Weight-Length and Length-Length Relationships of the Endangered Zebra Pleco Hypancistrus zebra ( Siluriformes , Loricariidae ) from the Xingu River, Amazon, Brazil

forbidden. Even with its capture and trade prohibited since 2004, the species continues to be one of the most trafcked fsh in the world. Te biometric knowledge of the species is essential for monitoring its natural populations and management in ex situ environments. Tis work aims to establish the weight-length relationship (WLR) and length-length relationship (LLR) of the species and to evaluate the condition factor ( K ) of H. zebra rescued from trafcking. Te study was conducted at the Laborat´orio de Aquicultura de Peixes Ornamentais do Xingu from February 2022 to June 2022, estimating the parameters of WLR and LLR of 1165 specimens divided into four simple linear regression (SLR) models: all specimens, only juveniles, only females, and only males. Te K of 190 specimens rescued from trafcking by the Brazilian Federal Police was evaluated. Te WLRs and LLRs were estimated using SLR. Te diference between the allometry coefcients ( b ) and between the residuals of the models (juveniles, females, and males) was evaluated using the ANOVA signifcance test. Te total length of the species ranged from 1.6 to 9.3cm. Te coefcient of allometry ( b ) of the WLR of the species is 3.22 and of the LLR is 1.04, demonstrating that H. zebra presents positive allometric growth. Te evaluation of the residuals of the models showed diferences between ontogenetic stages and between sexes. Te development of juveniles is isometric until the frst sexual maturation phase and difers in negative allometric for females and positive allometric for males, showing sexual dimorphism in the species. Among the specimens rescued from trafcking, 54.74% were underweight, and 13.16% were under 60% of the expected K value, demonstrating the poor conditions in which these animals are transported by trafcking.


Introduction
Hypancistrus zebra (Isbrücker and Nijssen [5]) is a smallsized species of pleco (Siluriformes, Loricariidae) endemic to the middle Xingu River, a tributary of the Amazon basin [1].Te distribution of natural populations of this species is limited to a short stretch of approximately 150 km of river in a region that presents a biodiversity hotspot [2,3].Te species inhabits regions of high environmental heterogeneity, characterized by the presence of rocky outcrops and the formation of strong currents [4].Tese environments consist of sections of rocky substrates, promoting the formation of crevices that are used as shelters for protection and reproduction [4].
Te species exhibits a pattern of oblique black and white stripes that extends throughout the body, including the fns [5].Additionally, it features "E"-shaped stripes on the snout region, characterized by a black line connecting the two nostrils, extending downwards, along with another black line passing between the nostrils [5].Its peculiar and attractive color pattern, rarity, and other ornamental fsh characteristics have placed the species among the most sought-after in the aquarium market worldwide [3].
Te demand for H. zebra in the international trade has intensifed the fshing pressure on its natural populations since its discovery in the late 80s, contributing to its insertion into the list of critically endangered species [6,7].From then on, its trade was prohibited, but the illegal capture and market continued, and today the species is one of the most trafcked in the world [3].It is estimated that 100,000 specimens are annually trafcked out of Brazil, with an average retail price of US$ 155 ( ± US$ 23 based on geographical location) per animal and an average wholesale price of US$ 100 ( ± US$ 94 over time) [3].
In addition to the fshing pressure on their natural populations, the construction of the Belo Monte Hydroelectric Plant brought considerable changes in its natural habitat, placing even more uncertainty about the future of the species [8].All these factors converge to the urgency of implementing conservation and monitoring technologies for their populations, drawing attention to the importance of knowledge about their biology and ecology [9].
Two biometric records are fundamental for environmental monitoring and ex situ management: the weightlength relationship (WLR) and the length-length relationship (LLR) of the species [10].Te allometry coefcient (b) calculated in the WLR establishes the proportion in which the total weight rate increases in relation to length, establishing three growth patterns (positive allometric, negative allometric, and isometric) [11,12].Te b also makes it possible to calculate the condition factor (K), indicating a rate, for which it is possible to evaluate the proportion at which the biomass of some specimens varies under certain conditions, and the LLR establishes the calculation to convert standard length to total length [13].Terefore, the objective of this study is to establish the WLR and LLR relationships of Hypancistrus zebra, and to evaluate the condition factor (K) of specimens rescued from trafcking.

