The colorful lives of squid

Your calamari, it turns out, may have come from a temporary transvestite with rainbows in its armpits.

Well, not armpits, but spots just below where the fins flare out. “Finpits,” cell biologist Daniel DeMartini nicknamed them. He and his colleagues have documented unusual color-change displays in female California market squid, popular in restaurants.

Squids, octopuses and cuttlefishes are nature’s iPads, changing their living pixels at will. DeMartini, of the University of California, Santa Barbara, saw so many sunset shimmers, blink-of-an-eye blackouts and other marvels in California’s Doryteuthis opalescens that it took him a while to notice that only females shimmered the finpit stripe. It shows up now and then during life, and reliably for about 24 hours after decapitation, DeMartini found.

The squid are color-blind, and what prompts their display is known only to them. But the researchers have figured out how it works.

The squid make rainbows when color-change cells called iridocytes lose water. Other kinds of color-change cells work their magic via pigments, but not iridocytes. “If you take a bunch of iridocyte cells in red, blue, green or yellow and you grind them up, then you wouldn’t see any color,” DeMartini says. Instead, little stacks of protein plates inside the cells turn colorful only when water rushes out of the stack. How closely the plates snug together determines whether the stack looks blue, scarlet or anything in between.

Another kind of cell called a leucophore makes a different girls-only special effect, a white streak between the fins. “I kind of ignored it for a long time,” DeMartini says. “I thought it was decaying or dying tissue.” He examined squid that were collected during a sexual gathering. Millions draw near shore for frenzied grappling, gripping and mating. In the fray, females get “pretty thrashed,” he says. “Maybe they need a break.” So displaying a white streak that could look like a male’s white testis showing through the transparent body may deflect male attention, he and his colleagues speculate in the Oct. 1 Journal of Experimental Biology.

How such fabulous colors evolved in the color-blind squid is “still kind of a brainteaser,” DeMartini says. The California species has only one visual pigment, he says, and mainly detects patterns of darks and lights.

RAINBOW MOM Female market squid produce long white capsules containing eggs, then add each capsule to a bouquet of others. Credit: Damian Davalos