BACKGROUND
elasmobranch fishes are among the top predators in the marine environment and thus play an important role in marine ecosystems, potentially regulating, through predation, the size and dynamics of their prey populations  <cit> .
an understanding of competitive and predatory processes is thus necessary to gain insight into the role of predators in influencing niche, community and food web structure, and ultimately ecosystem dynamics  <cit> .
in this sense, the theory about resource partitioning is frequently attributed to competitive or cooperative interactions.
this approximation predicts that spatial or temporal partitioning may increase the tolerance of niche overlap, and may reduce the pressure of competition among coexisting species  <cit> .
several studies have evaluated the influence of resource partitioning in teleost fishes to explain reduction in competition potential  <cit> .
these authors reported a positive relationship between habitat partitioning and the magnitude of diet overlap or competitive interactions, when the interacting species had the same diet preferences.
furthermore, ross  <cit>  found that the first resource that is fractioned is food, followed by habitat.
despite the ecological importance of elasmobranch fishes for the marine ecosystem, resource partitioning and competitive exclusions in this taxonomic group are poorly understood.
a substantial amount of data on the diet of different elasmobranch species has been reported to date.
elasmobranch fishes are often typified as opportunistic predators, with a wide trophic spectrum that includes plankton to marine mammals.
in general, oceanic elasmobranch species feed on squid and big fishes  <cit> , whereas the coastal and benthic species feed on crustaceans, mollusks and small or juvenile fishes  <cit> .
a few species feed on other elasmobranchs, birds, reptiles or marine mammals  <cit> .
ontogenetic variation in diet is well known  <cit> , with a strong tendency to ingest larger and more mobile animals with increasing size.
research on trophic relationships among sympatric species of elasmobranchs is scarce and results vary among studies  <cit> .
varying overlap values between coexisting species, successive sizes of a same species, or even between sexes have been reported.
moreover, the effect of habitat and feeding time on the diet of sympatric species is very poorly known  <cit> .
in the eastern pacific ocean of colombia  the study of elasmobranch diet is in its infancy  <cit> .
the existing studies have only considered three of  <dig> elasmobranch species reported for the area  <cit> .
furthermore, no research has addressed trophic interactions among sympatric fish species in coastal or oceanic environments.
therefore, the object of this study was to quantify and compare the diet and trophic interactions of coastal elasmobranch species from the eastern tropical pacific of colombia, and to suggest possible mechanisms for their coexistence.
RESULTS
feeding activity
a total of  <dig> specimens of five species  were analyzed.
all specimens were captured in shallow waters .
significant differences in the bathymetric distribution of the species were found  =  <dig> , p <<  <dig> ), with zapteryx xyster and raja velezi found in narrower depth ranges and under  <dig> and  <dig> m, respectively.
dasyatis longa and rhinobatos leucorhynchus were associated with the shallowest depths .
mustelus lunulatus, d. longa and rhinobatos leucorhynchus were captured mainly during diurnal bottom trawling, while z. xyster were captured mainly during nocturnal bottom trawling.
raja velezi was captured only at night.
only r. leucorhynchus and z. xyster showed significant differences in hourly feeding activity .
we did not carry out analyses of ontogenetic shifts in diet because we did not capture individuals from all sizes intervals.
sampling data of elasmobranchs caugth in the eastern tropical pacific ocean of colombia and range size of these species.
n: number of individuals captured.
p: p-value of k-w test.
diet composition
the proportion of empty stomachs varied widely among species, with a minimum vacuity index of  <dig> % in m. lunulatus and a maximum vacuity index of  <dig> % in d. longa .
the main prey item of mustelus lunulatus were stomatopods from the family squillidae  , with squilla panamesis being the main prey item .
an additional twelve prey categories were identified in the stomachs of m. lunulatus, which feed opportunistically on shrimps, crabs and gasteropods in low percentages.
feeding activity of elasmobranch captured in the eastern tropical pacific ocean of colombia.
number of stomachs analyzed , empty stomachs , vacuity index , mean number of prey per stomach , 95% confidence interval for χn , mean weight of prey per stomach , 95% confidence interval for χw .
prey items observed in stomachs of five species of elasmobranchs captured in the eastern tropical pacific ocean of colombia.
