This weekend some friends and I had an opportunity to look at some of the fish species in a local creek. It was a great time canoeing, seining, and relaxing; and we luckily saw plenty of fish representing many species.
You can get a lot of information about the ecology of animals based on their body type. For fish this is especially true, particularly in the orientation of their mouth. Take for example a fish like this Banded Darter (Etheostoma zonale):
Notice the mouth is on the lower half of the head. This is a pretty good indication that this fish lives on the bottom. Darters do in fact live on the bottom of creeks and streams where they move between and under rocks looking for insect larvae to eat.
Its not the greatest picture, but this next fish is known as a Blackstripe Topminnow (Fundulus notatus):
Notice the upturned mouth. As its name implies, these fish live along the surface where they feed on other types of insect larvae.
An upturned mouth wouldn’t be very useful for finding food on the bottom, just as a downturned mouth wouldn’t be useful for finding food on the surface. In response to the specific foods fish eat and the habitats they live in, fish have evolved a great diversity of mouth types and orientations. This specialization can tell you a great deal about the ecology and behavior of particular fish.
The Ligocki Lab 