Diel variation in the structure of fish assemblages in south western Amazon streams

: Aim: We investigate the influence of luminosity, habitat conservation and pluviometric periods in fish assemblages of in pasture and forest small streams in western amazon. Methods: Sampling was conducted every two months from July 2013 to April 2014 in nine first-and second-order streams using seine nets and dip nets during the day and night. Fish composition, richness and total abundance were determined for each sampling period. The PERMANOVA was used to evaluate the effects of land use, season, and photoperiod, on fish assemblages. Fish assemblage structure for each stream in the presence and absence of photoperiod was ordered by NMDS analysis. Results: In the light period, 3,484 specimens from 69 species were collected, while 4,574 specimens from 71 species where collected in the dark period. No significant differences in abundance and species richness were recorded between the presence and absence of luminosity periods, rainy and dry seasons and streams in forest and deforested areas. We found evidence of the dark phase composition and richness of exclusive species (22% of species collected were found at night), which were greater than in the light period (20% of species). Conclusion: Despite our failure to identify any nycterohemeral segregation, the results complement existing knowledge of regional ichthyofauna and help provide a better understanding of the distributional, behavioral and functional ecological patterns of fish assemblages.


Introduction
Diel activity patterns are among the most evident and detectable rhythms in animals and the main subject of many chronobiological naturalistic and experimental studies (Naruse & Oishi, 1996;Anras et al., 1997;Volpato & Trajano, 2006).These patterns have already been investigated in different ecosystems such as seagrass beds, coral reefs and mangroves (Nagelkerken et al., 2000;Pereira et al., 2010).Many organisms studied, from protists to plants and animals, provide strong evidence of a more or less tight control of activity patterns by internal clocks (Volpato & Trajano, 2006).
Aquatic organisms are not exceptions; many studies have been devoted to different kinds of rhythms these organisms show (Volpato & Trajano, 2006).In fishes, there is evidence of endogenous and exogenous circadian rhythms in some species, which affects activity patterns, vision and the growth of scales and otoliths (Boujard & Leatherland, 1992;Volpato & Trajano, 2006).It has been shown that the teleost circadian system encompasses multiple self-sustained oscillators and that at least two organs, the pineal organ and retina, contain oscillators (Volpato & Trajano, 2006).Photoreceptors in the retina and pineal organ and deep in the brain are believed to be involved in photosignal transduction to establish circadian rhythms in fish (Iigo & Tabata, 1996).Each of these organs may be involved individually in the entrainment of locomotor activity in certain fish species (Volpato & Trajano, 2006).
Diel activity rhythms are expressed by locomotor activity patterns associated with alternating phases (active period vs. rest period), exploitation of habitat, feeding and intra-and interspecific interactions (formation of shoals, agonistic behavior, territory defense, breeding and predator-prey interactions) (Kavaliers, 1980;Helfman, 1986).Taking into account the circadian rhythms, fish can be classified as active during nocturnal, diurnal or crepuscular (mixed types, for which between 35% and 65% of activity occurs in the dark phase) periods (Iigo & Tabata, 1996).For instance, in inland Neotropical waters, Siluriformes and Gymnotiformes are more active at night, whereas most Characiformes de nossa incapacidade de identificar qualquer segregação nictemeral, os resultados complementam o conhecimento existente da ictiofauna regional e ajudam a fornecer uma melhor compreensão dos padrões ecológicos distribucionais, comportamentais e funcionais de assembleias de peixes.

Study area
The Machado River is approximately 1,243 km long.It starts at the confluence of the Pimenta Bueno River and Comemoração River and flows in to the Madeira River in the north of the state of Rondônia (RO) (Fernandes & Guimarães, 2002).First to third order streams are predominat in the Machado River basin and have a total length of 27,497 km (Krusche et al., 2005).The climate is characterized by temperatures ranging from 19 to 33 °C and annual rainfall is around 2,500 mm (Krusche et al., 2005) and there are two well-defined seasons: the dry season (from late May to October) and rainy season (from November to May) (Fernandes & Guimarães, 2002).