%o is frequency of occurrence, %n is percentage by number, %w is percentage by weight
in d. longa stomatopods were the most frequent prey item  and had the highest percent by number , whereas fishes had the highest percent by weight  .
digested material was  <dig> % and vacuity index was  <dig> %.
the main prey items were squillidae  and fishes .
the diet of this species also included decapods and gastropods.
in rhinobatos leucorhynchus the most frequent , abundant  and with highest percent by weight  prey item was decapods, following by fishes  .
the main prey item was trachypenaeus sp.
.
however, stomatopods and crabs  were also found in the diet of this species.
the vacuity index was low .
only two prey items were identified in r. velezi, fishes and decapods.
fishes were the most frequent item , and also the most abundant by number  and weight , and collectively .
among the fishes, only batrachoididae could be identified .
six prey categories were identified in stomach contents of z. xyster .
fishes and penaid shrimp occurred most frequently  and were the most abundant by number , however shrimp were the most abundant by weight .
for this species decapods were the most important prey item with  <dig>  %iri.
diet breadth and overlap
use of a three-dimensional graphical representation of the diet  <cit>  facilitated the description of feeding styles of the five species studied .
the diet of m. lunulatus was specialized, with most individuals consuming squilla panamensis, although this species also feed on twelve other prey items.
in contrast, the diet of z. xyster was more heterogeneous and generalized because fishes, decapods and stomatopods were consumed by the majority of individuals.
these results were confirmed by levin's breadth index, which yielded the greatest diet breadth for z. xyster, while m. lunulatus was the most specialized predator .
levins' measure of niche breadth  and standarized bi  calculated with percentage by number  data.
n: total number of prey categories used.
following zares & rand's  <cit>  criteria, four biologically significant diet overlaps were established among the studied species .
a significantly high overlap was found between m. lunulatus and d. longa, which shared shrimps of the family squillidae.
significant overlaps were also found between rhinobatos leucorhynchus and raja velezi, and r. leucorhynchus and z. xyster, which shared penaeidae, and between raja velezi and z. xyster which shared fish and penaeidae.
the random overlap hypothesis was rejected , suggesting low overlap and low partitioning of resources  <cit> .
niche overlap between five species of elasmobranch caugth in the eastern tropical pacific ocean of colombia.
values were calculated using the pianka measure, indicating the extent to which the diet of species a overlaps with the diet of species b. significant diet overlap .
discussion
the composition of diets suggests that all elasmobranchs studied are species that feed on epibenthic invertebrates  and fish.
this result is highly associated with the source of the samples analyzed, i.e., elasmobranchs captured in bottom trawl fishery.
predators found during this activity, feed mainly on benthic infauna , epibenthic fauna , benthic prey  or demersal prey .
therefore, the results of this research are restricted to elasmobranchs that feed on prey species living on the sediment surface or inside the sediment.
however, our results contribute significantly to increase the knowledge on the feeding ecology of the other  <dig> species of fishes coexisting in the same habitat  <cit> , with similar food and space requirements.
mustelus species have been reported to feed mainly on crustaceans and fish  <cit> , although cephalopods would also be important in their diets  <cit> .
despite the large number of taxa found in the stomach contents of m. lunulatus, one species of stomatopod accounted for most of the prey consumed .
therefore, m. lunuatus in coastal waters of the colombian eastern tropical pacific have a specific dietary preference.
besides ellis et al.
<cit> , this is the only other report of feeding specialization in mustelus, a genus that has been classified as opportunistic polyphagous  <cit> .
the diet of raja velezi in the neritic zone of the eastern pacific ocean of colombia showed a low number of prey items in compared with other studies  <cit>  that found polychaetes, copepods, amphipods, myscidacea, stomatopods, cephalopods and bivalves.
however, the preference of r. velezi to feed on decapods and fishes detected in this study was similar to that reported for r. naevus  <cit> , r. clavata  <cit> , r. radiata  <cit> , and r. brachyura, r. montagui and r. eglanteria  <cit> , which also feed upon decapods or fish.
although r. velezi is a deep-water ray with a widespread distribution, our results are the first description of the diet of this species in the tropical pacific ocean.
rhinobatos leucorhynchus has been identified as a crustacean predator, with preference for penaeid shrimp  <cit> , although portunid crabs have also been reported as alternative prey  <cit> .
very little is known of the diet of dasyatis longa and zapteryx xyster and this study is the first record on the diet of these species.
similar to other benthic species dasyatis longa feed on crustacean decapods  and fishes.
the preferences for these dietary items have also been reported for dasyatis americana in the caribbean sea  <cit> .
zapteryx xyster showed the widest feeding spectrum of all elasmobranch studied.
this species included stomatopods, decapods and fishes in its diet and, was the most generalist species.
bornatowski et al.
