Body size variation in the sexually dimorphic scaphopod Rhabdus rectius (Carpenter, 1864) (Dentaliida: Rhabdidae)

ABSTRACT

Male-biased sexual size dimorphism typically evolves via sexual selection for larger males that are favoured by choosy females or are more successful in mate competition with other males. Among marine invertebrates that broadcast their gametes into the ocean for fertilisation, this form of sexual size dimorphism is rare because such species lack direct interactions among males or between the sexes. However, the broadcast-spawning tusk shell Rhabdus rectius was recently reported to show strong male-biased sexual size dimorphism. That pattern might imply interesting and undiscovered sexual selection in this species. We found instead that the distribution of body size variation (weight, shell length) was similar between males and females of R. rectius, and mean sizes were not different between the sexes. However, we noted a male-biased sex ratio (∼1:1.3) in our large sample of individuals. Many live scaphopods (and several dead shells) showed partial or complete boreholes drilled by predatory gastropods. Boreholes were observed on males and females in similar proportions. We collected scaphopods along with multiple individuals of one likely scaphopod predator, the small moon snail Euspira pallida, and in the lab we observed successful attacks by moon snails on tusk shells.

KEYWORDS:

• Sexual conflict
• SSD
• sex ratio
• sperm competition
• Naticidae

Acknowledgements

We thank Janice Pierce and John Richards of the R/V Alta at the Bamfield Marine Sciences Centre for taking us to, and dredging up, the mud. We are grateful to Christina Bowhay and her classmates in the 2018 Marine Invertebrate Zoology course at BMSC for their sharp eyes and gentle fingers used to capture scaphopods and snails. Thanks to two anonymous reviewers for thoughtful criticisms and suggestions that greatly improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Michael W. Hart http://orcid.org/0000-0003-0012-1016

Additional information

Funding

This work was supported by Natural Sciences and Engineering Research Council of Canada: [grant number RGPIN-2014-05404].