Fish collections
A series of 18 samplings was conducted during three rainy and three dry months: June, August and October 2013 (dry season) and December, February and April 2014 (rainy season).Each sampling included the fish abundance in the nine first-and second-order streams in the middle section of the Machado River basin (Table 1) during high luminosity period (12:00-13:00 hours) and once again during dark period (19:30-20:30 hours).
To prevent the potential negative effects of sampling, samples of the absence luminosity period were conducted 20 m upstream and 10 days after high luminosity period samplings.We highlight out that both the samples collected during absence luminosity period, as in the high luminosity period, obeyed the minimum distance of 20 m.Failing to collect samples in the same location.Noting that the sites are similar in terms of habitat condition and heterogeneity.
In total, 108 samples were obtained (Table 1).Fishes were collected using a seine net (1.5 × 2 m, 2 mm mesh) and dip net (0.5 × 0.8 m, 2 mm mesh) along a 50 m stretch for three people for one hour.Flashlights were used for the nocturnal sampling to find and capture the fishes following Bührnheim (1999).
Fish specimens were euthanized using a lethal dose of clove oil and immediately preserved in a 10% formalin solution.After two days they were transferred to 70% alcohol and identified by specialists.Voucher specimens were deposited in the fish collection at the Federal University of Rondônia, Porto Velho, RO, Brazil (Vouchers: UFRO-ICT 023980 to 024000).
In order to better understand the state of those environments, we describe in Table 2 the physical conditions of each sampling site.Environmental variables at each sampling site were characterized following visual observation; substrate composition (i.e., the relative proportion of each substrate component) was visually estimated according to Cummins (1962), and the volume occupied by the submerged vegetation (roots, leaves and stems of the submerged terrestrial vegetation) on each side of the stream was estimated based on the height and width of the vegetation on the banks.

Data analysis
Fish composition, richness (S) and absolute abundance (N) were determined for presence and absence of luminosity periods.The Shapiro-Wilk normality test and Levene's test were applied to determine whether the assumptions used in the parametric (t test) or nonparametric (Mann-Whitney U test) analyses of abundance (log 10 ) and species richness (log 10 ) for each photoperiod.The Permutational Multivariate Analysis of Variance (PERMANOVA, Anderson, 2001) was used to evaluate the effects of land use (forest and pasture), season (rainy and dry seasons), and photoperiod (presence and absence of luminosity), on biological assemblages, and the interactions between these factors (Anderson, 2001).PERMANOVA is a multi-factorial ANOVA based on any measured distance using permutation methods (Anderson, 2001).The analysis was based on dissimilarity coefficients using Bray-Curtis index (richness and abundance data, were previously Log-transformed) and under 10,000 permutations.
Fish assemblage structure for each stream in the presence and absence of photoperiod was ordered by nonmetric multidimensional scaling (NMDS) analysis, a method for bi-or tridimensional arrangement which represents the association among samples in a similarity matrix (Clarke & Warwick, 1994).The method is considered robust and suitable for ordination ecological data (Minchin, 1987).To quantify the similarity between sites we used the Bray-Curtis distance measure applied data previously Log-transformed (Clarke & Gorley, 2001).We performed Mann-Whitney tests with NMDS axis 1 and 2 scores, to detect differences in fish assemblages in the presence and absence of photoperiod.
The PERMANOVA and Mann-Whitney test were performed in the R environment (R Development Core Team, 2013).The NMDS analyses was carried out using PAST version 2.17 (Hammer et al., 2001).Differences were considered significant when p ≤ 0.05.
Sampling during the presence of luminosity period accounted for 3,484 specimens, 4 orders, 15 families and 69 species.During the absence of luminosity period, 4,574 specimens belonging to 4 orders, 20 families and 71 species were collected.No statistically significant differences were found in abundance (U = 869.0,p = 0.90) or species richness (U = 867.0,p = 0.89) between periods of presence and absence of luminosity.Considering all the samples together, Characiformes had the highest abundance and species richness, followed by Siluriformes, Perciformes and Gymnotiformes (Table 4).
The fish assemblages no differed significantly between seasons, type of land use and luminosity, both when considering the abundance and richness of species (Table 5).NMDS analysis of samples based on species abundance did not reveal any separation on axis 1 and 2 retained for interpretation (Stress = 0.41).Scores on axis 1 (U = 820.50,p = 0.58) and axis 2 (U = 845.00,p = 0.74) of the NMDS no differed with respect to period for fish assemblages.

