The density of L. siriboia (31.7 ± 34.1 burrows.m −2) estimated by counting open burrows on Fortalezinha beach is similar to the values found by Silva and Martinelli-Lemos (2012) for the Amazon coast using physical removal (suction pump – ghost shrimp pump). Counts of burrow openings have also been used to indirectly estimate the numbers of ghost shrimp (Hanekom et al. 1988; Griffis and Suchanek 1991; McPhee and Skilleter 2002a; Sumida et al. 2020). Whereas this indirect method may cast doubt over previous unconfirmed estimates of population sizes (Griffis and Suchanek 1991; Rotherham and West 2007), it is recommended when determining absolute abundances of cryptic and burrowing marine invertebrates is impractical and may also lead to considerable environmental disturbance (McPhee and Skilleter 2002a).

Although there may be some concern when comparing abundance estimates of crustaceans that tunnel deeply into the seafloor, beaches or mudflats, the density of L. siriboia in the present study was higher than in other areas of the Brazilian coast (eg Rodrigues and Shimizu 1997; Botter-Carvalho et al. 2007, 2015; Moschetto et al. 2020). The abundance and distribution patterns of callianassids in intertidal areas are mainly driven by sediment grain size (Witbaard and Duineveld 1989; Botter-Carvalho 2002, 2007; Oliveira et al. 2017), temperature and water salinity (Posey 1986; Berkenbush and Rowden 1998; Silva and Martinelli-Lemos 2012), beach morphodynamics (Pezzuto 1998; Alves and Rodrigues 2000), food availability (Rodrigues and Shimizu 1997), and extraction for bait use (McPhee and Skilleter 2002b; Botter-Carvalho et al. 2007; Moscheto et al. 2020). High abundance of L. siriboia generally occurs in areas with fine and rich organic matter and flat slopes (Souza and Borzone 2003), as in most sandy beaches of Algodoal-Maiandeua Island (Silva 2015), where estuaries contribute to a large input of sediments, dissolved nutrients and organic material to the beaches (Araujo da Silva et al. 2009). On the Amazon coast, the equatorial tropical climate maintains relatively high and stable temperatures, which support high levels of productivity throughout the year (Costa et al. 2011). In addition, Fortalezinha beach is conserved, with a low level of anthropic disturbance, and ghost shrimp populations are not harvested on the island.

Significantly higher densities of L. siriboia occurred in Area 1, and in both areas they increased towards the lower intertidal zone. This difference appears to be related to differences in the natural characteristics of each area. Area 1 is more sheltered and closer to a tidal channel (Furo Velho), which naturally allows the deposition of finer sediments. Conversely, Area 2 is situated in a more exposed part of the beach with a high influence of currents and waves, which resuspend more bottom sediments and may increase turbulence, erosion rates and prevent the deposition of finer sediment grains (Pereira et al. 2012; Silva 2015). Increasing densities towards the low intertidal zone were also recorded in a previous study with L. siriboia and Upogebia vasquezi in Amazonian coastal areas (Silva and Martinelli-Lemos 2012). The benthic organisms that inhabit sandy beaches are usually distributed differently across the intertidal zone, as a response to abrupt environmental gradients present in this area (Defeo and McLachlan 2005; Schlacher and Thompson 2013a, 2013b). This increase towards the sea is probably a response to the increase in sediment moisture, which decreases the risk of desiccation (in the HT zone), and the dependence of feeding activities in most macrofaunal species on tidal submergence (McLachlan and Jaramillo 1995; Armonies and Reise 2000). These environmental gradients are even stronger on Amazonian beaches, where extensive intertidal zones (≈ 300 m) and semi-diurnal macrotidal (> 6 m tidal range) regimes predominate (Pereira et al. 2012).