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[Miles]

Preliminary data on the feeding habits of the freshwater stingrays Potamotrygon falkneri and Potamotrygon motoro (Potamotrygonidae) from the Upper Paraná River basin, Brazil

The commercial rays capture is usually done with a line with multiple hooks (a long line called "espinhel") or with a harpoon in the area of study. The latter method, used in the present work, gives a better result in terms of captured individuals by capture effort and allows the sampling of specific microhabitats. The commercial capture of stingray is only done by accident (the rays are caught in the long line when trying to eat the bait intended for other fish species) and the harpoon capture is only done when specifically asked for, since no commerce of rays for human consumption exists in the general studied area (M. J. Vilela, pers. comm.). Currently only some of the professional fishermen eat ray meat, since it does not present any commercial importance. Although it occurred more frequently in the past, even today the rays captured by professional and amateur fishermen in the general area of study usually have their stings removed, after which they are reintroduced in the river (D. Garrone Neto, pers. comm.).

A total of 25 stingrays were captured, being 15 P. motoro and 10 P. falkneri. Potamotrygon motoro specimens presented a total length between 257 and 457 mm, disk length between 168 and 265 mm, and weight between 380 and 980 g. Potamotrygon falkneri specimens presented 531 to 835 mm of total length, 254 to 445 mm of disk length, and a weight of 940 to 6550 g.
MFK NOTE:
Wild Motoro Disc Sizes- 6.6" to 10.4"
Wild Falkneri Disc Sizes-10" to 17.5"

Although the sampled specimens of P. motoro were smaller than the other species, Thorson et al. (1983) reported that this species reaches a maximum length of 600 mm (23 Inches) and 8.5 kg of weight. Although the literature about P. falkneri is scarce, a maximum length of 892 mm (35 Inches) was cited for this species (Britski et al. 1999).

The two species showed a diversified diet, a common characteristic of fluvial fish (Weatherley 1963, Lowe-McConnell 1999), ingesting 14 different food items, including Mollusca, Crustacea, aquatic Insecta and fishes, with predominance of aquatic insects both in diversity and abundance. Despite presenting high diversity, Mollusca and Crustacea were consumed in low percentages. Only one individual of each species ingested fish. The two species differed in relation to the predominant type of aquatic insect ingested: P. motoro fed mainly on Ephemeroptera (Baetidae) and Diptera (Chironomidae), whereas P. falkneri fed mainly on Trichoptera (Odontoceridae) and Hemiptera (Naucoridae).

According to Achenbach & Achenbach (1976), the feeding habits of Potamotrygonidae rays change during ontogenetic development, with young rays feeding on small mollusks, crustaceans, and insect larvae, while adults feed on fishes and crabs. The ingestion of fishes of the family Loricariidae was also reported by this author. For P. motoro, it seems that the majority of captured individuals were juveniles, with the sampled individuals having 1/3 and 1/8 of the size and weight, respectively, of the largest individuals reported in the literature (data from Thorson et al. 1983). In addition, according to Achenbach & Achenbach (1976), reproductively mature individuals of P. motoro present disk sizes between 300 (11") and 350 (13") mm, larger than the disk sizes presented by the individuals collected in this study. In this way, the insectivorous diet of P. motoro may represent an ontogenetic diet variation. In the case of P. falkneri, the absence of reported data concerning the length of adults leaves this possibility open.

The relative index of importance (RII) reinforced the feeding differences between the two ray species. Potamotrygon motoro ingested mainly Ephemeroptera (aquatic insects) and P. falkneri, with a more diversified diet, ingested a high percentage of Mollusca, Hemiptera and Trichoptera. Despite these differences, P. motoro and P. falkneri showed some similarities with data from other stingray species. Potamotrygon orbigny from the Venezuelan llanos also showed preference for aquatic insects, mainly Diptera (Chironomidae) and Ephemeroptera (Lasso et al. 1996). In contrast, a carnivorous diet based on fish and shrimp was reported for Paratrygon aireba (Lasso et al. 1996).

The use of a volumetric method in the fish diet analyses through the measurement of the displacement in a graduate test tube is difficult when the items possess small size or are present in small amounts (Hyslop 1980). In these cases, the biovolume, defined as the area occupied by each food type (Esteves & Galetti Jr. 1995), can be used as an indirect measure of volume. In the present study, the advantage of the biovolume method was reinforced through the comparison between the values of the relative index of importance, calculated with the biovolume and the relative volume values.

