Sea slug (Phylidae varicosa). Photo by B. Dupont, Wikimedia Commons
Audio of sea slugs and octopuses
Sea slugs (Phylum Mollusca, Class Gastropoda) are ocean-dwelling relatives of terrestrial snails and slugs that, like terrestrial slugs, have given up their protective shells in order to reduce metabolic costs (shells are expensive to make and very expensive to carry around!) and to gain more mobility in their habitats. Many sea slugs are predators that feed on very specific prey species (like certain jellyfish, sea anemones, bryozoans, etc.) while others are grazers that feed on similarly very specific algae. The highly specialized nature of their food preferences makes sea slugs quite efficient in their intact ecosystems, but also makes them quite vulnerable to any disturbances that might negatively effect their food species.
To compensate for the loss of their protective shells, sea slugs have developed a variety of secondary metabolic chemicals that make their flesh taste foul. They also can accumulate, stinging-protein nematocysts that they acquire from their jellyfish or other cnidarian prey. In order to advertise their foul taste and stinging capabilities, sea slugs are often brightly colored and tend to swim about slowly and openly in their habitats.
Elysia marginata, Photo by S. Childs, Wikimedia Commons
Elysia marginata is a sea slug called the “ornate sap-sucking slug.” It is found in algae beds around the Hawaiian Islands and throughout the Indo-Pacific Oceans. It has sedentary and mobile forms and feeds quite specifically on the green algae, Bryopsis. Elysia marginata is green in color in part due to its ability to assimilate into its own tissues the chloroplasts from the algae that it consumes. Further, these chloroplasts remain functional in the sea slug and carry out symbiotic photosynthetic energy fixation inside its body! The slugs, then, to use a phrase popularized by a sea slug researcher, are “solar powered!”
In addition to being powered by photosynthesis, these ornate sap-sucking seas slugs have another superpower that was recently described in Current Biology (March 8, 2021). Sayaka Mitoh, a Ph.D. candidate at the Nara Women’s University in Japan and lead author of the Current Biology paper, was tending the university’s vast collection of living sea slugs when she came across a gruesome scene in the Elysia marginata tank: the ornate sap-sucking sea slug had been decapitated, but, as Ms. Mitoh watched, the severed head of the sea slug was now moving about the tank and was actively gobbling up algae!
Closer examination of the sea slug’s body and head suggested that the decapitation was self-inflicted. There was a “breakage plane” in the slug’s neck that could be weakened to the point that the slug was then able to simply rip its own head off! There are many examples of animals that are able to quickly shed tails and legs and other limited body parts in order to escape from a predator or a trap, but the ornate sap-sucking sea slug’s ability to shed its entire body was unprecedented in Nature!
Researchers at Nara Women’s University monitored the decapitated sea slug’s head found that it completely regenerated a new body in about three weeks. The discarded body, on the other hand, while responsive to stimuli for a few weeks, did not move about and did not regenerate its missing head. It decomposed in about three weeks. The research team also identified one other sea slug that like E. marginata, could accomplish self-decapitation: the closely related and similarly “solar powered,” Elysia atroviridis).
Elysia atroviridis. Photo by I. Diver, Wikimedia Commons
Examination of the shed bodies from E. atroviridis revealed that they were all full of parasites. This suggests that the decapitation was a method to cleanse the individual sea slug of a potentially debilitating parasite load. Parasites, however, were not found in any of the shed E. marginata bodies, so this hypothesis is still being developed. The fact that the two seas slug species that can shed their entire bodies are also two of the species that maintain functioning chloroplasts in their tissues, suggests that the photosynthetic symbiosis may be critical to sustain the “body-less” seas slug head while it regrows its critical organ systems.
