Appraisal of the usefulness of operculum growth marks for ageing Hexaplex trunculus (Gastropoda: Muricidae): Comparison between surface striae and adventitious layers

Abstract

This study estimated the age of Hexaplex trunculus (Gastropoda: Muricidae) by analysing operculum growth marks. Surface striae and adventitious layers were counted in opercula of H. trunculus from the Ria Formosa Lagoon (southern Portugal) and from the Bizerte Lagoon (northern Tunisia), and in aquacultured individuals used for validation of the ageing technique. Operculum growth features were also compared between H. trunculus populations under contrasting environmental conditions (namely seawater temperature). Despite high inter-individual variability, all H. trunculus populations displayed a clear increasing trend in the number of striae and layers with increasing specimen size. The operculum marginal growth revealed that opercular deposition is not annual. Instead, a few growth marks are formed in the operculum each year, which was confirmed with reared specimens. In both ageing methods, considerable variation in the number of counts at a determined shell length indicates that operculum growth marks should be interpreted cautiously because they might not correspond accurately with individual age. Because of the subjectivity inherent to either method, counting striae and layers in opercula only provides rough age estimates of H. trunculus.

Keywords:

• Adventitious layers
• age estimation
• Gastropoda
• Hexaplex trunculus
• Mollusca
• operculum
• surface striae

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

This study was undertaken in the ambit of the bilateral cooperation project: ‘Comparative study of the fisheries of gastropod molluscs with commercial value (banded murex – Hexaplex trunculus and purple dye murex – Bolinus brandaris) in the Portuguese and Tunisian coasts’, funded by the Fundação para a Ciência e Tecnologia – FCT (Portugal) and by the Ministère de la Recherche Scientifique, Technologie et Développement des Compétences (Tunisie). Paulo Vasconcelos was supported by a post-doctoral grant (SFRH/BPD/26348/2006) awarded by FCT (Portugal) and Ines H. Gharsallah was supported by a research grant awarded by INSTM (Tunisia). Thanks are due to João Cúrdia for photographing the opercula and acetate peels. The authors are grateful to the editor-in-chief (Dr Franz Uiblein), subject editor (Dr Roy Kropp) and four referees for valuable comments and suggestions that greatly improved the manuscript.

Subject editor: Roy Kropp

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark