Brachyplatystoma juruense (Boulenger, 1898)

Silva, Fabíola Katrine Souza Da, Cajado, Ruineris Almada, Oliveira, Lucas Silva De, Santos, Darliane Campos Dos, Avila, Leandro Cortes & Zacardi, Diego Maia, 2024, Early development of Brachyplatystoma juruense (Boulenger, 1898) (Siluriformes: Pimelodidae), Zootaxa 5481 (1), pp. 101-118 : 103-109

Identification of larvae and juveniles of Brachyplatystoma juruense

In the laboratory, larvae and juveniles were separated from other organisms and debris before being quantified and identified at the species level. To assist in the identification of larvae and juveniles, specimens were skeletonized (removal of tissues, muscles, and organs) was performed on adult specimens of B. juruense ( Figure 2a–b ), allowing the counting of vertebrae and consequently comparison with the myomere count of the larvae. Additionally, for this study, individuals of the other Brachyplatystoma species were analyzed for the number of vertebrae, the quantity of which corroborated with Lundberg & Akama (2005). This procedure is required for understanding the adult characteristics such as the numbers and position of spines and rays in all fins. Vertebral counts include all elements of the Weberian complex (four vertebrae); the compound center of the caudal skeleton is counted as one element.

The larvae and juveniles were identified according to the regressive development sequence technique proposed by Ahlstrom & Moser (1976). This technique consists of identification based on a sequence of individuals at different stages, from juveniles or larvae in more advanced stages to less developed larvae, based on morphological, meristic, and morphometric characteristics. Firstly, with the aid of a stereomicroscope, juvenile specimens were identified by comparing their fin ray information and morphology, which are consolidated at this developmental stage. Following the identification of juveniles, the process continues with the analysis of larvae, where similarities to juveniles’ characteristics are observed, thereby justifying the use of the term regressive sequence. Subsequently, individuals were classified according to the degree of development, following Ahlstrom et al. (1976) modified by Nakatani et al. (2001), into larval period (flexion and postflexion) and juvenile period.

Description of larvae and juveniles of Brachyplatystoma juruense

The description was based on the degree of development (developmental stages) and observation of the main morphological events, as well as meristic and morphometric characters. For each period and stage, individuals that best represented the characteristics were photographed and illustrated, following the technique described by Faber & Gadd (1983) and Shibatta (2016, 2017).

For the morphometric characterization of initial development, the following body measurements, expressed in millimeters (mm), were used: head depth (HD), body depth (BD), head length (HL), snout length (SNL), standard length (SL), eye diameter (ED), and snout length to the origin of the anal (SNA), dorsal (SND), pectoral (SNP), and pelvic (SNV) fins, according to Ahlstrom et al. (1976). Additionally, finfold depth in the posterodorsal region (adipose fin—AFD), snout length to the end of the anus (SNAN), finfold length in the posterodorsal region (adipose fin—AFL), interorbital distance (ID), and head width (HW) were measured. Morphometric measurements (in millimeters) were performed using a binocular stereomicroscope (Leica S9i—precision 0.01) coupled with a digital camera for image capture and analysis—(Leica LAS EZ software). For meristic characterization, the number of pre-anal (PAM), post-anal (PSM), and total (TM) myomeres, as well as the number of rays and spines in the pectoral (P), pelvic (V), dorsal (D), anal (A), and caudal (C) fins, were counted when possible. Only the main rays of the upper and lower lobes of the caudal fin were considered ( Figure 3a–c ).

Data analysis

For the analysis of morphometric proportions (expressed as percentages), the variables HD, SNL, and ED were related to head length, while BD, HL, SNAN, SNA, SND, SNP, and SNV were related to standard length. Additionally, the variables DI and AFD were related to head width and finfold length in the posterodorsal region (adipose fin), respectively, to better understand the morphometric proportions throughout the initial development of B. juruense . The body proportions for body depth (BD/SL), head length (HL/SL), and eye diameter (ED/HL) were established using the criteria suggested by Leis & Trinski (1989). We considered the short anus range for individuals with SNAN <50% of SL and the long anus range when SNAN ≥ 50% of SL. The adipose fin was considered low when AFD <30% of SNAN and high when AFD ≥ 30% of SNAN, and the interorbital distance was classified as narrow when ID <40% of HW and wide when ID ≥ 40% of HW.

