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1.
The demand for mud crab Scylla serrata (Forsskål 1775) in the global market has increased, hence there is growing momentum to farm the species in Africa. Aquaculture production in Kenya and elsewhere in East Africa currently relies on juvenile seeds sourced from the wild. Wild-seed collection calls for management of the juvenile crab industry founded on knowledge of the species’ ecology, so as to achieve a sustainable seed supply and recruitment to the capture fishery. This study investigated the tidal, diurnal and seasonal occurrence of juvenile crabs in three habitats (intertidal-flat boundary zones, inside the mangroves, and in channels) in small creeks (Mida, Kilifi and Mtwapa) and Gazi Bay, on the coast of Kenya. Sampling was done with scoop nets and seining at receding tides and via burrow searches at low spring tides (day and night). Juveniles in the mangrove/intertidal-flat boundary zone were found sheltered under mangrove leaves or debris, or in shallow burrows during low spring tides, whereas at receding tides they could be seen moving out with the tide or searching for sheltering substrate or burrows. Catch per unit effort at high-abundance sites varied between 59 and 68 crabs fisher?1 day?1. More juvenile crabs, sized 10–80 mm internal carapace width (ICW), occurred at night during the receding tide. Net-seining was effective in the collection of juvenile mud crabs <30 mm ICW, whereas burrow sampling was effective for gathering larger crabs. However, collection of juveniles by seining reduced the quality of the crabs caught due to frequent loss of chelipeds, as compared with retrieving individual crabs by searching burrows. Some juveniles collected in the intertidal-flat boundary-zone habitat were either in the process of moulting or had just moulted, indicating the significance of this habitat for mud crabs at this physically delicate life stage.  相似文献   

2.
We investigated the diversity of patterns of habitat use by juveniles of coral reef fishes according to seasons and at two spatial scales (10–100 m and 1–10 km). We conducted underwater visual censuses in New Caledonia's Lagoon between 1986 and 2001. Co-inertia analyses highlighted the importance of mid-shelf habitats at large spatial scale (1–10 km) and of sandy and vegetated habitats at small spatial scale (10–100 m) for most juveniles. Among all juvenile species, 53% used different habitats across seasons (e.g. Lutjanus fulviflamma and Siganus argenteus) and 39% used different habitats as they grow (e.g. Lethrinus atkinsoni and Scarus ghobban). During their ontogeny, at large and small scales, respectively, 21% and 33% of the species studied showed an increase in the number of habitats used (e.g. L. fulviflamma, L. atkinsoni), 10% and 3% showed a decrease in the number of habitats used (e.g. Amphiprion melanopus, Siganus fuscescens), 23% and 3% showed a drastic change of habitat used (e.g. S. ghobban, Scarus sp.) whereas 46% and 61% showed no change of habitat used (e.g. Lethrinus genivittatus, Ctenochaetus striatus). Changes in habitat use at both small and large spatial scales occurred during the ontogeny of several species (e.g. S. ghobban, Scarus sp.). Results pointed out the different spatial and temporal scales of juvenile habitat use to account for in conservation decisions regarding both assemblage and species-specific levels.  相似文献   

3.
Estimates of abundance and size of three commercially exploited grunt species indicate ontogenetic changes in habitat utilization concentrate their juveniles within the lagoon of the Bay of La Parguera, Puerto Rico. Eleven biotopes, defined by four benthic structures (reef, mangrove, vegetation beds and unconsolidated sediments) and three geographic zones (inner lagoon, outer lagoon and bank shelf) were sampled randomly by visual surveys. French, bluestriped and white grunt (Haemulon flavolineatum, Haemulon sciurus and Haemulon plumeri) were common in the bay and appeared to exhibit similar life history patterns of cross-shelf migration and habitat selection. Recently settled grunts were dispersed over vegetated and unconsolidated soft-bottom sediments of the bay. The juvenile stage occurred in highest densities in shallow lagoon biotopes among the submerged prop-roots of mangrove stands and on inshore reefs. Length data indicates that grunts migrate offshore to adult habitat via increasingly deep reefs. Indices of biotope nursery function based on standing stock estimates of juveniles identified three biotopes, all within the inner lagoon as essential habitat for juveniles of 5–10 cm length interval. This concentration of juveniles within biotopes of the lagoon could represent a bottleneck to recruitment for grunt stocks. Evidence that quantity and quality of lagoon nurseries may limit recruitment indicates that these areas represent a key component of a marine protected area designed to restore fisheries within the bay.  相似文献   

