Ecological importance of passive deposition of organic matter into burrows of the SW Atlantic crab Chasmagnathus granulatus

Abstract

The burrowing crab Chasmagnathus granulatus is the most abundant bioturbator in estuarine intertidal sediments from southern Brazil to central Argentina. This crab is a deposit feeder that excavates and maintains large semi-permanent open burrows with funnel shaped entrances. In this study we showed that the funnel shaped burrows with low aspect ratio are the most common and, with field experiments, we demonstrated that these burrows are also the most efficient in the capture of organic matter. As shown by C isotopic signatures, the origin of trapped detrital material is Spartina densiflora. Burrows are distributed in the upper part of estuaries and saltmarshes, mostly in areas of low energy, and cover extensive areas between the marsh vegetation and the open estuary. Through sampling of crab densities and use of satellite images, we estimated the number of burrows of different shapes in the Bahia Blanca estuary (38°50' S), one of the largest estuarine intertidals in the SW Atlantic. After combining this information with the trapping efficiency of burrows of different shapes, we estimated that within 100 d, a crab bed could capture the entire annual production from a marsh area of similar size. Therefore, we suggest that these extensive burrow beds may be considered large macrodetritus retention areas, reducing the amount of organic matter exported from marshes but locally increasing the sediment organic matter content.

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

“…Nevertheless, the presence of crabs shifted sediment characteristics to yet finer grain size and higher OMC at all tidal levels this study). Higher OMC and finer sediment indicate higher sediment stability (Grant and Daborn, 1994), and less sediment transport by tides within C. granulatus crab beds (see Botto et al, 2006). Such conditions should facilitate suspension feeding clams in terms of growth and body condition (Holland and Dean, 1977a;Murphy, 1985;.…”

“…Due to the high crab digging activity, single burrows move laterally by up to 3.2 cm d − 1 and overall sediment excavation rates may be up to 5.9 kg m − 2 d − 1 (Iribarne et al, 1997). At the same time, the large semipermanent funnel-shaped burrow entrances act as detritus traps and cause an increase in sediment organic matter content (Botto et al, 2006). Consequently, there are distinct changes in sediment chemistry (Gutiérrez et al, 2006), composition and transport (e.g., Palomo et al, 2004) and in bottom topography (Escapa et al, 2004;Perillo and Iribarne, 2004) within crab beds.…”

“…The first two mechanisms are likely to disturb the clam during feeding, whereas the latter may force it to invest more time and effort into burrow rebuilding and is even assumed to be the major cause of clam shell damage (see Lomovasky et al, 2005). However, the crabs may positively affect clams (as found between the polychaete Clymenella torquata and the bivalve Gemma gemma; Weinberg, 1984), given that their funnel shape burrows are passive traps for sediment and detritus Botto et al, 2006) which could also enhance food availability for the clams.…”

Crab Bioturbation and Herbivory May Account for Variability in Carbon Sequestration and Stocks in South West Atlantic Salt Marshes

“…This could be related to the larger number of crab burrows in that zone (Montemayor et al, 2011). In fact crab burrows act as passive traps increasing detrita entrapment in the sediment (Botto et al, 2006). Moreover, the C content trapped inside the burrows as a result of tidal sediment deposition is greater than the C content in the sediment excavated by crabs and exposed in the surface resulting in a net decrease in the amount of C that can be exported from the marsh by tidal processes (Gutiérrez et al, 2006).…”

Larval salinity tolerance of the South American salt-marsh crab, Neohelice (Chasmagnathus) granulata: physiological constraints to estuarine retention, export and reimmigration

“…Boschi 1964;Boschi et al 1992;RuVino et al 1994;Bas et al 2005). Due to its intense burrowing and omnivorous feeding activities, this abundant semiterrestrial species shapes regional habitat and community structures in the intertidal and supratidal coastal fringe, exerting signiWcant eVects on sediments, plants, benthic invertebrates, Wsh and migratory shore birds (Iribarne et al 2000Daleo et al 2003;Bortolus et al 2004;Martinetto et al 2005Martinetto et al , 2007Botto et al 2005Botto et al , 2006 and earlier papers cited therein).…”