<cit>  reported similar results for z. brevirostris from south brazil.
macpherson  <cit>  and cortés  <cit>  suggested that the relatively big body size of elasmobranch makes it easy for them to expand their feeding spectrum, preying simultaneously on the pelagic and benthic communities.
our study shows that diet breadth was reduced, and prey item distribution by number and weight were similar.
moreover, all studied elasmobranch fed on three main items: squillidae, penaeidae and fish.
this can be explained by the benthic habits and similarity of sizes and dental structure between m. lunulatus and d. longa, as well as between rhinobatos leucorhynchus, raja velezi and z. xyster.
the complete dietary overlap detected between m. lunulatus and d. longa, their similar geographical distribution in the study area, and their diurnal activity, suggest that these species could show a competitive exclusion trend for food resources  <cit> .
however, the dietary overlap would be reduced when the deep distribution is considered because m. lunulatus occupies deep waters and d. longa shallow waters.
in contrast the dietary overlap detected between rhinobatos leucorhynchus, raja velezi and z. xyster would be significant, since these species have morphological  and behavioral similarities.
the dietary overlap between rhinobatos leucorhynchus – raja velezi and r. leucorhynchus – z. xyster, based on the presence of trachypenaeus shrimp and fishes, could be compensated by differential diel feeding activity.
rhinobatos leucorhynchus feed mainly during the day whereas r. velezi and z xyster feed manly at night.
furthermore, 89% of rhinobatos whose stomachs were analyzed were caught at a deep of 15– <dig> m, whereas all raja and zapteryx specimens were caugth at a depth of  <dig> m. in contrast, the dietary overlap between r. velezi and z. xyster, species with similar feeding activity and bathymetric distribution, suggests that they share only a fraction of their feeding niche, protecting a portion as an uncontested space refuge  <cit> .
in this sense, mantis shrimp would be the feeding refuge of z. xyster.
dietary overlap between z. xyster and r. velezi was fish unidentified and decapods; however this food category was so indeterminate that the evidence of effective dietary overlap is weak.
furthermore, this wide category of prey item overestimate dietary overlap and underestimate niche breadth.
values of overlap based on simulations suggest that the species studied showed low dietary overlap .
in this study only five species of elasmobranchs were examined.
in the study area at least another  <dig> species of elasmobranchs  <cit>  and six species of fishes  <cit>  that share morphological and ecological characteristics with the studied species have been reported.
however, all these species and the elasmobranchs that we studied would conform apackaging throughout a feeding axis, and the temporal dynamics of this packaging could be determined by:  variety and productivity of available resources,  short-term and long-term environmental variability, and  niche overlap  <cit> .
therefore, if the packaging is modulated by resource availability  <cit>  then the shrimp trawl fishery in the neritic zone of eastern pacific of colombia has severely affected the natural availability of prey for elasmobranch communities  <cit> , affecting their structure.
CONCLUSIONS
shrimps  and benthic fishes were the main component of the diet of the elasmobranchs studied.
diet breadth and overlap were relatively low.
determination of food resource partitioning among the batoid species studied was not possible.
however, we identified partitions in other niche axes .
it is possible to assume that diffuse competition could be exceeding the biunivocal competition among the studied species.
therefore, this assemblage would have a strong tendency to trophic guild formation.

