Some more interesting reads I came across.. enjoy! 
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 scientific link;
http://www.scielo.br/scielo.php?scr...1676-06032007000100027&lng=es&nrm=iso&tlng=es
So - Motoros are primarily insectivorous it looks like, especially when juvenile. Hence, the love for black worms. It also looks as if Potamotrygon in general eat very few fish and carnivorous type food sources, in comparison to the aquatic insects digested in the wild... perhaps we overfeed our rays a little too much meat ?
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 scientific link;
http://www.scielo.br/scielo.php?scr...1676-06032007000100027&lng=es&nrm=iso&tlng=es
So - Motoros are primarily insectivorous it looks like, especially when juvenile. Hence, the love for black worms. It also looks as if Potamotrygon in general eat very few fish and carnivorous type food sources, in comparison to the aquatic insects digested in the wild... perhaps we overfeed our rays a little too much meat ?
