The surprising similarities in shrimp & insect brains (and why it matters)

A new study from neuroscientists has discovered a structure in shrimp’s brains which was thought to be found exclusively in insects. The discovery sheds new light on their shared evolutionary past and changes what we know about crustacean intelligence.

shrimp swimming
Shrimps swimming around in a laboratory tank

If you have ever tried pulling the head off a barbequed shrimp, then chances are you’ve probably also got your fingers covered in their sticky grey brains. It has been a longstanding belief that these brains are fairly simple and unremarkable, but a new study from the University of Arizona has revealed they share a surprising similarity with insects, which may well change how we think about these colourful crustaceans. The similarity in question is the presence of a specialised structure known in neuroscience as ‘mushroom bodies’. It shows us that these structures are much more deeply rooted in the shared evolutionary history of arthropods than previously believed, but why is sharing neurological features with insects anything to write home about?

What are ‘mushroom bodies’?

The neurological structures known as ‘mushroom bodies’ are arranged in pairs within brain tissue and are each made up of two defined parts. A clump of neurones organised into a column-like base, called the lobe, capped off by a dome-like structure, called the calyx, thus giving it the shape it was named after. Each mushroom body acts as a relay for information sent from the animal’s sensory organs to converge. From there the information is passed on to neurons that supply thousands of intersecting nerve fibres, which is believed to be essential for computing and storing memories. Mushroom bodies are found throughout terrestrial arthropods, including arachnids (spiders & scorpions) and myriapods (millipedes & centipedes), but most notably in the hexapods (ants, flies & other six legged insects). These brain structures have been extensively studied by neuroscientists in insects such as cockroaches, fruit flies and honey bees.

honey bee
Mushroom bodies have been extensively studied in honey bees and other hexapods

Similarities in shrimp

Ever since the discovery of mushroom bodies researchers have maintained the assumption that they are only present in insects and not crustaceans, despite them both being part of the arthropod family. However this misconception has now been thoroughly dispelled by a new study, released in the journal eLife, led by neuroscientist and entomologist Dr Nicholas Stausfield from the University of Arizona. Along with two of his former students Gabriella Wolff and Marcel Sayre, now both with different institutions, he has shown that mushroom bodies are indeed present in multiple shrimp species. He initially discovered the structures in mantis shrimp in 2017, but has now proved that they are found more widespread among the colourful crustaceans.

scan
Mushroom bodies (highlighted in pink) have now been discovered in the brains of multiple shrimp species

The team were able to identify mushroom bodies in several shrimps using specialised fluorescent antibodies, designed to seek out and attach themselves to known mushroom body proteins in fruit flies. This allowed the researchers to create a map of the crustacean brains with mushroom bodies highlighted by these ‘antibody probes’. Once they located the mushroom bodies they could then analyse the individual structures, and whilst they were not identical to the ones found in insects, they were remarkably similar for animals so far removed from each other in evolutionary history.

Exploring in 3D

The reason that for so long scientists assumed that crustaceans did not possess mushroom bodies was because the larger species, including crabs and lobsters, were found to not have them. That finding was indeed correct, but begs the question why some crustaceans would have this classic arthropod feature and others wouldn’t. The answer according to Dr Stausfield lies in how they explore the world around them. “The mushroom bodies contain networks where interesting associations are being made that give rise to memory” he told Science Direct, “it’s how the animal makes sense of its environment”. It appears it is the need to navigate a complex 3D environment which has caused certain crustaceans to possess these unique structures. Similarly to how honey bees must remember landmarks to navigate their way to and from the hive. Whereas other crustaceans like crabs and lobsters are for the most part constrained to the simplistically flat sea floor and so have no need for such a structure.

lobster
Lobsters and other larger crustaceans have no need for mushroom bodies as their habitat lacks the 3D complexity of shrimps

An evolutionary secret

This new study shows that mushroom bodies are deeply rooted throughout the arthropod family including hexapods, arachnids, myriapods and now crustaceans. These groups are believed to have derived from a common ancestor around 480 million years ago. There was then a major split into the arachnids and the mandibulates, which in turn split into the crustaceans and hexapods further down the line. It shows us that mushroom bodies must have evolved much further back in time than previously believed and could therefore have likely been present in all early forms of complex life. Dr Straufield believes that “this research moves us closer to answering the ultimate question – what was the earliest brain like?”

arthropod tree
The evolutionary tree of the arthropods. Mushroom bodies have now been discovered in each of these separate groups

Intelligence in the deep

This new study is just another example of how we are learning more and more about the brains and behaviours of marine species. As we search for other form of intelligent life in the universe it is becoming increasingly likely that we may just find it in our own oceans instead of among the stars. Octopuses, dolphins, turtles, whales, fish and now even crustaceans have all shown signs that there is much more going on inside their heads than we initially gave them credit for. You may think that this is a bit of a jump, but the mushroom bodies found in shrimp, as well as some marine worms, are astonishingly similar to structures in our own brain that make up the hippocampus, responsible in part for our own memories regarding navigation. These underwater creatures may not have a form of intelligence we can recognise or even comprehend, but they are definitely smarter than we thought.


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