 elasmobranch fishes are among the top predators in the marine environment and thus play an important role in marine ecosystems, potentially regulating, through predation, the size and dynamics of their prey populations. an understanding of competitive and predatory processes is thus necessary to gain insight into the role of predators in influencing niche, community and food web structure, and ultimately ecosystem dynamics. in this sense, the theory about resource partitioning is frequently attributed to competitive or cooperative interactions. this approximation predicts that spatial or temporal partitioning may increase the tolerance of niche overlap, and may reduce the pressure of competition among coexisting species. several studies have evaluated the influence of resource partitioning in teleost fishes to explain reduction in competition potential. these authors reported a positive relationship between habitat partitioning and the magnitude of diet overlap or competitive interactions, when the interacting species had the same diet preferences. furthermore, ross found that the first resource that is fractioned is food, followed by habitat. despite the ecological importance of elasmobranch fishes for the marine ecosystem, resource partitioning and competitive exclusions in this taxonomic group are poorly understood. a substantial amount of data on the diet of different elasmobranch species has been reported to date. elasmobranch fishes are often typified as opportunistic predators, with a wide trophic spectrum that includes plankton to marine mammals. in general, oceanic elasmobranch species feed on squid and big fishes, whereas the coastal and benthic species feed on crustaceans, mollusks and small or juvenile fishes. a few species feed on other elasmobranchs, birds, reptiles or marine mammals. ontogenetic variation in diet is well known, with a strong tendency to ingest larger and more mobile animals with increasing size. research on trophic relationships among sympatric species of elasmobranchs is scarce and results vary among studies. varying overlap values between coexisting species, successive sizes of a same species, or even between sexes have been reported. moreover, the effect of habitat and feeding time on the diet of sympatric species is very poorly known. in the eastern pacific ocean of colombia the study of elasmobranch diet is in its infancy. the existing studies have only considered three of elasmobranch species reported for the area. furthermore, no research has addressed trophic interactions among sympatric fish species in coastal or oceanic environments. therefore, the object of this study was to quantify and compare the diet and trophic interactions of coastal elasmobranch species from the eastern tropical pacific of colombia, and to suggest possible mechanisms for their coexistence.  feeding activity a total of specimens of five species were analyzed. all specimens were captured in shallow waters. significant differences in the bathymetric distribution of the species were found =, p << ), with zapteryx xyster and raja velezi found in narrower depth ranges and under and m, respectively. dasyatis longa and rhinobatos leucorhynchus were associated with the shallowest depths. mustelus lunulatus, d. longa and rhinobatos leucorhynchus were captured mainly during diurnal bottom trawling, while z. xyster were captured mainly during nocturnal bottom trawling. raja velezi was captured only at night. only r. leucorhynchus and z. xyster showed significant differences in hourly feeding activity. we did not carry out analyses of ontogenetic shifts in diet because we did not capture individuals from all sizes intervals. sampling data of elasmobranchs caugth in the eastern tropical pacific ocean of colombia and range size of these species. n: number of individuals captured. p: p-value of k-w test. diet composition the proportion of empty stomachs varied widely among species, with a minimum vacuity index of % in m. lunulatus and a maximum vacuity index of % in d. longa. the main prey item of mustelus lunulatus were stomatopods from the family squillidae, with squilla panamesis being the main prey item. an additional twelve prey categories were identified in the stomachs of m. lunulatus, which feed opportunistically on shrimps, crabs and gasteropods in low percentages. feeding activity of elasmobranch captured in the eastern tropical pacific ocean of colombia. number of stomachs analyzed, empty stomachs, vacuity index, mean number of prey per stomach, 95% confidence interval for χn, mean weight of prey per stomach, 95% confidence interval for χw. prey items observed in stomachs of five species of elasmobranchs captured in the eastern tropical pacific ocean of colombia. %o is frequency of occurrence, %n is percentage by number, %w is percentage by weight in d. longa stomatopods were the most frequent prey item and had the highest percent by number, whereas fishes had the highest percent by weight. digested material was % and vacuity index was %. the main prey items were squillidae and fishes. the diet of this species also included decapods and gastropods. in rhinobatos leucorhynchus the most frequent, abundant and with highest percent by weight prey item was decapods, following by fishes. the main prey item was trachypenaeus sp.. however, stomatopods and crabs were also found in the diet of this species. the vacuity index was low. only two prey items were identified in r. velezi, fishes and decapods. fishes were the most frequent item, and also the most abundant by number and