Spatial Dynamics and Life History Characteristics of an Aggregating Species, Petrolisthes Elongatus

Abstract

Intertidal boulder fields are heterogeneous mosaics of different micro-habitat patches providing high structural complexity and spatial variability of environmental conditions that shape the heterogeneity of abundance of species. The influence of the spatial scale at which ecological patterns are measured has gained much attention over the last two decades. The adaptations to environmental conditions as well as the ability to disperse vary among species and, consequently, result in species-specific responses to heterogeneous environments. Understanding the causes and consequences of spatial patterns of abundance of organisms is a central problem in ecology and insight in underlying mechanisms may have important implications for fisheries management and conservation biology. The competing demands organisms are subject to require compromises to optimise fitness and facilitate the persistence of a population. Reproductive trade-offs, for example, may be largely the result of adaptive processes, constrained by phylogenetic effects. Generally, trade-offs are regarded as a key to understand the evolution of life history characteristics. The present study investigates the spatial dynamics and life history adaptations of an aggregating species, the New Zealand half crab, Petrolisthes elongatus. This species has a biphasic lifecycle and occurs in geographically isolated benthic metapopulations that are associated with cobble beaches, where it reaches extremely high densities. Even though sampling was conducted exclusively in the Wellington region, sites at Cook Strait as well as within Wellington Harbour exhibited considerable differences in habitat structure and temperatures as well as wave exposure and food supply. Cobble size was an excellent predictor of both population density and mean carapace width on almost every scale of observation and, therefore, appeared to be an important indicator of habitat quality for P. elongates because cobble size is closely related to availability and size of interstitial space between boulders the crabs utilize as shelters against predation and desiccation pressure. Crab densities were significantly (6-fold) higher within the harbour compared to south coast sites, presumably due to 1) the different availability of shelter space, 2) lower settlement and recruitment rates at Cook Strait, and 3) higher food supply within the harbour. Population structure was mainly influenced by settlement and recruitment, but also by rock size (i.e. shelter size), size selective predation and a higher mobility of larger crabs. Under laboratory conditions, predation rates among small crabs strongly depended on rock size, whereas large crabs were generally rarely to never preyed upon. Migrated distances did not exceed 100cm over 25h in most cases but were different at different sites. While structural complexity could affect mobility (i.e. easier movement among larger rocks), local density and resulting levels of intraspecific competition were possibly the main drivers of limited mobility of adult crabs. P. elongatus exhibits a high phenotypic plasticity of reproductive traits in response to the variable environmental conditions across sites. Female crabs at Island Bay (Cook Strait) showed the highest fecundity, higher percentage of ovigerous females, higher proportion of clutch to body weight (i.e. reproductive output) and higher egg numbers among large individuals compared to similarsized individuals at other sites at the harbour entrance (Breaker Bay) as well as within the harbour (Hutt River mouth). Larvae of P. elongatus strongly responded to waterborne settlement cues released by conspecific adults both in the laboratory and in the field. The average numbers of settlers was 4-fold higher in basket traps deployed in the field if conspecific adults were present. Therefore, settlement patters are strongly influenced by conspecific density, even on a small spatial scale (tens of centimetres). Recently metamorphosed individuals may still respond to conspecific cues, presumably providing guidance to conspecific adults that protect juveniles from predation and desiccation pressure in addition to shelters between boulders. Under laboratory conditions, individual growth rates as well as mortality depended on density but not on food level. Differences in density dependent growth rates and mortality varied with body size and were both more pronounced among smaller crabs. Limb loss significantly influenced the increase in body weight over time, especially among small individuals; however, no differences could be detected regarding changes in body size due to few moults over the course of the experiment. Crabs with removed chelipeds gained more weight over 7 weeks relative to uninjured individuals, presumably due to the limb regeneration process. Reproductive output, however, appeared not to be affected by limb loss and the associated costs of regenerating chelipeds; however, sample sizes were low to reliably detect such differences. The results of my thesis underline the importance of the consideration of spatial scale in ecological studies in order to meaningfully compare results with other studies. Additionally, the present study contributes to elucidate the influence of environmental conditions on density dependence and reproductive traits of mobile intertidal invertebrates.