An illuminating partnership for squid - glowing squid
The squid Euprymna scolopes, a denizen of the shallow waters surrounding the Hawaiian archipelago, provides a shining example-literally-of symbiosis in action. The cephalopod's symbiotic partners are the bioluminescent bacteria Vibrio fischeri.
These bacteria colonize a specialized cavity called the light organ, located on the squid's underside, and allow the animal to emit a diffuse glow toward the seafloor. The result appears to be counterillumination, which eliminates telltale shadows, thus enabling a variety of fishes and cephalopods to move and hunt undetected at night.
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With the aid of its microbial tenants, E. scolopes may "camouflage itself against the moonlight and starlight," explains Margaret McFall-Ngai of the University of Hawaii in Honolulu. In return, she says, the animals seem to offer the bacteria comfortable homes.
Several researchers discussed aspects of this squid-bacteria collaboration at last week's Symbiosis 96! meeting in Bar Harbor, Maine. For example, McFall-Ngai's group, which recently moved from the University of Southern California (USC) in Los Angeles, studies changes brought about in the light organ when V. fischeri appears.
At hatching, notes McFall-Ngai, the squid are free of bacteria. The immature light organs have cells with cilia, or threadlike extensions, that propel bacteria from the surrounding water into the light organ. Once colonized, the light organ undergoes dramatic changes in shape that allow it to function as a bacterial hotel and squid night-light. McFall-Ngai's group has found that the ciliated cells die off within days after the bacteria arrive. McFall-Ngai and her colleague Jamie S. Foster have now shown that this cell death does not stem from a direct assault by the bacteria. Instead, the microbes seem to signal the ciliated cells to undergo apoptosis, a kind of cellular suicide. The investigators plan to determine the nature of that signal and why it doesn't affect the cells to which the bacteria are attached.
Edward G. Ruby, another USC scientist who is moving to Honolulu, heads a group studying how the squid control their bioluminescence, which follows a daily rhythm. "During the daytime, they become considerably dimmer," he says.
The squid turn down their wattage in part by limiting the amount of oxygen that reaches the light organ. The bacteria need oxygen to produce light.
Moreover, "about 90 percent of the bacteria are expelled each morning," says Ruby. That strategy may seed the water with bacteria for new hatchlings, he notes.
Both separately and in collaboration with a group led by Paul V. Dunlap of the Woods Hole (Mass.) Oceanographic Institution (WHOI), Ruby's team has identified mutant strains of V. fischeri that have trouble initially colonizing the squid, establishing a normal-sized community in the light organ, or persisting in the animal. The scientists are now searching for the mutated genes that cause those bacterial difficulties.
Researchers from WHOI and the Marine Biological Laboratory in Woods Hole are struggling to generate laboratory colonies of E. scolopes. The researchers have found that they can transport squid from Hawaii to aquariums at Woods Hole with relative ease and that the animals will lay eggs that hatch. Yet no matter what the scientists fed them, the hatchlings died. "We were trying to present them with small prey in great abundance," notes Dunlap.
Michael F. Claes of Northeastern University in Boston then pointed out that juvenile squid naturally feast on adult shrimp, even those much bigger than themselves. On that diet, Dunlap reports, two-thirds of the captive hatchlings live to the age where they can reproduce.
The eggs produced by these squid do not seem to be of the same quality as those of the first generation in captivity. They have a poor fertilization rate, and those that hatch produce squid that have trouble surviving. More research on rearing squid is needed before a stable laboratory colony of E.
scolopes can be maintained, says Dunlap.
Once laboratory squid are plentiful, the alliance between E. scolopes and V.
fischeri should serve as a research model for other animal-bacteria symbioses, including those that occur inside humans, says McFall-Ngai.
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