Worlds in Collision: Fishes of the Costa Rica-Panama Border

Author: Ron Coleman and Samantha Hilber

Our authors take you on a journey to this complex Central/South American divide, home to diverse and fascinating species that evidence the region’s geological changes over time.

When Worlds Collide

“Look! There’s an Aequidens—and there’s a Tomocichla sieboldii—together in the same river!” Here was something we had wanted to see for a long time. In this one river in the far south of western Costa Rica, the RÍo Coloradito, we could see the collision of two great cichlid faunas: the “Aequidens” group from South America, coming in contact with the cichlids of Central America. Both of these species were breeding in this river, literally side by side, oblivious to the immense geological and biological forces that had shaped their world over the past several million years or so, making this possible.

Complex Geology

As many people are aware, North America and South America were not always positioned the way that they now appear on the globe. Plate tectonics have literally floated these massive chunks of continent around like slow-moving icebergs over the past quarter billion years or so.

While the general patterns are now well known (e.g., South America was once attached to Africa while North America was once attached to Europe), the minor details at the edges of these giants are much less clear. It just so happens that many of the fishes that intrigue cichlid keepers live on those edges, and the complex ancestry of New World cichlids, particularly those in Central America, is buried in those details.

Continental Drift

The idea of continental drift is not that old, and this is important to keep in mind. When workers like Regan and others first described and summarized the fauna of Central America, they had in their minds the idea that Central America had been there, as is, for a very long time. To suggest to them that the land and the rivers on it had literally appeared a scant 15 million years ago would have been peculiar, to say the least. And yet, we now know this to be the case.

It was only in 1912 that Alfred Wegener proposed continental drift, and in fact, it wasn’t until the 1960s that it had become widely accepted. Some Websites (e.g., now plot the movements of the major continental plates around the globe in captivating animations that chart the paths of these giants—but what about the crumbs at the edges of the plates? Costa Rica and Panama are just such places, sitting on the western edge of the Caribbean Plate, with the North American Plate to the north, the South American Plate to the southeast, the Cocos Plate to the southwest, and the Nazca Plate to the south. With so many heavyweights coming in close contact, one would think that the geological history of this region is complex—and it is.

New Bodies of Water Emerge

Coates and Obando (1996) give a detailed account of the drama that has unfolded in this part of the world. In essence, here is the storyline: as two large continental masses (North and South America) moved together, the western edge of the Caribbean Plate, lying on the eastern edge of the junction of the Americas, got rammed from the southwest by a fast moving chunk of rock called the Cocos Ridge (part of the Cocos Plate). As this plate subducted (moved under) the western edge of the Caribbean Plate (a region known as the Chorotega block of the Costa Rica-Panama microplate), land was thrust upward out of a shallow sea.

Combined with the worldwide lowering of sea levels, a ridge of volcanic mountains rose above the sea roughly 16 million years ago. At first these were just a chain of islands between North and South America, but eventually they rose enough to create a complete barrier, cutting off the Pacific Ocean from the Caribbean Sea. These two bodies of water have since evolved remarkably distinct properties (temperature, salinity, etc.) and the parallels and distinctions among the fishes and invertebrates between the two sides of the Central American Isthmus have been the subject of study for over 100 years.

The final closing of the barrier was a mere 2 million years ago, yet the consequences for most terrestrial fauna have been profound. Huge migrations of mammals moved north or south along the isthmus. For example, this is why we have opossums, a South American group, in North America. But what about the cichlids?

The Cichlids of the North

As one moves south through Mexico toward Costa Rica, we see large-scale changes in the cichlid fauna. What is obvious to many is the fact that cichlids don’t extend far into the southern United States. On the eastern end of the United States-Mexico border, the genus Herichthys pushes up into Texas and the Rio Grande system, represented by the Texas cichlid H. cyanoguttatus. You can see this fish in the states if you go to places like the San Marcos River or San Antonio. These are not actually native; they were transplanted decades ago from the Rio Grande or other locations that are now well established. Texas cichlids have also moved into the area around New Orleans in Lake Pontchartrain.

The Herichthys Group

The Herichthys group is well represented in Mexico but fades as one heads further south. The large “Theraps” types, such as Paratheraps synspillum, are common in southern Mexico. In this area, the small cichlids are composed of various Thorichthys species like the firemouth Thorichthys meeki.

