Mutualism of the Month: Bruce's hinge-beak shrimp and its two hosts

Bruce's hinge-beak shrimp Rhynchocinetes brucei.  Photo courtesy of qm.qld.gov.au.

Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is the relationship between the the hinge-beak shrimp Rhynchocinetes brucei and its two hosts: the bubble-tip sea anemone Entacmaea quadricolor and the long-spined sea urchin Diadema setosum.

R. brucei is a small shrimp, less than 1.5 cm long, commonly found in shallow rocky reefs from Hong Kong to the Great Barrier Reef.  It plays an important role in shallow reef communities as both a predator of small invertebrates and as a food source for predatory fish.  R. brucei associates with both the sea anemone E. quadricolor and the sea urchin D. setosum.  Associations with these hosts seem to be facultative (i.e. optional) for the shrimp because individuals are also found inside small holes and crevices in the reef.  

The bubble-tip sea anemone Entacmaea quadricolor .  Photo courtesy of messersmith.name. 

R. brucei are more frequently associated with sea urchins even though sea anemones are more abundant in some locations.  It is likely that anemones and urchins don’t provide the same amount of protection; when sea anemones retract their tentacles shrimp become more vulnerable to predation.  In comparison, sea urchins are structurally more complex than sea anemones, thus providing better protection.  An added benefit of associating with motile urchins is access to new feeding grounds as the host moves.  In contrast, anemones are sedentary and therefore food resources around the anemone can become quickly depleted.  In order for the shrimp to continue to eat, they must venture farther and farther from their host, thus becoming more and more susceptible to predation.

The long-spined sea urchin Diadema setosum .  Photo courtesy of wikimedia.org.

A series of host-choice experiments found that sea urchins are the preferred host of R. brucei and that the shrimp prefer to associate with the host species they were originally collected from in the wild (Ory et al., 2013).  This behavior is called host imprinting.  Host imprinting may prevent shrimp originally associated with urchins from interacting with unfamiliar sea anemones that are likely to sting and attempt to consume them.  Shrimp must go through a lengthy acclimation period with a new anemone during which shrimp are susceptible both to predation by fish and stinging by the anemone.

It is still unknown as to what benefits the anemone and urchin get from R. brucei.  In tropical reefs around the world shrimp, crabs, and fish are frequently found taking refuge within the tentacles of sea anemones and the spines of sea urchins.  Anemonefish are known to provide nutrients to anemones through excreting waste which is high in nitrogen.  Perhaps R. brucei is also providing its host with elevated levels of nutrients.

An anemonefish living in its host anemone.  Anemonefish are known to provide nutrients to their host.  Does R. brucei?  Photo courtesy of messersmith.name. 

In summary, by using urchins and anemones, the shrimp can occupy areas of the reef where refuges are absent.  Urchins provide greater benefits to the shrimp compared to anemones by either providing better protection or enhancing access to feeding grounds.  Also shrimp imprint on their original host which may help in relocating their host after feeding forays.

References: 

Ory, N.C., Dudgeon, D., and Thiel, M. 2013. Host-use patterns and factors influencing the choice between anemone and urchin hosts by a caridean shrimp. Journal of Experimental Marine Biology and Ecology 449: 85-92.

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