Effect of disturbance on foraging: whelk activity on wave-exposed rocky shores with minimal tidal range

For slow-moving rocky intertidal predators such as whelks, frequent stress can be detrimental. Most studies on the foraging behavior of whelks have been conducted in regions where tidal amplitude is >1 m and on relatively small species. We studied the effects of wave action, desiccation, and water temperature on the foraging behavior of a large (approximately 80 mm) rocky shore whelk, Stramonita haemastoma, at the small-tide (<40 cm) but wave-exposed Israeli Mediterranean coast. We hypothesized that in such conditions foraging activity of this large predator will be greatly limited. Field observations show that when wave height exceeds 0.5 m, activity of whelks decreases by at least threefold relative to that seen under calm-sea conditions, and most whelks remain inside shelters. Wave height data from offshore moorings demonstrate that such conditions occur along the coast >85% of the time. Whelks move into shelters or completely disappear when exposed to prolonged desiccation conditions that can last for days along the shore during spring and autumn. Foraging activity is greater at night than in daytime and might be related to lighter winds and calmer sea conditions at night. Laboratory experiments show that elevated water temperatures during winter do not result in increased predation rates relative to those measured during spring. This indicates that a seasonal endogenous biological rhythm may exist. Our findings demonstrate that the foraging activity of large whelks can be extremely constrained by high rates of disturbance, that is, frequent wave action, and less frequent desiccation. We claim that high rates of disturbance in the eastern Mediterranean explain why large whelks are scarce in intertidal habitats where food is plentiful but suitable shelter is rare.Communicated by G.F. Humphrey, Sydney     

Feeding ecology of a size-structured predator population,the South American sun-star Heliaster helianthus

Patterns of feeding in a population of Heliaster helianthus (Lamarck), a common and dominant species of starfish indigenous to the Pacific South American coast, were investigated in an intertidal habitat in central Peru from October 1986 to April 1987. The H. helianthus population comprised individuals of 3.5 to 30.2 cm body size (diameter) with two modal size classes. The number of rays ranged between 18 and 40, and individuals with 31 to 33 rays accounted for ca. 42% of the total population. There was a higher rate of increase in ray number with body size amongst small individuals(<13.0 cm diam). H. helianthus is capable of feeding on more than one prey item at a time (average of 5.6 to 13.2 prey items handled, with several predators observed to hold >100), and both the number of prey individuals captured and the total prey biomass were significantly correlated with predator size. Amongst a total of 1132 feeding observations, the largest number of predators (an average of 85.4% of those feeding) were preying on the mussel Semimytilus algosus whilst another mussel, Perumytilus purpuratus, ranked second with 21,9% of predators feeding. The proportion of S. algosus in the diet increased from 65.4% in the smallest predator size-group (10.9 cm diam) to 91.2% in the largest (19.0 cm). In contrast, P. purpuratus and barnacles were more highly represented in the diet of small H. helianthus. The smallest size-group (10.9 cm) had low dietary overlap with larger sizes and less specialized prey utilization. Two geographically separated populations of H. helianthus in Peru and Chile showed contrasting patterns of prey utilization. S. algosus and P. purpuratus comprised 85.5 and 6.5% by number in the diet of the Peruvian population, respectively, whilst corresponding figures for the Chilean population were 8.3 and 60.5%, with barnacles attaining a higher share (22.6%). However, the total number of prey individuals per feeding predator was almost the same in Peru and Chile, with 10.0 and 10.7 individuals, respectively. H. helianthus individuals of different sizes occupy slightly different microhabitats within the intertidal area, which, coupled with differential spatial distribution of prey species, results in the predator population being able to utilize a wide range of resources.     