4.
Information on the environmental characteristics of the juvenile habitat of many deposit-feeding sea cucumber species is limited, despite most fished species exhibiting rapid localised depletion. The current study combined large and small scale surveying techniques within a New Zealand harbour to identify areas with high densities of juvenile Australostichopus mollis, a commercially valuable aspidochirote holothurian. Data from detailed surveys were used to relate densities of juveniles and adults with measures of physical habitat characteristics including depth, sediment facies type, grain size range, as well as measures of chlorophyll-a, phaeopigment, carbon and nitrogen content of surface sediment. Results revealed a highly localised distribution of juvenile A. mollis focused on one site associated with an area of high adult density. Sites of high juvenile A. mollis density were characterised by sediment qualities favouring epibenthic detritivorous deposit feeding, including high nitrogen content, high phaeopigment:chlorophyll-a ratio and small grain size. The high-density juvenile site had facies that were further characterised by the presence of large shell fragments (>10 cm length) of the horse mussel (Atrina zelandica), which may provide a unique settlement microhabitat for early juveniles. Unlike some other sea cucumber species, juvenile A. mollis shows no distinct spatial separation from adult sea cucumbers, no association with dense macroalgae and no clear preference for shallower depths than adults. Overall, the results illustrate the highly localised pattern of recruitment of this species to a widely distributed adult population, which may help to explain the lack of previous observations of juveniles in this species. These results indicate the importance of identifying and protecting what appear to be very specific juvenile habitats in deposit-feeding sea cucumbers to ensure continuing recruitment to exploited populations.  相似文献   

5.
There is little information on the movement and connectivity patterns for many species. The movement by shells occupied by the hermit crab Pagurus bernhardus and the organisms encrusting these shells was investigated on the south coast of Wales (UK). Hermit crabs shells moved considerable distances along the shore over 1 month periods, moving a minimum mean distance (±SE) of 148 (±8) and 174 (±9) m from release sites in the January and August respectively. Hermit crab-inhabited shells were also found to travel across habitats (sandy areas) that are unsuitable for both hermit crabs and epibionts. Through the examination of 150 of the most abundant shells used by P. bernhardus (Nucella lapillus), twelve epibiont species were found (10 animal and 2 algal species) and choice experiments demonstrated that hermit crabs preferred epibiont covered shells (84%), compared with bare shells (16%). The distance that shells were moved and the preference of hermit crabs for epibiont encrusted shells, coupled with the ability for epibionts to cross unsuitable habitats, may provide a dispersal advantage for epibiont species.  相似文献   

6.
By the consumption of algae, parrotfishes open space for young coral settlement and growth, thus playing a central role on the maintenance of coral reefs. However, juvenile parrotfish ecology is often overlooked due to the difficulty discerning species during this phase. Herein, we present the first attempt to investigate changes in habitat use and diet that happen to juveniles of the Redeye parrotfish Sparisoma axillare, focusing on four zones within an algal‐dominated reef: the macroalgal beds, back reef, reef flat, and fore reef. Smaller S. axillare juveniles (<5 cm) preferred to inhabit the macroalgal beds and the reef flat, whereas juveniles larger than 5 cm were more abundant in the back and fore reefs due to distinct post‐settlement habitat conditions. Aggressive interactions with the territorial damselfish Stegastes fuscus were the primary driving factor of juvenile distribution and feeding rates. Attack rates increased with juvenile size and the lowest bite rates were observed in zones with higher densities of territorial damselfish. In previous studies, the persistence of parrotfish recruits in habitats dominated by damselfish was reduced, but newly settled parrotfish occurred more densely within the damselfish domain by behaving as a cryptic reef fish. As these juveniles grew, their bite rates increased, a change associated with a shift from cryptic to roving behavior. Feeding preferences were determined by substrate cover, where juveniles fed on available food sources in each habitat. Juveniles relied on jointed calcareous algae in habitats dominated by these algae, a pattern not observed for thick leathery algae. Filamentous algae were the preferred food for smaller fish; for individuals greater than 10 cm, a higher ingestion of sand was observed. Most studies evaluating the functional role of parrotfish do not consider species feeding preferences. However, the potential for a species to turn an impacted reef back to a coral‐dominated phase is influenced by their food selection, which is dependent on the algal species composition.  相似文献   