The Archocentrus Group

As we move south from Mexico and Guatemala the Thorichthys disappear and are replaced by another highly successful group of small cichlids, the “Archocentrus” types, which include the convict cichlid “Archocentrus” nigrofasciatus. There is substantial discussion right now about the naming and systematics of this group because they cover a diverse range of habitats from Guatemala on down through Nicaragua, Costa Rica, and Panama. Exactly what a species is and just how different populations have to be before they are designated a separate species are key issues in these debates. River faunas naturally exhibit clinal variation (meaning that individuals from one part of a river do not look identical to individuals from another part; the differences becoming greater the further apart we go) making the designation of species tricky and possibly subjective.

The Astatheros Group

The other dominant group of cichlids in Costa Rica include the “Astatheros” group, with such species as Amphilophus (Astatheros) alfari, A. bussingi, and A. rostratus.

Other Groups

Nicaragua and northern Costa Rica have a few intriguing cichlids that do not clearly relate to other groups, such as Hypsophrys nicaraguensis and Neetroplus nematopus (now possibly combined in the same genus Hypsophrys).


There is also a small group that has intrigued us for many years, which includes Tomocichla tuba, T. sieboldii, and the more recently discovered T. asfraci. T. tuba extends up to eastern Nicaragua into the “mosquito coast,” where it has received very little study (the name of the region tells you why). It occurs throughout eastern Costa Rica down to the northeastern tip of Panama. It is replaced by T. asfraci, a beautiful orange and black fish in northeastern Panama. T. tuba does not occur to the west of the spine of mountains running roughly north-south, which divide Costa Rica into east and west.

On the west of those mountains, and specifically in the southwest, we find T. sieboldii, an enigmatic fish that superficially resembles a small tuba, complete with a distinct breeding mask—but it lacks the specialized breeding style that distinguishes the tuba species. T. tuba spawns in the open, laying extraordinarily large eggs, which hatch into large wrigglers that transform into large, bumblebee-colored fry capable of swimming in the fast-moving waters of eastern Costa Rica, unmatched by any other cichlid. These fish have been the focus of our investigations for many years.

T. sieboldii do not lay large eggs. Instead they lay moderately sized eggs in caves, like so many other Central American cichlids, and the fry, though wonderful as all cichlid fry are, are not capable of the world-champion swimming status attained by tuba fry, nor do they sport the distinctive vertical bands of the tuba fry (though the fry of T. asfraci do have this distinctive coloration).

Predatory Cichlids

The predatory cichlids are also puzzling. In much of Central America, the top of the fish food chain is held by the genus Parachromis. Fish like the wolf cichlid Parachromis dovii, at around 28 inches total length and with a toothy mouth that can swallow other fish in one gulp, hold top position in northern Costa Rica and Nicaragua. The jaguar cichlid (P. managuensis) and its relatives, like P. motaguensis, play similar roles as one moves north.

Southwestern Costa Rica is a bit of a puzzle. There are no Parachromis species in this area. This is not because there are no fish to eat—in fact, the abundance of fish in these rivers seems much higher than any rivers we have encountered in the north and east. We have seen schools of literally hundreds and possibly thousands of tetras in tiny rivers and creeks in the southwest. In fact, we had great difficulty taking photographs of fishes not because the water was murky (like in the north) but because almost inevitably a tetra would dart through the frame just as we were shooting a photograph. We must have taken hundreds of photos of blurry tetra tails (thank goodness for digital cameras and the ability to erase unwanted images). Worse yet, the tetras seemed to delight in wedging themselves between my mask and the viewfinder of the camera! Who or what eats all of these fishes?

And so, we have been drawn from our typical study sites in the northeast of Costa Rica, centered around La Selva Biological Station near Puerto Viejo de SarapiquÍ, to push ever further south and west in Costa Rica to see what was going on with the cichlids of that region.

Cichlids of the South

In a previous article, we described our encounters with T. sieboldii and “Archocentrus” sajica. A. sajica is the equivalent of the convict cichlid, yet is found only in southwest Costa Rica. We also met Amphilophus diquis, a cousin to the “Archocentros” alfari we knew up north. Last year, we pushed to the very southwest of Costa Rica to a river called the RÍo Coloradito, just a few miles from the Panama border. This region has a truly frontier feeling, which is not without its problems (see below).

