|Cichlids in the San Marcos River, Texas|
The Texas Cichlid
A texas cichlid in an aquarium
In 1987, I travelled to San Marcos, Texas, to study the Texas cichlid, Herichthys cyanoguttaum. The Texas cichlid isn't actually native to this river, in fact, it was only found natively along the extreme southern edge of Texas, in the Rio Grande. But, it was introduced a long time ago to the San Marcos along with a number of other localities in Texas. Normally, Texas waters get too cold in winter for cichlids to survive. However, the San Marcos, like other rivers in the area, arises from a hot spring and maintains a constant temperature year round of approximately 72oF.
The San Marcos River
Cichlids versus Sunfish
Redbreast sunfish, Lepomis auritus
The rivers in this area are interesting not only because they are one of the few places in the United States to see cichlids in the wild (the other good spot being Florida), but also because in this river you can see the interactions between cichlids and sunfish.
Sunfish (Family Centrarchidae) are one of the dominant families of fishes in North America and include the sunfish, like bluegill and redear, and the basses, including smallmouth, largemouth, rock bass and their various relatives.
The intriguing thing is that sunfishes have spread down throughout North America but into Mexico. Conversely, cichlids have spread up Central America into Mexico, but not into the United States. This is not a political issue. It is a fascinating biological issue.
The simplest explanation has to do with temperature tolerance. Sunfish are much more able to withstand the cold winters of North America, whereas cichlids cannot. But why then haven't sunfish spread down into Mexico? Besides which, in my opinion, as an evolutionary biologist, temperature tolerance is a relatively simple thing to evolve and it seems unlikely that fish as adaptable as sunfish couldn't evolve the ability to live in warmer waters or equally, that cichlids couldn't evolve the ability to live in colder waters. For example, there are cichlids in southern South America that live in very temperate waters.
A texas cichlid in the wild
Cichlids and sunfish are not closely related, other than both families being in the Perciformes. But, as many people have noted, they do have a certain resemblence to each other. Both groups are noted for extensive parental care, biparental in the Central American cichlids and solitary male parental care in the sunfish. Both are "intelligent" fish, meaning they take in a great deal of information, process it and adjust their behaviour accordingly.
I suspect that the interface between these two major families is no accident. The question is: is the exact location of the interface a chance event or is there more to it than that? Unfortunately, I have yet to prove my ideas so you will have to stay tuned.
Recently, the Texas cichlid has started to gain a foothold in the waters around New Orleans, Louisiana. For the latest news, see this news release by Dr. Robert Cashner