Having been deeply fascinated with the Pterophyllum genus for many years, I have found myself waist deep in research, analysing data, images and footage and even taking trips into the jungle to better understand the majestic freshwater Angelfish and the factors which influence their distribution and taxonomy. One area of recently increasing interest to me is the Rio Negro in Brasil. I have always been curious about the distribution of Pterophyllum leopoldi and the existence of disjunct populations along both the Rio Negro and Rio Solimões and in the last few years I have dedicated countless hours to uncover more information in an attempt to discover why exactly this species has such an unusually separate distribution and it turns out that the situation is far more complex than I imagined!
In late 2007, my dear friend Marcelo Fernandez returned from a trip to the Rio Negro region where he had the opportunity to collect Heckel Discus (Symphysodon discus) and various other species. His trip took him along a tributary of the Negro River known as the Rio Demini and among many species collected he found an Angelfish variant which was unlike most others. Within the Pterophyllum genus it is only P. leopoldi which has eight longitudinal bars across its body; all other species have just seven. But unusually, a large majority of the angelfish collected by Fernandez along the Demini River displayed eight clear bars despite appearing to be related and most closely identifiable to Pterophyllum scalare. Since this anomalous population very closely resembled P. scalare, Fernandez made no connection and assumed he had come across the more common congener.
Much later, at the beginning of 2014, my friend and well known Amazon explorer Heiko Bleher released a publication containing his unofficial summary of the Pterophyllum genus based on collections made during his own expeditions throughout almost the entire Amazon basin. Within this publication he also clearly identified an 8-barred angelfish from one juvenile individual found along the Rio Demini and this really sparked my interest again. From Blehers distribution map it appeared that the ‘new’ eight-barred angelfish was sandwiched between populations of P. leopoldi known from further upriver in the Rio Marauiá, where they live in sympatry with P. scalare (argued to be P. cf. altum by some) and the other population of P. leopoldi found in the lakes of the lower Solimões River in the region upstream of Manacupuru. Observing the distribution of both species on paper aroused a great deal of curiosity in my mind and although the lower Solimões population likely have no recent significant influence on the presence and distribution of the 8-barred angelfish in the Negro, my next discovery would make for some interesting historical contemplation.
Towards the end of 2014 a video was published online showing some sub-aquatic footage in the Paraná Ataú, which is a collection of annually flooding permanent interconnected waterways adjacent to the Rio Negro and situated just upstream from the mouth of the Rio Preto and its tributary, the Rio Padauari (both white water rivers). This footage was taken by some acquaintances of mine called Charly and Seder. Seder operates an ecotourism company out of Barcelos, along the Rio Negro and Charly, a fish importation business in France. In this very clear footage, I noticed quickly that the Pterophyllum species they encountered all displayed eight longitudinal bars and after some detailed conversation with Charly about the exact location I was able to determine that this 8-barred variant can be found in at very least the area between the Rio Demini and the Paraná Ataú; even though the information associated with his video upload erroneously indicated that the collection area was in the Rio Padauari until recently.
Naturally, the first question that comes to mind after looking at all this information is “could this be some form of natural hybrid?” and it is not a silly question, since there are instances of different Symphysodon species coming together during high water and forming hybridised populations such as those in the Lago Amanã and several other places. My issue with this is that as mentioned, P. leopoldi and P. scalare were found living in sympatry by S. Willis and team in 2010 just a short distance upriver with no evidence of hybridisation and so why would it happen in one river but not in a very closely associated river? Anyway, that is not to say that it could not have happened perhaps by some instance of reduced species density, possibly occurring during the expansion of a previous distribution boundary so this population could indeed be the result of hybrid speciation and only genetic analysis will reveal the truth, but there is one final factor makes things all the more puzzling.
The known distribution of the 8-barred angelfish that I have managed to determine from my research, and the populations of P. leopoldi and P. scalare living in sympatry further upriver in the Marauiá are separated by a population of angelfish consisting of only P. cf. scalare in the Rio Daraá; so the distribution of P leopoldi is interrupted prior to the beginning of the 8-Barred angelfish distribution boundary. Now whilst this possibly indicates that there have been no recent interactions between these two areas, it is not necessarily indicative that there has not been some degree of interaction between them in the past which could have resulted in this speciation.
This unusual distribution of Pterophyllum species in the Rio Negro, including the presence of the new 8-barred variant could be the result of prolonged competition between the two already taxonomically described species. I envision a situation where one species had been reduced to negligible numbers and was subsequently forced to resort to hybridisation. Since the 8-barred variant is larger in size than the average P. leopoldi it potentially has an advantage over them and since it also has superior camouflage in the form of eight bars instead of the seven compared with P. scalare, this could mean that they then went on to out-compete both of the other species in this area and become the dominant variant. This speculative theory is supported by the fact that both P. leopoldi and P. scalare occupy the same biological niche and in sympatry form part of the same biocoenoses within their biotopes.
There are many different speculations which run through my mind and by no means do I have an answer to this thought-provoking issue, but what I do know, I want to make available to the wider fish community in hope that a more concerted effort may one day be initiated to help us reveal some answers.
Bleher. H, Discusbook 04, January 2014, Aquapress Publishers, pp 72-88