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Some symbionts show large structured aggregations 28, while others live in heterosexual pairs 14, 32, 33, 34 or solitary 5, 35, 36. Host-use patterns vary widely depending on the ecology of hosts and symbionts, as reported in marine decapods 14, 28, 29, 30, 31. Such a behavior was reported in anemonefish 25, 26, 27, but it is unknown in marine invertebrates. However, it is even more energetically expensive when one individual or one pair of symbionts must control more than one host, which is rare in marine environments. Defending small, simple and sparsely distributed hosts requires less investment of time and energy 5, 22, while the energetics of defending complex and densely distributed hosts becomes too high, forcing symbionts to share hosts with conspecifics 23, 24. In symbiotic marine invertebrates, host bodies or structures associated with them (e.g., shells, tubes, burrows) are often the territory being defended 16, 18, 21. Therefore, the necessity of preserving such a resource predicts widespread territoriality among symbionts in the marine environment 16, 17, 18, 19, 20. For marine symbiotic fish and invertebrates, hosts are vital resources providing protection from predators, competition and environmental stress, food, and mating and larvae hatching spaces. Symbiotic organisms develop persistent, intimate associations with their hosts, frequently with a high degree of specificity, to the extent that one of the associates can no longer survive as free-living 15. This behavior generally involves intra- and interspecific contests 11 and has more commonly been studied in vertebrate systems, but also documented in marine invertebrates 12, 13, 14. Territories are fixed areas used exclusively by one or more individuals, from which they attempt to exclude potential competitors via agonistic or aggressive behavior 7. Many animals typically have specific home range areas in which individuals perform most daily activities, but some also defend territories 8, 9, 10. Resource scarcity has traditionally been considered among the main ecological factors leading to the establishment of resource monopolization and guarding behaviors in invertebrates, including territoriality 7. External factors controlling behavioral expressions are well known for terrestrial and marine taxa, including fish 3, social shrimps 4 or symbiotic crabs 1, 5, 6. Predictive models show a parallel evolution of resource-guarding behavior when possible intruders are in low abundance, but also when resources are aggregated and/or sparse 3. Many organisms show resource-guarding behaviors to control resources such as shelter, food, offspring or mating associates 1, 2. Overall, this is the first report of such a complex symbiotic behavior for an annelid polychaete. Our experiments proved that territoriality led to host monopolization by a single symbiont, provided the first empirical evidence that symbiont body injuries were caused during territorial contests, and allowed us to first suggest that a marine symbiotic invertebrate may control a territory extending beyond its host, even including neighboring hosts. Symbiont behavior included territoriality, expressed through conspecific recognition and intraspecific aggressive interactions (pursuit and escaping, hiding, choosing position, aggressive fighting, and targeting a specific bite zone). Hosts and symbiont populations had low densities, lacked size relationships and showed higher prevalence when denser. Then, by experimentally manipulating the competitor-to-resource ratio, we analyzed symbiont behavior and we assessed whether the 1:1 uniform distribution observed in nature could be driven by agonistic territorial behavior. appendiculatus from Nhatrang Bay (Vietnam). Based on field and laboratory observations, we described the symbiotic association between Ophthalmonoe pettibonae and Chaetopterus cf. Most commonly, they live solitary on hosts, likely resulting from territorial behavior, yet little is known of the precise nature of the involved interactions. Among marine invertebrates, polychaete worms form symbiotic associations showing a wide variety of host use patterns.