CRAB TRACES ON MUDDY TIDAL FLATS OF THE MIRA RIVER ESTUARY, PORTUGAL

Abstract Intertidal burrowing crabs produce distinctive biogenic sedimentary structures on tidal flats and salt marshes of the mesotidal Mira River estuary in southwestern Portugal. This study shows that the morphology and spatial distribution of crab burrows vary depending on the activity of the producer, its age, tidal cycles, tidal range, time of the day, and substrate consistency. The Atlantic shore crab Carcinus maenas was observed during the daylight hours roaming the tidal flats and marshes at neap low tide (1.29 m) as well as occupying burrows and cavities. A variety of traces and tracks are attributed to C. maenas, including cheliped scratch marks, tracks and trackways, burrows/cavities with associated root penetration structures (rhizoliths), and fecal pellets. Scratch marks are understudied in neo- and paleoichnology and are often interpreted as structures resulting from crab grazing and feeding activity (fodichnia/pascichnia), and occasionally from mating behavior. Digit traces and trackways predominantly concentrate around burrows and cavities on the upper tidal flat and resemble Coenobichnus currani, representing crab locomotion (repichnia). With morphology regarded as the most important criteria for ichnotaxonomy, wide burrow apertures and cavities found on the tidal flats are most similar to Psilonichnus upsilon, P. tubiformis, and Spongeliomorpha-type crustacean burrows, representing permanent- to semi-permanent crab domichnia. Finally, cylindrical, semi-consolidated pellets are interpreted as crab fecal pellets (coprolites) similar to Favreina. Carcinus maenas is a prolific bioturbator in intertidal and supratidal habitats; however, the preservation of its traces in the rock record is dependent on the preservation potential of the host sediments.