The blind Mexican cave fish lacks vision and thus cannot see a wall right in front of its face. However, they rarely hit walls while swimming. So the question arises: how do they prevent themselves from hitting obstacles that they cannot see?
Scientists have determined that they use what they call hydrodynamic imaging. A. fasciatus uses hydrodynamic imaging by creating a flow field around themselves and sensing the perturbations in that field.
(A. fasciatus avoiding a head-on collision with the wall)
(A physical model created to visualize the velocity (A-E) and pressure (F-J) contours used for hydrodynamic imaging against a head-on wall by A. fasciatus)
To create the flow field, A. fasciatus swims in short controlled bursts. By doing so, they create a flow field (velocity and pressure) around them. As the fish approaches the wall, the flow field deforms, which the fish senses with its lateral line, and adjusts their swimming trajectory accordingly for.
(A. fasciatus avoiding contact with wall parallel to its body)
(A physical model created to visualize the velocity (A-E) and pressure (F-J) contours used for hydrodynamic imaging against a parallel wall by A. fasciatus
And as you can imagine, the same principle applies for the fish when swimming parallel against a wall; the fish senses the deformation in the flow field with its lateral line and adjusts its swimming trajectory accordingly.
(A. fasciatus with its lateral line function removed lacks the ability to perform hydrodynamic imaging)
The above figure shows A. fasciatus specimens whose lateral lines were made dysfunctional. As you can see, they lack the ability to perform hydrodynamic imaging and need direct contact, be it with their face, pectoral fin, or other parts of their body, in order to sense the wall.
You catch three different species of cichlid (Family: Cichlidae) from Lake Tanganyika, a lake in Africa.
(Ophthalmotilapia heterodonta Photo from here)
(Lamprologus callipterus Photo from here)
(Telmatochromis bifrenatus Photo from here)
You want to know which of these fish is a carnivore, herbivore, and omnivore. How do you do it?
There is one way you can tell by looking at the structure of the head, but I will discuss a way to tell that perhaps did not cross your mind: the length of the intestine. This method actually pertains to other animals too!
Carnivores generally have an intestine that is less than or equal to the length of their body. This is because meaty foods are easily digested and the nutrients are easily sucked out by the intestine.
Herbivores generally have a very long intestine that is 2-20x the length of the their body. This is because plant matter is made from hard-to-digest matter like cellulose and lignin, making it is difficult to suck out the nutrients. The long intestine allows the herbivore to suck more nutrients out of the food before they poop it out.
Omnivores generally have a medium length intestine that is 1.3-4x their body length.
So which one was which?
Lamprologus callipterus is the carnivore.
Ophthalmotilapia heterodonta is the herbivore.
Telmatochromis bifrenatus is the omnivore.
A post on head structure in relation to feeding to come.