7.
The habitat in which predator–prey interactions take place may have a profound influence on the outcome of those interactions. Cannibalism is an intriguing form of predation whereby foraging by predators may contribute to the regulation of their own populations.This is particularly interesting in the case of invasive species, like the widely distributed European green crab (Carcinus maenas). This study explores how habitat complexity influences cannibalism rates in green crab populations of Prince Edward Island, Atlantic Canada. Both laboratory and field experiments were conducted to measure feeding rates by individual adult green crabs on a standard number of smaller conspecifics. In the laboratory, experimental treatments mimicked unstructured to increasingly structured habitats: water, sandy bottom, oyster shells, mussel shells, oyster shells with sandy bottom and mussel shells with sandy bottom. In those trials, adult green crabs consumed several times more juveniles on unstructured habitats than on the most structured ones, with a gradual decrease in predation rates across increasingly complex habitats. Field inclusion experiments used the same approach and were conducted in sandy bottoms, sandy bottoms with a layer of oyster shells and sandy bottoms with a layer of mussel shells. These trials showed similar patterns of decreasing feeding rates across increasingly complex habitats, but differences among treatments were not significant. These results support the idea that complex habitats have the potential to mediate predator–prey interactions, including adult–juvenile cannibalism in green crabs.  相似文献   

8.
Vulnerability to predation may be high for many megafaunal taxa in deep‐sea sedimentary habitats where physical heterogeneity is low. During ROV observations in a bathyal sediment plain off Central California, juveniles of the lithodid crab Neolithodes diomedeae were frequently observed on or under the holothurian (sea cucumber) Scotoplanes sp. A, and are hypothesized to benefit from this association as a nursery or refugium from predation. Ninety‐six percent (n = 574 of 599) of the juvenile N. diomedeae observed (density varied from 0.02–0.75/m2 among sites and seasons) in the study area were associated with Scotoplanes sp. A. Of the 2596 Scotoplanes sp. A observed (density varied from 0.48 to 25.90/m2), 22% were attended by at least one juvenile crab, and rarely two crabs (n = 4). Solitary N. diomedeae were rarely observed. This decapod–holothurian symbiosis appears to be largely commensal, with juvenile crabs (carapace width = 0.03–0.31 ×  holothurian length) observed on or beneath Scotoplanes sp. A in a habitat with few refugia from epibenthic predators. Other hypotheses may explain or enhance the potential benefits of the association for N. diomedeae, such as elevated food availability due to the activities of Scotoplanes sp. A. The relationship may be mutualistic if there is a benefit for the holothurian, including the removal of epizoic parasites. Ultimately, the nursery or other effects on the population dynamics of N. diomedeae may be minimal in low‐relief, sediment‐dominated habitats, as very few sub‐adult crabs were observed in the study area and were likely consumed upon outgrowing their refugia. While sedimentary habitats may be a sink for N. diomedeae populations, growth of juvenile crabs during their association with Scotoplanes sp. A should increase energy flow to its predator populations. This association has not been reported previously but may be expected in sediment‐dominated habitats where these species overlap.  相似文献   

9.
The distributions of juvenile brown shrimp, Penaeus aztecus, and white shrimp, P. setiferus, (57–72 mm TL) were monitored in partially vegetated laboratory enclosures to examine selection for vegetative structure. Simulated Spartina culms, composed of green straws, were used as structure in the experiments. Brown shrimp selected for this structure during the day but not at night. White shrimp showed no strong selective preference for structure during the day or night.Selection in the field was measured by examining densities of the two species of shrimp (40–80 mm TL) in a Galveston Bay, Texas, salt marsh. Brown shrimp were more abundant in vegetated habitats (Spartina alterniflora) than in adjacent non-vegetated areas. Differences in white shrimp densities between the two habitats were variable, and no consistent pattern was observed.laboratory predation experiments were conducted in non-vegetated and vegetated cages using Atlantic croaker (Micropogonias undulatus) as predators. In non-vegetated cages there was no significant selection for either species of shrimp by the fish. In partially and completely vegetated cages, fish selected white shrimp over brown shrimp. These data suggest that the presence of vegetation alters prey selection by Atlantic croaker, and this interaction may result in greater mortality of white shrimp compared with brown shrimp in salt marshes.  相似文献   

