EPOW - Ecology Picture of the Week

Explanation:   Has the (aquatic) world gone mad?  Is this catfish swimming upside down?  Well, yes.   

Like most catfish, this species has whiskers and a mouth positioned ventrally (on the bottom of the body) by which to bottom-feed on small organisms and detritus.  But this species has the interesting habit of swimming upside down along the top of the water column, feeding on the "bottom" of the water surface for floating organisms and food particles.  

We are at the amazing Stewart Grant Tropical Fish Farm in Senga Bay, Lake Malawi, in the country of Malawi in southern Africa.  Most of the fish bred here are cichlids, small lake fish that inhabit Lake Malawi.  Cichlids, and our catfish friend here, are one of the most striking examples of adaptive radiation ... the evolution of many and unique species from one or only a few original species.  There are something on the order of a thousand cichlid species in Lake Malawi.  And most or all are endemic, found nowhere else.  

This cichlid farm was created to hold or breed 250 species of cichlids and catfish, as a sustainable source for aquarium fish sold commercially throughout the world. 

So, again, exactly why is this Upside-Down Catfish swimming upside down?  With over 1,000 species to compete with, perhaps this peculiar feeding adaptation helps this species take advantage of an otherwise unexploited food resource.  To be a different species, each must distinguish itself somehow, behaviorally and reproductively.  

In the above photos, note the cryptic coloration pattern on the dorsum (the back).  The dark splotches on the lighter background may help "hide" the fish and break up its body outline when viewed from below, as it feeds upside down on the surface.

Some other interesting facts of members of this upside-down family:  All species of this catfish family have three pairs of barbels and most are scaleless except for the head.  Most have small spines that can inflict wounds.  Many species squeak when alarmed, and are called Squeakers in southern Africa.  And a few species can emit small electrical signals (probably used to detect live prey).   

Holding tanks for fish.
Ciclid Farm, Senga Bay, Lake Malawi
(click images for larger versions)

Addendum:  And no, the Upside-Down Catfish doesn't swim upside down because it's in the southern hemisphere ... ;^)