Random walk, zonation and the food searching strategy of Terebralia palustris (Mollusca, Potamididae) in Kenya Marco Vannini a, * , Stefano Cannicci a , Elisha Mrabu b , Rocco Rorandelli a , Sara Fratini a a Dipartimento di Biologia Evoluzionistica ‘‘L. Pardi’’, dell’Universita ` di Firenze, Italy b Kenyan Marine Fisheries Research Institute (KMFRI), Mombasa, Kenya article info Article history: Received 24 May 2008 Accepted 9 September 2008 Available online 11 October 2008 Keywords: mangrove Molluscs Terebralia palustris random walk strategy food searching Kenya abstract Terebralia palustris is a common mud-whelk present at a particularly high density in all Indo-West Pacific mangroves. Young snails feed on nothing but mud while larger specimens are able to feed on fallen leaves too. In Kenya (Mida Creek) under the canopy, competition for mangrove leaves can be very high due to the high density of Sesarmidae crabs. On open exposed muddy platforms, no Sesarmidae occur but the leaf density is very low because the leaves are only randomly present as they are deposited and removed twice a day by the tide. However, the snail density is always very high, raising the question as to whether the snails use a special searching strategy to optimize their resource finding rather than a purely random movement. By analyzing the snails’ movements on a uniform area at different levels and comparing them with simulated random paths, we could show that the snails’ movements are not purely random. The distribution of different size classes of T. palustris in Mida Creek was known to be quite odd: the same simulation approach suggests that the zonation asymmetry could reasonably be due to the stochastic recruitment of juveniles in space and time and maintained by a substantial long-lasting spatial inertia. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Terebralia palustris is a common Indo-Pacific gastropod living on muddy intertidal platforms, especially among mangroves. Within the mangroves it occupies almost the whole mangrove belt from just below the average High Water Spring level to somewhere between the Neap and Spring Low Water levels. In Kenya, within such a wide zone, it can be found under the Avi- cennia marina trees (on the upper levels), under the Rhizophora mucronata or R. mucronata-Ceriops tagal trees (on the relatively lower zones) or in open areas, and on more or less extended muddy platforms bordering the seaward edge of the mangrove belt (Fratini et al., 2004). The young T. palustris are known to feed on mud alone because their radula is insufficiently developed for them to feed on the thick mangrove leaves. However, starting from about 50 mm in length, they become active leaf eaters (Houbrick, 1991; Slim et al., 1997; Fratini et al., 2004). Leaf density should be much higher in the planted zones than on the open muddy platforms where scattered leaves are only found when carried and dropped by the outgoing tide. T. palustris can only detect leaves when crawling on it. Obviously, the probability of creeping on a fallen leaf is relatively higher under the canopy than on the open areas. However, in the planted zones there is a large population of Sesarmidae crabs that also feed on mangrove leaves, leading to a high level of inter-specific competition (Dahdouh- Guebas et al., 1998; Fratini et al. 2000). In the study area (Mida Creek, Kenya), T. palustris appears to be equally densely distributed in both environments: areas under the canopy with their higher density of leaves and of competitors can host an average density of 30–40 animals m 2 (Fratini et al., 2004) with peaks of over 200 animals/m 2 whereas areas with no competitors and rare leaves have recorded peaks of 160 animals m 2 (personal observations). Different areas of Mida Creek appear to be dominated by different densities and different size classes of animals (Fratini et al., 2004) a finding that is reminiscent of the observations of other authors (Plaziat, 1977; Wells and Lalli, 2003). The non-uniform zonation of T. palustris in Mida Creek was attributed to the possibility that the animals tended to remain, may be for years, within the area where they had landed as a small swarm of veligers. In this way the non-uniform spatial distribution could be interpreted as the effect of differential recruitment, i.e. years of local intense recruitment separated by years with no recruitment at all, combined with a substantial spatial inertia. * Corresponding author. E-mail address: vannini_m@unifi.it (M. Vannini). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2008.09.020 Estuarine, Coastal and Shelf Science 80 (2008) 529–537