Locomotory behavior and habitat selection in littoral gastropods on Caribbean limestone shores

Journal of Shellfisheries Research, Jan, 2005 by Thomas W. Bates, David W. Hicks

ABSTRACT Microhabitat selection and diurnal and nocturnal movement patterns in two tropical littoral gastropods (Nerita versicolor and Tectarius antonii) were compared in relation to shore position allowing for an assessment of the influence of physical stressors (heat and desiccation) on their behaviors and vertical distributions. Monitoring of individually marked specimens indicated that movement, and presumably feeding activities of tropical littoral gastropods, is initiated during periods of decreased desiccation and thermal stress. Near continuous movement was observed in snails inhabiting moist eulittoral (midshore) habitats. In contrast, snails inhabiting the upper eulittoral fringe (high-shore) exhibited movement only during periods of rain or when shores were moistened by increased wave action. Twenty-four hour surveys indicated a preference for nocturnal activity in mid- and high-shore habitats. The propensity for increased activity during conditions of reduced desiccation stress was also supported by an experiment in which wetting of the substrate (simulating inundation) encouraged activity in mid- and high-shore species. Mid- and high-shore species exhibited a preference for sheltered microhabitat and avoided exposed surfaces.

KEY WORDS: habitat selection, Littorinidae, locomotory behavior, Neritidae, rocky shores

INTRODUCTION

Rocky shore intertidal habitats are subject to varying intensities of physical stress, particularly with respect to temperature extremes and desiccation, which increase with increasing shore height (reviewed by Lewis 1964, Newell 1979, McMahon 1988, McMahon 1990). This gradient of physical stress is partially responsible for the zonation of species in rocky shore habitats because each vertical zone is set apart by a unique suite of selective pressures (McMahon 1990). Littoral gastropods have accumulated a number of specialized adaptations in response to periodic tidal emergence, many of which correlate with vertical shore position (reviewed by Underwood 1979, McMahon 1988, McMahon 1990). Indeed, many morphologic, physiologic, and/or behavioral adaptations have been interpreted as responses to desiccation and heat stresses encountered during tidal emergence.

Shell morphologies, which have been suggested to mitigate effects of thermal and desiccation stress include shape, pigmentation and/or ornamentation, size, and presence of opercula (Vermeij 1973, Heath 1975, Britton 1995). Such morphologies are generally assumed to ameliorate the effects of heat stress while eliminating the need for evaporative cooling and allowing for extended periods of emergence while conserving water (McMahon 1990, Britton 1995, Lang et al. 1998). Physiologic adaptations among gastropods in response to emersion-induced thermal and desiccation stress include metabolic thermal regulation (McMahon & Russell-Hunter 1977) and increased thermal (McMahon 1991, McMahon 2001) and desiccation (Britton 1992) tolerance. Behavioral adaptations to reduce the effects of thermal and desiccation stress in tidally emersed gastropods include selection of favorable microhabitats (Kensler 1967, Raffaelli & Hughes 1978), spatial distribution (Chapman & Underwood 1996), individual posture (Britton 1995), evaporative cooling (McMahon 1988), production of mucus holdfasts (Bingham, 1972), and restricting movement to periods of reduced stress (Peckol et al. 1989, Lang et al. 1998).

While behavioral adaptations in littoral gastropods have been explored, very few studies have examined locomotory rates for individual species, particularly with respect to shore position. Only by simultaneously comparing species from varying shore heights can the adaptive significance of behavioral responses in relation to shore position be assessed in a biologically meaningful manner. In this study, locomotory behavior and habitat selection were used tO examine the role of physical stressors in structuring behavioral adaptations in gastropods from eulittoral and eulittoral-fringe areas.

METHODS

Study Area

Comparisons of locomotory behavior and habitat selection in eulittoral and upper eulittoral-fringe species (referred to hereafter as mid and high-shore species, respectively) were examined on natural limestone hard shores of the eastern Yucatan Peninsula, Caribbean Sea.

Most Caribbean rocky shores are remnants of ancient coral reef communities consolidated over time by intense geologic pressure (Kaplan 1988). The resulting limestone rock is continuously weathered by wave action creating pits, cracks, and tide pools that provide a range of habitats for many littoral organisms (Kaplan 1988). The zonation scheme used to describe tropical hard-shore communities is based on the distinct gradations in rock color from weathered white rock surfaces in the highest zone, to distinctly colored growths on rock surfaces in low shore zones (Stephenson & Stephenson 1950, Kaplan 1988). The white zone is the uppermost region above the upper eulittoral-fringe and in many localities is occupied by upland and/or maritime vegetation. The gray and black zones of tropical hard shores correspond to the upper eulittoral-fringe of more traditional classification schemes. The gray zone is the highest and often widest portion of the upper eulittoral-fringe and because of its height on shore, remains dry for most of the year, except when inundated by rainfall and/or tropical storm surges. The black zone occupies the lower portion of the upper eulittoral-fringe and remains mostly dry due to a lack of wave spray,and daily tidal inundation. However, during spring high tides and storm surges, the black zone can remain wet for extended periods. Below the upper eulittoral-fringe (gray and black zones) is the yellow zone. This is the true intertidal zone (=eulittoral) and exhibits the greatest species diversity. Kaplan (1988) describes a pink zone below the eulittoral corresponding to the lower eulittoral fringe of more traditional classification schemes (Stephenson & Stephenson 1950).