Length-weight relationship and diet of the catfish Cambeva guareiensis (Siluriformes; Trichomycteridae)

.

Scientific data (e.g., biology) is scarce for most Cambeva species.For C. guareiensis, there are studies only recording its distribution in streams (e.g., Azevedo-Santos et al. 2020a) and describing a case of albinism (Azevedo-Santos et al. 2020b).Data on population biology and feeding ecology of the species need to be available in the scientific literature.This is justifiable especially by the eminent growth of functional studies that need basic information.For example, knowing the diet of species may be a useful trait for future investigations about anthropic impacts on fish assemblages.
Here we provide the length-weight relationship of C. guareiensis.In addition, we described and compared the diet of the species in different periods (dry and rainy).

Methods
Sampling was done in September 2020 (dry season) and February 2021 (rainy season), and with the permission of the Brazilian Institute of Environment and Natural Resources (license number: SISBio 76135-1).Fish were caught with a hand net (1 mm mesh), in a stretch of ~ 200 meters of the Corrente stream.Sampling activity lasted ~ 50 minutes in both seasons.The collected specimens were euthanized with eugenol (exposed for more than 10 minutes -lethal dose) and fixed (with formalin 10%).Specimens were identified based on the original description provided by Katz & Costa (2020).Vouchers (10 specimens that were transferred to alcohol 70 %) were deposited at the UFRJ (Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil).

Length-weight relationship
The specimens were removed from formaldehyde, washed in water and then weighed (0.1 g) and measured (standard length; 0.1 mm).Later they were transferred to alcohol (70%) for diet analysis.
For the calculation of the length-weight relationship (LWR, hereafter), we considered together specimens collected in dry and rainy seasons.For the analyses, we used a linear regression "logW = loga + blogL" (Froese 2006;p. 243).Through a plot (i.e., log-log) the outliers for Cambeva guareiensis were excluded (sensu Froese 2006).

Diet
For stomach content analysis, we made an incision in all specimens from the anus to the isthmus to remove the stomachs.The stomach contents were placed in Petri dish and examined using a stereomicroscope (Stemi 305).
The animal content present in the stomach of the trichomycterids was identified only to the level of Order or Family based on Costa et al. (2006) and Mugnai et al. (2010) because they were often in an advanced state of digestion (preventing the determination at lower levels).Sometimes contents could only be identified as terrestrial insects.Pieces of plant leaves and roots were also found (vegetal fragments).We used the abundance of the food items found in the stomach of the trichomycterids as the variable to quantify the diet of the species.
With the data obtained, we built a matrix with the abundance of each item consumed by each specimen.Individuals without content, or with an advanced digestion process, were not included in the analyses.From the constructed matrix, we compared the food of the individuals in both sampling seasons (dry vs. rainy), in order to verify possible seasonal variations.With data converted to log (x+1), we used a permutational multivariate analysis of variance (PERMANOVA), with 9999 permutations, and Bray-Curtis distance.For analysis, we used the software PRIMER 6.1.13PERMANOVA+ 1.0.3 (Clarke & Gorley 2006).Lastly, we also described the diet of the species using stomach contents during the studied period.

Length-weight relationship
There were collected 58 individuals of Cambeva guareiensis (UFRJ 12717).The standard length of the specimens ranged from 24.0-62.5 millimeters and the weight from 0.18-3.70grams.All sampled specimens were examined in the LWR (see Table 1).

Diet
Of the 58 specimens examined, 26 individuals sampled during dry season and 12 during rainy season had empty stomachs.The species ingested high amounts of aquatic insects (Figure 1), with predominance of Diptera of the families Chironomidae (56% of consumed items) and Simuliidae (14%).They also consumed aquatic insects of Trichoptera, comprising 12% of items, and vegetal fragments (5.8% of items).The PERMANOVA did not indicate a significant difference in species diet between dry and rainy seasons (Pseudo-F = 1.614, p = 0.159).

Length-weight relationship
The value of b found for Cambeva guareiensis was 3.054

Diet
Insects were the main food item found in the stomach contents of the individual examined, especially aquatic insects.Other studies (e.g., Casatti 2002;Chará et al. 2006;Scott et al. 2007;Manoni et al. 2009;Rondineli et al. 2009) showed that other species of the same family or genus that C. guareiensis also feed on this group.The low prevalence of vegetal items, when compared with insects, and the morphology of the digestive system of C. guareiensis, suggests that leaves and roots were accidentally ingested during prey capture or that plants are not an important item in the diet of this species.However, only with the analysis of the diet-without observations in situ of the feeding behavior of the C. guareiensis (e.g., Casatti 2002)-, is impossible to confirm the first possibility suggested.
We found no statistically significative differences in the diet of C. guareiensis between different seasons (i.e., dry vs. rainy).Similar results were found by Manoel & Uieda (2019) for other species of the genus Cambeva in one impacted stream.A possible explanation is that Corrente stream has a similar characteristic such as those reported by these authors.Another explanation is that the availability of insects in the aquatic ecosystem may modulate the presence of this item in the diet of trichomycterids (sensu Scott et al. 2007).Therefore, the absence of seasonal changes in the diet of C. guareiensis may be related to low or no seasonal variation in the availability of aquatic insects in the stream, which was the predominant group among the food items.Both lines of investigation deserve attention in futures studies in Guareí River basin.
Here we contribute to the knowledge about the biology (LWR) and feeding ecology of C. guareiensis.These baseline data are extremely important, for example, for future studies on functional diversity.Trichomycteridae is a large and varied family of fishes (see de Pinna & Wosiacki 2003 with numerous species with little or no biological and ecological data.Therefore, studies in both fields should be encouraged for other species of the family. (2.858-3.250).Other species of the family Trichomycteridae, such as Trichomycterus candidus (Miranda Ribeiro, 1949), T. pirabitira Barbosa & Azevedo-Santos, 2012, and T. piratymbara Katz, Barbosa & Costa, 2013 (identification corrected herein to T. sainthilairei Katz & Costa, 2021, UFRJ 12719), had similar patterns (Azevedo-Santos et al. 2018).In fact, many stream fish species are expected to have b values in a similar range as that found for C. guareiensis and other trichomycterids (Froese 2006).

Table 1 .Figure 1 .
Figure 1.Relative abundance (%) of insects and vegetal fragments consumed by the analyzed fish in the dry and rainy seasons.C-Coleoptera and D-Diptera.