To assess allometric variation, analytical regression models were used in which morphometric variables (response variables) were plotted against standard and head lengths (predictor variables), and their relationships were described by different growth models ( Kováč et al. 1999). The hypothesis of continuous isometric growth was tested through a simple linear regression model. In addition, alternative hypotheses were tested: gradually allometric growth (quadratic regression) and discontinuous isometric growth (piecewise regression). The ideal models for each morphometric variable related to the standard length and head length were determined through F-tests. The significance level adopted was p <0.05 (Sokal & Rolf 1981; L’avrinčíková et al. 2005). These statistical analyses were performed using R software version 4.1.1 with the segmented package ( Muggeo 2008).

Material examined

The larvae and juveniles of B. juruense analyzed are stored in the Coleção de Ovos e Larvas de Peixes (https:// specieslink.net/col/CROLP-LEIPAI/) at the Laboratório de Ecologia do Ictioplâncton e Pesca em Águas Interiores (LEIPAI) of the Universidade Federal do Oeste do Pará (UFOPA). The individuals of B. juruense had a standard length ranging from 3.93 (flexion) to 25.77 mm (juvenile). A total of 96 individuals were analyzed, including 91 larvae (35 in flexion and 51 in postflexion) and five juveniles. The catalog numbers of the examined samples are as follows: Brachyplatystoma juruense , Amazonas River, Santarém, Pará, Brazil: LEIPAI00824, 5, 3.93–20.86 mm SL; LEIPAI00825, 1, 19.19 mm SL; LEIPAI00826, 24, 4.77–25.77 mm SL; LEIPAI00827, 7, 6.14–16.22 mm SL; LEIPAI00828, 9, 5.11–15.47 mm SL; LEIPAI00829, 1, 4.86 mm SL; LEIPAI00830, 1, 6.03 mm SL. Japurá River, Tefé, Amazonas, Brazil: LEIPAI00831, 5, 5.29–7.68 mm SL .

At CROLP-LEIPAI, there is also comparative material with larvae of the congeners of B. juruense : Brachyplatystoma capapretum , Japurá River , Tefé, Amazonas, Brazil: LEIPAI00832 , 1, 12.69 mm SL. Amazonas River , Santarém, Pará, Brazil, LEIPAI00833 , 1, 9.11 mm SL. Brachyplatystoma filamentosum , Amazonas River , Santarém, Pará, Brazil: LEIPAI00823 , 2, 7.80–10.96 mm SL. Brachyplatystoma rousseauxii , Solimões River , Tefé, Amazonas, Brazil: LEIPAI00472 , 1, 28.65 mm SL; LEIPAI00473 , 1, 29.29 mm SL; LEIPAI00474 , 1, 28.03 mm SL; LEIPAI00475 , 1, 16.09 mm SL; LEIPAI00476 , 1, 17.88 mm SL; LEIPAI00477 , 1, 12.68 mm SL. Brachyplatystoma tigrinum , Japurá River , Tefé, Amazonas, Brazil: LEIPAI00822 , 1, 17.95 mm SL. Brachyplatystoma vaillantii : LEIPAI01094 , 1, 13.21 mm SL.

Results

The description of each stage is illustrated in figure 4a–g, and the meristic and morphometric data summarized in table 1.

