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November 11, 2011

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She-male shrimp are serial killers, but faithful

Nov. 11, 2011
Courtesy of BioMed Central
and World Science staff

Like the os­ten­ta­tiously mur­der­ous cou­ple por­trayed in the 1994 film Nat­u­ral Born Killers, a type of shrimp rou­tinely kills its peers—but spares a part­ner to which it’s faith­ful.

So re­veals a new study in­to the clean­er shrimp Lysmata am­boinen­sis, which al­so are hermaphrodites, that is, they have both male and female sex­u­al or­gans. Pub­lished in the re­search jour­nal Fron­tiers in Zo­ol­o­gy, the study found that clean­er shrimp in any group larg­er than two, vi­ciously at­tack and kill each oth­er at night un­til only two re­main.

Clean­er shrimp Lysmata am­boinen­sis (Cour­tesy BioMed Cen­tral)

L. am­boinen­sis are “protan­dric sim­ul­ta­ne­ous her­ma­phro­dites.” This means they start out as males but, as they grow, they al­so de­vel­op female re­pro­duc­tive or­gans. They can’t self-ferti­lize, like some other ani­mals, though. 

In­di­vid­u­als mat­ing as females can only do so in the few hours af­ter molt­ing, or shed­ding their out­er cov­er­ing. But they can re­pro­duce as males at all oth­er times, even when in­cu­bat­ing eggs. The 6 cm (2½ inch) long, yel­low-and-or­ange shrimp live by eat­ing par­a­sites and dead skin from “client” fish in the In­do-Pa­cif­ic or Red Sea. In re­turn for their beau­ty treat­ment the fish re­frain from eat­ing their clean­ers.

Ja­nine Wong and Nico Michiels of the Uni­vers­ity of Tübin­gen in Germany put L. am­boinen­sis in­to aquar­i­ums in groups of two, three or four. Shrimp in each tank were about the same size and had lim­it­less ac­cess to food as well as a pe­rch. Af­ter six weeks re­search­ers found that in all groups larg­er than pairs, one or more shrimp had been killed dur­ing the night—just af­ter shed­ding their old skin, when they were vul­ner­a­ble.

“In the wild, mo­nog­a­my is only seen for shrimp which have adopt­ed the sym­bi­ot­ic ‘clean­er’ lifestyle,” Wong said. “For these shrimp, com­pe­ti­tion for food is likely to be the driv­ing force be­hind their mo­nog­a­my—more shrimp equals less food per shrim­p—and, since body size is linked to the num­ber of eggs laid, a large group would de­crease each in­di­vid­u­al’s po­ten­tial to pro­duce off­spring.”

Con­firm­ing this idea, “we found that shrimp molt­ing was de­layed in the larg­er two group sizes, de­spite the freely avail­a­ble food,” she added. And “once the group size had re­duced to two, the rate of molt­ing in­creased for the re­main­ing shrimp.”

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Like the ostentatiously murderous couple portrayed in the 1994 film Natural Born Killers, a type of shrimp routinely massacres its peers—but spares a partner to which it’s faithful. So reveals a new study into the cleaner shrimp Lysmata amboinensis, which also are hermaphrodites, that is, they have both male and female sexual organs. Published in the research journal Frontiers in Zoology, the study found that cleaner shrimp in any group larger than two, viciously attack and kill each other at night until only one pair remains. L. amboinensis are “protandric simultaneous hermaphrodites.” In plain English, this means they start out as males but, as they grow, they also develop female reproductive organs. But they can’t self-fertilize. Individuals mating as females can only do so in the few hours after molting, or shedding their outer covering. But they can reproduce as males at all other times, even when incubating eggs. The 6 cm (2½ inch) long, yellow and orange shrimp live by eating parasites and dead skin from “client” fish in the Indo-Pacific or Red Sea. In return for their beauty treatment the fish refrain from eating their cleaners. Janine Wong and Nico Michiels of the University of Tübingen in Germany put L. amboinensis into aquariums in groups of two, three or four. Shrimp in each tank were about the same size and had limitless access to food as well as a perch. After six weeks researchers found that in all groups larger than pairs, one or more shrimp had been killed during the night—just after shedding their old skin, when they were vulnerable. “In the wild, monogamy is only seen for shrimp which have adopted the symbiotic ‘cleaner’ lifestyle,” Wong said. “For these shrimp, competition for food is likely to be the driving force behind their monogamy—more shrimp equals less food per shrimp—and, since body size is linked to the number of eggs laid, a large group would decrease each individual’s potential to produce offspring.” Confirming this idea, “we found that shrimp molting was delayed in the larger two group sizes, despite the freely available food,” she added. And “once the group size had reduced to two, the rate of molting increased for the remaining shrimp.”