weight, and collectively. among the fishes, only batrachoididae could be identified. six prey categories were identified in stomach contents of z. xyster. fishes and penaid shrimp occurred most frequently and were the most abundant by number, however shrimp were the most abundant by weight. for this species decapods were the most important prey item with %iri. diet breadth and overlap use of a three-dimensional graphical representation of the diet facilitated the description of feeding styles of the five species studied. the diet of m. lunulatus was specialized, with most individuals consuming squilla panamensis, although this species also feed on twelve other prey items. in contrast, the diet of z. xyster was more heterogeneous and generalized because fishes, decapods and stomatopods were consumed by the majority of individuals. these results were confirmed by levin's breadth index, which yielded the greatest diet breadth for z. xyster, while m. lunulatus was the most specialized predator. levins' measure of niche breadth and standarized bi calculated with percentage by number data. n: total number of prey categories used. following zares & rand's criteria, four biologically significant diet overlaps were established among the studied species. a significantly high overlap was found between m. lunulatus and d. longa, which shared shrimps of the family squillidae. significant overlaps were also found between rhinobatos leucorhynchus and raja velezi, and r. leucorhynchus and z. xyster, which shared penaeidae, and between raja velezi and z. xyster which shared fish and penaeidae. the random overlap hypothesis was rejected, suggesting low overlap and low partitioning of resources. niche overlap between five species of elasmobranch caugth in the eastern tropical pacific ocean of colombia. values were calculated using the pianka measure, indicating the extent to which the diet of species a overlaps with the diet of species b. significant diet overlap. discussion the composition of diets suggests that all elasmobranchs studied are species that feed on epibenthic invertebrates and fish. this result is highly associated with the source of the samples analyzed, i.e., elasmobranchs captured in bottom trawl fishery. predators found during this activity, feed mainly on benthic infauna, epibenthic fauna, benthic prey or demersal prey. therefore, the results of this research are restricted to elasmobranchs that feed on prey species living on the sediment surface or inside the sediment. however, our results contribute significantly to increase the knowledge on the feeding ecology of the other species of fishes coexisting in the same habitat, with similar food and space requirements. mustelus species have been reported to feed mainly on crustaceans and fish, although cephalopods would also be important in their diets. despite the large number of taxa found in the stomach contents of m. lunulatus, one species of stomatopod accounted for most of the prey consumed. therefore, m. lunuatus in coastal waters of the colombian eastern tropical pacific have a specific dietary preference. besides ellis et al., this is the only other report of feeding specialization in mustelus, a genus that has been classified as opportunistic polyphagous. the diet of raja velezi in the neritic zone of the eastern pacific ocean of colombia showed a low number of prey items in compared with other studies that found polychaetes, copepods, amphipods, myscidacea, stomatopods, cephalopods and bivalves. however, the preference of r. velezi to feed on decapods and fishes detected in this study was similar to that reported for r. naevus, r. clavata, r. radiata, and r. brachyura, r. montagui and r. eglanteria, which also feed upon decapods or fish. although r. velezi is a deep-water ray with a widespread distribution, our results are the first description of the diet of this species in the tropical pacific ocean. rhinobatos leucorhynchus has been identified as a crustacean predator, with preference for penaeid shrimp, although portunid crabs have also been reported as alternative prey. very little is known of the diet of dasyatis longa and zapteryx xyster and this study is the first record on the diet of these species. similar to other benthic species dasyatis longa feed on crustacean decapods and fishes. the preferences for these dietary items have also been reported for dasyatis americana in the caribbean sea. zapteryx xyster showed the widest feeding spectrum of all elasmobranch studied. this species included stomatopods, decapods and fishes in its diet and, was the most generalist species. bornatowski et al. reported similar results for z. brevirostris from south brazil. macpherson and cortés suggested that the relatively big body size of elasmobranch makes it easy for them to expand their feeding spectrum, preying simultaneously on the pelagic and benthic communities. our study shows that diet breadth was reduced, and prey item distribution by number and weight were similar. moreover, all studied elasmobranch fed on three main items: squillidae, penaeidae and fish. this can be explained by the benthic habits and similarity of sizes and dental structure between m. lunulatus and d. longa, as well as between rhinobatos leucorhynchus, raja velezi and z. xyster. the complete dietary overlap detected between m. lunulatus and d. longa, their similar geographical distribution in the study area, and their diurnal activity, suggest that these species could show a competitive exclusion trend for food resources. however, the dietary overlap would be