Resource utilization and reproduction of the hermit crab Clibanarius virescens (Crustacea: Decapoda: Anomura) in South Africa. by Michelle Wait Submitted in fulfilment of the requirements for the degree of Philosophiae Doctor in the Faculty of Science at the Nelson Mandela Metropolitan University Submitted: March 2010 Promoter: Dr. D. S. Schoeman Co-promoter: Prof. D. Baird Abstract Clibanarius virescens (Krauss) is a common and abundant hermit crab along the East coast of South Africa. Despite this, its biology is poorly known, both in South Africa and throughout the rest of its range in the West Indo-Pacific region. This study focuses on the descriptive analysis of the shell resource used by C. virescens and of the effects of this resource on crab populations. The underlying hypothesis of the study is that biogeographically imposed gradients in the morphology of intertidal gastropod shells used by hermit crabs affect hermit crab population parameters. The gradient of potential change in the shell resource was captured by sampling at 12 localities, encompassing the range of C. virescens in South Africa. Seasonal changes in shell use, population structure and reproduction at a single locality (Cape Recife) were recorded over a period of 13 months. The breeding season of C. virescens at the southern extreme of its range extended from December to June. The population structure shows some seasonal change, but no clear trends emerge. The shell resource changed substantially in nature over the region studied. Based on shell use, localities clustered into separate southern and northern groups with a break occurring between Dwesa and Coffee Bay. Southern localities were characterised by use of Burnupena cincta , B. lagenaria and B. pubescens . Northern localities were characterised by the use of Morula granulata , M. nodosa and Peristernia forskalii . Intertidal shells used by C. virescens show fewer adaptations to predation in southern localities than shells from northern localities. Southern shells i are relatively large, light and have wider apertures than those from northern localities which are generally smaller, heavier and have decreased aperture widths. Shell parameters affect population size-distributions as southern crabs were larger and heavier than northern crabs. C. virescens show sexual size dimorphism in which male crabs uniformly dominate the larger size classes at all localities. Differences in the sex ratio between males and females show more variable patterns. Most southern localities show no difference in the number of male and female crabs, but most northern localities show a skewed sex ratio in favour of females. Shell use and population size distributions of females affect reproductive output. Southern females produced significantly larger clutches than northern females. Reproductive output was related to crab mass and shell volume. Shell use patterns conform to the biogeographical regions in the range of the study and have a clear effect on both the population size distributions and reproductive output of Clibanarius virescens . Key words: Biogeographical gradient Clibanarius virescens Hermit crab Reproductive output Shell use Size-frequency distribution South Africa ii Acknowledgements I have so many people in so many places to thank for assistance with this work. I have hauled this PhD through three institutions and one career change, during which it was not always clear whether I was keeping it going or it was keeping me going. At the University of Port Elizabeth (now part of the Nelson Mandela Metropolitan University) thanks go to (in chronological order): Dr Brent Newman (now at the University of Zululand) for suggesting that hermit crabs would make an interesting PhD – you were right. Dr Dave Schoeman (now at the University of Ulster) for firstly being willing to take me on as a student and then for continuing to encourage me as your own career took you to three different institutions in three different countries. Without your help, especially in the last few years, this would not have been possible. Words cannot describe how much I owe you. Prof Dan Baird (now at the University of Stellenbosch) for finding the initial funding to keep me at UPE at the start of this study and for once again being willing to take an academically orphaned student and administer my progress, as you have done for so many others. Thanks to all the people who helped with the sampling at Cape Recife and who were willing to go out in all weathers for the simple reward of doughnuts: Shirley Parker- Nance, An van Cauter, Mariette Wheeler and Petro Erasmus are the regular helpers that I remember – it was so long ago… For help on the first two sampling trips I must thank Cathy Wiid, Abigail Kamineth, Dave Schoeman (again) and Petro Erasmus. For technical assistance in all matters involving vehicles, tents and laboratory equipment thanks must go to Piet du Toit, Maggie Hawkins, Brian Seale and Anton Cloete. Thanks to Cathy Wiid for straightening out all the accounting. iii At the University of Durban Westville (now part of the University of KwaZulu – Natal): Thanks go to Prof Biseswar for the papers and advice given, and also to the technical staff, Jay and Nirvana Naidoo and Thumba Ganesan for help with laboratory and sampling equipment. Thanks also to Sean O'Donoghue for help on the Wild Coast sampling trip. At Rhodes University: Thanks to Prof Allan Hodgson for allowing me such free use of the drying ovens and balances, to Liesl Knott for all the chats and to Terry Butterworth for always fixing things so quickly when the switches tripped. Thanks also to the technical staff of the Computer Science and Information Systems Departments for giving me space to work when our section of town lost power two days before the submission of this manuscript. Funding for this study was acquired through seed grants, two from UPE and one from UDW. Beautiful equipment was purchased through a National Research Foundation Thuthuka Programme grant for researchers in training (Gun 2053915). It was sad to leave the microscope behind. I hope you enjoyed it, Solly. The shortest but most important words must go to those to whom I owe the most. Thank you to my parents, Ria and Cedric Wait, who have always supported me in my studies. Truly, my house is built upon a rock. Dameon Wagner has stood with me through all the above and so much more - lesser men would have quailed. iv Table of Contents: Abstract ......................................................................................................................... i Key words:.......................................................................................................................ii Acknowledgements .............................................................................................................iii Table of Contents: ............................................................................................................... v List of Figures:.....................................................................................................................vi List of Tables: ....................................................................................................................xvi Chapter 1: General introduction ..................................................................................... 1 1.1 Overview of selected literature ................................................................................. 2 1.2 Clibanarius virescens ............................................................................................... 7 1.3 Biogeography......................................................................................................... 13 1.4 Thesis structure and aims ...................................................................................... 17 Chapter 2: General method and sampling localities ................................................... 19 2.1 Introduction ............................................................................................................ 19 2.2 Sampling localities ................................................................................................. 21 2.3 Sampling................................................................................................................ 23 2.4 Laboratory analysis ................................................................................................ 25 2.5 Data analysis ......................................................................................................... 29 Chapter 3: Population structure and reproduction of Clibanarius virescens during an annual cycle at Cape Recife ..................................... 30 3.1 Introduction ............................................................................................................ 30 3.2 Methods ................................................................................................................. 35 3.3 Results................................................................................................................... 38 3.4 Discussion.............................................................................................................. 62 Chapter 4: Trends in shell use by Clibanarius virescens along the east coast of South Africa .................................................................................. 70 4.1 Introduction ...........................................................................................................