Embryonic developmental ecology of freshwater snail Lymnaea acuminata (Lymnaeidae: Gastropoda)

The egg type of Lymnaea acuminata was determined as iso-lecithal and the cleavage is spirally holoblastic type. The development of L. acuminata was observed in details. Uncleaved zygote just after laying was found to contain a relatively yolk-free zone, the animal pole and the yolk-rich region, the vegetal pole. No polar bodies were present in eggs examined immediately after they had been laid. The first polar lobe and polar body were extruded out from the zygote within 15-25 min. These were reabsorbed after 12-15 min. The formation of the second polar lobe was followed within the next 48 min. The first cleavage division occurred about 115-130 min after the formation and re-absorption of the second polar lobe, retaining for 7-10 min. The 2-celled embryos underwent the second equal division of cleavage the within next hour and the embryos reached the 4-cell stage. At the end of the 5th h, the 4-celled embryos underwent the third cleavage and the cleavage was horizontal (i.e., equatorial). At the end of the 9th h, the embryos at the 8-celled stage reached the 16-celled stage by the 4th cleavage. The 4th spirally cleaved embryos usually underwent fifth and sixth cleavages within 24-26 and 27-29 h of incubation, respectively. After 2-3 days of incubation, the developing embryos attained the trochophore stage. At the beginning of the 4th day of incubation, embryos became slightly elongated, curved foot muscle and shell gland were developed through the extension of velum and the embryos turned into the early veliger. At the beginning of the seventh day, the miniature snail possessed all the structures found in a newly hatched individual. Interaction between water physico-chemical parameters and some breeding parameters have been observed.