…a guest blog from Mark Millan, a neuroscientist specialising in the study and improved treatment of disorders of the brain.

Following on from our blog “From Bugs to Brains.…and….From Green Spaces to Better Mental Well-Being” early last year, we are joined by neuroscientist Mark Millan once again to explore whether bugs have brains.

Which animals have brains?

Posing the question “Which animals have brains”, whether to school children, to people down the pub or even to biologists, triggers a surprisingly wide range of responses. Cats and dogs (familiar pets) invariably get the nod, as do the biggies from bears to bison, all mammals of course, and birds generally come up trumps as well. Conversely, answers vary for frogs and fish, and things get even trickier regarding “bugs“, probably since it is hard to imagine how an honest-to-goodness brain could be squeezed into an animal as small as a grain of rice. To sort this out, we need to initially address a related issue: “What exactly is a brain?” This sounds pretty straightforward, yet the question is harder to answer than one might think – even for neuroscientists, who tend to side-step it. Here, we keep things simple in proposing a modern definition of brains that works for animals of all shapes and sizes: from ants to antelopes, and from the diminutive shrimp to the mighty sperm whale.

What is a brain?

One standard definition of a brain is “the part of the nervous system surrounded by the skull” and, yes, the protective skulls of vertebrates do indeed enclose brains – including, thankfully, those of you, me and other humans. However, this definition implies that if you don’t have a skull, then, tough luck, you can`t have a brain, which is rather galling for the myriad invertebrates defended by Buglife. So, let´s try a more flexible and precise definition, and one which evokes function – brains don’t just sit there, they do stuff: “The brain is a prominent centre of interacting neurons and other cell types, usually located at the front of the body: it integrates information from the environment and the body, and coordinates the physiology and behaviour of the whole organism”. This definition ditches the skull, and a priori fits both vertebrates and invertebrates. Moreover, it is preferable to the misleading assertion that invertebrate brains “surround the oesophagus“, which is not really the case. They are far more variable in design and location and, if they surround anything, it is generally the pharynx (throat, lying in front of the oesophagus). Moreover, some sit above the pharynx, some in front, and some even behind it…

Why bother with a brain? The “Bilaterians”

Invertebrates possessing brains all belong – like vertebrates – to the so-called “Bilaterians”, one of the Five Great Groups of Animals (Picture 1). Bilaterians display left-side/right-side symmetry, as well as front-to-back body organization. However, not all Bilaterian invertebrates are lucky enough to possess brains, and whether they do – or do not – depends on two interrelated factors. First, brains are energetically expensive and challenging to make and maintain. Hence, if not essential, building them simply isn´t worth the hassle and cost. Second, lifestyles differ. Highly-mobile invertebrates that fly, run, swim, crawl, beetle etc. about – generally in search of lunch, or to avoid becoming lunch themselves – need brains. By contrast, those that stay put, such as filter-feeders hidden in mud-embedded tubes or shielded by hard shells, don´t bother with brains – they would be a pointless luxury. Interesting examples are found in the two major groups of Bilaterian invertebrates, the “Spiralians“ and the “Ecdyzoans“, both of which sound like aliens confronted by Captain Kirk in Star Trek.

Bilaterian Invertebrates with and without brains: the “Spiralians”

Several classes of Spiralian live on the sea floor, doing nothing much else but waving their tentacles (lophophores) around to catch little particles of food. As you will surmise, they lack brains but, in compensation, have been awarded charmingly evocative names like moss animals, horseshoe worms and lampshells. You are unlikely to run into them – unless you are deep-sea diver – but you may well have met other brain-bereft Spiralians, if only on a plate in restaurants; clams, mussels, oysters and scallops, all Molluscs.

Their Cephalopod cousins, the squid and the octopus, are also found both in the sea and in restaurants: contrariwise, they possess chunky brains (and eyes) reflecting their highly-active lifestyles, both as predators and as potential prey of their arch-enemy, fish. Having said that, the brain of the surprisingly smart octopus resembles an amorphous, squashed lump of putty with two enormous optic lobes stuck on the sides, and with a whole bunch of other neurons diverted to its legs (Picture 2). At around 500 million neurons, the octopus brain is the biggest of all invertebrates, but Spiralians can also lay claim to the world´s smallest brain: there are two competing candidates: 1), Symbion, a miniscule creature about 0.1mm long which lives as a non-paying tenant on the mouthparts of lobster and 2), Dinophilis ciliatus which cruises between grains of sand in the tidal zone of beaches, profiting from a brain composed of a mere 42 neurons – out of 330 cells for the entire animal. Funnily enough, they are both one-track brains, devoted purely to reproduction.

Spiralians also make up a large part of the highly-diverse “Worm-world“. Those boasting bone fide brains include marine ragworms, aquatic flatworms, leech and the humble earthworm, a welcome denizen of gardens. Earthworms (which have bilobed brains situated behind their heads) are good news for plants in aerating, enriching and breaking up the soil, but that can hardly be said of snails and slugs which – though a lot slower than Cephalods – likewise possess impressive powers of spatial memory and orientation, not least in targeting our favourite flowers and veg.

