Researchers have shared rare and astonishing footage of a tiny sea slug feeding. In the footage, the slug approaches some algae, clambers on top of it and starts chomping away.
The close-up film shows such a high level of detail that you can see the slugs’ tiny chops moving as it munches and fluid inside the algae’s cells being slurped up like a smoothie. The findings are published in Venus (the Journal of the Malacological Society of Japan).
The feeding behaviour of a group of sea slugs called sacoglossans is fascinating because these slugs are solar powered. When they feed on algae, “they steal chloroplasts [tiny structures that perform photosynthesis] and keep them working inside their own bodies, a phenomenon known as kleptoplasty,” says study author Taro Maeda, a researcher at the Institute for Advanced Biosciences, Keio University.
Scientists have studied this strange phenomenon for many years. “Even so, no one yet fully understands how the trick works,” says Maeda.
The researchers were particularly interested in a species called Plakobranchus ocellatus, sometimes known as the ringed sap-sucking slug. “We wanted to see exactly how those chloroplasts are drawn into the slug at the moment of feeding,” he explains.
Capturing this incredible video took a lot of preparation. First, the scientists had to find the slug. Then, using DNA testing they figured out that it eats green algae called Rhipidosiphon lewmanomontiae.
But there was a problem. When the scientists looked around the slug’s habitat for this type of algae, they couldn’t see it anywhere. Eventually, they realised it had been hiding in plain sight, growing in crevices such as those around the edges of loose stones. “Only after returning to the field again and again did we start to learn where it likes to grow,” says Maeda. “Until we knew where to look, it was genuinely hard going.”
To make things even harder, this type of algae looks almost identical to a closely related species. “So even after collecting a sample, we couldn't be sure what we had until we got it back to the lab for a DNA test,” he says.
Once they brought together the sap-sucking slug and its preferred snack, it was still hard to tell whether the little animal was chomping on the algae as you can’t easily see its tiny mouth moving. “The slug feeds agonisingly slowly, its mouth is usually hidden, and underwater you almost never see a clear, telltale change,” says study author and biologist Shinichi Inagaki.
To get around this, they set up a macro camera pointing directly at the slug’s mouth. This revealed exactly how the little slug feeds on the alga, which is made up of “slender tubes packed tightly together into a fan shape,” says Inagaki. “The slug punctures the wall of each tube, one by one, to drain the chloroplasts.”
“Then it sucks and spits, sucks and spits, drawing out as much of the green fluid as it can,” he says. “When that tube is empty, it picks another and takes its fill.”
Then it sucks and spits, sucks and spits, drawing out as much of the green fluid as it can.
Because the cell wall is left almost intact once the slug has gulped down its insides, the alga seems to stay the same shape. “There are almost no obvious ‘bite marks’ to see,” says Maeda, another reason it’s so hard to observe feeding in these animals.
Thes scientists’ special imaging system allowed them to see the cytoplasm (fluid) moving inside the thin, thread-like cells incredibly clearly.
It looked just like “when one sucks juice through a straw,” says Euichi (Luigi) Hirose, a professor at Ryukyu University and co-author of a related study on sea slug feeding in Galaxea, Journal of Coral Reef Studies.
Then, something weird happens. The sea slug does a headstand – and the experts don’t know why. “This one is hard to explain,” says Inagaki. “My hunch is that it's simply about getting comfortable.”
Then, something weird happens. The sea slug does a headstand.
Surprisingly, when the scientists offered the slugs a closely related type of green algae collected far away from its habitat, the slugs were happy to chow down. “We hadn't expected that,” says Inagaki.
“Sea slugs, such as Plakobranchus ocellatus, are more than just strange and beautiful reef animals,” says Hirose. “They may be living clues to one of life’s greatest evolutionary mysteries.” That is, how photosynthesis first began in complex organisms.
Although these little critters may be out of sight, out of mind for many, the experts hope they won’t be lost from our seas. “In a changing ocean, protecting such a sea slug may ultimately mean protecting an entire ecological partnership millions of years in the making.”
Top image credit: Inagaki & Maeda, figshare, CC BY 4.0
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