Fine-scale spatial and temporal variation in settlement of the intertidal mussel Perna perna indicates differential hydrodynamic delivery of larvae to the shore

Scales of spatial and temporal variation in the settlement of early and late plantigrades of the mussel Perna perna to artificial substrata were measured on the south coast of South Africa. One study examined daily settlement over one month at three sites of similar tidal height and aspect. The sites were separated by 100′s of m with 5 replicated artificial settlement substrata approximately 1 m apart within each site. Early (< 0.5 mm) and large late plantigrades (1 - 3.5 mm) dominated samples with few individuals of 0.5-1 mm. Settler abundance showed clear peaks, synchronised among sites and apparently following spring tides, though the relationship between tide and settlement was not significant. Two-way ANOVA indicated significant spatial variation, which varied with time, at the 100 m-scale among sites. A second study investigated the local effects of substratum aspect. Seaward facing surfaces had consistently lower settlement (but higher adult cover) than horizontal and landward facing surfaces, this effect was significant for large late plantigrades. This suggests that either juveniles migrate after settlement, or that post-settlement mortality differs markedly among aspects. The high degree of temporal variation at the scale of days and weeks was expected, but the results also suggest a degree of pulsing in settlement, with maximum intensities around spring tide. A previous study of nearshore waters indicated a common pool of larvae at scales of 100 s m. Thus, consistent ranking of sites for all size classes of settlers indicates predictably differential hydrodynamic delivery of larvae to sites separated by 100 s m. The effect of aspect on settlement highlights the importance of smaller scale hydrodynamics, suggesting that the stronger turbulence experienced by seaward facing surfaces may affect larval settlement negatively.     

Spring-neap cycle as a major driver of temporal variations in feeding of intertidal fishes: Evidence from the sea catfish Sciades herzbergii (Ariidae) of equatorial west Atlantic mangrove creeks

On macrotidal coasts, short- to medium term variations in feeding of intertidal fishes are influenced by several interacting time scales. To identify the driver of major variations in the feeding habits of intertidal fish, we used the pemecou sea catfish Sciades herzbergii (Ariidae), an abundant intertidal benthic second order consumer, as a model species. We analyzed the influence of the spring-neap and the day-night cycle on intertidal abundance, stomach fullness, diet composition and food consumption of S. herzbergii using block nets set at slack high tides in two mangrove creeks in north Brazil. At spring tides, intertidal abundance, stomach fullness, and total daily consumption of S. herzbergii were on average 8.8, 1.9, and 3.8 times higher than at neap tides, respectively. At spring tides, Uca spp. and Grapsidae (mostly Pachygrapsus gracilis) dominated the diet, irrespective of the time of day. Other important food items were Insecta and the semiterrestrial crab Ucides cordatus. At neap tides, Capitellidae contributed to the diet of nightly inundations while no specimens were caught at daytime. Creek location had no effect on any variable. Results from our study area and evidence from other studies suggest that the spring-neap tide pulse is likely the major driver of short- to medium term variations in feeding of intertidal fishes. This has important implications for feeding-related issues on macro- and probably also on mesotidal coasts: (i) juvenile fishes may have fortnightly growth spurts, (ii) intertidal prey populations may suffer regular fluctuations in mortality, (iii) studies of the feeding ecology of intertidal fishes should cover the combined effects of the tidal, diel, lunar and seasonal cycles, and (iv) the modeling of food webs should consider the differences between highly dynamic spring tide and quieter neap tide conditions.     

Activity patterns, feeding and burrowing behaviour of the crab Ucides cordatus (Ucididae) in a high intertidal mangrove forest in North Brazil

Activity patterns, feeding and burrowing behaviour of the economically important semi-terrestrial mangrove crab Ucides cordatus (Ucididae, L. 1763) was studied in a high intertidal Rhizophora mangle forest stand in Bragança, North Brazil. Video observations in the rainy and dry season were conducted over 24 h cycles at different lunar phases to investigate the behaviour of these litter-feeding crabs outside their burrows. During the rainy season, crabs stayed inside their burrows for 79% and 92% of the time during day and night, respectively. Time spent for feeding, burrowing and other activities outside their burrows was significantly longer during the day with 9.9% (night: 1.7%) and at waning and waxing moon with 9% (full and new moon: 0.9%). At neap tides (no tidal inundation) foraging and feeding activities outside burrows were clearly light-dependent, increasing at dawn and decreasing at dusk. Highest activities during daytime relate to the visual localisation of food. During the dry season, crabs spent less time inside burrows at neap tides than during the rainy season (80% and 91%, respectively). However, time spent for feeding activities was similar during both seasons. During almost all observation periods crabs collected leaf litter, but rarely fed on it outside burrows. At neap tides nearly all available litter was collected, suggesting that the U. cordatus population is litter-limited during these times. At spring tides (regular tidal inundation) the surface activity of U. cordatus was tide-dependent. Crabs closed their burrow entrances 2-3 h before flooding and re-emerged as soon as the tide retreated. During the day, burrow maintenance was the second most frequent behaviour after feeding. Agonistic interactions were regularly observed and were mainly related to burrow defence. The mean foraging radius of the crabs was only 19 cm (max: 1 m) underneath high Rhizophora mangle trees where crab densities were high. The results point to a high competition for burrows and show that U. cordatus is territorial. It is concluded that several exogenous factors, in particular light, leaf litter availability, flooding of burrows and the presence of conspecifics are important in controlling the crabs' activity patterns.     

