Eukaryon, Vol.13, March 2017, Lake Forest College
Review Article
Cephalopod Encephalization
nervous system.
Despite the mollusks including some of the most intelligent invertebrates on earth, the evolutionary pathway of their nervous systems
of cephalopods apparently developed separately four times. Researcher
Kevin Kocot and his colleagues at Auburn University examined the genetic
sequences of eight main branches of the mollusk phylum in an attempt to
redetermine their phylogenetic relationships. Until Kocot’s findings, it was
believed that the two groups with the most highly organized central nervous systems — the cephalopods and the gastropods — were the most
closely related. Both of these groups have highly centralized nervous systems compared to the other mollusks and invertebrates in general (Zullo,
L., & Hochner, B., 2011).
Amanda Gibbs
Department of Environmental Studies
Lake Forest College
Lake Forest, Illinois 60045
Cephalopods are members of the molluscan class Cephalopoda, which translates to “foot- headed.” These animals are exclusively marine and can be characterized by their bilateral body symmetry, a
prominent head, and a set of tentacles and arms that have evolved from
the primitive molluscan foot. There are two extant subclasses of cephalopods: Coleoidea, which includes octopuses, squid, and cuttlefish; and
Nautiloidea, made up of Nautilus and Allonautilus. The Coleidea subclass
is thought to be made up of the most intelligent invertebrates, and are an
important example of advanced cognitive evolution in animals. The intelligence of Cephalopods has an important comparative aspect in understanding intelligence in animals because the nervous system of these animals is significantly different from that of vertebrates (Bonnaud-Ponticelli
L, Bassaglia Y., 2014).
Figure 1. (A) Octopus brain, (B) Octopus nervous system,
(C) Squid nervous system, and (D) Giant fiber system of squid.
Evidence now suggests that is incorrect. Kevin Kocot analyzed
the genetic sequences that were common to all mollusks and looked for
differences that have accumulated over time. Less related species have a
greater number of differences in their genetics. According to Kocot’s analysis, the gastropods are most closely related to the bivalves, which have
very rudimentary nervous systems and arguably no brain. Further, the
cephalopods come from one of the earliest branches, meaning that their
evolutionary development predates that of snails, clams, and the others.
This means that the central nervous systems of gastropods and cephalopods evolved independently and at different times (Zullo, L., & Hochner, B.,
2011).
One of Kocot’s colleagues, Lenoid Moroz stated that traditionally
neuroscientists and biologists think complex structures such as nervous
systems can only evolve once. That their research is proving otherwise is
a remarkable feat. “We found that the evolution of the complex brain does
not happen in a linear progression,” Moroz said. Instead, parallel evolution
can result in similar levels of complexity across numerous groups. The results of this study found that the nervous system evolution among mollusks
happened over at least four independent events. The four groups that the
researchers found had independently evolved nervous systems include the
octopus, the freshwater snail genus Helisoma, and two seaslug genuses,
Tritonia and Dolabrifera (Kocot, K.M., Cannon, J.T., Todt, C., et al., 2011).
The development of the nervous system of Cephalopods is unmatched
by any other invertebrate. Paired ganglia (as seen in other mollusks) are
present in cephalopods, although the cephalization of this class of invertebrates is dramatic. Most of the ganglia have moved forward and become
concentrated as lobes that form a larger brain which encircles the organism’s gut, with fewer small ganglia clustered in the rest of the nervous
system (see Fig. 1B). Approximately fifteen structurally and functionally
distinct pairs of lobes have been identified in the brain of octopuses (many
identified in Fig 1A). A number of the lobes of the octopus brain correspond
to certain ganglia of other molluscs, which generally lack lobes (see Fig
1C). For example, the lobes of the supraesophageal complex parallel the
cerebral and buccal ganglia of the squid. A large portion of the brain of all
cephalopods in encased in a cartilaginous cranium (Brusca, R. C., & Brusca, G. J., 1990). This development can be clearly contrasted with the basic
molluscan nervous system, which was derived from the basic protostome
plan of an anterior circumenteric arrangement of of ganglia and paired ventral nerve cords (Fig. 2). In the simplest mollusks, ganglia are poorly
developed, and only a simple nerve ring surrounds the esophagus with
small cerebral ganglia on each side. Transverse commissures connect longitudinal nerve cord pairs, which give rise to a ladder-like structure for the
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Eukaryon, Vol.13, March 2017, Lake Forest College
Review Article
Much of evolutionary theory has been guided by Occam’s Razor; it is simpler to assume that something so complex like a brain could only evolve
once in a single group that all members of a group with a similar brain
were from a common ancestor. Mollusks appear to be pointing us to a
different story of evolution. While evolution does not have any set goals,
it does appear that certain ideas and structures have enough evolutionary
importance that they keep coming back and back again. Nevertheless, the
advanced nervous system organization of cephalopods has a great impact
on their behavior that separates them from the other mollusks, regardless
of phylogeny.
