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The burrow morphology, burrowing behavior and feeding mechanisms of the thalassinidean shrimp Upogebia omissa were studied. Twenty burrow casts were made in situ with epoxy resin, and an overall 'Y' shape was most frequently observed. Several burrows consisted of a single, oblique tunnel; burrow diameter was positively correlated with burrow length, maximum depth and distance between openings. Additionally, burrow length was positively associated with maximum depth, indicating that as burrow length increased burrow depth increased; i.e. burrows spread vertically rather than horizontally. Total sediment displacement by the burrows accounted for 2.6 % of sediment to a depth of 30 cm. The sediment-water interface increased by 290 % in areas containing burrows compared to areas without burrows. Solitary adults were trapped in 15 casts. Three casts were interconnected, but each burrow contained only 1 shrimp. Regression analysis revealed shrimp length to be positively correlated with burrow diameter and area. In aquaria, U. omissa displayed a dual feeding behavior: the shrimp filtered suspended particles, and also fed &redly on the sediment. Filter-feeding occurred mainly within U-shaped tunnels, while deposit-feeding was observed in association with the oblique tunnels of the burrow. Although filter-feeding has been considered as the main trophic mode in upogebiids, we present evidence that U. omissa may have a more pronounced deposit-feeding behavior than described for other species of this group. Based on our findings, we suggest that current models on the functional morphology of burrows in relation to feeding should be used cautiously in inferring the trophic behavior of these shrimp.

The tidal flats at Praia do Araça, Brazil have muddy siliciclastic sediments on the surface and a layer of heavily packed shells down to 30–40 cm depth. The most obvious element of the infauna is the thalassinidean shrimp Axianassa australis. Several animals were captured with a yabby pump. Burrow openings were characterized by a low mound (1‐2 cm high and 10–30 cm in diameter at the base) with one or two simple holes nearby (20‐70 cm away). Counts along two transects showed a mean density of Axianassa burrow openings of 1.4 m−2 (range: 0–7), mounds ranged in density from 0 to 3 m−2 (mean 1.25). Three nearly complete (and several incomplete) resin casts showed a unique burrow shape, with spiral shafts leading to wide horizontal galleries from which several evenly proportioned corkscrew‐shaped spirals branched off, leading to further horizontal galleries at greater sediment depths. Burrows had up to 15 such spirals and a total length of over 8 m. The total burrow depth was between 106 and 130 cm. The role of the spirals and the similarity of Axianassa burrows to the trace fossil Gyrolithes are discussed.

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