By Vladimir Brezina
This week’s Photo Challenge has got me thinking about the color Purple.
The most vivid purple that I’ve ever seen was not the purple of flowers, nor of sunsets. Rather, I see it in my lab. One of the experimental animals we work with is a large marine slug from California, Aplysia californica, popularly known as the sea hare. When disturbed, the sea hare releases a cloud of ink that has the most intense, rich purple color.
Those of a classical bent will recall Tyrian Purple, also known as Royal or Imperial Purple, a dye greatly prized in antiquity, which was made from a similar ink produced by several Mediterranean snail species:
Tyrian Purple was expensive: the 4th-century-BC historian Theopompus reported, “Purple for dyes fetched its weight in silver at Colophon” in Asia Minor. The expense meant that purple-dyed textiles became status symbols, and early sumptuary laws restricted their uses. The production of Tyrian purple was tightly controlled in Byzantium and was subsidized by the imperial court, which restricted its use for the colouring of imperial silks…
Interestingly, the purple color of the sea hare’s ink, as well as the purple tinge of its skin, actually derives from its preferred diet of red seaweed. If sea hares in the lab are fed green seaweed, their skin eventually turns green. I wonder if they then release green ink?
What is the biological function of the ink? We all know about squid, which release a dense cloud of ink (black in that case) into which they quickly vanish when danger threatens. But sea hares, despite their name, are sluggish. Their ink cloud is sparse to begin with, and when it disperses, the sea hare is still there.
It turns out, however, that the ink, together with other secretions that are released at the same time, provides a chemical, rather than a visual, defense. A 2005 paper by Kicklighter et al. analyzed the chemical composition of the secretions and their effect on attacking predators such as spiny lobsters. The secretions contain a complex mix of chemicals that elicit multiple, conflicting behaviors in the lobster. Indeed, while some of the chemicals are aversive, as one might expect, others actually stimulate lobster feeding behavior and mimic a food source. The result is that the lobster is not just coated with sticky goo and repelled, but, if it persists, it is diverted to attack a phantom food stimulus—the inky cloud—while the real food item slips, slowly, away:
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In multiple such sea hare–lobster pairings, Kicklighter et al. found that, if the sea hare was allowed to release its defensive secretions, it escaped from the lobster 60% of the time. But if it had had its secretory glands removed, it escaped only 19% of the time—most of the time, without its chemical defenses, it was eaten.
Reference:
Kicklighter CE, Shabani S, Johnson PM, Derby CD. Sea hares use novel antipredatory chemical defenses. Current Biology 15:549-554, 2005.
Vladimir Brezina (RIP) 
Johna Till Johnson
Wind Against Current 