Many cephalopods display rapid escape behaviors that depend
on their system of giant motor fibers. The fibers control powerful and synchronous contractions of the muscles of the mantle (the sheet-like organ
that makes up the dorsal wall of the body), particularly in squids. The portion of the nervous system that is responsible for this behavior is a pair
of large first-order neurons, which are located just behind the eyes of the
squid and extend into the mantle (see Fig. 1D). These particular neurons
are located specifically in the lobe of the visceral ganglia, and within this
collection of neurons, second-order giant neurons establish connections
and extend to the stellate ganglia, projecting further into the mantle, away
from the tentacles. Finally, at the stellate ganglia, third order giant neurons
connect and project and innervate the muscle fibers of the mantle. This set
up and behavior are seen throughout the cephalopod class; however, the
abilities of the octopus are more widely varied than those of squids, as over
sixty percent of an octopus’ nerves extend throughout its incredibly strong
and flexible eight arms (The Encyclopedia of Astrobiology, Astronomy, and
Spaceflight , 2013).
Figure 2. Basic molluscan nervous system
D
Due to the encephalization of these ganglionic masses, the octopus’ central nervous system is more similar to vertebrate brains than
to the ganglionic chain seen in its close relatives like the gastropods and
bivalves. The size of the cephalopod nervous system lies within the same
range as vertebrates’ nervous systems. When compared to lower molluscs,
cephalopods show extreme changes in their number and organization of
nerve cells. For example, Aplysia (a sea slug) has about 20,000 neurons in
its nervous system, where an octopus has half a billion (The Octopus: More
Complex than a Simple Mollusk Should Be, n.d.)
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One article showed that the nervous system of an octopus can
be morphologically separated in to three main sections. Two of these, the
optic lobes and the nervous system of the arms, are located outside of the
brain capsule in the mantle. The central brain has around 45 million cells. A
number of stimulation and lesion experiments have helped assign possible
functions to many of the lobes in the octopus brain. Certain areas of cephalopod brains have been very interesting for evolutionary convergence due
to their “strikingly similar” morphological organization to parts of vertebrate
brains that have similar functions. Researchers suggest that evidence supporting that the architectural similarities are the result of convergent evoltuion, they might highlight the importance of connectivity as opposed to cell
structure or cellular properties with regard to brain function (Evolutionnews.
org, n.d.).
Recent research suggests the arms of the octopuses may have
“minds” of their own. Studies have show that each individual arm has an
independent nervous system, and that the centralized brain serves simply
to delegate orders, though the arm itself is responsible for deciding exactly
how the order will be carried out. Essentially, the brain is able to give a
quick
assignment to the arm and then is no longer required to think about it,
allowing the arm’s nervous system to take over. This has been demonstrated in scientific studies: researchers severed the nerves in the arms
of octopuses, disconnecting them from the rest of the body and brain. The
researchers would then tickle the arm of the octopus, which elicited a response a as though the nerves were not severed (The Encyclopedia of
Astrobiology, Astronomy, and Spaceflight , 2013).
There does not appear to be any clear somatotopic arrangement in the motor areas in the cephalopod CNS, which is frequently seen
among vertebrates and insects. This further supports the belief that there
is a widespread distribution of sensory areas throughout higher nervous
centers, as seen in the previous example, rather than strict centralization
of neural command. Specifically, in octopus species, motor control appears
to be organized hierarchically into three levels: higher motor centers, intermediate motor centers, and lower motor centers. A number of studies
have shown that stimulation of the higher motor centers are capable of
producing “discrete and complex responses, movements, and behavioral
responses” that are characteristic of the organism’s repertoire. This sort of
hierarchical functional organization seems to be generally similar to that
of vertebrates and arthropods, however in the octopus a lot of this control
extends into the PNS (Harmon, K., 2012).
By having built up a set of “peripherally controlled stereotypical
motion primitives” the octopus is able to bypass a number of mechanical
constraints, allowing for virtually unlimited degrees of freedom. Still, there
is a substantial amount of communication between the PNS and CNS of
the octopus, particularly regarding sensory-motor information, allowing cooperation between the systems to create complete and elaborate motions.
Because of this, there is a reduction in the complexity of the movement
command. Additionally, this allows the central brain to deal mainly with
“global control parameters” and with overall coordination of movement. It is
suggested that the lack of a somatotopic motor representation in octopuses paralleled the evolution of their unique body plan. With an incredibly active body and eight long and flexible legs, it in important to ensure that the
appropriate information processing and reactions are carried out (Hochner,
B., Shomrat, T., & Fiorito, G., 2005).
This comparatively complex nervous system of the octopus is
also responsible for carrying out the appropriately complex behaviors of
the animal. Documented instances of these behaviors include individuals
giving impressions of flounder, mimicking coral, unscrewing a jar and eating the crabs housed inside, and a number of other “intelligent” behaviors.
