Wild Israel: Chameleons of the sea

Cuttlefish change color, but can't see it.

By DIANA BARSHAW
August 6, 2009 14:11
Wild Israel: Chameleons of the sea

Cuttlefish bones 88 248. (photo credit: Diana Barshaw)

Cuttlefish (Sepiida) are an order of marine mollusk in the class Cephalopoda which also include octopuses, squid and nautiluses. Like squid, cuttlefish have eight arms and two longer tentacles that shoot out at fast speeds to catch prey, and like most other cephalopods they change colors rapidly both for camouflage and for communication with each other. Cuttlefish (deeyonun in Hebrew) have larger brains than any other invertebrate, with complex eyes which enable them to see well enough to be visual hunters. Many of the adaptations of this whole group have contradictory aspects. Usually organisms with large brains specialized for learning are long lived, but cephalopods have life spans shorter than two years. Equally strange, while cuttlefish change color with great accuracy, they are color blind. Beach walking is one of the favorite leisure activities of the people who live in Haifa. If you start at the Dodo Beach promenade, you can walk south for approximately 10 kilometers, practically all the way to Atlit. Throughout my years of beach walking, I have found all manner of interesting creatures and skeletons. There are, of course, the myriad shells of clams, oysters and snails, the occasional fish carcasses and the seasonal abundance of beached jellyfish, but also there are frequent cuttlefish bones. These are perhaps best known as the source of calcium for pet birds, but to me they show that cuttlefish live in great numbers off the coast of Haifa, although I have yet to find the living creatures. The cuttlefish bone, made of aragonite like all mollusk shells, is the only hard part of a cuttlefish except for its beak. It is used by cuttlefish to regulate their buoyancy much like a diver uses a buoyancy vest. When the cuttlefish need to be more buoyant they fill their bone with air, and when they need to be heavier they empty the air and fill the spaces with water. The bone of each species has a different shape, size and pattern of ridges so a taxonomist can identify the species of cuttlefish from the bone without seeing the living animal. These visual animals can detect polarized light and use it to catch prey that might otherwise be invisible to them. Dr. Nadav Shashar from Ben-Gurion University took common European cuttlefish (Sepia officinalis), the species that lives off the shore of Haifa, and placed them one by one in a round tank. At the opposite side of the tank there were silvery fish for the cuttlefish to choose. The experimental group of fish were fitted with filters that caused them to reflect light that was not polarized, while the control group were also fitted with the same light filters, but in a different orientation so that they reflected polarized light exactly as if they had no filters near them at all. The cuttlefish overwhelming chose the fish that reflected polarized light. Perhaps the reason that cuttlefish are color blind is because it is difficult physiologically to detect both color and polarization and the polarization detection is of greater importance to a marine animal. RECENTLY, A different species of cuttlefish, Sepia elongata, was studied off the coast of Eilat. Scientists knew this species existed since 1839 from identification of its bone, but only recently has the living animal been found. So far, fewer than 25 individuals of this species have been observed in the field. Dr. Anne-Sophie Darmaillacqa of the Universite de Caen and Shashar were given permission by the Israel Nature and Parks Authority to capture three individuals of this rare, protected species. They used them to do a preliminary study on the ability of this new species to see polarized light, but found that while the animals did have the physiological properties in their eyes to observe polarization, they did not respond to polarization behaviorally as the common cuttlefish did. It will take further experiments to find out why. Darmaillacqa and several other scientists worked on another aspect of cuttlefish vision, the ability to imprint on what they first see. Imprinting was first described by Konrad Lorenz, who showed that a goose will follow the first moving thing it sees after it hatches. Of course, usually the first thing a goose sees is its mother, but Lorenz had geese that were imprinted on him, and they followed him around exactly as if he was the mother goose. The common cuttlefish's preferred prey are shrimp. However, after Darmaillacqa visually exposed newly hatched cuttlefish to crabs, she found that they changed their preference to crabs. However, the story doesn't end there. Cuttlefish develop in eggs that are black to begin with but then slowly become clear. Darmaillacqa thought perhaps the eggs became clear so that the cuttlefish could see out, as if through a window, before they hatch. She repeated her previous experiment, this time exposing the clear eggs to crabs. Amazingly, the cuttlefish from the eggs that were shown the crabs before they hatched preferred to eat crabs, while those that were not shown crabs retained their usual preference for shrimp. So cuttlefish observe their environment before they leave their eggs and learn to prefer whatever is abundant around them: probably a good idea for a small, vulnerable animal which is not taught how to hunt by its parent. In my last article I wrote about chameleons, a family of lizards which change color both for camouflage and for communication and catch their prey by shooting out their sticky tongues. The similarities between these two groups of species is undeniable, as cuttlefish are often referred to as the "chameleons of the sea." Yet cuttlefish and chameleons are just about as unrelated genetically as two organisms can be and still be in the kingdom of the animals. Yet they are related in a way: They are closely related functionally because both hunt by ambush, and the requirements for such a lifestyle are the same whether you are a mollusk or a lizard. Next time you take a walk on the beach and find a cuttlefish bone, try to imagine seeing through cuttlefish eyes. The writer has a PhD in evolutionary biology. www.DianaBarshaw.com


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