Materials and Methods
Te study was conducted at the Laboratório de Aquicultura de Peixes Ornamentais do Xingu (LAQUAX), from Universidade Federal do Pará (UFPA), Altamira city, Pará state, Brazil.Measurements were performed between February 2022 and June 2022.All procedures were authorized by the Committee for the Use and Experimentation of UFPA (CEUA No. 1921240222) and IBAMA (No. 801598/2019).
All the fsh used for collecting biometric data were rescued from trafcking through enforcement operations conducted by the Brazilian Federal Police and were housed at LAQUAX under the faithful custodian agreement (No. 801598/2019).Te weight-length and length-length relationships were calculated based on biometric data from 1165 animals kept under suitable conditions for 2 years, simulating the limnological patterns of the Xingu River.Te condition factor was assessed in 190 animals rescued immediately from trafcking.
A total of 1165 animals were used.Te fsh were anesthetized with 25 mg.L −1 of eugenol.Te total and standard lengths were measured with a digital caliper (0.01 mm) make ZAAS Precision, model digital 0-300 mm.Te total weight of each specimen was measured with a digital scale with a Precision of 0.001 g, make Laborana, model EL.Sex was determined by macroscopic analysis of external secondary sexual characteristics (Figure 1) [14], and the age of frst sexual maturation was established based on the size of the specimens (Figure 2) [15].
After evaluating the assumptions of simple linear regression, the parameters intercept (a) and angular coefcient (b) of WLRs and LLRs were estimated by regression analysis on log-transformed data [13].Te degree of association between the variables was calculated by the correlation coefcient (r) [10].Te verifcation of outliers was determined from the coefcient of determination less than 0.95 (R 2 ) [16].
Te growth type was determined by the angular coefcient (b), and the diference between the residuals of the regression models was calculated using the one-way ANOVA signifcance test comparing (juveniles and females), (juveniles and males), and (females and males) [17].
Subsequently, the condition factor (K) was verifed for 190 animals seized from trafcking by the Brazilian Federal Police at the airport in the city of Altamira, the starting point for the trafcking route [3].Te ideal K (K ≥ 1) was calculated by the ratio of the observed weight (W o ) by the estimated weight (W e ) and the (W e ) specifc to each model, were calculated from the equations generated by the trend lines of the scatter plots of the regressions [13].

Results
We analyzed the biometric data of 1165 specimens of the species Hypancistrus zebra, divided into two ontogenetic stages (juveniles and adults) and between sexes (Table 1).Te total length of the observed animals ranged from 1.62 to 9.30 cm, and the weight ranged from 0.04 to 7.40 g (Table 1).
All regressions were highly signifcant (P < 0.001), with the correlation coefcient R 2 ranging from 0.9003 to 0.9848 for WLRs (Table 2) and from 0.9016 to 0.9860 for LLRs (Table 3).Te allometric coefcient (b) of the WLRs and the residuals of the models of all regressions varied signifcantly (P < 0.001), demonstrating a diference in growth between ontogenetic stages and between sexes (Table 2).
Te regressions with their respective equations for the trend lines of the WLRs of the species are represented in (Figure 3), for juveniles only in (Figure 4), for females in (Figure 5), and for males in (Figure 6).All regressions were represented by scatter plots with a signifcance level of 95%.Regressions with their respective equations for the trend lines of the LLRs of the species are represented in (Figure 7), for juveniles only (Figure 8), females (Figure 9), and males (Figure 10).All regressions were represented by scatter plots a with 95% signifcance level.
We analyzed the condition factor (K) of 190 species seized from trafcking, showing that 54.74% of the specimens analyzed were below the expected weight (K < 1), with 13.16% of the specimens presenting biomass below 60% of the condition factor.

Discussion
Giarrizzo et al. [10] established the length-weight relationship of 135 fsh species from the middle Xingu River.Among the sampled species, nine specimens of Hypancistrus zebra were evaluated without distinction of sex or ontogenetic phase.To improve this, our study used a more signifcant sample of 1165 specimens, divided by sex and ontogenetic stage, and applied the coefcient (b) to establish the condition factor (K) of 190 animals rescued from trafcking in the city of Altamira.
Te values of (b) of the WLRs observed in the species were within the expected range of 2.5-3.5 [18].As suggested in [11], several fsh species may show diferences between the values of the angular coefcient of WLRs, between ontogenic stages, and between sexes.For example, H. zebra showed a diference between juveniles compared to adults and between females and males.
Te juveniles of the species grow isometrically until they reach the age of frst sexual maturation.Upon reaching adulthood, growth varies between the sexes.Females show negative allometric growth, while males develop a pattern of positive allometric growth, exhibiting sexual dimorphism in the body shape of H. zebra.Tis change in body anatomy occurs due to variations in the still developing secondary sexual characteristics in juveniles.Males tend to be more slender, while females require a larger abdominal space to accommodate mature gonads.
Among the 190 specimens of H. zebra rescued from trafcking, we observed that 54.74% of the animals were underweight, with 13.16% below 60% of the condition factor.Taking into consideration that the city of Altamira is the beginning of the wild specimen trafcking route and that up  to 70% of all trafcked animals are sent to China in suitcases for a long period of time [3], it is likely that 54.74% of the animals were at risk of death or would arrive very weak to their fnal destination, and probably 13.16% would not arrive alive.
According to [3], the mortality rate of trafcked H. zebra is highest during the frst stages of the journey, which goes from capture to arrival in Bogotá (Colombia) or Iquitos (Peru).Tese animals are usually packed in bags with low oxygen or adequate insulation and can reach temperatures Length-lenght relationship (Species) y = -0.17+ x, R 2 = 0.99   above 32 °C.In addition, the animals that die midway through the journey can increase the amount of ammonia in the enclosed environment.A condition factor with K < 1 may indicate inadequate nutrition or unbalanced homeostasis; thus, fsh with K below the expected level are more susceptible to death or sufering irreparable physical injuries [13].Sousa et al. [3] reported that the average mortality of trafcked H. zebra is 50%; this data corroborate with the sampled data of 54.74% of animals below the expected condition factor.

