Biffarius arenosus had a mean δ13C of –15·4 ± 0·2‰ and a mean δ15N of 5·9 ± 0·1‰ (n = 38), and Trypea australiensis had a mean δ13C of –16·3 ± 0·3 and a mean δ15N of 7·6 ± 0·1 (n = 20). The δ13C signatures of the only mangrove species present (Avicennia marina) and the most abundant saltmarsh plant (Sarcocornia quinqueflora) indicated that they were not major food sources. Seagrasses, predominantly Heterozostera tasmanica, had mean δ13C and δ15N values of –11·7 ± 0·2‰ (n = 65) and 3·9 ± 0·2‰ (n = 62), respectively. Seagrass epiphytes had mean δ13C and δ15N values of –17·9 ± 0·4‰ and 4·6 ± 0·3‰ (n = 27), respectively. A mixture of seagrasses and their epiphytes was the most likely source of organic carbon for B. arenosus. Benthic microalgae, such as diatoms, were a possible food source, but phospholipid biomarkers indicated a meagre abundance of diatoms in the sediments, and microscopy of shrimp guts revealed few or no diatom frustules. For T. australiensis, food sources were less easily distinguished than for B. arenosus, but they could include seagrass epiphytes plus the green macroalgae Enteromorpha spp. and/or the brown alga Chordaria cladisiphon.