Like many other cephalopods, the California market squid, Doryteuthis opalescens, is able to camouflage itself to blend in with the environment. But researchers have discovered that this milky white and purple squid can also manipulate light to direct its own iridescence in multiple ways. The findings were published in the Journal of Biological Chemistry.
Reflectins are aptly-named proteins unique to the light-sensing tissue of cephalopods like squid. Their skin contains specialized cells called iridocytes that produce color by reflecting light in a predictable way. When the neurotransmitter acetylcholine activates reflectin proteins, this triggers the contraction and expansion of deep pleats in the cell membrane of iridocytes. By turning enzymes on and off, this process adjusts (or tunes) the brightness and color of the light that’s reflected.
Daniel Morse and a team of University of California Santa Barbara researchers examined the different subtypes of reflectin in tunable, adaptive iridocytes and in non-tunable, static iridocytes. They wanted to see how optical output is affected by the structure, distribution, and abundance of reflectins.
The adaptive iridocytes in the skin of the California market squid are able to tune color through most of the spectrum. UCSB
As it turns out, different subtypes of reflectin have different roles, and the squid's ability to tune its colors is connected to the presence of specific reflectin sequences. "We found three major types of reflectins – A1 and A2, B and C – which in various combinations determine slightly different iridocyte function," UCSB’s Daniel DeMartini says in a statement. "A-type reflectins are found in static iridocytes; B are enriched in fully tunable ones, which contain A as well. We also found reflectin C, a new type that is probably important for anchoring proteins to membranes."
Additionally, location may also be important: Responsive iridocytes are located on the back of the squid, while more passive ones are on its underside.
"The discovery reported in this paper reveals the subtlety and power of the reflectin proteins to fine-tune the colors of living cells with a beauty that reminds us of paintings by Monet,” Morse adds.
A close-up view of the iridocytes, showing their potential to tune to a broad range of colors. UCSB