The response of meiofauna and microphytobenthos to engineering effects of fiddler crabs on a subtropical intertidal sandflat MONICA CITADIN, 1,2 TÂNIA MARCIA COSTA 2 AND SÉRGIO A. NETTO 1 * 1 Marine Science Laboratory, University of Southern Santa Catarina UNISUL, Tubarão, SC, Brazil (Email: sergio.netto@unisul.br) and 2 Laboratory of Ecology and Animal Behaviour, São Paulo State University UNESP, Biosciences Institute, Coastal Campus, São Vicente, SP, Brazil Abstract Fiddler crabs are key bioturbators on tidal flats. During their intense bioturbation process, they manipulate large amounts of sediment, altering the physical state of existing materials. We investigated whether different types of sediment bioturbation produced by fiddler crabs modulate meiofaunal assemblages and microphytobenthic content. We hypothesized that sedimentary structures produced by burrowing (the burrow itself and the excavation pellets) and feeding (feeding pellets) generate different microenvironments compared with areas without apparent signs of fid- dler crab disturbance, affecting both meiofauna and microphytobenthos, independent of the sampling period. Our re- sults indicate that the engineering effects of burrow construction and maintenance and the engineering effects of fiddler crab foraging modulate meiofaunal assemblages in different ways. Overall, meiofauna from burrows and excavation pel- lets was more abundant and diverse than at control sites, whereas feeding pellets contained poor meiofaunal assem- blages. By contrast, only foraging effects were detected on microphytobenthos; independent of the sampling period, Chl a and phaeopigment content were higher in the feeding pellets, but similar among burrows, excavation pellets and control sites. The present study demonstrates that the different engineering effects of fiddler crabs are an important source of habitat heterogeneity and a structuring agent of meiofaunal assemblages on subtropical tidal flats. Key words: bioturbation, excavation pellets, feeding pellets, nematodes, Uca. INTRODUCTION Faunal bioturbation is every transport process carried out by fauna that directly or indirectly affect sediment matrices (Kristensen et al. 2012). The bioturbation def- inition is related to sediment reworking by animals that generates changes in chemical gradients and relocates feeding resources and microorganisms. This sediment restructuring mediated by biological process can also promote physical alterations, by changing the balance of material transport, and affecting the structure and or functioning of assemblages. Thus, bioturbation plays a fundamental role in ecosystems (Kristensen et al. 2012; Meysman et al. 2006). The conspicuous pantropical and subtropical fiddler crabs (genus Uca; Crane 1975) are allogenic ecosystem engineers (sensu Jones et al. 1994) of estuarine tidal flats, as they change the environment through the alteration of the physical state of pre-existing material. This is performed by two main activities of fiddler crabs, the burrow construction and maintenance, and foraging. The crabs burrow constantly to build or maintain their burrow, manipulating large quantities of sediment (Sayão-Aguiar et al. 2012). The burrowing activities produce the burrow itself and the excavation pellets (the manipulated sediment from digging or maintaining the burrow; Fig. 1). During burrowing, fiddler crabs in- crease habitat heterogeneity both horizontally and verti- cally (Hoffman et al. 1984; Botto et al. 2008). On a small scale, burrowing increases sediment water content and the turnover of carbon and nitrogen, while it de- creases bulk soil density (Wang et al. 2010). Fiddler crabs feed by extracting the organic matter from the sediment surface (Crane 1975; Kristensen 2008). During the feeding process, which usually occurs next to the burrow, the crab transfers particles from the sediment to the mouth with the aid of the cheliped (the small one in the case of the males) (Wolfrath 1992; Dittmann 1993; Takeda et al. 2004). In the buccal cav- ity, the edible particles are separated by flotation feeding: organic matter floats in the water from the branchial chamber and is then ingested (Quinn 1980; Dittmann 1993; Kristensen 2008). Inorganic and heavy particles are deposited on the bottom of the buccal cavity and dropped as feeding pellets (Fig. 1; Crane 1975; Dittmann 1993; Kristensen 2008; Takeda et al. 2004; Wolfrath 1992). The engineering effects of fiddler crab foraging, as well as of burrow construction and maintenance, may be particularly relevant to small benthic organisms, such as meiofauna and microphytobenthos. The meiofauna (small metazoans between 45 and 500 μm in size) *Corresponding author. Accepted for publication November 2015. © 2016 Ecological Society of Australia doi:10.1111/aec.12346 Austral Ecology (2016) 41, 572–579