Experimental exclusion of the burrowing crab Macrophthalmus japonicus from an intertidal mud flat: effects on macro-infauna abundance

Abstract

Abstract:Bioturbators affect other benthic animals by two processes: sediment reworking and changes in chemical properties. Crab exclusion experiments were conducted on an intertidal mud flat in the Tama estuary to examine the effects of the burrowing crab Macrophthalmus japonicus (Ocypodoidea) on the densities of sympatric macro-infauna in 2010 and 2011; and on sediment parameters in 2011. In 2010, approximately 1 month after M. japonicus was excluded, the densities of Corbicula japonica and Hediste sp. or spp. were significantly greater than those of the control, with M. japonicus present. Concerning sediment parameters: chlorophyll-a content, water content and total nitrogen content at the surface significantly increased and subsurface oxidation-reduction potential significantly decreased in the exclusion treatment compared with control conditions. These results appear to indicate that sediment reworking by M. japonicus was the main process affecting the sedimentary environment and density of C. japonica and Hediste sp. or spp. However, the magnitude of the effects of M. japonicus apparently varies from year to year because there were no significant differences in the densities of any macro-infauna species in 2011.

Effects of the bioturbating crab <i>Macrophthalmus</i> <i>japonicus</i> on abiotic and biotic tidal mudflat characteristics in the Tama River, Tokyo Bay, Japan

“…The number of M. japonicus individuals, the abiotic sediment parameters, MPB parameters, and meiobenthos density were compared between treatments by analysis of deviance using generalized linear mixed models (GLMM), the explanatory variables being treatment, tidal flat, and treatment tidal flat interaction, with year considered as random because the effects of bioturbators can fluctuate between years , Tanaka et al 2013. Based on the data characteristics, the models for number of M. japonicus individuals were fitted with a log link function and a Poisson error structure, and the models for all abiotic and biotic sediment characteristics were fitted with an identity link function and a Gaussian error structure.…”

“…Although other crab species (e.g., Helice tridens, Sesarmops intermedium, Chiromantes dehaani) occurred in neighboring reed beds, they rarely encroached on the tidal flats and mixed with M. japonicus. Similarly, the burrowing polychaete Hediste diadroma occurred in the study areas (Tanaka et al 2013), but sediment reworking by this polychaete was trivial compared to that of M. japonicus. Macrophthalmus japonicus was therefore the main bioturbator in the study areas.…”

“…The abiotic sediment characteristics subsequently analyzed included organic matter (total carbon content (TOC), total nitrogen content (TN) and C/N ratio), sediment grain size (median grain size and silt-clay content (particles <63 µm)) and water content. For the analyses, sediment samples were taken with a plastic corer (5 cm diameter) to a depth of 1 cm, based on a preliminary investigation that indicated that M. japonicus did not affect sediment characteristics at >1 cm depth (Tanaka et al 2013). Sediment samples for meiobenthos were taken with a plastic corer (2 cm diameter) to a depth of 5 cm.…”

“…Because the burrow entrance readily collapses from tidal water movements, M. japonicus constructs a new entrance every two or three days (Otani et al 2010). Accordingly, this species is a major bioturbator on mudflats, and it is classified as a regenerator since it does not change the sediment grain size distribution (Tanaka et al 2013). In addition to frequent burrowing, the crab reworks the sediment by surface-deposit-feeding, consuming mainly MPB ).…”

“…In addition to frequent burrowing, the crab reworks the sediment by surface-deposit-feeding, consuming mainly MPB ). This active reworking of the sediment affects the macrofauna of intertidal mudflats: Tanaka et al (2013) have shown, for example, that sediment disturbance caused by M. japonicus has negative impact on the filter-feeding bivalve Corbicula japonica and on the surface-depositfeeding polychaete Hediste diadroma. It has not yet been established, however, precisely how M. japonicus modifies the abiotic and biotic sediment characteristics of tidal mudflat ecosystems.…”