Conclusions
Hypancistrus zebra is considered critically endangered in Brazil and by the IUCN, and the results obtained in this study will be useful for work related to the conservation of the species.Te WLR, besides facilitating the conversion of length into weight, allows the calculation of the condition factor (K) of animals in in situ and ex situ environments, helping in the environmental monitoring of the species and the management of these animals in artifcial biotopes.Furthermore, study presents the LLR of the species, enabling the conversion of standard length into total length, and establishes a new maximum length for the species from a measured marker of 9.30 cm.

Figure 1 :Figure 2 :
Figure 1: External sexual dimorphism in two adult zebra plecos.Te female (SL � 5.67 cm) presents smaller odontodes on the opercular region (A) and on pectoral spine (B) while the male (5.92 cm) has hypertrophied odontodes on the pectoral spine (C) and (D) opercular region.

Figure 3 :
Figure 3: Weight-length relationship (WLR) of Hypancistrus zebra based on the equation y � (LOG (W)) � LOG (a) + (b) x � (LOG (L)), where (a) is the intercept, (b) is the angular coefcient, and R 2 is coefcient of determination.(L) is the LOG of total length in cm, and (W) is the LOG of weight in g.

Figure 4 : 9 Figure 5 :Figure 6 :
Figure 4: Weight-length relationship (WLR) of juvenile Hypancistrus zebra based on the equation y � (LOG (W)) � LOG (a) + (b) x � (LOG (L)), where (a) is the intercept, (b) is the angular coefcient, and R 2 is coefcient of determination.(L) is the LOG of total length in cm, and (W) is the LOG of weight in g.

Figure 7 :
Figure 7: Length-to-length ratio (LLR) of Hypancistrus zebra based on the equation y � (LOG (SL)) � LOG (a) + (b) x � (LOG (TL)), where (a) is the intercept, (b) is the angular coefcient, and R 2 is coefcient of determination.(SL) is the LOG of standard length in cm, and (TL) is the LOG of total length in cm.

Figure 8 :
Figure 8: Length-to-length ratio (LLR) of juvenile Hypancistrus zebra based on the equation y � (LOG (SL)) � LOG (a) + (b) x � (LOG (TL)), where (a) is the intercept, (b) is the angular coefcient, and R 2 is the coefcient of determination.(SL) is the LOG of standard length in cm, and (TL) is the LOG of total length in cm.

Figure 9 :
Figure 9: Length-to-length ratio (LLR) of female Hypancistrus zebra based on the equation y � (LOG (SL)) � LOG (a) + (b) x � (LOG (TL)), where (a) is the intercept, (b) is the angular coefcient, and R 2 is coefcient of determination.(SL) is the LOG of standard length in cm, and (TL) is the LOG of total length in cm.

Table 1 :
Biometric data for the species (all), juveniles only, females only, and males only.N is sample size, (TL) is total length in cm, (W) is weight in g, (Min.)equals minimum, and (Max.) is maximum.

Table 2 :
Weight-length relationships (WLRs) of the species (all), juveniles only females only, and males only, based on the equation LOG (W) � LOG (a) + (b) LOG (TL), where (a) is the intercept and (b) is the angular coefcient.N is the sample size, (TL) is total length in cm, and (W) is weight in g.CL is the confdence limits, and R 2 is the coefcient of determination.

Table 3 :
Length-to-length relationships (LLRs) of the species (all), females only, and males only, based on the equation LOG (TL) � LOG (a) + (b) LOG (SL), where (a) is the intercept and (b) is the angular coefcient.N is the sample size, (TL) is total length in cm, and (SL) is standard length in cm.CL is the confdence limits and R 2 is the coefcient of determination.