Small-scale temporal and spatial variability in foraging behaviour of the mid-shore gastropod Nerita yoldii on seasonal, tropical, rocky shores

Abstract

Describing temporal variation in the foraging behaviour of intertidal gastropods is important to gain an understanding of the environmental conditions that constrain their activity patterns. Foraging behaviour in the tropical gastropod Nerita yoldi was mainly determined by the tidal cycle: individuals foraged when they were awash, or emersed when the rock was wet, migrating downshore to forage and upshore when they returned to refuges. Apart from tidal influences, foraging in N. yoldii also varied with season and tidal state. Individuals were generally more active in summer than in winter and on neap than spring tides; although small-scale daily variation existed. Such temporal variation may be caused by several interacting environmental variables, especially air temperature, pressure and irradiance, which influence physical conditions as well as individuals' metabolism. Individual foraging patterns were also highly labile between seasons: most individuals would stop and remain stationary on the open rock surface in winter, but returned to refuges after foraging in summer, a pattern probably driven by physical stress. The foraging behaviour of N. yoldii, therefore, is predictable on a coarse scale, in terms of when they are active, but highly labile in duration and foraging range, which indicates the importance of local, daily variation in environmental factors controlling foraging on shores that have strong temporal changes in abiotic conditions. KEY WORDS: Nerita · Foraging behaviour · Tropical shoreResale or republication not permitted without written consent of the publisher Aquat Biol 16: 177-188, 2012 Such lability in foraging behaviour represents important adaptive traits in organisms (Mery & Burns 2010). Climatic seasonality, for example, is a large time-scale temporal pattern that affects foraging behaviour owing to changes in physical stress, predation pressure and food availability as well as associated changes in tidal and diurnal conditions. As a result, such variation in foraging behaviour can make it difficult to define precise behavioural patterns of a species (Chapman & Underwood 1992a, Ng & Williams 2006, Santini et al. 2011.The gastropod genus Nerita Linnaeus 1758 (Neritacea) has a high abundance and diversity in tropical regions (Vermeij 1973). Most species, such as N. textilis (Chelazzi et al. 1983) and N. polita (Chelazzi 1982) are zonal shuttlers (Chelazzi et al. 1988), which forage from a refuge either up-or downshore during each tidal cycle before returning to their resting level (Chelazzi et al. 1988). Previous observations on the behaviour of Nerita have not, however, investigated longer-term variations in foraging patterns, such as monthly or seasonal variation, al though a longerterm study of N. plicata, N. textilis and N. undata in Kenya showed these species to be more active on spring than neap tides (Ruwa & Jaccarini 1988).Nerita are important herbivores in the Hong Kong rocky intertidal zone (Hill 1980). In contrast to other tropical localities where the foraging beh...

Where three snail species attach while emersed in relation to heterogenous substrate temperatures underneath intertidal boulders

“…Nerita atramentosa , D. concameratum and B. nanum all retreated to the lower surfaces of boulders when emersed, with upper surfaces at low tide rarely occupied by any mobile biota (N. Janetzki, 2016, personal observation). This avoidance of dry, sun-exposed surfaces is consistent with observations made for a variety of gastropod species on seashores in Panama ( Garrity, 1984 ) and Hong Kong ( Yeung & Williams, 2012 ). It is thought that the protected lower surface of boulders, relative to more exposed substrates (e.g.…”

Untangling the roles of microclimate, behaviour and physiological polymorphism in governing vulnerability of intertidal snails to heat stress

“…This small-scale variation in temperature provides a spectrum of thermal environments that can provide refugia to avoid lethal or sublethal heat stress [18]. The ability of an organism to benefit from those refugia depends on environmental factors [53], the rate at which it can move [54], its size relative to the microhabitat [21], and the frequency at which the organism can access shelter during unpredictable extreme conditions [55]. Thus, the 'grain size' of the thermal environment will vary among interacting species [21].…”

“…Small-scale (i.e. between days) variability in duration of feeding and inactivity as a result of changing environmental conditions has been demonstrated for a number of intertidal species, suggesting these animals are highly plastic in their responses to daily environmental changes [53,60,61]. rspb.royalsocietypublishing.org Proc.…”

Why are ‘suboptimal’ temperatures preferred in a tropical intertidal ectotherm?

“…Thin acetate sheets (0.25 mm) were used as the bottom substrate of the runways to avoid direct contact of the snails with the aluminium block. Movements were measured in air to mimic scenarios when the snails are emersed by the tide and moving on drying rocks until stopping at locations where they will remain stationary for the remaining emersion periods (Yeung & Williams, 2012). After each recording, the runways were cleaned using 75% ethanol and water to remove mucus trails before the next trial to ensure independence among individuals.…”

“…Similar to other tropical nerites such as Nerita polita and N. textilis which are active during emersion periods (Chelazzi, 1982;Vannini & Chelazzi, 1978), N. yoldii are also active during tidal emersion at both day and night times (Yeung, 2006). N. yoldii undertakes foraging excursions from refuge to sun-exposed habitats when emersed by the tide and, as the tide recedes, the number of active snails reduces and activity ceases when individuals are completely immersed by the next tide (Yeung & Williams, 2012). At the boulder shore in Lung Kwu Tan, temperature differences between the snails' foraging areas and refuges can exceed 20°C; with temperatures at sun-exposed habitats averaging 30°C (and reaching 58°C) during the hot and wet season, while rock temperatures at refuge habitats (areas shaded by boulders) averaged at 28°C and never exceeded 40°C (see Supporting Information for quantification of the general thermal environment).…”