Unlike the cool, forested rivers and streams of the northeast, many of these rivers are slower-moving and exposed (the forest was cut down in recent decades to make way for agriculture), making the water sometimes quite warm (over 80°F), a far cry from the 72°F common in the northeast. As you get near the coast, particularly in rivers that have a very direct connection to the ocean, the reality of large crocodiles becomes significant. These are serious crocs—the kind that eat people and deter even avid cichlid biologists.

The RÍo Coloradito

After having looked at many rivers using our typical sampling scheme—namely to drive, find a promising river of the right size (too small and there are no fish, too large and there are big crocs) under a highway bridge, jump in and look around, take data using a GPS and other gadgets, then drive on—we found one river that is almost magical: the RÍo Coloradito. In this river, we see not only the last vestiges of the cichlids from the north, including Amphilophus diquis and the most beautiful T. sieboldii I have ever seen, but we also see the cichlids from South America poking up into their northern-most extension.

Aequidens coeruleopunctatus

Aequidens coeruleopunctatus lives and breeds in this river. This particular species is an elongate version of one of the mainstays of New World cichlid hobbyists, namely A. pulcher, the blue acara. It has similar spangling on the head and the slightly bent black line along the flank. Interestingly, other major groups of South American cichlids such as the pike cichlids (Crenicichla spp.) or the tiny Apistogramma species have not journeyed north. The eartheaters (geophagines) only make it as far as Panama, as with Geophagus crassilabris, but this species does not get up near the Costa Rican border.

OtherRÍo Coloradito Groups

The cichlids are not the only group that tells you that something has changed. In this river and others in the region, you see suckermouth catfishes, a hallmark group from South America. Although they have been introduced elsewhere, they are an enormous South American group of fishes that just pushes up into Panama and this small part of Costa Rica. There are catfishes elsewhere in Costa Rica, but they are the night-active Rhamdia types that you seldom see, and frankly, they are not all that attractive. The suckermouths Hypostomus panamensis, the original plecostomus, are present in large numbers in the south.

Other catfishes are also here: Pimelodella chagresi cats are sometimes seen in schools of hundreds in places like the RÍo Olla Uno. It was a real treat to see Rineloricaria uracantha, one of the elongate twig-like catfishes, another group from South America.

Danger Awaits

Discovery has its price. As mentioned previously, the southwest of Costa Rica is a frontier region and our nonchalant, park-at-the-side-of-the-road-and-snorkel style has its risks in a region marked by strong contrasts between the rich and poor. Twice we have had our vehicle broken into. In the worst instance, on a trip when I (Ron) was working alone, the thief smashed the back window with a rock and then grabbed what he could, including my passport, laptop computer, digital camera—and my pants! It was only because I was so enthralled with watching the fish while snorkeling in the river below that I did not notice the commotion above.

As I returned to my car, I was greeted by two policemen. I thought that they wanted to give me a ticket for parking on the side of the highway—which would have been really strange given that people park pretty much anywhere, anytime in Costa Rica—but that would have been so much better than the next few days of trying to get a new passport, returning a damaged rental car, etc. Fortunately (and why we keep going back to this country), there are so many wonderful people in Costa Rica who are willing to help.

A local person, Nancy Campos—who just so happened to have spent some time in our hometown of Sacramento, California—was so bothered that I had been robbed in her country near her little town that she helped me with the police. Picture a tourist wearing snorkeling gear who spoke very little Spanish, trying to explain why he was watching little fish under a bridge while his car was being broken into, and is due to fly home in a couple of days. But in the end, she actually managed to get almost all of my stuff returned to me, including my passport, computer, and camera. I am forever indebted to Nancy and the wonderful people of Costa Rica. I am not so thrilled about the thief, but you have to wonder about someone who would take their own picture on a stolen camera!

Such is life and such things could happen anywhere, but it is only in very few places where you can see the consequences of large-scale processes like plate tectonics, writ in small print in the nest of an Aequidens coeruleopunctatus positioned inches from the nest of a Tomocichla sieboldii. This is truly where north meets south.


Coates, A. G. and J. A. Obando. 1996. “The geologic evolution of the Central American isthmus.” Evolution and Environment in Tropical America. Eds. J. B. C. Jackson, A. F. Budd, and A. G. Coates. University of Chicago Press, Chicago. pp. 21–56.

Fuentes, G. N. and R. C. Cashner. 2002. “Rio Grande cichlid established in the Lake Pontchartrain Drainage, Louisiana.” The Southwestern Naturalist 47:456–459.

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