Morphological characterization of the larval period

Flexion stage ( Figure 4a, b, c ): The standard length ranges from 3.93 to 9.23 mm (n = 35; mean: 6.76 mm ± 1.48 mm). The notochord is visible through transparency and has its final section flexed due to the formation of the hypural plate and caudal-fin rays. The body has a fusiform shape. The mouth is located in a terminal position, the snout is rounded, and the nostril is paired, with a distal tubular shape. The eye is small (6.53% of HL), spherical, and pigmented. The operculum is visible. There are three pairs of barbels: one pair of maxillary barbels developing on the side of the head near the corner of the mouth (premaxillary), and two pairs of mental barbels located on the ventral region of the head, below the lower jaw. At the beginning of the flexion stage, both pairs of barbels extend to the stomach, not reaching the anus, while at the end of flexion, the maxillary barbels extend to the anal fin, and the two pairs of mental barbels reach the anus. The length from snout to anus is short (46.10% of SL), not reaching the mid-body region. At 3.93 mm SL, pigmentation is restricted to dendritic chromatophores located at the base of the mental barbels and in the isthmus. From 5.00 mm SL onwards, the dorsal region of the head begins to show pigmentation. The finfold surrounds the body dorsoventrally and, towards the origin of the adipose fin, it has a relatively high triangular shape (>30% of SNAN). The pectoral-fin bud appears from 5.00 mm SL, and the first rays emerge at 7.68 mm SL. The caudal fin initially has a heterocercal shape, but becomes bifurcated at 7.68 mm SL, with the first two rays of the upper lobe larger than the others, forming a long filament. The number of myomeres varies from 58 to 59 (mode = 59) (22 pre-anal and 36 to 37 post-anal).

Postflexion stage ( Figure 4d, e, f ): The standard length ranges from 9.50 to 19.99 mm (n = 51; mean: 14.04 mm ± 2.93 mm). At the beginning of the stage, the pelvic-fin bud emerges. The characteristics related to body shape, mouth, snout, nostril, eye, operculum, and anus reach remain similar to the flexion stage. The maxillary barbels and the lateral mental barbels, at 12.82 mm SL, extend beyond the caudal peduncle and have different lengths, with the maxillary barbels being longer and the medial mental barbel extending beyond the anal fin. At 19.19 mm SL, plates containing numerous villiform teeth are visible on the premaxilla and dentary. The coloration pattern is similar to the previous stage; however, a greater quantity of irregular dendritic and punctate chromatophores can be observed in the occipital region of the head, snout, base of the maxillary barbels, dorsal fin, and origin of the adipose fin. Additionally, filiform pigments are found along the first rays of the upper and lower lobes of the caudal fin. The finfold is partially absorbed, leaving only remnants in the anterior region of the anal and adipose fins. The adipose fin is triangular and depth, representing more than 30% of its base. The pectoral fin has a serrated spine, and in the dorsal fin, the first ray surpasses the height of the spine. The first ray of the lower lobe of the caudal fin modifies to form a long filament, similar to the upper lobe. At 19.99 mm SL, the fin rays are not fully formed. The total number of myomeres is 59 segments (22 pre-anal and 37 post-anal).

Morphological characterization of the juvenile period

The standard length ranges from 20.33 to 25.77 mm (n = 5; mean: 22.21 mm ± 2.32 mm). These individuals are characterized by the complete formation of all structures (e.g., teeth, fins and rays). The mouth is terminal, and the head is dorsoventrally flattened. Pigmentation consists of irregular dendritic and punctiform chromatophores distributed as follows: (1) in the dorsal and lateral region of the head; (2) up to the halfway point of the maxillary barbels; (3) in the dorsal region of the body, concentrated below the dorsal and adipose fins; (4) on the spine and the first rays of the dorsal fin; (5) on the hypural plate; and (6) at the base of the caudal-fin rays and along the first rays of the upper and lower lobes of the caudal fin. The dorsal fin is located anterior to the vertical passing through the origin of the pelvic fin. The adipose fin is located anterior to the vertical passing through the origin of the anal fin. The caudal fin is elongated, bifurcated, and pointed, with long filaments at the tips of the upper and lower lobes. The rays of all fins are fully formed and segmented, including branching and the appearance of ceratotrichia.

