(CN) – Scientists have unlocked genetic secrets of the elusive octopus, finding striking differences with other invertebrates and more than 3,500 genes believed to be specific to the alien-like creature.
Octopuses, shy and intelligent, live for about 8 years. They perform remarkably well on intelligence tests involving mazes and other challenges, and are said to be able to recognize the scientists who work with them.
Now an international team of scientists has sequenced the genome of the California two-spot octopus – the first cephalopod ever to be fully sequenced. They discovered novel genes, unlikely rearrangements, and some evolutionary solutions that appear remarkably similar to those found in humans.
“The octopus appears to be utterly different from all other animals, even other mollusks, with its eight prehensile arms, its large brain and its clever problem-solving capabilities,” said co-senior author Clifton Ragsdale, associate professor in the Departments of Neurobiology and Organismal Biology and Anatomy at the University of Chicago.
“The late British zoologist Martin Wells said the octopus is an alien. In this sense, then, our paper describes the first sequenced genome from an alien,” Ragsdale said.
Octopuses, along with squids, cuttlefish and nautiluses, are cephalopods, an order of predatory mollusks with an evolutionary history of more than 500 million years, long before plants moved onto land.
The octopus has long been recognized as a remarkable creature, with its eight sucker-lined arms that can regrow, cameralike eyes, a large and elaborate brain, and skin that has the ability to camouflage by changing its color and texture.
Cephalopods are the most intelligent invertebrate and have demonstrated learning behaviors and elaborate problem-solving abilities, such as figuring out how to open a food-filled jar.
Ragsdale and his colleagues sequenced the California two-spot octopus, a relatively small species that dwells off the coast of Southern California, to study the genetics of these specialized traits. They published their findings in the Aug. 12 issue of the journal Nature.
The research reveals striking differences between the genome of the octopus and other invertebrates, including huge expansions in two families of genes previously thought to be unique to vertebrates, and a complex gene arrangement.
The octopus’s genome is nearly the size of the human genome and much larger than those of other sequenced invertebrates such as snails or oysters. Of its 33,000 genes, approximately 3,500 are found only in the octopus. Many of these unique genes are active in the brain, suckers, retina and in its sophisticated camouflage system.
The most notable of the expansion of genes was in the protocadherins, a family of genes that are involved in developing complex neural networks such as those found in mammals.
These genes were previously thought to be expanded only in vertebrates, but the researchers found that the octopus genome contains 10 times more of these genes than other invertebrates and more than twice as many as mammals.
“We expected to be surprised,” Ragsdale said of the findings. “These are such remarkable animals.”
Another gene family discovered in the genome – the zinc finger transcription factors – are involved in embryonic and nervous tissue development. The octopus genome contained the 1,800 of these transcription factors, which is the second-largest gene family discovered so far in animals.
The researchers also found that while the octopus gene families are similar to other invertebrates, their genetic arrangements are drastically different.
“With a few notable exceptions, the octopus basically has a normal invertebrate genome that’s just been completely rearranged, like it’s been put into a blender and mixed,” said Caroline Albertin, co-lead author and graduate student at the University of Chicago.
“This leads to genes being placed in new genomic environments with different regulatory elements, and was a completely unexpected finding,” Albertin said.
The study identified hundreds of octopus-specific genes, large numbers of them in the nervous system, retina and suckers. Many of the gene families are scientifically interesting.
The suckers, for example, express a set of genes that resemble receptors for the neurotransmitter acetylcholine, but the proteins these genes code for lack the ability to bind acetylcholine and are suspected to function as chemosensory receptors for the octopus’s ability to taste with its suckers, according to the research.
Six octopus reflectins – genes associated with light processing and camouflage – were identified and found to be relatively different from those in other cephalopods.
This suggests a single gene was present in a cephalopod ancestor and evolved independently across different species, which backs up the researchers’ estimates that the octopus and squid lineage broke away from each other about 270 million years ago.
Ragsdale, Albertin and their colleagues are studying the molecular and genetic mechanisms responsible for octopus development.
“The octopus genome makes studies of cephalopod traits much more tractable, and now represents an important point on the tree of life for comparative evolutionary studies,” Ragsdale said. “It is an incredible resource that opens up new questions that could not have been asked before about these remarkable animals.”
A separate study published this week in the journal PLOS One revealed that a rare octopus – the larger Pacific striped octopus – exhibits some unique hunting, social and sexual behavior.
Although its name suggests otherwise, this species is the size of a tennis ball. The species was first observed more than four decades ago, and remains without a formal scientific description.
Richard Ross, senior aquarium biologist and cephalopod expert from the California Academy of Sciences, and two colleagues began studying captive larger Pacific striped octopuses in 2012 and came up with some surprising results.
“Personally observing and recording the incredibly unique cohabitation, hunting, and mating behaviors of this fascinating octopus was beyond exciting – almost like watching cryptozoology turn into real-life zoology. It reminds us how much we still have to learn about the mysterious world of cephalopods,” said Ross, who did the study with Roy Caldwell, a UC Berkeley professor of integrative biology, and Dr. Christine Huffard of Monterey Bay Aquarium Research Institute.
One of their most surprising observations was the species’ intimate mating strategy, which challenged what is known about conventional octopus reproduction.
Mating pairs of this octopus were observed to cohabitate in the same cavity for at least a few days and share meals in an unusual beak-to-beak position, the researchers said.
While male octopuses usually mate with females at arms’ length, for a quick getaway should the female get aggressive or hungry, this species’ mating pair grasp each other’s arms sucker-to-sucker and mate beak-to-beak, as if kissing.
Though females in other octopus species tend to die after producing a single clutch of eggs, female larger Pacific striped octopuses can lay many clutches of eggs over the course of their adult lives, the study found.
The hunting habits of this octopus are also unique. Whereas most octopuses tackle their prey with all eight arms, this species subtly taps its prey on the shoulder and startles it into its arms.
“I’ve never seen anything like it,” Caldwell said. “Octopuses typically pounce on their prey or poke around in holes until they find something. When this octopus sees a shrimp at a distance, it compresses itself and creeps up, extends an arm up and over the shrimp, touches it on the far side and either catches it or scares it into its other arms.”
Caldwell said the peculiar behaviors of the larger Pacific striped octopus are a testament to how little is known about most octopuses, especially considering that most research is based on observations of just a handful of the more than 300 species of octopus worldwide.
“There are a lot of species of octopus, and most have never even been seen alive in the wild and certainly haven’t been studied,” he said.
In answer to the inevitable question, the plural of octopus can be octopuses, octopi or octopodes.
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