Cynodon gibbus, (Spix & Agassiz, 1829) (Spix & Agassiz, 1829)

Cajado, Ruineris Almada, Zacardi, Diego Maia, Silva, Fabíola K. Souza, Oliveira, Lucas Silva & Giarrizzo, Tommaso, 2024, Early ontogenetic development of Cynodon gibbus (Characiformes: Cynodontidae) in the Amazon River basin, Neotropical Ichthyology (e 240012) 22 (3), pp. 1-19 : 5-8

RESULTS

Forty-eight larvae were studied at various stages development (two yolk-sacs, 35 preflexion, 10 flexion, and one postflexion) with a SL ranging from 5.73 to 21.57 mm. No phenotypic differences were observed among individuals collected from different rivers and environments (e.g., channel and lake).

Morphological and meristic characterization. Yolk-sac stage ( Tab. 1 ; Fig. 2A ). The standard length ranges from 5.73 to 5.81 mm (5.77± 0.04 mm). The yolk-sac is elliptical, and the notochord is straight and visible owing to transparency. The body is elongated in a fusiform shape, and the snout is rounded. The mouth is semi-open in a subterminal position. The nostrils are simple, the otolith is visible, and the opercula partially cover the gill cavity. The eyes are spherical, brown, and poorly pigmented. Moreover, the terminal part of the intestine slightly overtakes the middle region of the body. The color pattern consists of two small punctate pigments in the fontanelle, dendritic at the end of the yolk, in the rectum, in the region of origin of the anal fin, and in a dashed shape at the upper and lower ends of the caudal peduncle. The embryonic membrana (finfold) encompasses the entire dorsal and ventral region of the body, being interrupted only by the anus, with no clear differentiation between dorsal, caudal, and anal fins. The pectoral fin button is visible but small. The myomeres are, clearly visible, and 51–52 in number (25 preanal and 26–27 postanal).

Preflexion stage ( Tab. 1 ; Fig. 2B ). Individuals have an SL ranging from 5.94 to 8.64 mm (6.99± 0.62 mm). The notochord is straight and visible owing to transparency. Some specimens with an SL of 6.78 mm had yolk remnants. The body is elongated in a fusiform shape; the snout is pointed; and the mouth is subterminal, large, oblique, and has numerous conical teeth externally. The nostrils are simple, and the eyes are spherical, fully pigmented, and black in preserved individuals. The swim bladder is visible, inflated, and oval shaped. The intestine is vertically striated, and elongated, and the anal opening reaches the midline of the body. The pigmentation is similar to that of the previous stage. Dendritic melanophores are observed in the apical region of the swim bladder, in the embryonic membrane just below the stomach, but are scarce at the origin of the anal fin. The finfold still surrounds the body dorsoventrally, being interrupted only by the anus. The pectoral fin bud is small and surrounded by a membrane, not reaching the swim bladder. Total number of myomeres ranges from 51 to 53 (25–26 preanal and 26–27 postanal).

Flexion stage ( Tab. 1 ; Figs. 2C–D ). The SL ranges from 8.62 to 16.33 mm (12.25±2.56 mm). The final SN is flexed by the emergence of the hypural bones. Morphologically, the body, snout, mouth, nostrils, eyes, and position of the anus show no change in relation to the previous stage. The swim bladder assumes a triangular shape. The gill apparatus is covered by the operculum, a feature not observed in earlier stages. The pigmentation is analogous to that of the preflexion stage, but pigments appear on the hypural plate and on the caudal rays. At this stage, the delineation of the unpaired fins is observed. Individuals with 15.22 mm SL have the first dorsal and anal fin rays. Anal fin is located just after the beginning of the dorsal fin base, making them overlap. The finfold is still visible in the ventral region, from the stomach to the anus and in the region post dorsal fin, origin of the adipose fin, and caudal peduncle. At the end of this stage, the caudal fin shows well-developed rays, and its shape is changed from truncated to forked. The pectoral fin still lacks rays. The total number of myomeres ranges from 52 to 53 (25–26 preanal and 26–27 postanal).

Postflexion stage ( Tab. 1 ; Fig. 2E ). The SL of the analyzed individual is 21.57 mm. The notochord and swim bladder are visible owing to transparency. The pigmentation pattern is similar to that of the previous stage but more intense, particularly in the caudal peduncle and in the caudal rays. Some pigments weakly radiate from the base of the anal fin into the rays. All fins, except for the pelvic one, are in the final developmental phase, including ray segmentation. Remnants of the finfold can be observed in the ventral region anterior to the anus and dorsally situated between the dorsal and anal fins, and extending toward the caudal peduncle. The adipose fin is outlined, and the pelvic fin is only visible as a bud. Most of the rays of the pectoral fin are completely formed (i,13). The anal fin is long and has numerous rays (ii,78), beginning slightly posterior to the midpoint of the body, and perpendicular to the dorsal fin, which has ii,11 rays. The caudal fin has 18 rays (9 in each lobe, superior and inferior). Notably, all unpaired fins are segmented and branched. The total number of myomeres is 53 (26 preanal and 27 postanal).

Morphometric relationships. Throughout development, the body varies from very long to long (8.5 to 15.20% of SL), and the eyes are small (14.45 to 22.66% of HL); the head is initially small but becomes moderate in the preflexion stage (15.68 to 20.09% of SL) ( Tab. 1 ). The proportions of most morphometric variables increased with development, except for SnD/SL, SnL/HL, and SnA/SL, which decreased with development ( Tab. 1 ).