The number of spines and rays of the fins is: dorsal I + 6, anal 18, pectoral I + 11, pelvic I + 5, and caudal II + 14 principal rays ( Figure 4g , Table 1 ).

Morphometric relationships

Brachyplatystoma juruense has a small eye (4.02% to 11.17% of HL) and a narrow interorbital distance (30.59% to 37.78% of HW). The head varies from moderate to large (20.99% to 41.36% of SL), the body from long to moderate (10.82% to 28.08% of SL), the reach of the anus short (42.24% to 47.93% of SL), and the finfold in the dorsoposterior region (adipose fin) high (32.15% to 46.33% of SNAN). Body proportions including snout length, head length, and the distances from snout to pectoral, anal, dorsal, and pelvic fins increased throughout development ( Table 1 ).

Body growth

The initial growth is distinct for different body parts of B. juruense . The eye grows at a faster rate compared to the head (ED/HL). This indicates that B. juruense prioritizes the development of its eye, characterized by positive allometric growth (quadratic regression) ( Table 2 , Figure 5a ). For the head length (HD/HL) and body height (BD/ SL), as well as the lengths of the snout (SNL/HL) and head (HL/SL), growth was discontinuous isometric (piecewise regression). Initially, these body parts grow proportionally to the HL and SL until the end of the postflexion stage, at which point there is an abrupt change (breakpoint) in growth rates, resulting in alterations in relative proportions as B. juruense develops. The growth rates for HD, BD, and HL decreased, indicating that these structures grow slowly in postflexion (after the breakpoint). However, for SNL, the growth rates increased, meaning that the snout develops more rapidly, demonstrating priority in the growth of this structure in postflexion ( Table 2 , Figure 5b, c, d, e ). The snout length to the pectoral (SNP), anal (SNA), dorsal (SND), and pelvic (SNV) fins grows proportionally to SL during all stages of development, following a pattern of continuous isometric growth (simple linear regression). Thus, the relative size of these body parts remains constant throughout the early development of B. juruense ( Table 2 , Figure 5f, g, h, i ).

Figures

FIGURE 2. Brachyplatystoma juruense (410.00 mm SL). (a) After collection in the natural environment and (b) After skeletonization. Vertebra count = 58 elements. Scale bar = 10 mm.

FIGURE 3. Morphometric measurements in Brachyplatystoma juruense. (a) Dorsal view of the head, (b) Lateral view of the head and body in larvae, and (c) Lateral view of the head and body in juveniles. Head depth (HD), body depth (BD), head length (HL), snout length (SNL), standard length (SL), eye diameter (ED), snout length to the origin of the anal (SNA), dorsal (SND), pectoral (SNP), and pelvic (SNV) fins, finfold depth in the dorsoposterior region (adipose fin—AFD), snout length to the end of the anus (SNAN), finfold length in the dorsoposterior region (adipose fin—AFL), interorbital distance (ID), head width (HW), number of pre-anal (PAM), and post-anal (PSM).

FIGURE 4. Early development of Brachyplatystoma juruense: (a) Early flexion larvae—3.96 mm SL, (b) Flexion larvae—5.29 mm SL, (c) End flexion larvae—7.68 mm SL, (d) Early postflexion larvae—9.50 mm SL, (e) Postflexion larvae—12.82 mm SL, (f) End postflexion larvae—19.19 mm SL, and (g) Juvenile—25.77 mm SL. Bars = 1 mm.

FIGURE 5. Body growth during the early development of Brachyplatystoma juruense. (a) Eye diameter; (b) Head depth; (c) Snout length; (d) Head length; (e) Body depth; (f) Snout length to the origin of the pectoral fin; (g) Snout length to the origin of the anal fin; (h) Snout length to the origin of the dorsal fin; and (i) Snout length to the origin of the pelvic fin.