Internal embryonic brooding and development in the southern Australian micro-snail Tricolia rosea (Angas, 1867) (Vetigastropoda: Phasianellidae: Tricoliinae)

Abstract

ABSTRACTInternal embryonic brooding has been suggested as an adaptation to enhance reproductive success in minute gastropods. It is rare in vetigastropods, previously known in only two species of Spectamen Iredale, 1924 (Solariellidae) from South Africa. Herein it is confirmed in the temperate Australian micro-snail Tricolia rosea (Angas, 1867), with up to 46 embryos observed within a gravid female. Embryos are brooded to an advanced stage, possessing a translucent, calcified shell and a calcified operculum. The initial protoconch is colourless, spirally sculptured and delineated by a consistent axial demarcation at the 0.75 whorl mark, when it measures 320 µm across. Beyond this, the second part of the protoconch is tinted pink, the strong spiral sculpture continues but the ribs and interstices are broader, with smoother surface microsculpture. At the 1.1 to 1.125 whorl mark the protoconch measures 400 µm across when transition to smooth teleoconch sculpture occurs. No brooded embryos possessed teleoconch sculpture. The potential relationship of protoconch morphology to embryonic development, hatching, feeding and release are considered. The mechanism of fertilisation is unknown, but embryos in a brood are at the same developmental stage. Unanswered questions in embryonic development and problems with protoconch terminologies in vetigastropods are also discussed.