, traces acquire additional morphological details such as levees and median furrows. On stiff and very stiff substrates, the crustaceans do not penetrate into the sediment, but move by jumping, with the consequence of producing jumping traces instead of furrows. Some lebensspuren obtained in the experiments, especially furrows, are similar to some trace fossils attributed to several non-arthropod animal groups such as annelids, bivalves, and gastropods. This study helps clarify the interpretation and taxonomy of trace fossils and in reconstruct of substrate properties during their formation.