Body growth relationships. The depth of the head and the length of the snout tend to increase in proportion to HL and exhibited continuous linear growth (linear regression). The remaining growth relationships are better explained by the quadratic model and exhibit positive allometry, except for SnA/SL, which showed negative allometric growth, decreasing its distance to the snout during development ( Tab. 2 ; Figs. 3A–G ). The variables SnD/SL and SnV/SL were not tested due to the small number of individuals in which these variables were measured, making the application of regression models unfeasible.

Figures

FIGURE 2 | Larval development of Cynodon gibbus. A. Yolk-sac (5.81 mm SL); B. Preflexion (6.44 mm SL); C. Early flexion (9.70 mm SL); D. Late flexion (15.22 mm SL); and E. Postflexion (21.57 mm SL). Scale bars = 1 mm.

FIGURE 3 | Body ratios (mm) between head length and snout length (A); head length and eye diameter (B); head length and head depth (C); standard length and body depth (D); standard length and head length (E); standard length and distance from snout to pectoral fin (F); and standard length and distance from snout to anal fin (G) during the early development of Cynodon gibbus. Ysl = yolk-sac, Pre = preflexion, Fle = flexion, Pos = postflexion.

Tables

TABLE 1 | Variables analyzed (mm), minimum values (Min), maximum values (Max), mean (Mean), standard deviation (SD), and morphometric relationships (%) for Cynodon gibbus larvae. SL = standard length, HL = head length, SnL = snout length, ED = eye diameter, HD = head depth, BD = body depth, SnD = snout–dorsal fin distance, SnA = snout–anal fin distance, SnP = snout–pectoral fin distance, SnV = snout–pelvic fin distance, BDA = body depth toward the anus, AF = absent fin, NV = rays not visible or absents, n = number of individuals.

Variables	Yolk-sac (n = 2)		Preflexion (n = 35)		Flexion (n = 10)		Postflexion (n = 1)
(mm)	Min–Max	Mean±SD	Min–Max	Mean±SD	Min–Max	Mean±SD	Min–Max	Mean±SD
SL	5.73–5.81	5.77±0.04	5.94–8.64	6.99±0.62	8.62–16.33	12.25±2.56	21.57	–
HL	0.90–0.90	0.90±0.00	1.12–1.61	1.27±0.11	1.60–3.50	2.45±0.60	4.98	–
SnL	0.19–0.25	0.22±0.03	0.26–0.51	0.35±0.06	0.41–0.88	0.58±0.13	1.25	–
ED	0.19–0.20	0.20±0.00	0.18–0.32	0.22±0.03	0.27–0.61	0.41±0.10	1.01	–
HD	0.67–068	0.67±0.01	0.64–1.02	0.79±0.08	0.95–2.12	1.40±0.38	2.75	–
BD	0.76–0.87	0.81±0.06	0.55–1.16	0.71±0.12	0.85–2.16	1.43±0.43	3.15	–
BDA	0.36–0.40	0.38±0.02	0.32–0.58	0.40±0.05	0.54–1.40	0.93±0.32	2.61	–
SnD	AF	AF	AF	AF	4.64–8.68	6.70±1.27	11.29	–
SnA	3.43–3.45	3.44±0.01	3.75–5.35	4.24±0.38	5.29–8.91	7.07±1.25	11.53	–
SnP	AF	AF	1.10–1.70	1.31±0.12	1.67–3.38	2.45±0.56	4.98	–
SnV	AF	AF	AF	AF	AF	AF	9.75	–
Morphometric proportions (%)
ED/HL	21.65–22.66	22.15±0.50	14.45–20.21	17.66±1.44	14.79–19.30	16.83±1.29	20.34	–
HD/HL	74.22–75.67	74.94±0.73	51.68–71.73	62.34±4.20	50.52–64.04	57.23±4.32	55.20	–
HL/SL	15.43–15.65	15.54±0.11	15.68–20.09	18.23±0.84	18.35–21.44	19.82±1.03	23.08	–
SnL/HL	20.87–28.13	24.50±3.63	18.92–36.89	27.82±3.85	17.74–32.46	24.29±4.65	25.14	–
BD/SL	13.02–15.20	14.11±1.09	8.50–13.40	10.16±1.21	8.70–14.19	11.57±1.65	14.58	–
BDA/SL	6.15–7.02	6.59±0.44	4.84–6.70	5.71±0.50	5.49–9.21	7.47±1.21	12.10	–
SnD/SL	AF	AF	AF	AF	52.36–60.15	54.93±2.16	52.34	–
SnA/SL	59.45–59.83	59.64±0.19	58.07–62.47	60.58±1.16	54.55–62.35	58.16±2.27	53.46	–
SnP/SL	AF	AF	16.34–20.92	18.73±0.96	18.68–20.92	19.92±0.75	23.08	–
SnV/SL	AF	AF	AF	AF	AF	AF	45.19	–
Myomeres
Preanal	25	25 (n = 2)	25–26	25 (n = 15)	25–26	26 (n = 6)	26	–
Postanal	26–27	26 (n = 1)	26–27	26 (n = 16)	26–27	27 (n = 5)	27	–
Total	51–52	51 (n = 1)	51–53	52 (n = 11)	52–53	53 (n = 4)	53	–
Number of rays
Dorsal	NV	NV	NV	NV	8–10	8–10	ii,11	–
Anal	NV	NV	NV	NV	36	36	ii,78	–
Pectoral	NV	NV	NV	NV	NV	NV	14	–
Pelvic	AF	AF	AF	AF	AF	AF	NV	–
Caudal	NV	NV	NV	NV	15–18	15–18	18	–