The question of how one defines intelligence has certainly been a hot and
controversial topic in cephalopod research, but it cannot be denied that
octopuses can learn, process complex information, and behave in the complex ways already mentioned. This intelligence is the product of “hundreds
of millions of years of evolution under radically different conditions than the
ones under which our own brains evolved,” and for that reason their intelligence is different than that of humans (Kocot, K.M., Cannon, J.T., Todt, C.,
Citarella,, 2006).
Over many years, octopuses have continued to demonstrate
additional signs of intelligence: they have proven to have a memory that
surpasses other invertebrates. N.S. Sutherland, an Oxford biologist in the
1950s, showed that octopuses could be taught to select one shape over
another to receive a reward. Later, Canadian biologist Jennifer Mather observed octopuses playing with toys she put in their tanks: the animals
Eukaryon, Vol.13, March 2017, Lake Forest College
Review Article
declaration classifies them) do not depend on
an organism maintaining a particular brain
structure such as the cortex seen in humans
and other mammals. In fact, there are a number of different brain regions activated when
we experience various emotions .
Despite brain structures such as
the cerebral cortex being highly conserved
through evolution, the complex behaviors of
other organisms like cephalopods have demanded that our conceptions of consciousness be reconsidered. New science has
considered the octopus and found it to be
conscious. The next goal is figuring out what
the “octopus experience” is (Tricarico E, Amodio P, Ponte G, Fiorito G., 2014).
This self-awareness is not equal
among all mollusks, and many argue that bivalve consciousness is nonexistent. For that
reason, many vegan or vegetarian individuals will eat bivalves. An argument for this is
that oysters and mussels have rudimentary
nervous systems. The bivalve nervous system has two pairs of nerve cords and three
pairs of ganglia. Further, there is no obvious
cephalization and are presently no published
descriptions of behavioral or neurophysiological responses to injury. This suggests that the
nervous systems of bivalves operate without
the presence of endogenous opiates or opiate
receptors that are involved in the perception
of pain.
Because cephalopods are so
phylogenetically distant from birds and mammals, they offer a unique point of comparison for general intelligence and
allow researchers to find some common denominations that appear essential for consciousness and cognition. Social behavior, though not necessary for the evolution of such advanced cognition, does seem to create
a byproduct of self-awareness and awareness of other individuals. Such
social behavior, coupled with reports of observational learning in octopuses, seem to suggest the presence of this self- awareness (Vitti, J.J., 2012).
Figure 3. Mussel nervous system
would inspect the objects and push them around with blasts of water. “They
are playing,” Mather claims, “Clams do not play. Humans do.”7 These octopuses continue to surprise researchers. Apparently, their “amazing body,
eyes, and behavior” seem “far too complex for a soft-bodied invertebrate”
and for having been the descendants of clams and snails, which lack tentacles, camera eyes, and such behavioral complexity (Mather, J. A.,2008) .
Jennifer Mather published a paper, “Cephalopod consciousness: Behavioural evidence,”in 2008 in which she suggests cephalopods may have
a form of primary consciousness. She proposes three conditions which
support this. The first is that the connection between brain and behavior
observed in cephalopods under a developmental context is comparable
to that of mammals and birds. Next, because cephalopods are highly dependent on learning as a response to visual and tactile cues, they may
have a domain-general learning (learning through the development of a
global knowledge that is internalized from experience), allowing them to
form simple concepts. Finally, Mather argues that cephalopods are aware
of their position in large spaces and within themselves and further have
a working memory of their foraging areas. Thus, she believes, “if using a
‘global workspace’ which evaluates memory input and focuses attention in
the criterion” for having a primary consciousness, cephalopods appear to
have it (Marion Nixon; J.Z. Young, 2003).
Further, in 2012, a prominent group of researchers specializing in diverse neuroscience fields came together to create the Cambridge
Declaration on Consciousness. The group came to the conclusion, based
off of a substantial amount of empirical evidence, that “the absence of a
neocortex does not appear to preclude an organism from experiencing affective states” and that “the weight of evidence indicates that humans are
not unique in possessing the neurological substrates that generate consciousness.” Octopuses (in addition to all mammals and birds) were one
organism claimed by this group to possess these neurological substrates
(Evolutionreview.org, 2016).
Despite these claims, many people — including scientists — do
not accept the existence of consciousness outside of humans, and it appears to be merely brain anatomy differences that perpetuate these beliefs.
Evidence has suggested that nerve networks seen in cephalopods are involved in attentiveness, sleep, and decision making. Further, research has
demonstrated that emotions (or “neural substrates” as the aforementioned
Note: Eukaryon is published by students at Lake Forest College, who are
solely responsible for its content. The views expressed in Eukaryon do not
necessarily reflect those of the College.
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