Chuck Bednar for redOrbit.com – Your Universe Online
The unique underwater light show produced by the creature known as the “disco clam” could be an attempt by the invertebrate to scare off predators or attract prey, a new study claims.
The clam, Ctenoides ales, can be found in the dark caves of the Indo-Pacific Ocean, and while some lucky divers have experienced its flashy display, the reasons behind it are unknown. Now, however, University of California, Berkeley researchers may have solved the mystery.
Dive instructor and graduate student Lindsey Dougherty, who presented her findings at the 2015 annual conference of the Society for Integrative and Comparative Biology in West Palm Beach, Florida, said in a recent statement that she first became fascinated by the creatures when she saw their light show during a dive in Indonesia.
“It was on that trip I first saw the disco clam, and immediately fell in love,” she recalled. When she returned to the US, however, she was stunned to find out that it had not been explained why these clams produce their flashing display. She teamed up with UC-Berkeley professor Roy Caldwell and undergraduate student Alexandria Neibergall to learn more about the behavior.
Ctenoides ales is an active, filter-feeding mollusk that lives in small caves or crevices of the coral reefs in the Indo-Pacific Ocean, the researchers explained. Because their light show is so bright, it was initially believed to be the result of bioluminescence (the production of light within the tissue), but more in-depth study has revealed that far more unusual mechanisms are at work.
Dougherty and her colleagues used high-powered transmission electron microscopy to study the disco clams, and found that the flashes are actually caused by specialized tissues that form a double layer. The tissue reflects light on one side but absorbs it on the other, and when the clam rapidly rolls and unfurls it, the reflected light gives the appearance of flashing, they explained.
The tissues are so reflective that they are capable of producing a flash using only the low levels of blue light discovered in the caves they called home, the study authors noted. It is believed that these clams are the only bivalve species to have evolved this type of structural coloration.
The UC-Berkeley researchers set out to discover why such a mechanisms to developed. They had three different hypotheses about the phenomenon: it may have evolved to help attract mates, to scare off predators or to attract prey. Each of these hypotheses were tested on live claims in laboratory conditions, as the team worked to find out if the clams responded with the flashing in the presence of other clams, predators or prey, and if they could see one another.
Dougherty and her colleagues examined the structure and proteins in the eyes of the clams using a powerful microscope, and found that the creatures likely cannot see well enough to see displays by other clams. As a result, they concluded that it is unlikely that the disco clams are visually attracted to each other, and that the flashing is probably used to attract or repel other creatures.
They then tested the effect of the flashing on predators such as octopi, snails or mantis shrimp. They conducted what are known as “looming trials,” during which the scientists moved a fake predator in the direction of the claims in order to gauge their reaction. They found that the flash rate nearly doubled, from just under 2 Hz to nearly 4 Hz, Dougherty said.
The researchers also detected elevated levels of sulfur, the key ingredient in sulfuring acid, in the tentacles of the clam. A peacock mantis shrimp attacking the disco clam also appeared to recoil from its tentacles and enter a catatonic state, they noted. These findings suggest that the creatures may be producing an acidic mucus in order to repel potential predators.
Finally, phytoplankton prey were introduced into the tank, and as the clam’s tentacles sensed the presence of a threat, there was a significant increase in flash rate. Many types of plankton are phototaxic and attracted to light, the researchers explained, though it is unclear if the prey of the disco clam can also see their flashing. That will be the topic of a future field study.
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