| Resumo: |
The ontogeny of the first stages of development of hake (Merluccius hubbsi)considering yolksac larvae, preflexion larvae, larvae in flexion, postflexion larvae, transformation larvae and posttransition juveniles, is described. Fish larvae and posttransition juveniles were collected in the north Patagonian area, between 42° and 46°S, from surroundings of the coast to the 100 m isobath (Argentine Sea, SW Atlantic). The larvae were collected between December and February, and the posttransition juveniles, in April and June since 1989 to 2004. The morphometrics variables and pigmentation were recorded in 127 larvae y 41 posttransition juveniles ranging in total length (TL)from 2.0 to 35.0 mm. Additionally, the openning of the mouth was calculated and morphometric relationships were studied using an allometric test. Eighty-three hake larvae and 28 posttransition juveniles were cleared and stained, in order to count the number of vertebrae, myomeres and fin rays, and to determinate the osteological development of the caudal fin and the openning of the mouth. Additionally, a quantitative microanatomical analysis was made in 17 larvae from 2.4 to 17 mm TL. Relative surfaces of the central nervous system, eyes, striated musculature of the trunk, viscera and stomach cavity were estimated using a point counting method. The opening of the mouth and the pigmentation of the eyes were coincident with yolk resorption, finishing the yolk sac stage. At about 6.5 mm BL, the flexion stage began. The ending of notochordal flexion gave place to the beiginning of the postflexion stage (8.0- 18.0 mm BL). During the transformation stage (18.0- 24.5 mm BL), the pectoral fins development was completed. Posttransition juveniles stage began (24.5mm BL)when the fin-ray complements were completed, the vertebral column ossification finished and the squamation began. The fin-ray formation could be described by the following sequence: pelvic fins, first dorsal fin, second dorsal and anal fin, caudal fin and pectoral fins. The caudal complex was totally developed in larvae of 22.0-23.0mm BL. It was associated with the last 8 or 9 vertebrae and it had 37-40 rays. Vertebral column ossification started in 4.0 mm BL larvae. The definitive number of vertebrae (49-50)was observed in 8.5 mm BL larvae. The microanatomical analysis showed that the relative surfaces of the central nervous system and eyes represented the 41. of the total during the yolk sac stage. After the yolk resorption, a gradual and constant decrease of the nervous surface was observed which was replaced by an increment of the other systems. These differences were higher after the flexion stage, showing a clear preponderance of the muscular system over the other components. Additionally, a step grown of the muscular surface was observed. This could be an evidence of a saltatory ontogeny. In M. hubbsi a quickly ontogentic development was observed, in which the end of fin development occurred in 24,0- 25,0mm BL larvae. This velocity to reach a greater autonomy could be one of the factors that explain the big resilience of this species, favouring their recruitment.
Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales (Argentina)
|
