Tuesday, April 09, 2013

Fish embryonic development

After fertilization the eggs swell. The swollen eggs attach readily to the surface such as the real plants or artificial plants. Then the blastodish forms: 
The cleavage begins approximately 40 minutes after the fertilisation at 20 degree C.

The blastodisc forms(A).

A small celled morula is formed after 6 hours(B).

         The blastulation starts 9 hours after the fertilisation(C).

During the first day the germ layers are formed and the embryo outlines appear aroung teh vitelline curvature(D).

The head region then starts to differentiate: optic vesicles and the anterior part of the neural tube can be soon distinguished(E).

The first somites appear during the second day(F).

On the third day the eye lenses appear, blood circulation starts(heart beating starts to circulate colorless plasma and the caudal section of the embryo is detached from the yolk sac(G).

During the last days preceding the hatching, the embryo is mobile and its pigmentation gradully increases(H).

This is the real pictures from the larva development of ranchu goldfish. 

Ranchu fish larva development at 5 hrs post fertilization
Ranchu fish larva development at 12 hrs post fertilization
Ranchu fish larva development at 24 hrs post fertilization
Ranchu fish larva development at 50 hrs post fertilization

The newly hatched free embryos are passive: they lie on the bottom of the tank. In the second day after hatching they try to swim upwards, towards the light(positive phototaxis) and attach to the surface. A few days(3 days at 20 degree celsius) after hatching, the fish swim to the surface to fill the swim bladder and then switch to exogenous feeding when yolk reserves are reduced. The transformation period to exogenous feeding is considered to be a critical period in fish development and especially sensitive to environmental conditions, for example, temperature.