"The squid allows one particular bacteria to colonize it while excluding all other types. This bacteria only colonizes this one particular tissue in this animal, so it is very specific," said Crookes.
This recognition occurs anew every generation, as the light organs of newly hatched squid are not yet colonized by Vibrio fischeri.
The squid benefits from the relationship by reflecting the luminescence of the bacteria. In turn, the bacteria benefits by living in a nutrient-rich and competition-free environment, researchers believe.
Reflecting its glowing bacteria colonies, the squid can swim, forage, and search for mates at night while hiding its shadow or silhouette from predatory fish that bury themselves in the sand on the ocean bottom.
"If a predator sees a shadow or silhouette, it knows dinner has arrived," said Crookes. "So we think the light produced by the bacteria is used by the squid to kind of counter-illuminate, to mimic, the moonlight so the squid doesn't cast a shadow and doesn't create a silhouette."
Crookes, study co-author Margaret J. McFall-Ngai, researchers at the University of California at Los Angeles, and other collaborators investigated the structure and composition of the reflectins.
In addition to finding that the reflectors are made up of stacks and stacks of Frisbee-shaped plates, they discovered that the molecules that make up each plate consist of a unique set of proteins.
Unlike most proteins, which consist of 20 different amino acid subunits, Crookes and her colleagues found that the reflectin platelets are made up predominantly of just six amino acids.
The researchers also found that the sequence of the protein is divided up into five repeating units, so that perhaps only one fifth of the protein is responsible for helping to reflect light, said Crookes.
"Not only is the protein made principally of six components, but it's possible you can break it down to smaller pieces and still have it function the same way," she said.
Fully understanding the seemingly simple construction of the reflectins should make it easier to create synthetic versions, said Crookes.
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