How did the Amazon stingrays become stenohaline?

[ewurm]

I was looking at this presentation http://nersp.nerdc.ufl.edu/~pmpie/EPAPoster.pdf and was wondering why and how the Amazonian stingrays became intolerant of higher salinity. Most of the research I have found suggests that a formative tectonic event occurred which landlocked the rays into lakes that were formed, and over a very long time the salinity of these lakes was reduced to it's current state (about 10 million years by some estimates). The stingrays then adapted to their increasingly desalinized environment. The presentation above shows that the gill structure of the P. Laticeps makes it a stenohaline organism as opposed to the Dasyatis Sabina which is a euryhaline fish. That would mean that the rays evolved not only to be tolerant of freshwater, but completely intolerant of saltwater (the apparent maximum salinity they can withstand is 15 ppt and seawater is 35 ppt (Ronald Oldfield, Michigan Dept of Zoology.)

Has anyone seen any other theories how stingrays ended up in the Amazon? I would say the theory has merit, but I haven't really seen anything from a geological perspective that suggests a truly massive tectonic event that would close off an estuary or river, only the theory that it took place. Were they there all along? Did this tectonic event occur? Or did Chuck Norris put them there?

 

[FireMedic]

I support the Theory of Chuck:
There is no evolution, only species Chuck Norris has allowed to live.

 

[Big Daves Rays]

I support the Theory of Chuck:
There is no evolution, only species Chuck Norris has allowed to live.

Good theory....... I love chuch norris jokes.! another theory is that they didn't evolve, they were just made that way.

 

[HarlanAshmore]

maybe the rays just swam up the amazon river

 

[amazongirl]

The thinking is that when the Andes Mountains were built they became trapped in freshwater. There are many other fish found in the Amazon River that are typically saltwater fish, not just rays.

 

[rayman]

There are several theories wherefrom, when and of which ancestor the freshwater stingrays origin. I have summarized some possibilities that were object of discussion in the past in a really confusing map:

arrows:
white = ancestor lived already in freshwater
blue = ancestor came from the Atlantic Ocean
red = ancestor came from Pacific Ocean into pre-amazon before the uplift of the Andes
yellow = several ancestors

top right possible sister group

bottom left different possible timeframes for the incursion into freshwater

There is no clear picture on this topic and the research is still going on.
Probably they form a monophyletic group with a marine ancestor.

A great summary is what Daniel R. Brooks wrote in his paper: "Neotropical freshwater stingrays and their parasites: a tale of an ocean and a river long ago":

"We now think, that it is ancient in origin and that it is derived from a source that is no longer accessible to the Amazon or other major river systems of eastern South America. Consequently, we can surmise that potamotrygonids are a truly unique and irreplaceable component of Neotropical biodiversity and are important symbols of the fact that what we do not yet know about tropical biodiversity is overhemingly greater than what we do know."

Some sources to this topic:

Brooks, D. R. 1995. Neotropical freshwater stingrays and their parasites: a tale of an ocean and a river long ago.
In: Oetinger, M. I. and G.D. Zorzi (eds.). The Biology of Freshwater Elasmobranch. Journal of Aquariculture and Aquatic Sciences 7:52-61.

Dingerkus, G. 1995. Relationships of Potamotrygon stingrays (Chondrichthyes: Batiformes: Myliobatidae. Journal Aquariculture Aquatic Sciences 7: 32- 37.

Lovejoy, N.R., 1997. Stingrays, parasites, and neotropical biogeography: a closer look at Brooks et al.‘s hypothesis concerning the origins of neotropical freshwater rays (Potamotrygonidae).
Systematic Biology 46: 218-230.

Marques, F.P.L., 2000. Evolution of neotropical freshwater stingrays and their parasites: taking into account space and time.
University of Toronto (Canada), 325pp

 

[Miles]

I wonder if seasonal flooding played a part over a long period of time... Marine/Brackish rays would move into flooded estuaries only to become land-locked. This is more my theory of forced hybridization, but maybe over a long period of time this would play a roll in the evolution of the species.

 

[ewurm]

Thanks for the graph Rayman, I'll save that if you don't mind.

The thinking is that when the Andes Mountains were built they became trapped in freshwater. There are many other fish found in the Amazon River that are typically saltwater fish, not just rays.

That was noted as well. Apparently there are quite a few species of molluscs in the region, and the endangered freshwater dolphins.

