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Imperial Shrimp And Nudibranch: A Moveable Feast

Text and photography by Mark Strickland


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Imperial Shrimps Living on Nudibranch
As if living on the dining car of a train, a pair of Imperial Shrimp, Periclimenes imperator, lean over the side of their host, a nudibranch, Risbecia tryoni, to feed. Milne Bay, Papua New Guinea Image #: 012397

Finning as gently as possible, I slowly made my way across the muddy bottom, trying not to kick up clouds of silt. We were diving in Milne Bay, Papua New Guinea—one of the richest marine environments I’ve encountered anywhere. I was just settling down to photograph a ghost pipefish when my wife Suzy swam over, gesturing emphatically for me to stop whatever I was doing and follow her. I really wasn’t inclined to leave such a photogenic subject, but experience has taught me not to ignore such messages.  We swam a considerable distance until Suzy stopped and pointed to a pair of unusually attractive nudibranchs—ample justification for the long swim. Traveling single-file, nose-to-tail, the colorful slugs resembled some kind of miniature train, consisting of only locomotive and caboose. While it had no “passenger cars”, a closer look at the procession did reveal two very special travelers—a pair of tiny, brilliantly colored imperial shrimp clinging to the nudibranchs’ dorsal surface.

Moving closer, I was immediately impressed with how active these shrimp were. Apparently not content to just relax and enjoy the ride, the little crustaceans were constantly on the move, jockeying for position, crawling all over both nudibranchs and each other. More than anything, though, they hung out near their hosts’ muscular “feet”, sifting through the mucky substrate, eagerly consuming whatever food items came within range. Several times, the nudibranchs must have passed through especially productive areas, because the shrimp really picked up the pace of their feeding, stuffing themselves  as fast as their little mandibles could keep up. As the nudibranchs steamed slowly onward, beginning to leave these preferred areas behind, the shrimp appeared torn by indecision. Although reluctant to let such prime real estate slip past, they seemed unwilling to “jump train” and abandon their hosts.  Unable to influence the nudibranchs to circle back for another pass, they did their utmost to continue feeding, leaning out precariously to reach the best areas. At one point, a shrimp was so intent on browsing a particular bottom growth that it clamped down on the bush-like structure, holding on tenaciously as the nudibranchs continued on their way.  Forced to run from nose to tail of its mobile host, the shrimp found itself hanging off the “caboose”, desperately struggling to hold on to the growth, which was now bent over like a sapling in a hurricane. Only when the shrimp was about to be completely pulled off its host did it finally relinquish its grip!

What accounts for such a cozy relationship between the nudibranchs and shrimp? One apparent reason is protection. Many nudibranchs emit a noxious substance that discourages potential predators; by living with such an unpalatable host, the shrimp greatly reduce their chances of being preyed upon. To enhance this protection, the shrimp have adopted a color pattern similar to their host, creating excellent camouflage.  Another advantage for the shrimp involves feeding efficiency. At first glance, you might assume that the shrimps’ feeding opportunities would be restricted due to their residency on the nudibranchs. On further observation, however, it becomes clear that the opposite is true; by traveling with their hosts, the shrimp actually cover far more territory than they could on their own, and expend very little energy doing it. In addition, the shrimp often feed even closer to home, dining on their host’s fecal pellets. With such obvious advantages to the shrimp, it seems like the nudibranchs deserve some benefits from the association as well.  The deal may not be exactly equitable, but the shrimp do assist their hosts by keeping their anal-gill area free of fecal material. It is also possible that the shrimp may help control parasites, but little research has been done in this area.

For those who take the time to observe patiently, the complex interrelationships that exist in nature can be a never-ending source of fascination. And, as I learned with the nudibranchs and shrimp, sometimes the most amazing discoveries come in small packages!