Spatial dispersion patterns of Planaxis sulcatus: patterns and consequences

Abstract

Mobile, rocky shore species often exhibit distinct but dynamic spatial distribution patterns, such as the occupation of microhabitats and formation of aggregations. These patterns are likely to be influenced by the behavioural patterns of a species, their population dynamics and the consequent benefits of adopting particular patterns at certain times of the year. Using Planaxis sulcatus (Planaxidae) as a model species, this thesis documented spatial and temporal variation in the spatial patterns of this species, and attempted to identify the causes and consequences of these behaviours in the seasonally variable Hong Kong intertidal zone.

Activity patterns and snail behaviour during activity influenced the locations and types of spatial patterns formed. Planaxis was active and foraged whilst awash, with activity initiated by the ebbing or rising tides. Activity continued until the rock dried on an ebbing tide, or until snails were submerged by the rising tide. Snails were more active and moved for longer durations in summer than winter. When inactive, Planaxis occupied microhabitats such as cracks and crevices in summer and winter, but in the cooler months snails also formed aggregations on open rock surfaces and around microhabitats such as crevices. The extent of aggregation and number of snails in each aggregation showed strong seasonal variation, being greater in the transition period between summer and winter, and differed between sites, being the greatest at one site at Stanley. Seasonal differences in aggregation patterns may be linked with concurrent variation in Planaxis populations which, as a result of life histories events, had a greater abundance of snails and a larger number of recruits in the summer-winter transition, potentially increasing the chances and likelihood of aggregation formation.

Microhabitat occupation in the summer provided physiological benefits to the snails. Compared with snails on open rock surfaces during summer low tides, those in crevices remained cooler and suffered less osmotic stress; while individuals in rock pools experienced lower osmotic but not thermal stress. In summer, aggregated snails suffered higher thermal and osmotic stresses than solitary individuals. The winter aggregations, however, showed little physiological benefits; with aggregations of different sizes having no effect on snail body temperatures, and only weak indications of large aggregations relieving osmotic stress. Winter aggregations, therefore, may be linked to minimizing dislodgement risks due to increased wave action at this time.

A computer simulation incorporating seasonal differences in population densities, the likelihood and duration of activity, different topographies and three behavioural “rules” produced spatial patterns similar to those on the shore in terms of aggregation sizes, and the proportions of aggregating and crevice-occupying snails. These variables, therefore, play a role in determining the spatial patterns seen in Planaxis. This simple model did not, however, completely match field observations, suggesting that seasonal and spatial differences in distribution patterns involved more complex processes such as variation in local environmental conditions (temperature, wave action, shore topography) and/or biological factors (population densities, size structures, behavioural variation). Further investigations of these processes may better resolve our understanding of how these patterns form and their potential benefits.