10.
Timing, microhabitat selection and behavior from the onset of settlement to recruitment to the adult population of juvenile fishes of the genus Diplodus (Pisces: Sparidae) were investigated along a rocky coastline in the Central Mediterranean Sea. The settlement periods in Diplodus sargus and Diplodus annularis were concentrated in spring, between late May and early June, and the recruits leave the nursery grounds in late September–October. Juvenile fishes of Diplodus puntazzo and Diplodus vulgaris showed a partial time overlapping, sharing the same zones in winter and early spring, from February to May. Multiple correspondence analysis showed that sea breams settle in well‐defined habitats. The smallest juveniles of D. sargus and D. puntazzo settled primarily in the shallowest sheltered pebbly areas, located in sciaphilous crannies covered by red algae. Diplodus vulgaris settlers were observed on a wider range of substrata: rock on sand, gravel and pebbles without algal cover or large boulders, generally in deeper waters. The intermediate‐size juveniles of D. sargus, D. puntazzo and D. vulgaris showed a preference for rocky substrata with substantial algal cover, with arborescent structures (Phaeophyceae). Diplodus annularis juveniles showed high fidelity to seagrass beds (Posidonia oceanica). The home range increased over time in all species, highlighting a loss of substrate specificity: larger juveniles were even observed in deeper and different microhabitats outside nursery grounds. This study suggests that shallow infra‐littoral rocky communities with photophilic algae play a key role in recruitment of sparid fishes, affecting the distribution and abundance of juvenile fishes and therefore determining the renewal of populations and the structure of adult assemblages.  相似文献   

11.
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12.
Prey availability is one of the most important factors affecting the quality of nursery grounds. Estuaries play an important role as nursery grounds for juvenile stone flounder, but the mechanism behind the consistently high availability of prey has never been examined. This study investigates which prey is mainly selected by juvenile stone flounder (15–55 mm standard length) in the estuary of the Natori River, northern Japan. In a cage experiment, juveniles showed positive selection for the palps of the spionid polychaete Pseudopolydora kempi in March, and for the siphons of the bivalve Nuttallia olivacea in April, May and June in both sandy and muddy-sand habitats. This selective predation showed that sublethal predation on regenerable parts of invertebrates is important for stone flounder. Nuttallia olivacea, the dominant bivalve in the estuary, was more abundant and in better somatic condition in the sandy area in spite of the stronger siphon-cropping pressure by juvenile stone flounder. These results confirm that sublethal predation on highly abundant benthos plays an important role in forming estuarine habitats into areas of high prey availability for juvenile stone flounder, which leads to their high growth rate.  相似文献   

13.
The gastropod Terebralia palustris often dominates the surface of muddy to sandy substrates of intertidal mudflats and mangrove forests, where they clearly destabilize the sediment. In the present study, it was investigated whether and to what extent the behaviour of juvenile and adult snails differs among habitats (mudflat vs. mangrove stand) in a Sonneratia alba mangal at Gazi Bay, Kenya. For this purpose we: (1) examined their distribution along three land–sea transects; and (2) applied stable isotope analysis to determine the feeding patterns of different-sized snails from the mangrove and mudflat habitats. Additionally, we investigated if these gastropods exert an impact on microphytobenthic (diatom) biomass, and whether this is size-dependent. The latter objective was met by either enclosing or excluding different-sized snails from experimental cages on the intertidal mudflat and the subsequent assessment of a change in pigment concentration of the sediment surface. In agreement with several previous studies conducted in other mangroves and geographical locations, a spatial segregation was demonstrated between juveniles (more common on the mudflat) and adults (more common in the mangrove forest). On the intertidal mudflat juveniles avoided sediment patches characterized by highly saline water in intertidal pools and a high mud content, while adults tended to dwell on substrates covered by a high amount of leaf litter. Stable carbon isotope analysis of the foot tissue of snails sampled from the S. alba stand and the mudflat indicated a transition in food source when a shell length of 51 mm is reached. Considering the δ13C value of juveniles, it seems they might be selecting for microphytobenthos, which might explain their preference for the mudflat. The diet of size classes found in both habitats did not differ significantly, although juveniles inhabiting the mangrove forest were slightly more depleted in 13C compared to those residing on the mudflat. Assuming juveniles feed on benthic microalgae and considering the lower microalgal biomass inside the mangrove forest, this may be a consequence of a higher contribution of other, more 13C depleted organic carbon sources, like phytoplankton, to their diet. Experimental results indicate a negative, but insignificant, impact on benthic diatom biomass by juveniles (due to grazing) and adults (due to physical disturbance). This finding seems to be in agreement with the results of the stable carbon isotope analysis, strongly suggesting the selective feeding of juvenile T. palustris on benthic diatoms.  相似文献   

