Diet of the endemic bivalve Solen cylindraceus inferred from stable isotope analysis in the Kariega Estuary, South Africa

ABSTRACT

Sedentary filter-feeders such as the razor clam (Solen cylindraceus) are believed to indiscriminately exploit a carbon source derived from particulate organic matter (POM). POM demonstrates a high degree of spatial and temporal variability in composition, which may result in intraspecific dietary variation. This study aimed to investigate seasonal ontogenetic shifts in the feeding ecology of S. cylindraceus using dual stable isotopic (δ¹³C and δ¹⁵N) analyses in the oligotrophic temperate permanently open Kariega Estuary, South Africa. Mixing model results demonstrate that the different size classes of S. cylindraceus had a diet comprised mainly of POM. However, size grouping did have a significant effect on carbon and nitrogen signatures. Specifically, during winter, carbon isotopic signatures became more depleted with a decrease in size, while both winter and summer δ¹⁵N values became more nitrogen enriched with an increase in size. The observed trend in the ratios can likely be ascribed to the hydrodynamics of the system. Data from this study suggest that there may be some degree of variation among the different size cohorts, although this needs further investigation.

KEYWORDS:

• Carbon
• diet composition
• nitrogen
• razor clam
• size-related variations
• temperate permanently open estuary

Acknowledgements

A special thanks to Jess Joyner, Jaryd Ridgeway, Brendon Dredge and Matthew Machell-Cox for accompanying us on the various field trips. Additional thanks are given to the two anonymous reviewers for their invaluable input to improve the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The National Research Foundation (NRF, Pretoria/Grant UID – 88414) and Rhodes University (Grahamstown, Eastern Cape) provided the necessary funding for this project.