Why do caudal fin shapes differ?

[Martin]

I have noticed that some families of fish such as Cyprinidae,
containing such fish as Barbs, tend to have caudal fins that are
divided into an upper and lower lobe ie, for want of a better
description, they are forked. On the other hand other families such as
Cichlidae, containing such fish as Discus, Oscar,s and Angelfish tent
to have rounded, paddle like caudal fins. The body shape also tends to
differ, with the former tending to be more streamlined and the latter
having a rounded appearance when viewed from the side.

Are these differences due to chance factors of evolution or are they
adaptions to different environmental demands made on the different
families? I've noticed that the Cyprinidae are often found in moving
water whereas the Cichlidae are often to be found in still waters. If
habitat is the key, then I can understand why the Cyprinidae are more
streamlined but I still do not understand why the tail needs to be
different.

Martin

[Blue Gourami]

(Martin) wrote in message >...
> ...some families of fish... tend to have caudal fins that are
> divided into an upper and lower lobe... On the other hand... other families > tent to have rounded, paddle like caudal fins...
>
> Are these differences due to chance factors of evolution or are they
> adaptions to different environmental demands made on the different
> families?...
>
> Martin

Fish built for high speed have a highly forked tail with a narrow
caudal peduncle. Think of the tuna, which is one of the fastest
swimmers. On the other hand, slow-swimming (except when you are trying
to catch them in a net...) fish have a rounded tail with a wide caudal
peduncle. The different degrees of "forkiness" between these two
extremes indicate adaptations to the environment, for example fish
from flowing waters need to be stronger swimmers to go against the
current.

Blue Gourami