Embryonic and larval development of bering founder Hippoglossoides robus¬tus (Pleuronectidae) in the northern Okhotsk Sea

Abstract

The main stages of embryonic and early post-embryonic development of bering flounder in the northern shelf areas of the Okhotsk Sea are described for the first time, their timing is determined and the heat amounts necessary to achieve the stages are evaluated on the base of experimental data. The developing eggs had an average diameter of 2.33 ± 0.01 mm and volume — 5.66 ± 0.05 mm3. The yolk occupied 19.36–31.15 % (on average 24.40 %) of the egg volume that corresponded with the polyplasmatic type of eggs. Time of the eggs incuba­tion from the beginning of epiboly to hatching of prelarvae was 238 hours in the experiment under temperature 8.1 oC. The total time of embryogenesis was estimated as 276 h, and the heat amount required for the total embryogenesis — as 2236 degree-hours. At the stage of the blastula cells gastrulation, the process of axial convergence of the blastoderm directed to forming of an axial bud in the embryo. The Kupfer vesicle was developed and the body pigmentation appeared before closing of the blastopore, but segmentation of the body started after the closing. The muscle activity was detected only with start of the heart working, in opposite to other studied flounders which muscles became active earlier. The newly hatched prelarvae had the body length 4.85–6.00 mm, on average 5.44 ± 0.06 mm. Their transition to mixed and exogenous nutrition occurred at the age of 2 and 3 days, respectively, when their average size was 5.75 ± 0.17 and 5.83 ± 0.17 mm. Following to the experimental and cited data, morphology of prelarvae and larvae of bering flounder is distinguished by weak development of black melanophore on the dorsal side and its absence on the fin fold, as well as relatively early pigmentation of eyes with guanophore.