14.
Well-developed aerial roots of mangroves make it difficult to study how fish utilize the mangrove forest as a habitat. In the present study, we compared the differences in fish assemblages in three major types of habitats of mangrove estuary (vegetated area, treeless mudflat, and creek) of a mangrove bay in Hainan Island, China, at different seasons during two consecutive years. Three types of gears, centipede net, gill net and cast net, were used in the different habitats of mangrove estuary and sampling efficiencies among gears were evaluated. Centipede nets were used in all the three types of habitats and cast nets and gill nets in treeless mudflats and creeks. Fish assemblages were dependent on gears used. Centipede net could efficiently catch fish occurring both inside and outside of vegetated areas efficiently. A total of 115 fish species in 51 families were collected. In terms of numbers of species per family, Gobiidae was the most diverse (17 species), followed by Mugilidae (5 species). Almost all of the fish were juvenile or small fish and few predators were recorded, implying low predation pressure in the bay. ANOVA analysis showed that significant seasonal and spatial variation existed in species richness, abundance, and biomass, which were less in the vegetated areas than those of treeless mudflats and creeks. The attraction of vegetated areas to fish was less than that of creeks and mudflats. Many species were specific to a particular habitat type, 4 species occurring exclusively in the creeks, 45 species occurring exclusively in the treeless mudflats, and 5 species occurring exclusively in the vegetated areas. The results indicated that mangrove estuaries were potentially attractive habitats for juvenile and small fish, but this attraction was accomplished by a connection of vegetated areas, treeless mudflats and creeks, not only by vegetated areas.  相似文献   

15.
Predation by the big brackish-water isopod Saduria entomon on two age-classes (juveniles and adults) of the small deposit-feeding amphipod Monoporeia (syn. Pontoporeia) affinis was studied at different oxygen conditions (normoxia>11 mg/l O2, and moderate hypoxia 4 mg/l O2). Vulnerability of juvenile and adult M. affinis was studied in single and mixed age-class treatments. The proportions of juveniles and adults were varied at one total numerical density. The predatory effects were measured as numbers of individuals consumed and of total biomass intake (mg ash-free dry weight). The highest predation rate (in numbers) was detected in the single-prey treatment with juvenile M. affinis, both in normoxia and moderate hypoxia. No preference for any of the two size classes was found in the treatment with equal numbers of adults and juveniles. In normoxia, the total number of prey consumed decreased with increasing proportions of adults, while intake increased in terms of biomass. In moderate hypoxia, high proportions of adults reduced the predation rate of S. entomon to such an extent that also the intake in terms of biomass was significantly negatively affected.  相似文献   

16.
为从行为学角度解释中国蛤蜊(Mactra chinensis Philippi)的跑滩现象, 研究了规格、温度对其跳跃行为以及底质、流速和周期性干露对其潜沙行为的影响。结果表明, 中国蛤蜊运动能力与其规格和水温有关, 3 mm稚贝不具备跳跃能力, 5 mm以上具备跳跃能力; 跳跃频率和高度随水温的升高呈上升趋势。中国蛤蜊稚贝潜沙比例均随时间推移逐渐上升; 各时间节点中国蛤蜊稚贝在细沙底质的潜沙比例最高, 中沙底质次之, 粗沙底质最低。施加微弱水流后, 中国蛤蜊稚贝潜沙速度和比例显著增加; 流速增大到一定程度后部分稚贝被水流冲走, 潜沙比例随之下降。周期性干露会导致中国蛤蜊稚贝从底质中爬出, 干露时间越长, 爬出比例越高。干露后的稚贝重新放置于海水中, 部分稚贝会在水面漂浮一段时间后再下落至水底, 漂浮比例随干露时间的增加而增加。本研究从行为学角度初步查明了中国蛤蜊跑滩现象, 为解决中国蛤蜊防跑滩问题提供了思路。  相似文献   

