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Cake day: August 4th, 2023

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  • modeler@lemmy.worldtovegan@lemmy.worldThat is not vegan.
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    2 months ago

    As an aside, oysters are not bivalves, they are brachiopods. Brachiopods do have a nervous system - some even have eyes.

    What’s the difference and how do you tell a brachiopod from a bivalve? It’s the plane of symmetry. In bivalves the plane of symmetry is where the shells (also known as valves) join. So bivalves have two identical shells. Whelks and razor shells are bivalves. Brachiopods also have two shells, but the shells are normally quite different. The oyster for example has one big concave shell and one small flat one on top. The big shell has a hole at the apex (just next to the hinge) and a root-like anchor grows from it to bind the brachiopod to the matrix on which it lives. Brachiopods have an axis of symmetry from this root/foot that vertically separates each shell into two mirrored parts.



  • This is true for only red and green loght detecting proteins (opsins) - the blue opsin gene is on chromosome 7.

    The red and green detecting proteins have an interesting history in humans.

    Fish, amphibians, lizards and birds have 4 different opsins: for red, green, yellow and blue colours. And the blue opsin sees up into the ultra-violet. Most animals can see waaaay more colours in the world than we (or any mammal) can. So what happened that makes mammal vision so poor?

    It’s thought that all mammals descend from one or a few species of nocturnal mammal that survived the catastrophe that wiped out the dinosaurs at the end of the Cretaceous. The colour detecting cells (the cones) need a lot of light compared to ones that see in black-and-white (the rods) and therefore nocturnal animals frequently lose cones in favour of the more sensitive rods for better night vision. The mammals that survived the Cretaceous extinction had also lost the green and yellow opsins while keeping red and blue - basically the two different ends of the light spectrum.

    Consequently today most mammals still have only 2 opsins so your cat or dog is red-green colourblind.

    Why do humans see green? Probably because our monkey forebears, who lived in trees and ate leaves, needed to distinguish red leaves and red fruit (visible to birds) from the green background.

    But how did we bring back the green opsin? A whole section of the X chromosome (where the red opsin is coded) got duplicated in a dna copying mistake and then there were two genes for red opsins. As there are different alleles (versions), they could be selected for independently and so one red opsin drifted up the spectrum to be specific for green. So our green opsin is a completely different gene to the green opsin in fish, birds, etc. This kind of evolution happens a lot which is why, for example, there are many families of similar hormones like testosterone and estrogen. And steroids too.