Analyzing the diet of the species in function of the microhabitat occupied, a diet similarity among the two species was registered only in the rocky right margin microhabitat, suggesting resource partitioning. In spite of the small number of analyzed individuals, the data apparently indicate a more specialized diet for P. motoro, consuming more Ephemeroptera (Baetidae) in the three microhabitats, and a more generalized diet for P. falkneri, with a strong spatial variation. Resource partitioning, defined as any considerable difference in the use of resources among species, like differences in diet or in temporal and spatial use of the habitat (Ross 1986), constitutes an important mechanism allowing the species coexistence. When the alimentary resources are available in abundance, they can be shared among species; on the other hand, when they are scarce, there is segregation among species through ontogenetic changes in the diet and/or differences in spatial/temporal distribution (Arcifa et al. 1991). The trophic partitioning was indicated by many authors (Zaret & Rand 1971, Sabino & Castro 1990, Uieda et al. 1997, Casatti 2002) as the most important mechanism allowing the coexistence of species and can be occurring with these two syntopic ray species.



Full Article:
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1676-06032007000100027&lng=es&nrm=iso&tlng=es

[Miles]

Okay.. so alot has been brought forth to the community concerning long-term dietary needs for freshwater rays. It has been long thought that a high calorie seafood diet with lots of variety was the best thing to offer, but we are finding that it can have poor side-effects and is not as beneficial as freshwater based foods.

Why is bad you might ask? Well - Seafoods can be very high in proteins and fats, and although large adult rays are more likely to have the ability to digest and benefit from these type of diets, they can also be detrimental. Continious 'bulging' feedings of high fat seafoods can lead to fatty liver disease, which has been a hot topic now for a few months concerning rays.

Many seafoods and shellfish also contain thiaminase. Thiaminase destroys Thiamine (Vitamin B1). Goldfish & Rosie Reds (and most cyprinidae) contain a very high level of thiaminase. Thiamine deficiency can lead to a myriad of problems including neurodegeneration, wasting and death. Tissue Distribution in many living animals is dependent on Thiamine levels. Once Thiaminase is ingested, it has very long lasting effects and does not purge itself from the body. Lack of Thiamine (Vitamin B1) is the primary reason why fish would not be able to properly produce fleshy tissue.
http://www.jbc.org/cgi/reprint/196/1/289

After looking into the dietary pattern of potamotrygon in the wild (http://www.monsterfishkeepers.com/fo...+stingray+diet) you can see that they are very much adaptive to whatever their habitat provides them. Depending on seasonality, their diets can change depends on the food sources available. They tend to mainly consume aquatic insects, insect larvae, snails and shrimp. As they grow older, they tend to become a bit more piscivorous and predate on small fish and larger crustacea. I am guessing they also feed on aquatic type worms, similiar to that of the blackworms we often offer them in domestic aquaria. Even vegetation has been found in their diet in the wild.

Side Note: Some species of rays are pretty much insectivorous their whole life - as rays such as Reticulata and Orbignyi tend to have very small jaw/mouth structures that are not designed for crunching or biting, but more-so for grinding and mashing. While other species, like Motoro or Castexi, have very powerful jaw structures that will consume hard shelled crustaceas and eventually small fish when they are larger. Then the other genus, paratrygon, tends to be a complete ambush predator, consuming mainly live fish.

So what about pelletized food? Hikari even put a stingray on the package! Well - Hikari didn't do much research into the dietary needs of rays and if you actually take the time to do a comparison, you will find the hikari massivore pellets are very high in caloric intake and proteins. Likely, these are very hard for a ray to digest and could also lead to other long-term problems if overly fed as a daily staple diet. Obviously, red tail catfish and freshwater stingrays do not eat the same thing - as RTC will predate on other large fish and even mammals, while freshwater rays are more insectivorous than anything.

Heres another good MFK thread on these topics; Overfeeding is bad... mmmk?
http://www.monsterfishkeepers.com/fo...+stingray+diet

One thing to also consider is that rays are cold-blooded like reptiles and control their metabolism and digestion by moving in and out of warmer areas. We keep our rays in warm water in captivity, but I think the main reason this has been a recommendation for ray husbandry is the fact that the fatty high protein seafoods we offer might not be digestible without the aid of the higher temperatures. Obviously, small insects and worms will be easier to digest than the prior. However, a ray in the wild that eats a fatty/high protein meal would have the ability to move into the shallow and bask, allowing digestion to be aided.