Octopuses (Phylum Mollusca, Class Cephalopoda) have a more subtle superpower than the solar-powered, body shedding sea slugs: octopuses are extremely intelligent! Intelligence in animals is measured in many ways, but high on the list of criteria for a high powered animal-brain is the ability to escape from enclosures and entrapments, the ability to solve problems and puzzles, the ability to find food and avoid being someone else’s food, and the ability to learn and remember. In all of these categories, octopuses score extremely high on animal intelligence scales. Many lists of “most intelligent animals” put octopuses among the ten most intelligent animals in all of Nature! They are more intelligent than dogs or cats, but slightly less intelligent than chimpanzees, orangutans, elephants or dolphins! They are, without question, the most intelligent, invertebrate organisms in existence!
Octopus. Photo by Pseudopanex. Wikimedia Commons
Octopuses are active predators that live in marine habitats that are loaded with other predators. The evolution of their intelligence has undoubtedly been linked to their ability to find food and to avoid becoming food for someone else. Further, the evolution of their remarkable brains has followed very independent and very different pathways from the evolution of mammalian brains.
For example, in mammals the brain is a single organ well stuffed with highly interconnected neurons. In octopuses, there is one central brain but also eight auxiliary brains that are each associated with one of the octopus’s tentacles. Muscular control of the tentacles is independently accomplished by these “auxiliary” brains while sensory analysis, memory, learning, homeostasis and overall body control is a function of the larger, central brain.
On average, octopuses have a total of 500 million neurons in their nine brains. This is about the same number of neurons that are found in the brain of a dog (humans, just for comparison, have 100 billion highly interconnected neurons in their brains (200 times more neurons that the average octopus or average dog)!). About two-thirds of the octopus neurons are in the central brain while the rest are divided up between the auxiliary brains.
There is, though, another feature of brain neurons that are remarkably developed in octopuses: the types and sophistications of the cell surface proteins that functionally knit the brain’s neurons together! The genes that encode for these vital, cell surface proteins are called the “protocadherin” genes. In mammals, 70 of these protocadherin genes have been identified and 53 of these are found in humans. In a typical invertebrate there are only 16 or 17 of these protocadherin genes. In octopuses, though, there are 168 protocadherin genes! The neurons in each of an octopus’s nine brains are knitted together by these cell surface proteins in remarkably sophisticated ways!
Octopus vulgaris (a close relative of O. insularis). Photo by M. Sontag, Wikimedia Commons
Octopus insularis is an Atlantic Ocean dwelling octopus that can be found off of the east coast of South America. A team of researchers from several universities in Brazil studied the sleep patterns of O. insularis and made some interesting observations (their research was published in iScience (April 23, 2021)).
During its sleep cycle, O. insularis has two distinct phases that correlate roughly with the two sleep phases seen in mammalian sleep. There is a long phase that lasts 30 to 40 minutes in which the octopus is relatively still (tentacles wave up and down slowly), its body is pale in color and its eyes have slit-like pupils. This period resembles “slow-wave” sleep in mammals, the sleep period in which wastes are cleared from the brain and memories are consolidated. In between the octopus’s slow phases is a shorter period of sleep activity in which the octopus’s skin changes color and darkens, its eyes and tentacles twitch and jerk about and in which its tentacular suckers are flexed by their controlling muscles. This period lasts for about 40 seconds and seems to correlate to the Rapid Eye Movement (“REM”) phase of mammalian sleep. REM, of course, is when humans (and maybe other mammals and even other types of vertebrates) dream! The question that the octopus researchers then asked was, do octopuses dream? And, if they do, how can we tell what they are dreaming about?
Future research on these dreaming octopuses will involve the impact of sleep deprivation: do octopuses learn more slowly or remember less acutely if they are denied sleep? Also, when an octopus is awake and working on a task it changes its body colorations, possibly, in very related and recognizable ways. When an octopus is being taught a task and then goes into its active sleep phase, does it display similar “task” color patterns? Could these color change sequences tell us what the dreaming octopus is dreaming about?
Super-powered mollusks: sea slugs and octopuses! Isn’t Nature full of surprises?
A film recommendation: check out “My Octopus Teacher” on Netflix. It just won the Academy Award for best documentary and will make you fall in love with and in awe of an octopus!