Between-habitat differences in burrow characteristics and trophic modes in the southwestern Atlantic burrowing crab Chasmagnathus granulata

Burrow characteristics, food type, and feeding habits of the SW Atlantic burrowing crab Chasmagnathus granulata were compared between individuals living in mud flats and in Spartina-dominated marshes. Burrows were shorter (x = 19.7 cm, SD = 5.8, n = 54) and more dynamic (entrance displacement: x = 3.2 cm d(-1), SD = 1.7, n = 21) in mud flats than in Spartina-dominated areas (length: x = 41 cm, SD = 12, n = 78; no entrance displacement). Sediment turnover rate was much higher in mud flats (x = 5.9 kg m(-2) d(-1)) than in Spartina-dominated areas (x = 2.4 kg m(-2) d(-1)). Burrow shape differed between areas, being straight, near-vertical tunnels in the vegetated area, but oblique (average angle to vertical = 60 degrees, SD = 16 degrees, n = 110), and having a funnel-shape entrance and a much larger diameter in mud flats. Stomach contents also differed between habitats. Pieces of plants dominated contents in the vegetated area, while sediment (with polychaetes, diatoms, ostracods, and nematodes) dominated in the mud flats, indicating that crabs are mainly deposit feeders in the mud flats and herbivorous in the Spartina-dominated areas. This pattern suggests that the heuristic model relating burrow architecture to trophic modes previously proposed for fossorial thalassinidean shrimps applies to individuals of a C. granulata population. The burrow content showed higher organic content and vegetal parts in the vegetated area than in the other area. Burrows in the mud flat showed a significantly higher abundance of nematodes and ostracods. Due to their hydrodynamic characteristics and content, burrows in the mud flats may work as passive traps for sediment and organic matter. Given the extensive intertidal area inhabited by C. granulata in SW Atlantic estuaries, and the location of their burrows (between marshes and the open estuary), these burrows may work as traps for detritus, thus reducing the export rate of organic matter from marshes.