Early life history of two closely related gastropods, Rissoa auriscalpium and Rissoa italiensis (Caenogastropoda: Rissoidae)

Abstract

A comparative analysis of the early life history of two common gastropod forms living on the leaves of the Mediterranean seagrass Posidonia oceanica has been carried out, taking into account timing and phases of development. The forms have a different distribution along a depth transect at Ischia Island (Gulf of Naples, Italy), the more squat and ribbed being mainly present between 1 and 15 m depth, with a maximum density at 3 m, the other, between 10 and 30 m. They were distinguished as sibling species, Rissoa italiensis (Verduin, 1985) and Rissoa auriscalpium (L., 1758), on the basis of protoconch size. However, to date, the systematic relationship between them is still unclear. Electrophoretic studies showed lower genetic distance between the two forms along the transect, than among populations from other locations in the Mediterranean Sea. R. auriscalpium have planktonic development, with a higher number of smaller eggs, while R. italiensis have an intracapsular veliger, with hatching juveniles. As for the patterns of development, while the same stages are recognizable in the two gastropod forms up to the formation of the veliger, several differences in timing occur, along with differences in growth and death rates of the embryos. R. auriscalpium hatches as a veliger at the first phase of torsion, about 10 days after spawning, while R. italiensis completes its torsion and metamorphosis within the capsule, and then hatches as a juvenile about 18 days after spawning. The time of encapsulation is what seems to lead to similar developmental sequences with different reproductive patterns and ecological roles, depending on the organization of the elements in the capsule.