Marine Biodiversity

, Volume 48, Issue 3, pp 1541–1550 | Cite as

Diel and seasonal variation in heterobranch sea slug assemblages within an embayment in temperate eastern Australia

  • Meryl F. Larkin
  • Stephen D. A. Smith
  • Richard C. Willan
  • Tom R. Davis
Original Paper


Biodiversity surveys of marine species are largely conducted during the day, and often over relatively short timeframes, due to practical, operational, and budgetary constraints. As a consequence, surveys can underestimate biodiversity for some groups, such as heterobranch (formerly opisthobranch) molluscs, where abundances vary seasonally by orders of magnitude and different species are active at different times of the day. Here, we quantitatively assess day/night (diel) and seasonal variation in heterobranch sea slug assemblages using monthly, diel scuba surveys for 13 months at three sites in Port Stephens, New South Wales (NSW). All heterobranch sea slugs encountered in 50 × 5-m belt transects were recorded using identical survey methods both during the day and at night. Significant differences were detected between day and night assemblages, with species richness and abundance being consistently higher at night. Significant cyclical patterns were also detected in sea slug assemblages over the duration of the study, for both day and night assemblages, with species assemblages changing throughout the study period. The results demonstrate that marine diversity studies conducted only during the day, and those which do not account for all seasons, are likely to underestimate diversity and abundance of molluscs, particularly heterobranch sea slugs. It is, therefore, important that studies which aim to provide a comprehensive catalogue of molluscan biodiversity include not only day-time surveys, but also those conducted at night. They should also include temporal replication in order to capture ephemeral species.


Port Stephens Molluscs Opisthobranchs Nudibranchs Subtidal Observational Biodiversity Cyclicity Temporal variation 



This study would not have been possible without the generous assistance of many individuals. We would, therefore, like to acknowledge the following people for their assistance both above and below the water: Nicola Davis; Peter Davey; Matt Nimbs; Margo Smith; Ashley Smith; Cathie Shorthouse; Kate Tinson; Liying Guan; Ian Marriner; David Atkinson; Karen Rusten; Rob Rogers; Joao Carlos Mendes Junior; Dave Harasti; Nic Rewitt and Newcastle Dive Centre; and Emma Challen and Adam Shorter from Let’s Go Adventures.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

All applicable international, national and/or institutional guidelines for the care and use of animals were followed.

Supplementary material

12526_2017_700_MOESM1_ESM.docx (20 kb)
Supplementary Table S1 Table of all species found on diel surveys, with total number of individuals sighted (N) separated by those found at night, and those found during the day. (DOCX 19 kb)


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Copyright information

© Senckenberg Gesellschaft für Naturforschung and Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  • Meryl F. Larkin
    • 1
  • Stephen D. A. Smith
    • 1
  • Richard C. Willan
    • 2
  • Tom R. Davis
    • 1
  1. 1.National Marine Science CentreSouthern Cross UniversityCoffs HarbourAustralia
  2. 2.Museum and Art Gallery of the Northern TerritoryDarwinAustralia

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