Behavioural Competition in the Intertidal Shore Crab, Petrolisthes elongatus
Authors:   , Yeoh Li Hao 
Published: December 2021

<p>Competition is a well-documented ecological interaction that underpins community structures and much of population ecology. Physical characteristics such as size, age, sex and weaponry all have an important part to play in how an organism competes, and for many animals, competition is mediated by behavioural patterns. Outcomes of these competitive interactions are not only driven by these characteristics, but by the environmental conditions and external pressures that influence them.   The focal species of my study is Petrolisthes elongatus (H. Milne Edwards, 1837), a porcellanid crab that aggregates in high densities among cobblestone beaches along the intertidal shores of New Zealand and Tasmania. They utilize rocks and crevices as shelter spaces to protect themselves from environmental and predation pressures, displaying variation in physical characteristics, such as sexual dimorphism and autotomy, as well as high levels of behavioural complexity.   I used laboratory experiments with crabs collected from the field and placed them in shelter-limited tanks under the following comparisons; 1) adult and juvenile males, 2) males and females, 3) ovigerous and non-ovigerous females, and 4) autotomized and non-autotomized males. For each of these experiments I used three different environmental conditions; 1) a control high tide, 2) a low tide treatment (where water was drained from the experimental tank), and 3) with the presence of a predator, a juvenile spiny rock lobster (Jasus edwarsii). Each experiment was recorded for 8 hours, where time spent under shelter and shoving interactions among individuals were counted.  In the adult vs. juvenile and male vs. female experiments, smaller individuals spent a significantly more time under shelter than larger conspecifics, but increasing size resulted in more time spent under shelter in the autotomized vs. non-autotomized experiment. In all experiments, smaller individuals initiated the least amount of competitive interactions, and each size class was more likely to displace a smaller individual from a shelter, than a larger one. There was no significant difference in the time spent under shelter between males and females, but ovigerous females and autotomized males spent significantly more time under shelter than their respective competitors. Males also engaged in more shoving interactions than females, with smaller classes of males displacing larger classes of females from shelter spaces. Ovigerous females also outcompeted non-ovigerous conspecifics in the large majority of competitive interactions, and autotomized individuals engaged in significantly more contests than non-autotomized conspecifics. In all experiments, the presence of a predator had no effect on the number of shoving interactions and only resulted in an increase in time spent under shelter for individuals in the male vs. female and ovigerous vs. non-ovigerous experiments.  A field survey on body to cheliped size ratios, autotomy and claw punctures counts was also conducted in support of the shelter-competition experiments. Crabs were collected over a one-month period in November 2018, brought back to the lab to be sexed, measured (Carapace Width, BW; and Cheliped Length, CL) and then surveyed for autotomy and claw puncture wounds. Juveniles of both sex (BW =</p>