SEASONAL DIET SHIFT IN A TETRAGONOPTERINAE ( OSTEICHTHYES , CHARACIDAE ) FROM THE UBATIBA RIVER , RJ ,

In the present study, we describe feeding habits of Deuterodon sp. from the Ubatiba River and explore if diet changes according to a temporal cycle of dry and wet seasons. We observed that Deuterodon sp. fed on an extremely high diversity of items ranging from organic matter, sediment (sand plus quartz parts), algae, seeds and leaves to animal organisms, such as, crustaceans, oligochaets and several life stages of terrestrial and aquatic insects, indicating an omnivorous diet. An important shift in the use of feeding resources was also registered; animal and vegetal items had alternated importance between both seasons. Allochthonous vs. autochthonous items analysis showed predominance of allochthonous items during dry season while no significant differences were registered during wet season.


INTRODUCTION
The Ubatiba River (22 o 60'S and 42 o 48'W), together with its tributaries, Silvado, Caboclo, Fundo, Itapeteiú and Ludigério, composes a small fluvial system of the Brazilian East Basin in the State of Rio de Janeiro.Twenty-two species make up the Ubatiba River ichthyofauna totalling 14 families and/or subfamilies.The characiforms predominate among these families (Mazzoni, 1998) and, Deuterodon sp.(henceforth Deuterodon) stands out as one of the most abundant species.
It belongs to the Tetragonopterinae subfamily (Characidae), which is the most abundant and has the wildest distribution in the mentioned river (Mazzoni & Lobón-Cerviá, 2000).
Despite their importance in coastal streams, little information concerning Deuterodon biology is available.Sabino & Castro (1990) described D. iguape feeding habits in a coastal stream from Mata Atlântica and Mazzoni & Petito (1999) described the reproductive strategy of Deuterodon from the Ubatiba River (Maricá, RJ).One of the most important limiting factors for such studies is the taxonomical problem; which is discussed by Gomes (1994).
In this work we present a description of the feeding habits of Deuterodon from the Ubatiba River, as well as analyses the diet shift according to seasonal patterns.It concerns a preliminary study to the establishment of the trophic structure of such stream dwelling fish community.

MATERIAL AND METHODS
The Ubatiba River is a typical coastal stream 16 km long flowing to the Maricá lagoon.A seasonal rainfall marks the annual hydrological cycle; nonetheless, occasional storms are common all over the year as in other local coastal streams (Fig. 1).
Deuterodon specimens were electrofished (AC, 3-4 A, 900W - Mazzoni et al., 2000) during dry (June, July, August/1995) and wet (September, October, December/1995) months in a 100 m long locality in the medium reaches of the Ubatiba River.Sampled fishes were adult specimens (standard length > 2.8 cm; according to Mazzoni & Petito, 1999) and comprised a wide range of standard lengths, in order to avoid feeding variations related to ontogenetic development.Each collected specimen was conditioned in ice for transportation and dissected to gut removal.
Gut content were then, conditioned in alcohol 70 o GL and food items identified under stereomicroscopic, to the lowest feasible taxonomical level.In general, studies on the food of fishes describe numbers or volume of prey items.Owing to the small size of Deuterodon guts and, low numbers of items per gut, we considered the Numerical Method (Hyslop, 1980) to better represent the importance of each item in the fish diet.Complementary analyses were done under optical microscopic but, items registered through this method were not quantified.Our results are expressed as the relative abundance of quantified items (stereomicroscopic analyses), i.e., animal vs. vegetal and autochthonous vs. allochthonous items for guts pooled according to wet and dry seasons.χ 2 tests were applied in order to detect seasonal (dry vs. wet season) differences in food resource use.

RESULTS
A total of 96 specimens were analysed, being 44 from the wet season and 52 from the dry one.All sampled individuals were bigger than 2.8 cm (adult specimens according to Mazzoni & Petito, 1999), this suggests that no difference in feeding habits could be assigned to ontogenetic variation (Table 1).
As a whole, Deuterodon ingests many items ranging from organic matter, sediment (sand plus quartz parts), algae, seeds, leaves, crustaceans, oligochaets, nematodes, and several life stages of terrestrial and aquatic insects.
Most specimens (88) had contents in their guts and 13, 24 and 51 had, respectively, only animal, only vegetal and animal plus vegetal as food items indicating omnivory.Considering only quantifiable items Deuterodon's main food resource is animal (Fig. 2).

DISCUSSION
The Ubatiba River, unlike most Neotropical running waters, is essentially a nutrient-rich, highconductivity river, as a consequence of deforestation and cattle ranching on its surroundings (Mazzoni & Lobón-Cerviá, 2000).Deuterodon is one of the most abundant species and widespread distributed in the mentioned river, i.e., it feeds at several trophic levels and exhibits a broad flexibility to ingest practically all organisms available at the site.Nonetheless, its diet is based mainly on autochthonous animal items.Autochthonous Allochthonous Arcifa et al. (1991) found similar results for A. bimaculatus and A. fasciatus (Tetragonopterinaes related to Deuterodon) and detected mainly zooplankton items in their gut content, as also did Ortaz & Infante (1986).Nomura (1975) found exactly the opposite for Astyanax spp.from the Mogi-Guaçu River.Sabino & Castro (1990) and Escalante (1987) classified D. iguape and Bryconamericus ihering (both species related to Deuterodon) as carnivorous, finding mainly diptera larvae plus allochthonous insects as food resources.
The predominance of autochthonous items agrees with the way that Deuterodon feeds.They often forage near the bottom (RM personal observation).This feeding behaviour facilitates autochthonous items to be ingested in opposition to another stream dwelling fish species, Mimagoniates microlepis from the Ubatiba River, that swims and feeds in midwater ingesting large amounts of allochthonous items (Mazzoni & Iglesias-Rios, 2002).Nonetheless, the high occurrence of allochthonous items, registered during dry season, corroborates the hypothesis of various authors (e.g., Winemiller, 1989;Luiz et al., 1998) that allochthonous matter input is the most important means of food supply in streams, specially during dry seasons.
The main difference, between dry and wet seasons was the relative importance of animal versus vegetal items.During dry season we registered predominance of vegetal items (seeds) while in wet season animal items were predominant.Considering the seasonal occurrence of allochthonous vs. autochthonous items, we registered almost total presence of allochthonous items during dry season but high homogeneity in the use of both kinds of resources were registered during wet season.Seasonal shifts in the feeding habits of Tetragonopterinae species were commonly registered in the literature (e.g., Nomura, 1975;Esteves, 1996;Hartz et al., 1996).The trophic spectrum exhibited by Deuterodon and the ability to shift prey according to the seasonal cycle has been mentioned for other Neotropical fish species outside the Mata Atlântica coastal streams (Lobón-Cerviá & Bennemann, 2000).In general, such patterns are related to rainy vs. dry seasons (e.g., Goulding, 1980;Winemiller, 1989;Fugi et al., 1996;Almeida et al., 1997;Jepsen et al., 1997;Esteves & Aranha, 1999).
Temporal variation of daily mean rainfall values plotted by months, between January and December/1995 (data obtained from INMET).
Relative participation of quantifiable food items registered on gut contents of Deuterodon sp. from the Ubatiba River.Relative participation of animal and vegetal food items on the diet of Deuterodon sp. from the Ubatiba River, during dry and wet seasons.
Relative participation of autochthonous and allochthonous food items on the diet of Deuterodon sp. from the Ubatiba River, during dry and wet seasons.