Morphological features of the Black Sea turbot (Scophthalmus maeoticus) during the period of embryonic development

Black Sea turbot (hereinafter BST), Scophthalmus maeoticus (Pallas, 1814), is a valuable fish for commercial fishery and promising object of industrial mariculture. Potential fecundity of BST is very high, 3–13 million eggs; however, survival of its progenies during early development in the sea is unpredictable and low (mortality is up to 90 %). In nature fertilized pelagic BST eggs rise to the sea surface in 2–3 hours; BST develop in upper waters being part of neuston till hatching. BST on its early stages of development could be considered the most vulnerable as the embryo is exposed to diverse adverse effects. The survival and physiological state of the larvae at hatching till exogenous feeding depend on the norm of morphological characteristics of the embryos during their development. Our aim was to study the norm of the changes in BST morphological characteristics during embryogenesis. Morphological analysis of the BST embryogenesis stages from fertilization till hatching on the basis of detailed study of intact embryos (> 2000 eggs) sampled from different experimental batches incubated under experimental conditions is presented. Digital photos and videos of alive eggs were taken with Canon PowerShot A720 using binocular microscope MBS-10 at magnification 8×4 and under light inverted microscope Nikon Eclipse TS100, equipped with analog camera, at magnification ×4, ×10, and ×40. The morphological features of embryogenesis in BST before and after fertilization, cleavage, blastulation, gastrulation, epiboly, and neurulation and until hatching are presented by photos with detailed description of transforming embryological structures. Fertilized pelagic BST eggs covered by transparent shell vary from (1.26 ± 0.14) to (1.31 ± 0.15) mm in diameter, have homogenously distributed yolk and a single round transparent oil drop of 0.20–0.21 mm, positioned at the top of the yolk. Scale of timing of morphological changes is presented in relative time units (as a time interval from fertilization until the emergence of morphological structure in percentage of the total duration of embryogenesis, % RT). Cleavage starts at 2.5 % RT. Cell division desynchronizes between the 6th and 7th cleavage, at 128 blastomeres. Yolk syncytial layer controlling processes of epiboly, cells differentiation, and morphogenesis is formed during the 10th–11th mitotic cycle (12 % RT, about 512–1024 cells). From the germ ring registered at 21 % RT, the embryonic shield develops (at 25 % RT), and organize formation of embryonic axis from 20 to 50 % epiboly (31 % RT). During 70–75 % epiboly (40–45 % RT), the neural keel is formed; notochord and optical primordia become visible; Kupffer’s vesicle emerges at the start of segmentation. Optic cups develop, and more than 20 somites are observed at the end of epiboly (49 % RT). By 60 % RT the Kupffer’s vesicle disappears in tail bud formed; lens placodes are formed in optic cups. Notochord vacuolization, myotomes formation, and tail growth are observed by 65 % RT. The caudal part of the body separates from the yolk by 70–75 % RT. About 80 % RT neuromuscular activity starts; heart beating initiates; free tail covers more than 60 % of the yolk; differentiating xantophores give a pinkish hue to the embryo. By 90–95 % RT eye cups with lenses; three symmetric otic capsules with otoliths, melanophores, and xantophores present in the embryo with 33–38 body somites; it performs jerky movements. Prior hatching, the egg shell becomes elastic, stretches, and breaks in the head area. Hatching occurs 114–94 hours after fertilization at +14…+16 °С. By hatching, all organs are formed in bilateral symmetrical BST larva (standard length is (2.53 ± 0.13) to (2.91 ± 0.10) mm), three auditory chambers with otoliths exist, eyes are non-pigmented, intestinal tract is closed; within 3–5 days it develops at the expense of yolk. Description of morphological changes in the BST embryo at norm of development could be used for elaboration of criteria of developing BST eggs both in natural environment and under cultivation conditions.