Behavioural Competition in the Intertidal Shore Crab, Petrolisthes elongatus A thesis submitted in partial fulfilment for the requirements for the degree of Master of Science in Marine Biology Li Hao Yeoh 2020 School of Biological Sciences Victoria University of Wellington 1 Acknowledgements I would first like to thank my primary supervisor Nicole Phillips. Your un-ending support and guidance has been greatly appreciated during the course of my program. Thank you for the passion you bring to your work and I am truly grateful for your vast knowledge and incredible insight. I could not have done this without you. I would also like to extend my appreciation towards the community at Victoria University Coastal Ecology Lab (VUCEL), for all the cheerful and insightful conversations I have had with the many students, but especially to the technicians, John Van der Sman and Daniel McNaughton, who have been readily available and incredibly willing to help with whatever I may have needed. To my ever-supportive parents, thank you for giving me the opportunity to study what I love and for creating a positive environment to work in. To my eco pals; Dayanitha Damodaran, Brittany Florence-Bennett, Caitlyn Shannon, Baylee Wade, Jenna Mumford, Ronja Hauke and Rosella Nicolai, thank you for sharing this journey with me. It’s been amazing to see all of us work so hard to achieve something so great. Thank you to all my friends, especially my flat mates; Isaac Harper, Theo Moore, Sebastien Hamilton, and Josh van Bergen, who have helped and supported me in some way or other over the years. Thank you to those who have listened to my struggles, helped me enjoy life outside of work or tried to give me answers even when you had no idea what I was talking about. Finally, I want to thank God for this amazing opportunity and blessing. “For from him and through him and to him are all things. To him be glory forever. Amen” – Romans 11:36 2 Table of Contents Acknowledgements ............................................................................................................. 2 List of Figures and Tables .................................................................................................. 5 Abstract ................................................................................................................................ 7 Chapter 1 – Introduction ..................................................................................................... 9 The traits that predict a successful competitor – Physical ...................................................... 10 Bigger is better .......................................................................................................................................... 10 Older is better ............................................................................................................................................ 11 Sex as a predictor ...................................................................................................................................... 12 The traits that predict a successful competitor – Behavioural ................................................. 14 The influence of environmental conditions and predation pressures ...................................... 15 The rocky shore environment ................................................................................................. 16 Competition in marine benthic communities ......................................................................... 18 Sessile organisms ...................................................................................................................................... 19 Mobile organisms ..................................................................................................................................... 20 Porcelain crabs (Petrolisthes spp.) .......................................................................................... 21 Petrolisthes elongatus ............................................................................................................. 23 Measuring competitive interactions ........................................................................................ 26 Study objectives ...................................................................................................................... 27 Chapter 2 – Methods ......................................................................................................... 28 Data collection ........................................................................................................................ 28 Assessing the competitive behaviour of P. elongatus. ............................................................. 31 General approach ...................................................................................................................................... 31 Adult vs. juvenile males ............................................................................................................................ 34 Males vs. females ...................................................................................................................................... 35 Ovigerous vs. non-ovigerous females ...................................................................................................... 35 Autotomous vs. non-autotomous males .................................................................................................. 36 Footage analysis ........................................................................................................................................ 36 Assessing cheliped characteristics in the field ........................................................................ 36 Statistical analyses .................................................................................................................. 38 Chapter 3 – Results ............................................................................................................ 40 Low tide treatments ................................................................................................................ 40 Adult vs. juvenile males ........................................................................................................... 41 Time spent under shelter .......................................................................................................................... 41 Shoving interactions ................................................................................................................................. 42 Males vs. females .................................................................................................................... 44 Time spent under shelter .......................................................................................................................... 44 3 Shoving interactions ................................................................................................................................. 45 Ovigerous vs. non-ovigerous females ...................................................................................... 47 Time spent under shelter .......................................................................................................................... 47 Shoving interactions ................................................................................................................................. 48 Autotomous vs. non-autotomous males .................................................................................. 49 Time spent under shelter .......................................................................................................................... 49 Shoving interactions ................................................................................................................................. 50 Field survey data ..................................................................................................................... 52 Cheliped size ratios ................................................................................................................................... 52 Autotomy and claw puncture survey ....................................................................................................... 53 Chapter 4 – Discussion ...................................................................................................... 57 Low tide treatments ................................................................................................................ 57 Adult vs. juvenile males ........................................................................................................... 60 Time spent under shelters ........................................................................................................................ 60 Shoving interactions ................................................................................................................................. 62 Males vs. females .................................................................................................................... 64 Time spent under shelter .......................................................................................................................... 64 Shoving interactions ................................................................................................................................. 66 Ovigerous vs. non-ovigerous females .....................................................................................