17.
One of the present concerns of fish biologists involves defining and identifying nursery habitats in the context of conservation and resource management strategies. Fish nursery studies usually report upon nursery occupation during the latter juvenile stages, despite the fact that recruitment to nurseries can start early in life, during the larval phase. Here we investigated the use of a temperate estuarine nursery area, the Lima estuary (NW Portugal), by initial development stages of flatfish species before and after metamorphosis, integrating the larval and juvenile phases. The Lima estuarine flatfish community comprised twelve taxa, seven of which were present as pelagic larvae, six as juveniles and three as adults. There was a general trend of increasing spring–summer abundance of both larvae and juveniles, followed by a sharp winter decrease, mainly of larval flatfishes. The Lima estuary was used by Solea senegalensis, Platichthys flesus and Solea solea as a nursery area, with direct settlement for the two first species. In contrast, indirect settlement was suggested for S. solea, with metamorphosis occurring outside the estuarine area. Estuarine recruitment of S. senegalensis varied between years, with young larvae occurring in the estuary throughout a prolonged period that lasted 6–9 months, corroborating the protracted spawning season. P. flesus, the second most abundant species, exhibited a typical spring estuarine recruitment, without inter-annual variations. Developed larvae arrived in the estuary during spring, whereas the 0-group juveniles emerged in the following summer period. The present study contributes new insight to our understanding of the economically important S. senegalensis, and highlights the importance of integrating the planktonic larval phase into traditional flatfish nursery studies.  相似文献   

18.
Growth is determined by an organism's physiology, physical environment, and biological conditions, including food availability and any intra‐ and inter‐specific interactions that can affect feeding activity. To analyse how all these factors interact to produce final growth in the herbivorous/detritivorous crab Neohelice granulata, we performed field and laboratory experiments with juveniles and adults from three populations which differed genetically as well as in their physical environment and the organic matter (OM) content in the soil that serves as food. We evaluated (a) growth in the field: Juveniles of the three populations were cross‐transplanted in exclusion cages; (b) effect of the presence of adults on juvenile feeding: We measured the feeding activity of small juveniles in presence/absence of an adult male as potential predator in the field; and (c) effect of diet on cannibalism: We analysed the cannibalistic behavior of adult males from two of the populations in the laboratory after they had received protein‐rich (24%) and protein‐poor (3%) diets. In experiment (a), final size was similar for the crabs from all three origins but growth differed between sites. Experiment (b) showed that the presence of adults interfered with feeding activity at the two sites with lower weight indicators. In experiment (c), we observed that low protein diet increased the cannibalistic behavior of adult males, and this effect was more intense in crabs from the poorest food site. Our results contribute to understanding the set of factors and interactions involved in the response of individuals to the prevailing conditions in natural environments in order to maintain a growth rate, perhaps at the expense of different reserve accumulation. They also enable discussion of the limitations of approaches used in laboratory experiments.  相似文献   

19.
The present study explored ontogenetic shifts in habitat associations by coral reef fishes between recently settled juvenile and adult life stages (Moorea Island: Tiahura and Papetoai sites). Visual censuses highlighted four ontogenetic patterns in habitat associations: (1) no change in habitat associations between the juvenile and adult stages; (2) a decrease in the number of habitats used by adults compared to juveniles; (3) an increase in the number of habitats used during the adult stage; and (4) use of nursery areas by juveniles followed by an extensive movement to an entirely different adult habitat. The comparative analysis of spatial distribution of fish at Tiahura and Papetoai highlighted no-spatial variability in ontogenetic patterns (i.e., 10 of the 15 recorded species have spatial consistency in ontogenetic patterns). Overall, the shifts in habitat associations are of interest in the perspective of understanding flexibility and adaptation capability of coral reef fish, at least at the settlement time.  相似文献   

20.
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.  相似文献   

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