Analysis of size and shape in early larval stages of European seabass (PhD summary)

Analysis of size and shape in early larval stages of European seabass (PhD summary)

Dicentrarchus labrax (European seabass) is one of the most commercially important marine species in European and Mediterranean aquaculture. Its production, along with all other major fish aquaculture species, is severely affected by deformities. These have been known or strongly suspected to have their causes and first manifestations in the first larval stages. The existing identification methods are mostly taking place in late stages of larval development, and in the case of seabass usually at least 50 or 60 days after hatching (DAH) (Nikolakakis et al., 2018c). Thus, the first main goal of this PhD study was the identification of a protocol that can focus on the objective estimation and quantification of size and shape variation, and linking them to abnormalities, at a much earlier stage (from incubation until 2 weeks post hatching). This way it can produce a timely and interesting insight on the impact of factors that induce abnormal development.

This protocol was investigated on seabass larvae from DAH 2 until DAH 14. It is based on the use of distance morphometrics and outline analysis to determine the variation in total length and body shape that can be induced by a range of parameters. Examples of these parameters are anaesthetisation, handling, fixation, and mounting (comparison between live and fixed specimens). No size or shape changes occurred on the specimens fixed in 3% phosphate-buffered glutaraldehyde, even five months after fixation (Nikolakakis et al., 2014).

In the context of the study of the effects of bacterial load and its communities, a technique that has received considerable attention in recent years is the rearing of gnotobiotic animals in axenic environments. The aforementioned protocol can be a valuable tool in the quantification of the effects of any biotic or abiotic factor that has the potential to induce size and shape changes. Consequently, it was used in the investigation of the morphological effects of the application of axenity, egg disinfection agents and bacterial load on fish larvae. This investigation was the second scientific goal of the present study.

Regarding the effects of antibiotics-induced axenity, disinfected-axenic (DA) specimens exhibited larger bodies than both disinfected-xenic (DX) and non-disinfected-xenic (NX) on DAH 5 and 11. They also had a slower consumption of their yolk sac than DX. This is probably due to the reduced energy demands from lack of bacterial colonisation of the digestive tract, and other similar reduced energy expenditures to combat or assimilate bacteria. Towards the end of the experiment the DA larvae were thicker, but to a small degree more curved than DX and NX, which may be considered an abnormal shape, or a slightly more advanced ontogenetic stage. Egg disinfection with glutaraldehyde was associated with a more rapid consumption of the larval internal energy reserves. An adverse effect was that it induced smaller anal finfolds on DAH 0 and 11. This might be associated with a reduced respiratory ability, with consequent increased energy expenditures that could explain the increased yolk sac depletion rate (Nikolakakis et al., 2018a).

To study the effect of antibiotics-induced difference on bacterial load, the control treatment (“NA”) included larvae reared without antibiotics, while the second (“A”) included larvae reared with rifampicin, ampicillin, kanamycin, trimethoprim and gentamicin. The antibiotics had a significant size effect, yielding larvae with larger total length on DAH 7 and 14, larger bodies on DAH 7, and on DAH 14 larger anal body depth. On DAH 7, “A” specimens were more uniform in their dorso-ventral development, and on DAH 14 “NA” had more slender shapes. The beneficial total length and size effects and the witnessed shape effects might be associated with the low bacterial presence (Nikolakakis et al., 2018b).

 

References:

  • Nikolakakis S., Bossier P., Kanlis G., Dierckens K. & Adriaens D. (2014): Protocol for quantitative shape analysis of deformities in early larval European seabass Dicentrarchus labrax. Journal of Fish Biology vol. 84, issue 1, p. 206-224
  • Nikolakakis S., Dierckens K., Bossier P. & Adriaens D. (2018a): Phenotypic effects of antibiotic-induced axenity and egg disinfection in early larval European seabass (Dicentrarchus labrax L.). Aquaculture Research vol. 49, p. 2536–2552
  • Nikolakakis S., Dierckens K., Bossier P. & Adriaens D. (2018b): Effects of antibiotic-induced differences in bacterial load on growth and shape of early larval European seabass (Dicentrarchus labrax L.). Aquaculture Research vol. 49, p. 988-1000
  • Nikolakakis S., Bossier P. & Adriaens D. (2018c): The importance of shape analysis in the study of larval deformities and its application in the investigation of deformity-inducing bacterial mechanisms. Hatchery Feed vol. 6, issue 1, p. 21-24