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

* Dedication: This work is dedicated to the memory of the late malacologist Robert Robertson (1934–2018), whose work on Phasianellidae inspired this study. He was aware that brooding in Tricolia rosea had been confirmed, but sadly did not live to see this publication. An overview of his life’s work is given by Mikkelsen and Bieler (2018).

ABSTRACT

Internal 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.

KEYWORDS:

• Breeding season
• growth
• juvenile
• lecithotrophy
• pheasant shell
• planktotrophy
• protoconch terminology
• reproduction
• shell morphology
• viviparity

Acknowledgements

From Museums Victoria (Carlton campus) we thank Robin Wilson (Senior Curator, Marine Invertebrates), Joanne Taylor (Manager, Natural Sciences Collections), Melanie Mackenzie and Chris Rowley (Collection Managers, Marine Invertebrates) for allowing the study of well-preserved T. rosea material from their collections. Roger Curtain (Advanced Microscopy Facility, Bio 21 Institute, The University of Melbourne) provided SEM access and support. Lynton Stephens (Malacological Society of Australasia) generated many of the SEM images. Ken Bell (Honorary Associate in Invertebrates, Museums Victoria) reviewed the work and provided valuable feedback and guidance. Don Cram (Malacological Society of Australasia) provided assistance with the calibration of light microscopy measurements. Gemma Steele (Librarian, Museums Victoria), procured some of the reference material. The encouragement of Lisa Kirkendale (Western Australian Museum) has been greatly appreciated, as has the editorial support of Don Colgan (Australian Museum) and Rachel Przeslawski (Geoscience Australia). Finally, we thank two anonymous reviewers for their constructive comments and guidance, which have greatly improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

* Dedication: This work is dedicated to the memory of the late malacologist Robert Robertson (1934–2018), whose work on Phasianellidae inspired this study. He was aware that brooding in Tricolia rosea had been confirmed, but sadly did not live to see this publication. An overview of his life’s work is given by Mikkelsen and Bieler (2018).