Bilaterian Invertebrates with and without brains: the “Ecdysozoans”

As for Ecdysozoans, they likewise embrace species without brains. One striking example is the aptly-named penis worm which strongly resembles a…you guessed it…and merely displays an extensive nerve net embedded in the body wall. Smirk you may, but – brain or no brain – the penis worm has the last laugh: it dug its very first, protective marine burrow some 500 million years ago and has been flourishing there ever since – humans have yet to make it to 500 thousand years. Another Ecdysozoan that drew a short straw is the barnacle: evolution left it stranded and brainless on intertidal rocks. Conversely, Crustaceans like crabs and lobsters (back to the restaurant!) reveal well-developed brains. Apart from woodlice, Crustaceans never made it onto land where, in contrast, insects are omnipresent, just as they abound in the air and in fresh water.

Insects and their brains (Picture 2) are currently enjoying increased recognition, partly due to public awareness of the plight of bees which – like moths, beetles, flies and wasps – pollinate many food crops and other plants. A special part of the insect brain called the central complex explains their gift for navigation, while another part, the mushroom bodies (so-called due to their shape), underlies their striking powers of learning and memory. Honeybees, for example, recall the location of pollen-rich flowers, use landmarks to track the fastest route from and back to the hive, and waggle-dance to share information with other bees. Intriguingly, the three-part (front to back) insect brain resembles the tripartite brain of vertebrates. Honeybee and human brains may, then, have evolved from a common ancestral brain in the long-distant past, though not all experts agree. Irrespective of their origins, the sophisticated design of insect brains has just been unveiled in the first complete wiring-diagram of any brain: the 150,000 neuron/50 million-synapse strong brain of the fruit fly: a flipping nuisance to you and me, but a key tool for neuroscientists! This work highlights the distinctively efficient way insects pack the maximal number of neurons into the minimal amount of space.

Like Crustaceans, Insects belong to a larger group of brain-powered invertebrates called the Arthropods which includes the carnivorous milli/centipedes, a multitude of roundworms, and the cognitively-gifted spiders, whose brains may even swell out of their heads into their chests! A closely-related group worth a quick mention are the irresistibly engaging water-bears: their brains (and bodies) are virtually indestructible thanks to a suite of unique, protective proteins (Picture 2).

“Off-beat” Bilaterians and other Invertebrates

To conclude our tour, let’s briefly dive back into the sea to look at two off-beat classes of Bilaterians. Starfish belong to the pentameric (radially symmetrical) Echinoderms which possess nothing more than a set of nerve nets to control their five sucker-armed legs, which prise open clams and other shelly Spiralians. Only their bilateral, swimming larvae possess brains – they are lost in adults! We encounter a similar story in the amusingly-named sea-squirts (Urochordates). The similarly bilateral larvae are equipped with an upfront brain that helps them find a place to attach, yet once they settle down, they are in for a nasty shock: their brains get absorbed, and they have to make to do with nerve nets for the rest of their adult lives. Adolescence may seem a bit traumatic for the average teenager, but it could be a lot worse…

Finally, Bilaterians are only one of the Five Major Groups of Animal and you may be wondering about the other four, all invertebrates. Remarkably, neither Porifera (sponges), nor Placozoans (a single species, Trichoplax), nor Ctenophores (sea gooseberries) nor Cnidaria (jellyfish, corals and co), possess brains. Indeed, Sponges and Placozoans can’t come up with a single neuron between them. While Cnidaria and Ctenophora possess neurons and nerve clusters (“condensates“) that control specific functions and/or parts of their (usually circular) bodies, they do not possess genuine brains.

Summing-up: Protecting “Bugs” with – and without – brains!

To sum up, brains are unique to animals but, surprisingly, they are found in only one of the Five Main Groups, the Bilaterians, which houses humans and other brain-bearing vertebrates. As concerns Buglife-sponsored invertebrates, several groups, many lurking in the deep sea, do not make brains, principally since they don’t need them for their undemanding lives. Conversely, many familiar invertebrates, from snails to squid (Mollusks), crabs to crayfish (Crustaceans), assorted worms, and those extraordinary “Centres of Excellence in Miniaturization” – insects and spiders – possess highly capable brains. Thanks to the 4-million odd varieties of Arthropod, in numerical terms, most invertebrate species do indeed possess brains.

The above comments will hopefully encourage people to think more kindly of the bees, butterflies and other bugs we encounter in gardens and other green spaces – as well as their cousins under water! Despite the occasional inconvenience, the collective importance of invertebrates to ecosystems is indisputable, so enhanced protection is paramount. Nonetheless, let us likewise respect animals that have renounced brains – the brainless jellyfish is about to celebrate its 600 millionth birthday and still going strong 🙂!