Reproduction, embryonic and early post-embryonic development of saffron cod Eleginus gracilis (Gadidae) from the Tauyskaya Guba Bay (northern Okhotsk Sea)

Abstract

New information on reproductive biology, embryonic and early post-embryonic development of saffron cod Eleginus gracilis Tilesius is presented on the base of long-term observations in the Tauyskaya Guba Bay, northern Okhotsk Sea. The species begins to spawn in this area at the age of 2 years, with the body length of 14 cm, and reaches mass maturity (when at least 50 % of fish in the age group are matured) at the age of 3 years. All fish are matured at the age of 5 year and elder. Males prevail in the first three age groups, but females dominate in elder groups. The mature fish enter to the spawning grounds in the Tauyskaya Guba Bay in December. The spawning occurs in January-March under the water temperature of -1.5^оC. The saffron cod females spawn 7.2-172.0 thousand eggs, depending on their size, weight, and age (on average 56,600 eggs or 334 eggs per 1 g of body weight). Relative fecundity of the females increases until the age of 7 years, elder females have slightly lower but stable fecundity. The eggs at the beginning of the second stage of development have the diameter of 1.00-1.58 mm, on average 1.410 ± 0.005 mm, and are distinguished by almost transparent shell and colorless yolk, with the ochre colored plasmodisc. The animal pole (were plasmodisc is formed) is in the upper position. A micropyle on the outer shell is clearly visible as a small bulge with diameter slit surrounded by two concentric beads (this structure remains during entire embryogenesis and can be considered as a taxonomic sign of the species rank). Under the incubation temperature of 2.8^оC, the eggs reach the stage of two blastomeres in 11-12 h. At the first stages of partition, numerous small fat-like inclusions are visible in the blastomeres. The second and third cycles of the zygote division with forming of 4 and 8 blastomers pass in the age interval 14-18 h. The interphase between divisions increases from 3 h between the first and second divisions to 5 h between further divisions. The eggs reach the early stage III (large cell blastula) and late stage III (small cell blastula) after 30 and 41 h of their development, respectively. By the end of the stage III, the dome of blastula is maximally developed. At the age of 75 h, the stage of gastrulation and epiboly begins. The blastoderm covers 50-60 % of the yolk surface after 123 h. Bead-shaped embryo is formed after 177 h of development. In the next 309, 417, 459 h of incubation, the embryos undergo successively the stages of tail separation from the yolk sac, mobility beginning, and heart functioning. In experiments, prelarvae of saffron cod hatched after 849 h of incubation. Their size in the Tauyskaya Guba population is the largest known in scientific literature: the total body length varies from 5.30 to 6.25 mm (on average 5.82 ± 0.05 mm); besides, they are distinguished by greater number of pigment cells on the yolk sac and nearby. Intensive pigmentation of prelarvae is known for other fish species in the northern Okhotsk Sea, as well, that presumably is their adaptation for better accumulation of solar heat in conditions of late spring warming in this area.