Escape responses of four Chilean intertidal limpets to seastars

The Chilean intertidal Lottiidae limpet fauna consists of at least nine species, grouped into the monophyletic taxon Scurria and a single taxon preliminarily designated as `Lottia' orbignyi (Dall). Within the Scurria clade, the most basal species are Scurria variabilis (Sowerby), S. viridula (Lamarck), and S.zebrina (Lesson), and among the most crown groups are S. ceciliana (Orbigny) and S. araucana (Orbigny). This phylogenetic information, obtained by the analysis of molecular characters (16S mtDNA), provides a framework for a comparative study of behavioral characters. In this study, we analyze the escape responses of two basal limpet species (`L.' orbignyi and S. viridula) and two derived limpet species (S. araucana and S. ceciliana) in the presence of the limpet predatory seastar Heliaster helianthus (Lamarck) and the limpet non-predatory seastars Stichaster striatus Muller and Troschel and Patiria chilensis Lutken. Neither P.chilensis nor Stichaster striatus induced escape responses such as those observed with the predatory H. helianthus. Moreover, in the presence of H. helianthus, basal and derived limpet species differed significantly in the percentage of individuals responding, reaction time, and duration of the response. The basal species `L.' orbignyi and Scurriaviridula exhibited instantaneous and vigorous locomotor responses, whereas the derived species S. araucana and S. ceciliana displayed no locomotor responses. The same pattern was found in responses to H. helianthus homogenates. The results strongly suggest a correlation between phylogeny and anti-predator escape responses to the seastar H. helianthus. Alternative escape responses have evolved for basal and derived species within the Chilean clade. Received: 27 April 2000 / Accepted: 16 August 2000     

High intertidal community organization on a rocky headland in Maine,USA

A mosaic patchwork of the barnacle Balanus balanoides L., the mussel Mytilus edulis L., and the alga Fucus vesiculosus L. was found in the transitional region between the mid and high intertidal zones on a rocky headland on Mount Desert Island, Maine, USA. The development of the mosaic was observed by following recruitment and survival of B. balanoides in denuded patches located at the same tidal level along a 60 m stretch of shore. Barnacle recruitment was least under canopies of F. vesiculosus and greatest in open areas kept moist at low tide by surf. Barnacle survival after settlement was least under the F. vesiculosus canopy due to the whiplash effect of the algal fronds in the surf and greatest in open areas free from competition from mussels. In open areas, early mortality was correlated with settlement density. In areas of dense settlement (60 spat cm^-2) up to 90% mortality resulted within 5 months from crowding associated with growth. In older individuals crowding produced hummocks of elongated, weakly attached barnacles which were more prone to removal by surf than uncrowded barnacles. Mussels exerted competitive dominance over barnacles for space and the presence of mussel beds prevented further barnacle recruitment. Mussels suffered extensive mortality during winter storms when surf removed dense mats of weakly attached mussels. The patchy distribution of mussels and barnacles results from irregular rock substrata producing numerous environmental patches with respect to wave exposure and drainage at low tide, and from densitydependent mortality of both mussels and barnacles which creates patches of new colonizable space within each environmental patch.     

Comparison of mussel bed community between two intertidal mytilids Septifier virgatus and Hormomya mutabilis

Mussel bed community structure of two intertidal mytilids, Septifer virgatus (Wiegmann) and Hormomya mutabilis (Gould), whose beds were contiguous vertically on a rocky intertidal shore (Wakayama Prefecture, Japan), was compared between 1982 and 1983. In the upper S. virgatus bed, crustaceans and bivalves were dominant in terms of both number of individuals and biomass. There were three barnacle species representative of the epizoans, two isopods and one amphipod as mobile fauna, and two bivalves as infauna. The lower H. mutabilis bed supported virtually no epizoans or mobile fauna. Infaunal free-ranging polychaetes and sipunculids were dominant in terms of both number of individuals and biomass. The H. mutabilis bed contained a much greater amount of sediment than did the S. virgatus bed, and the interstices among individual H. mutabilis and among their byssal threads were filled with sediment. The biomass of six of nine species dominant in the S. virgatus bed was negatively correlated with the amount of sediment. Recruitment of these faunal assemblages into artificial mussel clumps was examined in mussel enclosure experiments, and a negative sediment effect in H. mutabilis clumps was detected for one isopod and one limpet species. Factors causing the differences between two mussel bed communities are discussed, focusing on the effects of sediment.     

Rhythmic vertical migration of the gastropod Cerithidea decollata in a Kenyan mangrove forest

In Mida Creek, Kenya (3°20′S, 40°5′E), at high water, the snail Cerithidea decollata dwells on the trunks of mangrove trees (Avicennia marina), while during low water it migrates to the ground, foraging at various distances from the trunk, where it aggregates again well before the incoming tide. Snails from the upper shore level are 150–200 m distant from those living at the lower shore level and they cluster at lower heights on trunks. In any case, sufficient height is usually attained to avoid being submersed. An experiment was designed (February and October 2005), exchanging individuals from different shore levels subject to different tide regimes, in order to test whether snails rely on internal information or on external, direct cues, to adapt their behaviour to local conditions. Results show that C. decollata mostly rely on internal information, presumably based on an internal clock. When individuals from upper and lower shore levels were exchanged, their internal clocks continued to govern when to ascend the home trunk and how high to climb for five to six successive tides, after which the behaviour was reset to the new local conditions.     

