Better understanding of host-microbe interactions and natural defenses is imperative for the successful production of marine larvae. In aquaculture eggs are kept in incubators with a microflora that differs in numbers and characteristics from that in the sea, and the eggs become heavily overgrown with bacteria shortly after fertilization. The adherent microflora on eggs may cause damage, whereas we do not know whether the egg epiflora may bestow some protection. Marine fish larvae ingest bacteria, and are thus primed with antigens before active feeding commences. Sequestering of intact bacterial antigens by newly hatched larvae may result in tolerance, but as yet we have little information about aquired tolerance of fish larvae to the microflora.
Most marine bacteria adhere well to fish mucus.
Numerous saccular cells were observed in the epidermis of the yolk-sac larvae, aqnd the numbers of these cells increased following addition of various bacterial strains to incubators. These findings may help to understand the protective role of the mucus layer of halibut during development. Addition of commensal apathogenic bacterial strains affected survival of the halibut larvae, and the best result was obtained with a commensal, apathogenic Vibrio salmonicida strain. Thus the use of microflora manipulation, competition, or antagonism may increase larval survival, depending on the strains used. Marine invertebrates may harbour bacteria that are pathogenic to other organisms, and thus serve as vectors for spreading of fish-pathogens.