The encapsular developmental sequence of the mesogastropod Turritella communis (Gastropoda: Turritellidae)

Abstract

The development of the lecithotrophic encapsulated larva of the internally-fertilizing, sublittoral gastropod Turritella communis Risso 1826 was documented using scanning electron microscopy and light microscopy. Encapsulated development was completed in 12 days at 15°C in the laboratory. Spawning occurred above ~10°C. Spawn masses consisted of numerous gelatinous egg capsules, each of which contained ~28 eggs, encased in albumen and fertilizing sperm. The eggs had an average diameter of 139 μm. Fertilization was accomplished by unpaired eupyrene sperm and occurred at the germinal vesicle stage. The developmental sequence followed the typical gastropod pattern, but was unique in a number of respects. Polar lobes were produced during meiotic maturation and early cleavage, with an especially large lobe occurring in association with the first cleavage, which was unequal. The blastula developed into a dorso-ventrally flattened placula at the 70-cell stage. Gastrulation occurred through invagination and epiboly acting together, and was uniquely accompanied by the development of ectodermal microvilli measuring ~ 1 μm in length. Extra-embryonic albumen began to be depleted after the development of the microvilli. It is proposed that the ciliated telotrochal cells gave rise to the pair of statocysts. Torsion was additionally observed to be facilitated by the growth of pre-mantle tissue on the right side and retardation of growth on the left side.