Foraging and homing behaviour in the high-shore, crevice-dwelling limpet Helcion pectunculus (Prosobranchia: Patellidae)

The foraging activity of the high-shore, crevice-dwelling limpet Helcion pectunculus (Gmelin, 1791) (Prosobranchia: Patellidae) from an exposed shore on the eastern Cape coast of South Africa was monitored. Activity was compared during spring and neap tides and between spring, summer, autumn and winter. Rhythms of activity in this limpet varied depending upon microhabitat; individuals inhabiting both east- and west-facing rock surfaces were active during nocturnal low tides, but limpets on west-facing rock surfaces were also active during daytime low tides, whilst in the shade. Individuals travelled further during foraging excursions in winter (xˉ=85.53 cm) than in either spring (xˉ=55.7 cm) or summer (xˉ=48.8 cm) and also during spring low tides (xˉ=89.8 cm) compared with neaps (xˉ=40.9 cm). H. pectunculus exhibited rigid homing to a fixed scar within a crevice, and feeding excursions were found to consist of three distinct phases: a relatively rapid outward phase, a much slower foraging phase and a rapid homeward phase. Whether or not these limpets graze throughout an excursion is not known. Foraging was always highly directional, with a mean vector which took limpets onto an area of the rock face with the highest microalgal biomass. The fact that H. pectunculus took advantage of optimal feeding areas and memorized their location to enable return visits suggests a learning component in herbivorous gastropod foraging behaviour. Received: 29 December 1997 / Accepted: 8 May 1998     

Movement patterns of the limpet Cellana grata (Gould) observed over a continuous period through a changing tidal regime

Time-lapse videography of limpets mounted with light-emitting diodes was used to monitor the movements of a population of the non-homing Cellana grata on a vertical gully wall in Hong Kong. Animals were monitored for >7 days to examine spatial and temporal variation in their behaviour as the tides transited from a semi-diurnal to an almost diurnal pattern. Movement was synchronised with the tides, irrespective of the day–night cycles. Limpets rested low on the shore and were stimulated to move by the rising tide. Individuals moved up shore with the flooding tide, maintaining themselves in the awash zone, and then down shore on the ebbing tide, until they reached a resting height when the tide then fell beneath them, exhibiting ‘zonal shuttling’. A tight coupling of limpet position to tide height persisted through the changing tidal pattern, and almost all animals displayed the same organisation of activity over all tides. Initiation of activity and maximum height reached were probably controlled by the tides, but the cessation of activity may have been controlled by an internal clock. The pattern observed is consistent with the threat of attack from aquatic predators coupled with the need to minimise physical stresses while exposed. It is also consistent with the avoidance of grazing lower on the shore where interspecific competition may be more intense. Limpets showed two peaks of activity per tide, corresponding to when the rate of change of tide height was the greatest, except when tides became much reduced during the transit to a diurnal pattern. Movement was triphasic: an initial rapid phase, followed by a slower phase in the high shore and then a rapid phase before the limpets stopped moving. This structure is common in limpets and in this case is likely to be a consequence of animals maintaining themselves within the awash zone. Tide height appears to determine foraging activity, but with modifications in the behaviour in response to factors operating at more local temporal and spatial scales.     

Microhabitat and resource utilization in cryptic rocky intertidal echinoderms at Aldabra Atoll,Seychelles

On a sheltered upper eulittoral rocky shore at Aldabra Atoll, 5 cryptic, tropical holothurian species Afrocucumis africana (Semper), Holothuria (Selenkothuria) parva Krauss, Polycheira rufescens (Brandt), H. (Thymiosycia) impatiens (Forskaal) and H. (Mertensiothuria) leucospilota Brandt, in decreasing order of abundance, co-occurred on intact beach-rock, whereas the only ophiuroid Ophiocoma scolopendrina (Lamarck) lived mainly on rubble. The 3 commonest holothurian species (A. africana, H. parva and P. rufescens) utilized significantly different microhabitats. All 5 holothurian species differed appreciably in body size, gut contents and feeding behaviour. The differing holochurian body sized could be related to both particle size classes of the gut contents of these species and microhabitats occupied. The amount of suitable microhabitats is probably more important to the number and abundance of cryptic species than the amount of detrital food resources available. The echinoderm assemblage appeared to be stable over time.     

