It is a curious thing to consider that UC Berkeley, a school notably lacking a marine biology program, has produced not one, not two, but three published studies on the venerable octopus within the last year. But then octopuses, too, are curious to consider. They have three hearts; blue, copper-based blood; regenerating tentacles; and a level of sentience unique among invertebrates. No one who has ever fallen down a YouTube rabbit hole of these eight-armed Houdinis escaping from tanks, unscrewing jars, or scuttling across the sea floor toting ready-made coconut shell shelters could fail to be intrigued.
In an attempt to better understand what makes an octopus an octopus, Berkeley scientists teamed up with researchers from the University of Chicago, the University of Heidelberg, and the Okinawa Institute of Science and Technology to complete the monumental task of sequencing the entire genome of the California two-spot octopus (Octopus bimaculoides). The results were published in the journal Nature in August 2015.
The octopus genome turned out to be surprisingly large, just a tenth smaller than our own.
The octopus genome turned out to be surprisingly large, just a tenth smaller than our own (2.7 billion DNA base pairs, versus the 3 billion base pairs in humans). Scientists initially hypothesized that the abundance of genetic material was explained by a whole genome duplication, an event thought to have happened twice in our own vertebrate lineage.
“We all thought it could be an interesting parallel between octopuses and humans,” explains Eric Edsinger, a former Berkeley postdoc on the study. Two years into the study, Edsinger says, they realized they were wrong. Instead, researchers found that most of the growth in the octopus genome was limited to a few specific gene families.
“One surprise was that many of [the genes] seemed more or less similar to their counterparts in ‘dumber’ animals like the limpet,” says Daniel Rokhsar, genetics professor at Berkeley and one of the lead authors of the genome study. “But there were several types of genes involved in building the nervous system during embryonic development, and in some cases these genes expanded dramatically.” One such class of gene, the protocadherins, regulates neuronal development and the short-range electrical interactions between them. The octopus has 168 of these genes, twice as many as any mammal.
Professor David Lindberg, a veteran taxonomist specializing in mollusks in Berkeley’s Department of Integrative Biology, speculates that these changes in the octopus genome came in response to an event during the Carboniferous period, some 375 million years ago, when mollusks began to float off the ocean floor and into the water column. “They had to adapt to live in a sensory world that’s three-dimensional rather than two-dimensional,” Lindberg says. “It probably required the amazing neural sophistication we now see in the genome.”
Scientists have witnessed amazing behavioral sophistication in octopuses as well. Another recent study, also published in August 2015, was coauthored by Berkeley’s resident marine invertebrate expert, Roy Caldwell. In the journal PLOS ONE, Professor Caldwell and his California Academy of Sciences collaborator, Richard Ross, described some very “un-octopus-like” behaviors in a species called the larger Pacific striped octopus (LPSO)—including a hunting technique in which the LPSO sneaks up on an unsuspecting shrimp, taps it on its back, and waits for the startled shrimp to jump straight into its ravenous embrace. This deliberate sort of stalking is unusual in cephalopods, says Caldwell, which generally pounce on their prey.
Although seldom studied, the LPSO did get a rare mention in the scientific literature back in 1982, when Panamanian marine biologist Arcadio Rodaniche also documented the species’s strange behaviors, including intimate “beak-to-beak” mating—a finding then largely dismissed by the scientific community.
But 34 years later, Caldwell and Ross were able to confirm Rodaniche’s observation. Normally, male octopuses mate by inserting a modified third right arm into the female’s mantle cavity to deposit their sperm—usually after mounting or at a distance, due to the penchant of many female octopuses for postcoital cannibalism. Not so the LPSO. Some specimens in Caldwell’s study even honeymooned, if you will, sharing a den for a day or two after their “face-to-face” sexual union.
Just how anomalous is this mating behavior? Caldwell says it’s difficult to know. “There are about 300 known species of octopus, and almost everything we know about their behavior and physiology comes from just a handful of species.” All he can say for sure is that “the LPSO is different than the other species we’ve studied.”
One thing all octopuses seem to have in common (along with their cephalopod cousins, the squid and cuttlefish) is a talent for camouflage, something the genome analysis helped shed light on. Scientists identified six genes that coded for proteins known as reflectins, which change the way light reflects from the octopus’s skin. Using this genetic Photoshop, the so-called chameleons of the sea are able to change texture, brightness, pattern, and color with breathtaking fluidity.
It’s a talent that has long struck biologists as paradoxical, however, since cephalopods have only a single color receptor (humans have three) and were therefore thought to be color-blind. Alexander Stubbs recalls that the problem began to bother him while he was doing fieldwork in Indonesia. There the Berkeley graduate student, who studies the physics of coloration and visual perception in animals, watched cephalopods such as the broadclub cuttlefish (Sepia latimanus) blend seamlessly into the scenery to avoid predators, but also flash conspicuous color displays when mating. If cephalopods couldn’t perceive colors, such displays seemed senseless.
To resolve the contradiction, Stubbs teamed up with his dad, Harvard astrophysicist Christopher Stubbs, to produce a computer model of the cephalopod vision system. The key, they hypothesized, lay in the animal’s strange pupils, which tend to be shaped like dumbbells, or like Us or Ws. The duo, who published their findings in the Proceedings of the National Academy of Sciences, think these off-axis pupils act as prisms, scattering white light into its component rainbow colors. The creature can then bring different wavelengths into focus by changing the shape of its eyeball, effectively adjusting the distance between lens and retina.
Alexander Stubbs says of the study, “I think it’s particularly cool because we show the viability of a totally different form of color vision that relies on the principles of physical optics rather than different photoreceptor types.” Given that complex-lens eyes evolved independently in cephalopods and vertebrates, Stubbs says, “it wouldn’t be terribly surprising if these different lineages solved the problem of color vision with different evolutionary trajectories.”
Not surprising, perhaps, but certainly curious.
Kaitlyn Kraybill-Voth is a California intern.