You may have been told before that Bichirs have poor eyesight. Comparatively speaking, yes, this is true, but how bad is it, and is 'bad' the right word to use?
Before we discuss their eyesight, let's talk about the other ways they see the world, as they have some more amazing senses.
The Sensory Canal (Line) System: Neuromasts & Lateral Line
On Polypterids you can see rows of tiny sensory pits. Some are neuromasts, some are ampulla. Adult specimens have more of these organs/pits than the juveniles. In the snout lines alone, adults usually have around 20, whereas juveniles only have 6-7 on each side. The neuromasts have relations to the lateral line systems and help to detect even the slightest vibrations using cells inside, making them very aware of their surroundings and which direction possible prey is. There are hair cells inside which are so sensitive that they even detect sound, although not clear by any means. On an unrelated note, Bichirs have an Otolithic organ, (the fish equivalent of ears) detecting sound better than their lateral line; which also uses similar hair cells.
The Sensory Canal (Line) System: Ampullae of Lorenzini
The ampulla or Ampullae of Lorenzini as they're more commonly known as, are electroreceptors which help detect changes in water temperature, facilitates social behaviour, navagates their environment and most importantly, aids in finding prey.
Open circles are pit line sense organs, dotted circles are neuromasts (37mm juvenile P. senegalus).
Tubular Nares
Bichirs have elongated, tubular paired, incurrent nares (essentially just inverted nostrils/nares filled with fluid). If that sounds a bit ridiculous, just think they're more or less barbels as they do the same job. These nares act as their sense of 'smell', or taste, depending on how you look at it. It helps them tell whether food is nearby or if something is safe to eat (not even Polypterids want to eat rotting food). Have you ever had an unexplained period of a few days or a week where your Polypterid wouldn't eat? These nares are so effective, that just adding food to your closed aquarium a few days in a row pollutes their sense of smell. If you add the same food to the tank, any Bichir which didn't detect you adding the food, will not even realise they're being fed. A quick waterchange remedies this, but if you have more Bichirs in the tank, they will certainly detect the others feeding.
There are many other senses I could write about, but hopefully this outlines some of their sensory systems; back to the initial question of eyesight. Their vision itself is very limited, only being able to distinguish figures with little detail beyond their own markings. Things are clearer for them in daylight, but think of their vision like an over exposed photograph. Their eyes are very sensitive to light, and to my knowledge, little much in the way of limiting the amount of light they receive; an argument for keeping them in dimly lit aquariums. When things get dark, it's a different story. Inside the eye's retina are tapetum crystals which increases avaliable light, blurring their vision slightly, but allows them to see in the dark. If you shine even a weak light on their eyes, the crystals bounce the light directly back to you, making their eyes appear to glow. This is observed mostly in nocturnal carnivores and deep sea animals. Gars also have this amazing feature too.
You could now argue that Bichirs don't really need their vision, they could map out their terrain and prey perfectly well without it. While that's also true, their limited vision is still very important, as it is a way of communicating to eachother and physically replying to their environment. Bichirs contain light reactive cells in their body called melanophores, which acclimate to whatever the fish sees in its surroundings. It's the reason we can adjust a Bichir's colour contrast by changing the colour of lighting or substrate. When a Bichir loses their sight completely, the fish will turn jet black from being inundated with all the melanin pigment cells. Not only does this put them at a disadvantage in the wild (likely being swiftly eaten), but they're unable to contract the melanophores which flare their markings, (making the fish visible in certain environmental conditions) losing the ability to relay information like warning signals, breeding opportunities or stress. Thankfully we can take good care of fish which lose their vision in the aquarium, whether that be from prior injury or infection or cataracts through aging.
To answer the question, from an adaptive point of view, any vision is better than no vision. With even the limited eyesight of Polypterids, vision of the environment directs the body how to appear, almost like a built in mirror. In that regard, their vision is better than mine, as I need an actual mirror!
Thanks for reading, hope you're all keeping safe!
Before we discuss their eyesight, let's talk about the other ways they see the world, as they have some more amazing senses.
The Sensory Canal (Line) System: Neuromasts & Lateral Line
On Polypterids you can see rows of tiny sensory pits. Some are neuromasts, some are ampulla. Adult specimens have more of these organs/pits than the juveniles. In the snout lines alone, adults usually have around 20, whereas juveniles only have 6-7 on each side. The neuromasts have relations to the lateral line systems and help to detect even the slightest vibrations using cells inside, making them very aware of their surroundings and which direction possible prey is. There are hair cells inside which are so sensitive that they even detect sound, although not clear by any means. On an unrelated note, Bichirs have an Otolithic organ, (the fish equivalent of ears) detecting sound better than their lateral line; which also uses similar hair cells.
The Sensory Canal (Line) System: Ampullae of Lorenzini
The ampulla or Ampullae of Lorenzini as they're more commonly known as, are electroreceptors which help detect changes in water temperature, facilitates social behaviour, navagates their environment and most importantly, aids in finding prey.
Open circles are pit line sense organs, dotted circles are neuromasts (37mm juvenile P. senegalus).
Tubular Nares
Bichirs have elongated, tubular paired, incurrent nares (essentially just inverted nostrils/nares filled with fluid). If that sounds a bit ridiculous, just think they're more or less barbels as they do the same job. These nares act as their sense of 'smell', or taste, depending on how you look at it. It helps them tell whether food is nearby or if something is safe to eat (not even Polypterids want to eat rotting food). Have you ever had an unexplained period of a few days or a week where your Polypterid wouldn't eat? These nares are so effective, that just adding food to your closed aquarium a few days in a row pollutes their sense of smell. If you add the same food to the tank, any Bichir which didn't detect you adding the food, will not even realise they're being fed. A quick waterchange remedies this, but if you have more Bichirs in the tank, they will certainly detect the others feeding.
There are many other senses I could write about, but hopefully this outlines some of their sensory systems; back to the initial question of eyesight. Their vision itself is very limited, only being able to distinguish figures with little detail beyond their own markings. Things are clearer for them in daylight, but think of their vision like an over exposed photograph. Their eyes are very sensitive to light, and to my knowledge, little much in the way of limiting the amount of light they receive; an argument for keeping them in dimly lit aquariums. When things get dark, it's a different story. Inside the eye's retina are tapetum crystals which increases avaliable light, blurring their vision slightly, but allows them to see in the dark. If you shine even a weak light on their eyes, the crystals bounce the light directly back to you, making their eyes appear to glow. This is observed mostly in nocturnal carnivores and deep sea animals. Gars also have this amazing feature too.
You could now argue that Bichirs don't really need their vision, they could map out their terrain and prey perfectly well without it. While that's also true, their limited vision is still very important, as it is a way of communicating to eachother and physically replying to their environment. Bichirs contain light reactive cells in their body called melanophores, which acclimate to whatever the fish sees in its surroundings. It's the reason we can adjust a Bichir's colour contrast by changing the colour of lighting or substrate. When a Bichir loses their sight completely, the fish will turn jet black from being inundated with all the melanin pigment cells. Not only does this put them at a disadvantage in the wild (likely being swiftly eaten), but they're unable to contract the melanophores which flare their markings, (making the fish visible in certain environmental conditions) losing the ability to relay information like warning signals, breeding opportunities or stress. Thankfully we can take good care of fish which lose their vision in the aquarium, whether that be from prior injury or infection or cataracts through aging.
To answer the question, from an adaptive point of view, any vision is better than no vision. With even the limited eyesight of Polypterids, vision of the environment directs the body how to appear, almost like a built in mirror. In that regard, their vision is better than mine, as I need an actual mirror!
Thanks for reading, hope you're all keeping safe!