Effects of brackish water incursions and diel phasing of tides on vertical excursions of the keystone predator Pisaster ochraceus

The physical factors that constrain the vertical foraging excursions of the keystone predator, the sea star Pisaster ochraceus, hold considerable interest because they indirectly shape the vivid patterns of zonation of rocky shore communities by impeding or enhancing the ability of P. ochraceus to traverse the intertidal zone. In this paper, we describe a study conducted in the Pacific Northwest of North America in which we examined, in the field and laboratory, the abiotic factors that can affect vertical excursions by P. ochraceus. Our field observations revealed that the extreme upward reach and average shore level height reached by P. ochraceus were significantly lower for daylight high tides than nocturnal high tides. Based on diver observations following a severe storm, it would also appear that these diurnal movements can be impeded by freshwater incursions into the intertidal zone; a regularly occurring event in the Pacific Northwest. As part of an experimental investigation into this phenomenon, we observed that sea stars maintained in tall cylindrical aquaria, without tidal flux, remained near the bottom during daylight and moved to the top of the column at night, suggesting that photoperiod alone can influence the cycle of vertical movement. Adding a freshwater layer to the aquaria restricted these vertical excursions. Our results suggest that on rocky coastlines susceptible to fresh water incursions, the suppression of foraging may be an important factor in the spatial and temporal variation in the intensity of predation. Furthermore, given the relative increase in frequency and intensity of freshwater incursions in the Pacific Northwest and the intolerance of P. ochraceus to lowered salinity, there is the long-term potential to significantly alter patterns of species zonation in this essential marine habitat.     

Movement patterns of the seastar Heliaster helianthus in central Chile: relationship with environmental conditions and prey availability

The rates and patterns of feeding and displacement of predators constitute two of the most important plastic behavioral responses that allow individuals to respond quickly to changes in abundance of their prey, predation risks and to rapid alterations in environmental conditions. In this study, we quantified seasonal and spatial variation in displacement (net changes in location in 12 or 24 h periods) and prey consumed of marked individuals of the keystone seastar Heliaster helianthus at six sites spanning 600 km along the coast of north-central Chile. We evaluated the hypotheses that: (1) at sites with low availability (cover) of the main prey, the mussel Perumytilus purpuratus, Heliaster displays larger displacements and consumes a greater proportion of other prey (e.g. mobile species) than at sites with high mussel cover, (2) daily displacements will be correlated with sea surface temperature (SST) and (3) increased wave action will reduce seastar daily displacement. Our results show that Heliaster displacement is higher at sites with lower availability of P. purpuratus; and at these sites, a larger proportion of Heliaster individuals are observed feeding, mostly on other prey (e.g. limpets), which could offset the higher costs associated with increased movement. In addition, wave forces affected the activity of Heliaster negatively. Contrary to our expectations, the daily displacements did not show any relationship with SST measured on the day or the previous days of the surveys, despite the fact that average displacement was generally higher in summer than in winter months. Future studies should examine Heliaster movement during single foraging excursions and determine whether these responses affect the growth and reproductive output of individuals. Such information is vital to understand how changes in prey abundance and environmental conditions alter the behavior and energy budget of this predator and its ability to control prey populations.     

Diel foraging patterns of the sea urchin Centrostephanus coronatus as a predator avoidance strategy

During the day, the diadematid sea urchin Centrostephanus coronatus occupies holes and crevices in shallow subtidal rocky substrata. Individuals emerge from these after sunset and forage on organisms attached to the surrounding rock surface. Each urchin travels <1 m from its shelter and returns to the same one before sunrise. The sheephead wrasse Pimelometopon pulchrum does not remove urchins from their shelters, but will attack and consume urchins placed in normal feeding locations during the daytime. The active periods of the sheephead and the urchin do not overlap; urchins begin foraging about 20 min after the diurnal sheephead retire in the evening and return to their shelters 1 to 2 h before sheephead resume feeding in the morning. We infer that the urchin's daytime crevice-dwelling and nocturnal foraging habits have evolved as a response to sheephead predation. Moreover, because shelters are limited in supply, shelter fidelity may have evolved to insure refuge from sheephead.     