reduced when the deep distribution is considered because m. lunulatus occupies deep waters and d. longa shallow waters. in contrast the dietary overlap detected between rhinobatos leucorhynchus, raja velezi and z. xyster would be significant, since these species have morphological and behavioral similarities. the dietary overlap between rhinobatos leucorhynchus – raja velezi and r. leucorhynchus – z. xyster, based on the presence of trachypenaeus shrimp and fishes, could be compensated by differential diel feeding activity. rhinobatos leucorhynchus feed mainly during the day whereas r. velezi and z xyster feed manly at night. furthermore, 89% of rhinobatos whose stomachs were analyzed were caught at a deep of 15– m, whereas all raja and zapteryx specimens were caugth at a depth of m. in contrast, the dietary overlap between r. velezi and z. xyster, species with similar feeding activity and bathymetric distribution, suggests that they share only a fraction of their feeding niche, protecting a portion as an uncontested space refuge. in this sense, mantis shrimp would be the feeding refuge of z. xyster. dietary overlap between z. xyster and r. velezi was fish unidentified and decapods; however this food category was so indeterminate that the evidence of effective dietary overlap is weak. furthermore, this wide category of prey item overestimate dietary overlap and underestimate niche breadth. values of overlap based on simulations suggest that the species studied showed low dietary overlap. in this study only five species of elasmobranchs were examined. in the study area at least another species of elasmobranchs and six species of fishes that share morphological and ecological characteristics with the studied species have been reported. however, all these species and the elasmobranchs that we studied would conform apackaging throughout a feeding axis, and the temporal dynamics of this packaging could be determined by: variety and productivity of available resources, short-term and long-term environmental variability, and niche overlap. therefore, if the packaging is modulated by resource availability then the shrimp trawl fishery in the neritic zone of eastern pacific of colombia has severely affected the natural availability of prey for elasmobranch communities, affecting their structure.  shrimps and benthic fishes were the main component of the diet of the elasmobranchs studied. diet breadth and overlap were relatively low. determination of food resource partitioning among the batoid species studied was not possible. however, we identified partitions in other niche axes. it is possible to assume that diffuse competition could be exceeding the biunivocal competition among the studied species. therefore, this assemblage would have a strong tendency to trophic guild formation. 
was the main prey item in the diet of rhinobatos leucorhynchus and raja velezi, and penaeidae shrimp were the main prey items in the diet of z. xyster. furthermore, fish were important in the diet of raja velezi, z. xyster and d. longa. stomach contents of specimens of five elasmobranch species caught in the central fishing zone in the pacific ocean of colombia were counted and weighed to describe feeding habits and dietary overlaps. therefore, this assemblage would have a strong tendency to trophic guild formation. the greatest diet breadth corresponded to z. xyster whereas m. lunulatus was the most specialized predator. diet breadth and overlap were relatively low. determination of food resource partitioning among the batoid species studied was not possible. the mantis shrimp squilla panamensis was the main prey item in the diet of m. lunulatus; tiger shrimp trachypenaeus sp. however, we identified partitions in other niche axes. finally, four significant diet overlaps between the five species were found, attributable mainly to squillidae, penaeidae and fish. shrimps and benthic fishes were the most important food types in the diet of the elasmobranch species studied. it is possible to assume that diffuse competition could be exceeding the biunivocal competition among the studied species.
 stomach contents of specimens of five elasmobranch species caught in the central fishing zone in the pacific ocean of colombia were counted and weighed to describe feeding habits and dietary overlaps.  twenty-one prey items belonging to four major groups were identified. decapod crustaceans were the most abundant prey found in stomachs. the mantis shrimp squilla panamensis was the main prey item in the diet of m. lunulatus; tiger shrimp trachypenaeus sp. was the main prey item in the diet of rhinobatos leucorhynchus and raja velezi, and penaeidae shrimp were the main prey items in the diet of z. xyster. furthermore, fish were important in the diet of raja velezi, z. xyster and d. longa. the greatest diet breadth corresponded to z. xyster whereas m. lunulatus was the most specialized predator. finally, four significant diet overlaps between the five species were found, attributable mainly to squillidae, penaeidae and fish.  shrimps and benthic fishes were the most important food types in the diet of the elasmobranch species studied. diet breadth and overlap were relatively low. determination of food resource partitioning among the batoid species studied was not possible. however, we identified partitions in other niche axes. it is possible to assume that diffuse competition could be exceeding the biunivocal competition among the studied species. therefore, this assemblage would have a strong tendency to trophic guild formation. 