 

[ewurm]

Was reading this article on anatomy differences and comparative analysis of the relationships of the freshwater rays and the marine species, and the intro details the two main theories on how stingrays evolved in the Amazon. The body of the article makes for a difficult read unless you have a degree in Biology and the vocabulary that goes with it. Link to the article and the portions I found relevant:

http://www.utsc.utoronto.ca/~lovejoy/pdfpubs/Lovejoy%20Zool%20J%20Linn%20Soc%20%2796.pdf

The neotropical freshwater family Potamotrygonidae appears to be the only stingray group that has
radiated in a non-marine environment. To assess the affinities of potamotrygonids to other rays, a
phylogenetic analysis was undertaken using 39 morphological characters from 18 stingray groups. The
single tree produced (CI = 0.80, RI = 0.88) suggests that neotropical freshwater rays are a
monophyletic group, and that within Potamotrygonidae, Paratrygon is basal to a clade composed of
Plesiotrygon and Potamotrygon. The sister group to potamotrygonids was determined to be amphi-American
Himantura — these taxa share synapomorphies of the ventral mandibular musculature and the
hyomandibular/mandibular articulation. The topology suggests that potamotrygonids are derived from
a freshwater-invading ancestor that was distributed along the northern coast of South America (Pacific
and Caribbean) prior to the emergence of the isthmus of Panama. This hypothesis conflicts with
parasite-based biogeographic scenarios of a strictly Pacific origin for potamotrygonids. General
systematic results concerning urolophids, dasyatids, and pelagic myliobatoid stingrays are also
discussed.

In red are the two principle theories associated with the evoultion of freshwater stingrays in the Amazon. The marine incursion theory is hard for me to grasp, because the idea of rays swimming up the Amazon and simply never leaving doesn't seem to fit an evolution model. Animals don't evolve over the course of a few years, they do it over the course of hundreds of thousands to millions of years when they face adverse conditions that force them to adapt and/or evolve, or when they have an advantage in evolving to benefit the species. I could see a euryhaline species of ray frequenting a freshwater source if it's normal food supply became more scarce. The continued scarcity of food would cause it to frequent freshwater continuously, and perhaps it adapted simply because the conditions or food source were more favorable in the freshwater environment than in their traditional saltwater environment. Once it became established in the freshwater environment it could have evolved into a stenohaline species. This leads me to wonder if the D. Sabina living in freshwater in Florida may eventually evolve to be more suited for the environment that they are living in. We'll never know.

On the other hand, the theory that rays evolved because of a tectonic event that formed the Andes mountains would make for a very reasonable explanation. The formation of the Andes causes the euryhaline invaders to be cut off in saltwater lakes which slowly turn to freshwater over time. I think this idea is at least from a layman's standpoint easier to see happening. A very slow change in salinity over a very long time would be easy for the rays to adapt and evolve as necessary.

The family Potamotrygonidae consists of exclusively freshwater stingrays distributed
throughout most of the major river systems of South America. Although marine
sharks and stingrays invade the large rivers of most continents (e.g. Compagno &
Roberts, 1982, 1984; Roberts & Karnasuta, 1987; Snelson, Williams-Hooper &
Schmid, 1988; Thorson, 1983), fully freshwater rays appear to have only diversified
in the Neotropics. The potamotrygonid family comprises some 20 species divided
into three genera (two of which are monotypic), and demonstrates a considerable
array of morphological variability (Rosa, 1985, 1990; Rosa, Castello & Thorson,
1987). All potamotrygonids possess a unique (among elasmobranchs) suite of
characteristics related to their obligate freshwater lifestyle. These include: the
inability to retain high blood concentrations of urea to counter water loss caused by
high salinity (Thorson, 1970; Thorson, Cowan & Watson, 1967); the degeneracy of
the rectal gland, a salt excretory organ (Thorson, Wooton & Georgi, 1978); and
modifications of the electroreceptive Ampullae of Lorenzini for freshwater operation
(Raschi & Mackanos, 1989).
The distribution of stingrays in freshwaters of South American is paralleled by
several other families of predominantly marine fishes, including anchovies (Engraulidae),
herrings (Clupeidae), needlefishes (Belonidae), flatfishes (Achiridae or Soleidae),
croakers (Sciaenidae), etc. (G´ery, 1969; Fink & Fink, 1979). Roberts (1972) has
tabulated some 14 families of mainly marine fishes with freshwater neotropical
representatives. In contrast, the Congo basin has only four. This difference in
diversity has been attributed to the accessibility of the two river basins — while the
Congo is largely above sea level and defended by rapids, the Amazon is low-lying
and relatively open to marine invaders (Roberts, 1972). Thus, marine taxa (including
stingrays) are thought to have independently invaded South America through the
mouth of the Amazon. Brooks, Thorson & Mayes (1981), however, provide an
alternative perspective. Based on studies of parasite systematics, they suggest that the
South American stingray parasites and hosts originated from the Pacific, before the
uplift of the Andes. This hypothesis, although derived from stingray parasites, has not
been tested using stingray phylogeny, largely because the affinities of potamotrygonids
to marine rays are poorly known.

 

[JD7.62]

My personal opinion is that the rays are of Pacific origin. If they were Atlantic origin then why are there many rivers that flow directly into the Atlantic (I know they all eventually flow into the Atlantic) that lack any rays? Rays seem to be more concentratd in the western parts of South American FAR from the Atlantic. Plus the Andes were formed very quickly.

Another personal opinion I have is, IF they swam UP the Amazon why arent there more FW rays in the world that have swam up other rivers?

Too bad sharks did survive the transition to FW after they were trapped too....