Influence of predation by a crab on the distribution of the size-groups of three intertidal gastropods in South Australia

Two South Australian rocky intertidal platforms were sampled in April 1981 and December 1982 (Marino Rocks) and in December 1982 (Lady Bay), respectively. Three snail species, Nerita atramentosa Reeve, 1855, Bembicium nanum (Lamarck, 1822) and Austrocochlea concamerata (Wood, 1828), exhibited shore-level size-gradients, with smaller individuals occupying the higher intertidal levels. The reef crab (Ozius truncatus M.-Edwards, 1834), whose distribution overlaps that of the gastropods, is an important predator of these gastropods. The shore-level distribution of shell sizes can be explained by differential selection against smaller individuals by reef crabs. The decreased density of small snails at lower intertidal levels may be the result of either their consumption by reef crabs and/or their movement to higher shore levels where crab predation is less intense.     

Intra-specific variability in the temporal organisation of foraging activity in the limpet<Emphasis Type="Italic"> Patella vulgata</Emphasis>

Patella vulgata is one of the most common limpet species on rocky shores throughout the north-eastern Atlantic. Foraging activity of this species has been described during various different diel and tidal phases and several factors have been suggested that might influence the timing of its activity. Among these, inclination of rock surfaces seems to have a clear role, with limpets on horizontal rocks being active during daylight at high water, while limpets on vertical rocks were active during night-time low water. However, by comparing the results of previous investigations it is difficult to separate clearly the effect of any single factor from the effects of their interactions. In the present paper we investigated the simultaneous influence of height on the shore, slope of rock surface and date of sampling (August–September and March–April) on the behaviour of a population of P. vulgata on the Isle of Man (UK). The study was carried out using modern telemetric methods, allowing the activity patterns of limpets to be recorded for long periods of time. The results showed that limpets concentrated their activity during two distinct temporal windows—daytime tide-in and night-time tide-out—while no significant activity was observed during daytime tide-out or night-time tide-in. The observed behaviour appeared to be driven by complex interactions among the three factors examined, and the influence of rock slope alone was not so strong as to override the effect of other factors completely. In addition, a key finding of this study was the conspicuous variability in the individual behaviour of P. vulgata. The greatest level of variability was observed in limpets on vertical surfaces on the upper shore, which showed an extreme switch in behavioural patterns, according to date of sampling (daylight/tide-in foraging during the spring and night-time/tide-out foraging during late summer). In addition, individuals apparently sharing the same set of conditions (inclination, height on the shore and sampling date) differed in their behaviour, and a given behavioural pattern was not always shared by all individuals from a given group. Finally, variability was also observed at the intra-individual level since the same individual could be active during different tidal and diel phases. In conclusion, although the existence of endogenous rhythms has previously been demonstrated in this species, our data clearly show that P. vulgata was able to modulate its behavioural patterns in relation to a variety of exogenous determinants.Communicated by R. Cattaneo-Vietti, Genova     

The importance of sandflat morphology to recruitment of the intertidal snail Nassarius pauperatus during ten consecutive years at three sites in South Australia

This project was desinged to confirm the temporal consistency of inter-site differences in recruitment of an intertidal gastropod, and to test the hypothesis that these differences are accounted for by differences in shore topography. Annual recruitment of the intertidal sanil Nassarius pauperatus (Lamarck) was estimated at two sandflats in Gulf St Vincent, South Australia, over a period of 12 yr (1979 to 1990). At one sandflat the gradient was slight, the zone containing N. pauperatus was wide, and there were extensive pools of water at low tide, whilst at the other sandflat the gradient was steeper, the zone containing N. pauperatus was narrower, and there were fewer or no pools. Although recruitment at both sites varied temporally, the density of recruits was always higher at the first site. These data, together with data covering 10 yr (1981–1990) from a sandflat where the gradient of the substratum was decreasing due to accretion of sediment, suggest that shore topography is an important determinant of relative recruitment for N. pauperatus in South Australia.     

