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The myelination of axons has plenty of advantages. It increases signal speed in axons, and thereby reduces reaction times. This is, of course, very good for the survival of the animal in question. Most invertebrates do not have such a myelin coating. I know, that evolution does not just evolve features, because they are useful, but it seems to me, that invertebrates are entirely outclassed by vertebrates, because of how important reaction times are to an animals survival. So if such a trait is not impossible to evolve, it would most likely have evolved multiple times in different animals. If it was as advantagious as I imagine, invertebrates without myelin would have been outcompeted by now. So there must be a significant disadvantage to having myelin coating. Can you explain to me, what that is exactly? Is it just the associated energy cost?

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    $\begingroup$ When you say "outclassed", I hope you realize the most numerous and diverse members of the animal kingdom are insects. And please think about your assumption that reaction time only depends on myelination... I think we could agree that flies or mosquitos have faster reaction times than your average mammal due to the architecture of their muscles and cuticular exoskeleton. The efficacy with which they respond with movement is very much quicker than the tugging of a bone by a partially contracting muscle pulling a tendon. $\endgroup$ – S Pr Jan 8 at 11:56
  • $\begingroup$ Apologies for the late reply, I kind of forgot about my question. I know, that insects are the most numerous. But at their size, intelligence is not as important as it is for bigger creatures, because of the limited number of neurons. $\endgroup$ – Jonathan Böcker Jan 10 at 13:47
  • $\begingroup$ I kind of forgot, that insects have increadible reaction times :) Thank you for pointing that out :) $\endgroup$ – Jonathan Böcker Jan 10 at 13:48
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Many invertebrates possess myelin. It is a misconception that invertebrates lack myelin.

The world speed record for a traveling bioelectric signal is held by the myelinated axons in the abdomen of the Penaeus shrimp!

Please take a look at this website or this review to find a highly recommended comprehensive website about invertebrate myelin from about a decade ago. Here you will find a list of references and additional readings if you are so inclined.

As an additional note: invertebrates also possess and very often make use of ensheathing glial cells, such as is the case with sensory neurons in lepidopterans (moths and butterflies) as depicted below. I've labeled the sheathing cells in pink and red. Please understand that it is currently still unclear whether these cells secrete myelin, though they are thought in some way to insulate the neuron. What you are looking at is a hair-shaped sensillum that is innervated by one or several sensory neurons; these are the basic functional units for sensing movement, proprioception, smell and taste and they exist across all invertebrates, often lining the cuticle and across the entire body, e.g. on legs, on the antennae, in genital tracts, etc. They are best studied in hexapod insects.

enter image description here

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  • $\begingroup$ Thank you very much for the detailed answer (and apologies for the late reply...) I am a complete novice, when it comes to biology, so I have to admit, that I don't fully understand your answer, but I will read through the linked sources more thoroughly, and then post follow up questions, if necessary. Thank you very much for your time :) $\endgroup$ – Jonathan Böcker Jan 10 at 13:51
  • $\begingroup$ Quick follow up question: In what way are intelligence of an animal and signal speed (and myelination) related. If you were to create a mouse with much less myelin in the brain, would it be less intelligent, or would only things like reaction speed decrease? Likewise, could we increase the intelligence of some invertebrates like octopuses by increasing the myelination in their brain? $\endgroup$ – Jonathan Böcker Jan 10 at 13:56
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    $\begingroup$ I think intelligence and myelination are not directly linked. Intelligence is a very complex (multifaceted) word that is typically poorly defined. In evolutionary biology, it typically means the ability to respond to entirely novel problems with a pre-existing machinery; it has little to do with the speed of transmission or speed of thinking. Sloths have myelin too! Maybe you are asking what the effects of hypo- and hyper-myelination would be. Take a look here, it may be of help. $\endgroup$ – S Pr Jan 14 at 9:38
  • $\begingroup$ thank you very much :) My idea was, that the brain of the octopus is specialized to achieve high reaction speeds etc despite the lack of myelin. Could it have more neurons, to achieve higher intelligence, because the size of the neurons could be reduced? Also, what are your thoughts on this paper: sciencedirect.com/science/article/abs/pii/0191886994900493 $\endgroup$ – Jonathan Böcker Jan 15 at 2:53

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