Perhaps someone was right in recommending rays be kept at 76-78 degrees, rather than the 80-84 most people keep them at. This could have more benefits on the digestion, metabolism, and nutrition intake than the warmer temps. Andersp90 will chime in hopefully, as he has interesting thoughts on this situation..

I have also heard growing a fish too fast can lead to long-term problems. "Power-Growing" is not always beneficial. It seems like our rays grow fairly quickly in captivity, and we have captive adults larger then any wild rays I have seen(BigMama). Perhaps it is more beneficial to grow them slower with a more controlled diet and temperature, then gorging them and turning the temperature way up to get them to eat more and grow more? Perhaps these high temps and high protein diets might show short-term benefits, but they are actually detrimental in the grand scheme of things. I look at high temps/high protein foods more of a conditioning agent/weight gainer/destressing tool rather then a long-term captive husbandry practice.

Like others have noted - Rays need some source of calcium through spines/guts/crustacea, etc.. they also need many other nutritional sources that seafood diets and/or single source diets can not provide. (ie: My rays only eat krill and blackworms - and I am worried they are not getting nearly enough varied nutrition) Mazuri makes a shark/ray vitamin supplement - but the trick is getting a smaller FW ray to eat it. A larger Motoro or Castexi, not a problem as you can probably stuff the pills within the food source. My small retics on the other hand, no chance. So how else would you provide this type of varied nutrition? Lets look..
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So - What's the other options of food sources?

Earthworms/Red Wigglers - Great varied nutrition as they consume nutrients and minerals from the dirt they live in. Nutrition could be variable based on the quality care of where the worms came from. Also has been some fear of parasites, but highly unlikely with no scientific proof of the theory. I have heard worms are super high in specific vitamins, such as vitamin K, in which other food sources have a very hard time providing.

Blackworms - I have heard these can lead to fatty liver disease also. The rays sure do love them, and they are great for bulking up. They naturally live in freshwater, and carried the same types of varied nutrition that other worms provide. A great food source, but overfeeding could still lead to problems.

Freshwater Fillets - They are suppose to contain less thiaminase, and also be more digestion friendly for freshwater rays. Keep in mind freshwater rays do not consume seafood in the wild so their digestion system is not as capable when it comes to breaking down those types of proteins and fats.
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So what else is there?

Any ideas for insect-based food cultures?

Maybe we should start looking high and low for a food source that reproduces rapidly but is more nutritionally effecient for the rays?

What is that tiny little water-flea looking bug that sticks to the glass, many people use it as a feeder type ?

What about small live fish, such as gambusia or guppies. They would have the highest level of nutrition as being alive provides the utmost freshness.. and with these, you get the spines/guts nutrition, too!

How come no one in the USA commercial produces live blood worms? I have only seen live bloodworms available in Europe and Asia.

Freshwater crayfish/crabs tend to scare people, because of parasite issues. Nic has a great QT method in place, but its a bit time consuming and not economically effecient. Also, only the larger aggro rays will chomp these crunchy morsels down.

What about a source of freshwater grass shrimp? From what I understand, most of the 'ghost shrimp' is not ghost shrimp at all, but rather Florida Grass Shrimp.. Perhaps someone in the stingray community can find a harvest locale or supplier of these FW grass shrimp. They have benefits such as gut-loading, and this is much like what a ray would eat in the wild. My old LFS owner claims he could get grass shrimp for a penny a peice.. a 1000 shrimp for 10 bucks? Sold. if someone were to start producing these on a major scale and selling them to the stingray community as a source of live food like black worms - it could be a hit!

Maybe someone (Mike) should partner with a fish food company and see about getting a stingray specific food on the market.. A soft-sinking pellet of some sorts, designed for the nutrition needs of rays? I know they have the Mazuri Gel and Mazuri Vits - but that is more Zoo/Aquarium applicable when the home hobbyist would need something more user-friendly. Would be awesome if you could get a company to produce a product for you, but then allow you to label, package, and sell it on your own. -I will take 2 bottles of H20CustomAquatic Ray Pellets please -


So what else is there? Some sort of freshwater insect? Who has ideas?

I have a feeling their might be an 'untapped' resource of a very good stingray food out there - we just need to find it.. but where?