Factors affecting variability of foraging excursions in a population of the limpet Patella vulgata (Mollusca,Gastropoda)

Paths moved by the limpet Patella vulgata L. were monitored on a sheltered vertical rocky shore in North Wales using time-lapse photography throughout nocturnal low tides in April and November 1992, roughly corresponding to periods of minimum and maximum gonad ripeness, respectively. Various motion parameters, including total duration, total length, and maximum distance reached from home were computed from 124 complete foraging routes obtained from 18 higher-zoned and 18 lower-zoned limpets. P. vulgata typically performed a single loop per night (average total length, ca. 70 cm), moving from the individual home scar to graze the surrounding algal grounds (average maximum distance, ca. 25 cm), then following its own trail back home. On the average, limpets moved from the home scar during three of four available nocturnal low tides and exploited about half the emersion time. Evidence for size-related variability in behaviour was found, as a positive correlation was assessed between both total duration and legth of the excursions, and shell length. In addition, zonation was proved to affect the limpets' foraging behaviour, since low-shore limpets moved faster and covered greater distances than high-shore ones. Moreover, consistent seasonal variation in foraging behaviour emerged, since in April the excursions were longer and longer-lasting than in November, but limpets exploited a larger fraction of potential activity phases in November than in April. The within-population variability in the temporal and spatial characteristics of the foraging excursions is discussed in relation to the available data on zonal and seasonal variation of food resources and in relation to physiological changes due to reproductive cycle.     

Amphipods and isopods in the rocky intertidal: dispersal and movements during high tide

Animals in the intertidal, both mobile and sessile, generally exhibit some zonation pattern, in which each species shows a preference for, or is confined to, some height levels. The study of zonation patterns is, however, almost exclusively based on surveys made during low tide, when many animals are relatively inactive. We studied zonation patterns of amphipods and isopods on rocky shores in southwestern Iceland, both by traditional sampling at low tide as well as by sampling during high tide. The distributional patterns seen at high tide differed significantly from that at low tide. One amphipod, Anonyx sarsi, was common around baits at all levels at high tide but absent from the intertidal at low tide. Several other species were either relatively more common or tended to be recorded higher, or in one instance, lower on the shore when the tide was in than at low tide. There was also evidence of some species changing habitats within the intertidal with the tidal cycle. Many species, however, moved little away from their respective zones occupied at low tide, and for some species, including some capable of rapid swimming, very limited mobility was indicated. We conclude that low-tide surveys of the intertidal give an incomplete picture of the community structure, and even key species may be missed in such surveys.Communicated by L. Hagermann, Helsingør     

How wave exposure, group size and habitat complexity influence foraging and population densities in fishes of the genus Halichoeres (Perciformes: Labridae) on tropical rocky shores

Wave exposure and habitat complexity have been used to explain variations in the distribution patterns and behavior of many reef fishes. This study analyzed the influence of both factors on density and foraging activity, and the influence of group size on foraging in three species of the genus Halichoeres (Labridae) on tropical rocky shores. It was shown that initial phases (IP) and terminal phases (TP) Halichoeres, were influenced by wave exposure, although foraging in Halichoeres brasiliensis TP was not influenced by wave exposure. The IP in all three species were positively associated with rugosity and algal cover while the TP was positively associated with depth. Habitat complexity also influences foraging in these species. Group size influenced foraging activity, except in H. brasiliensis TP. We also found variations in microhabitat patches used for foraging between species and differences in the stomach contents between species and phases, showing that functional roles change in response to ontogenetic shifts.     

Nocturnal feeding migrations of Nerita plicata,N. undata and N. textilis (Prosobranchia: Neritacea) on the rocky shores at Mkomani,Mombasa, Kenya

The vertical zonation of the three common rocky shore neritids at Mkomani, Mombasa, Kenya, Nerita plicata Linnaeus, N. undata Linnaeus, and N. textilis Dillwyn, as a function of feeding migrations and of size, was studied from 28 February to 24 March 1983. These snails perform feeding migrations at night starting at around mid-ebb tide and return to their resting positions with the flood tide. They remain in their resting positions throughout the day until the next nocturnal ebb tide. The direction of migration is sizerelated, with the larger snails of each species moving in the opposite vertical direction to the smaller ones, so that the populations as a whole exhibit no statistically significant net vertical displacement. The larger individuals of two of the species, N. plicata and N. undata, invariably move downwards to their feeding levels, while the smaller individuals move upwards; the larger individuals of N. textilis display a different pattern of migration, moving downwards on and around spring-tide days and upwards on and around neap tide days, while the smaller individuals move in the opposite directions. N. textilis rest above their feeding level around spring tides, and below that level around neap tides. It is demonstrated how these nocturnal migratory feeding rhythms are integrated into the spring-neap and seasonal cycles of the snails' daytime resting positions. The adaptive significance of these migrations is also discussed.