Protective benefit of sea anemone host to symbiotic palaemonid shrimps and predator-avoidance behaviours among palaemonid shrimps.

In symbiotic relationships, protection is believed to be the primary benefit that symbionts obtain from a host. Palaemonid shrimps are of notable interest when examining marine symbiosis, with over 500 described species forming symbiotic associations with other macrofauna. Instead of maintaining close contact with the host, several symbiotic palaemonid shrimps were found to leave their host temporarily when disturbed, suggesting that they may not completely rely on host protection and possess abilities to evade predators. This study examined protection obtained by symbiotic palaemonid shrimps from sea anemone host. It compared survival and behavioural responses of symbiotic shrimps with different levels of host association to free-living shrimps in the presence of a predator. A selected model predator, juvenile giant grouper, was introduced to the aquarium to investigate the protective benefit offered by the bubble-tip anemone. The survivorship of all three symbiotic shrimps examined, Ancylocaris brevicarpalis, Ancylomenes holthuisi, and Ancylomenes venustus, was significantly increased when associated with the anemone host, even though the latter two species form a loose association with their host. In the anemone treatment, most of the symbiotic shrimps showed a 100% survival rate, while all free-living shrimps were consumed. The stinging cells of the anemone serves as the major protective function rather than the physical complexity of its structure because the synthetic anemone did not provide detectable protection to the shrimps. Swaying motion and remaining motionless were the predominant behaviours for symbiotic cleaner shrimps and symbiotic non-cleaner shrimp, respectively, in responses to the predator. The symbiotic shrimps exhibited fewer tail-flipping responses as compared to the free-living shrimps, suggesting that the dependency on host protection reduced the self-protective capability of symbiotic shrimps. The present study highlights the importance of the protective benefit offered by sea anemone host to both closely and loosely associated symbiotic shrimps and reveals differences in predator-avoidance behaviours among palaemonid shrimps.