Characidae
Our results corroborating the study by Mojica et al. (2014) in Colombian Amazon streams, who found no significant differences in abundance and species richness between light and dark phase.A dual phasing capacity, which is characteristic of a highly adaptable circadian system, appears to be a common fish trait, especially in temperate species, and an adaptation to marked seasonal changes in photoperiod, temperature and food availability (Volpato & Trajano, 2006).In tropical regions, however, where seasonal changes are not as marked as in temperate zones, this capacity does not appear to be adaptive, corroborating the pattern observed in our study.In tropical areas with well-defined rain cycles, there may be important annual fluctuations in the quantity and quality of food available (Volpato & Trajano, 2006).
Temporal niche partitioning predicts that different species may be limited by the same resources but differ in terms of when they exploit the resource (Chesson, 1985).Previous studies have shown that the composition of fish populations can vary between light and dark periods (Volpato & Trajano, 2006).Arrington & Winemiller (2003) found that Characiformes were a dominant group in both periods, accounting for 80% and 97% of abundance during diurnal and nocturnal periods, respectively.This was also observed in the present study, where 82% and 84% of the abundance of Characiformes was represented in the diurnal and nocturnal periods, respectively, composed mainly by species K. heteresthes, S. notomelas and B. giacopinii (Characiformes order) which represented 60% of total abundance.These species need of high visual acuity, given that these are nektonic species that    collect food items drifting at mid-water and at the surface, predominantly in the main channel (Ceneviva-Bastos & Casatti, 2007;Carvalho, 2008;Brejão et al., 2013;Nogueira & Costa, 2014).Characiformes collected exclusively during nocturnal samplings belong to species that have large eyes, which presumably increase visual acuity in environments with limited light (Shand, 1997).This anatomical modification is described as an adaptation for foraging in deep water, where diurnal light is limited (Stewart et al., 2002) and capturing food in shallow waters with reduced nocturnal light (Arrington & Winemiller, 2003).
The variations are related to the different activities that these fishes perform, such as feeding, breeding and moving (Lowe-McConnell, 1999).Characiformes and Perciformes are visually oriented fish with diurnal habits and are found mostly in clear-water environments, while fish guided by chemical, electrical or tactile stimulus, such as Siluriformes and Gymnotiformes, are more active at night and found mostly in turbid waters (Matthews, 1998).Siluriformes from the genera Trichomycterus (Trichomycteridae) and Ancistrus (Loricariidae), which are generally nocturnal (Buck & Sazima, 1995;Casatti & Castro, 1998), were reported during the diurnal period, which is uncommon among small loricariids (Volpato & Trajano, 2006).
The absence of temporal segregation of fish assemblages in our study, corroborates the results of Arrington & Winemiller (2003).Many fish species have a daily movement pattern from marginal areas to open waters and vice versa and they are often members of different spatially defined assemblages (Matthews, 1998;Arrington & Winemiller, 2003;Costa et al., 2011).This spatial segregation makes it more difficult to detect nycterohemeral differences when assemblages of fish from specific habitats are analyzed.The lack of segregation between the periods analyzed is accounted for by the high similarity in the fish assemblages in both periods, as a large number of individuals from some taxa (e.g., Characiformes, Characidae and Perciformes) are active during the day and night (Barthem, 1987;Sazima & Machado 1990;Hahn et al., 1999;Costa et al., 2009;Duarte et al., 2010).
Therefore, for fish assemblages in which most of the species are active during both periods, no significant differences between these periods can be expected.Additionally, Willis et al. (2005) described a low degree of species replacement between diurnal and nocturnal periods (absence of light/dark period variation), especially during the dry season in structurally complex environments.In these environments, fishes make fewer movements toward marginal areas (refuges) at night time (Willis et al., 2005), by the fact that structurally more complex environments show a higher stability of resources intrahabitat, resulting in the greater concentration of resources results in less movement between marginal areas and open waters for foraging during the day (Willis et al., 2005;Pelicice et al., 2005).This is corroborated by the structural conditions of the small streams analyzed in our study, which are composed mainly of rocks, logs, branches, exposed roots and emergent and submerged macrophytes (see Table 2).
The homogeneity of the assemblages in the light and dark periods, can also be related the artifact collection resulting from differences in sampling techniques and data analysis mainly in the case of independent studies (Volpato & Trajano, 2006).We use methods of active collection, which consists in catching fish with use of instruments that affects the environment by altering the structure of the microhabitat (e.g., submerged litter banks, trunks) (Uieda & Castro, 1999).The greater fish capture efficiency with active methods results in a higher probability of record low abundant species and cryptobiotic habits (Ribeiro & Zuanon, 2006), active or inactive (sheltered) in each photoperiod.
Despite our failure to identify any segregation based on photoperiod, we provided evidence of the nocturnal composition and richness of exclusive species (S = 20; 22%) of Machado river.While only a few studies have analyzed the fish assemblages in Amazonian streams in absence/presence of luminosity periods, it is reasonable to suppose that temporal niche partitioning could explain the high fish diversity in these assemblages.Hence, more studies on the temporal segregation of Amazonian fish are needed to understand the distributional, behavioral and functional ecological patterns of these assemblages.Diel variation in the structure of fish assemblages... Brasiliensia, 2016, vol. 28, e16 with the taxonomic identification.Statistica serial number: AX505B150718FA.

Table 1 .
Identification of collection site, abbreviation, land use in the adjacent matrix, geographical coordinates and number of samples of the nine streams in the Machado river basin, from July 2013 to May 2014.

Table 2 .
Physiographic descriptors of each stream in the Machado River basin, July 2013 to May 2014.

Table 3 .
Abundance data for species collected in presence of luminosity (PL) and absence of luminosity (AL) in each stream in the Machado River basin, July 2013 to May 2014.

Table 5 .
PERMANOVA (Permutational Multivariate Analysis of Variance, Pseudo-F value) for fish assemblages in stream in the Machado River basin, July 2013 to May 2014.

Table 4 .
Data of numerical abundance (N) and richness (S) for each fish order in periods of presence and absence of luminosity of the nine streams in the Machado river basin, July 2013 to May 2014.