For symbiotic crustaceans, theory predicts that monogamy is adaptive when species inhabit scarce, relatively small and morphologically simple hosts in tropical environments where predation risk away from hosts is high. We tested this prediction in the shrimp Paranchistus pycnodontae, which inhabits the mantle cavity of the winged pearl oyster Pteria penguin in the Coral Triangle. In various symbiotic crustaceans, males are smaller than females, and this sexual dimorphism has been used as evidence of sex change (protandry) in these organisms. Preliminary observations in Pa. pycnodontae suggested that males were smaller than females. Thus, we first investigated the sexual system of Pa. pycnodontae to determine if the species was protandric. Morphological identification and size frequency distributions indicated that the population comprised small males, small immature females and larger mature females, which was confirmed by dissections. No transitional individuals were found. Thus, Pa. pycnodontae is a gonochoric species with reverse sexual dimorphism. Pa. pycnodontae inhabit as heterosexual pairs in the mantle cavity of hosts more frequently than is expected by chance alone. Pairing was size assortative; both carapace length and propodus length of the major cheliped were positively correlated between males and females forming pairs. Males occur with females in the same host, independent of the female gravid condition or of the stage of development of the brooded eggs. Lastly, the major cheliped did not exhibit positive allometry in males. All the available information suggests that Pa. pycnodontae has adopted a socially monogamous mating system with males and females forming exclusive pairs from other adults. Symbiotic shrimps can be used as a model system to understand behavioural diversity within socially monogamous marine invertebrates.