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

Abstract

Burrow characteristics, food type, and feeding h a b~t s of the SW Atlantlc burrowing crab Chasmagnathus grdnulata were compared between ~n d i v~d u a l s living in mud flats and In Spartinadominated marshes. Burrows were shorter ( X -19.7 cm, SD = 5 8, n = 54) and more dynamlc (entrance displacement: X = 3 2 cm d-l, SD = 1 7, n = 21) in mud flats than In Spartlna-dominated areas (length X = 41 cm, SD = 12, n = 78, no entrance displacement). Sediment turnover rate was much higher in mudbetween areas, being straight, near-vert~cal tunnels in the vegetated area, but oblique (average angle to vertical = 60'. SD = 16", n = 110), and having a funnel-shape entrance and a much large]-diameter in mud flats Stomach contents also differed between habitats. Pleces of plants dominated contents in the vegetated area, while sediment (with polychaetes, diatoms, ostracods, and nematodes) d o m~n a t e d in the mud flats, lndicatlng that crabs are mainly deposit feeders In the mud flats a n d herb~vorous in the Spart~na-dominated areas. This pattern suggests that the heuristic model relatlng burrow arch~tecture to trophic modes prev~ously proposed for fossorial thalassinidean shrimps applies to individuals of a C granulata populat~on 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 hydrodynalnic 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 estuanes, and the locatlon of their burrows (between marshes and the open estuary), these burrows may work as traps for detritus, thus reduclng the export rate of organic matter from marshes.

Experimental studies on predation and cannibalism of the settlers of Chasmagnathus granulata and Cyrtograpsus angulatus (Brachyura: Grapsidae)

“…Additional evidence suggests that burrows are the preferred recruitment habitat for this species; they act as Amegalopae trapsB under the hydrody-Ž . namic conditions in Mar Chiquita Iribarne et al, 1997 , and megalopae metamorphosed earlier in the presence than in the absence of a chemical stimulus from adult con-Ž . specifics Gebauer et al, 1998 . Juveniles always caused higher settler mortality than adults.…”

“…C. granulata dig and inhabit semi-permanent burrows in mudflats and Ž . Spartina densiflora grasslands Boschi, 1964;Spivak et al, 1994;Iribarne et al, 1997 . Juveniles live near the conspecific adults in separate burrows, but usually juvenile and Ž adult burrows are interconnected.…”

“…size: preference for shrimp and crabs cannibalism declined and consumption of clam Ž . and polychaetes increased as the crabs reached larger size Visser, 1997 . C. granulata are also herbivores, although parts of conspecific crabs have been Ž observed in the stomachs Olivier et al, 1972b;D'Incao et al, 1990;Iribarne et al, . 1997 .…”

The effect of the SW Atlantic burrowing crab Chasmagnathus granulatus on the intertidal razor clam Tagelus plebeius

“…The burrowing grapsid crab Chasmagnathus granulatus occurs at high densities in estuaries and embayments of the SW Atlantic (southern Brazil to northern Patagonia in Argentina; Boschi, 1964;Spivak et al, 1994;Iribarne et al, 1997Iribarne et al, , 2005. This species (up to 4 cm carapace width) inhabits tidal flats and salt marshes dominated by Spartina spp.…”

“…This species (up to 4 cm carapace width) inhabits tidal flats and salt marshes dominated by Spartina spp. and Sarcocornia spp., with densities up to 60 crabs m − 2 (Iribarne et al, 1997;Bortolus and Iribarne, 1999). The crab excavates and maintains semi-permanent open burrows (Spivak et al, 1994;Iribarne et al, 1997).…”