Behavioural basis of intertidal zonation in Eurydice pulchra Leach

Eurydice pulchra Leach freshly collected at neap and spring tides exhibits an endogenous circatidal rhythm of swimming when kept without sand in constant conditions in laboratory aquaria. When provided with sand in the laboratory, tidally-phased swimming may occur spontaneously at some times of the lunar month, predominantly after the times of highest spring tides but not at neaps. In these cases, therefore, there appears to be clearly demonstrated for the first time in a marine animal a circasemilunar pattern of emergence from the substratum which is coupled to a circatidal rhythm of swimming; the isopods in the laboratory emerge from the sediment only when the swimming rhythm and emergence rhythm are at their maxima. In the field at neap tides some isopods emerge to swim at high tide, dependent upon their responses to water agitation, their feeding state and responses to light. At and just after spring tides in the field large numbers of the isopods swim at high tide, triggered by the endogenous circasemilunar rhythm of emergence which in the field is probably cued by the effects of increased water agitation. The main function of the endogenous circatidal rhythm of swimming appears to be to permit the isopods to return to their preferred zone in the sand before the fall of the tide. The coupled circasemilunar rhythm of emergence which induces greater swimming at spring tides reduces the risk of animals being stranded high on the shore during neap tides.     

Tidal rates of settlement of the intertidal barnacles Chthamalus stellatus and Chthamalus montagui in western Europe: the influence of the night/day cycle

Chthamalus montagui and Chthamalus stellatus are abundant barnacles in western Europe. Tidal settlement of Chthamalus in SW Ireland and SW Portugal was studied in relation to a night and day factor and at different temporal (dates) and spatial (shores and sites) scales. Based on the identifiable cyprids and metamorphs, Chthamalus settlement in SW Ireland was comprised mainly of C. stellatus but was composed of C. montagui only in SW Portugal.In SW Ireland and SW Portugal, settlement rates of Chthamalus (mean number of settlers per 25 cm²±S.E.) were higher after one day tidal cycle (597±158.7 in SW Ireland, 144±23.6 in SW Portugal) than one night tidal cycle (55±12.1 in SW Ireland, 13±2.2 in SW Portugal), but significant differences were only detected in SW Portugal. Different models were proposed for explaining this pattern related to night and day variability of the physical processes responsible for transporting cyprids to shore (1), and/or of pre-settlement behaviour (2) and/or of settlement behaviour of cyprids (3).Spatial patterns of tidal settlement of both species or at both locations seem similar with small scale variability (between sites, 5 to 30 m apart) in settlement of Chthamalus being the only spatial scale at which variability was detected in both locations.The composition of Chthamalus cohorts settling during one tidal cycle differed considerably between locations/species: settlers of C. stellatus were mainly cyprids; settlers of C. montagui during the day (when most settlement occurred) were essentially metamorphs.     

Settlement patterns of Chthamalus spp. at Praia da Oliveirinha (SW Portugal)

Recruitment of Chthamalus spp. is intense during spring and summer but the number of settled cypris larvae is very low when compared to the high number of Chthamalus spp. metamorphs. In the summer of 1997 at Praia da Oliveirinha (SW Portugal), two hypotheses were tested: 1) the cypris larvae of Chthamalus spp. metamorphose in one tidal cycle; and 2) the settlement of Chthamalus spp. is higher during diurnal high tide compared with nocturnal high tide. The results indicate that cypris larvae of Chthamalus spp. can metamorphose in one tidal cycle. The hypothesis of higher settlement during diurnal high tide could not be tested at the mid and upper levels of C. montagui distribution due to a significative heterogeneity of variances. At the lower level, the results support this hypothesis. In spite of these data having been collected on a single shore and on a few sampling dates, these results suggest that more studies using different spatial and temporal scales are needed to see if this pattern is general.     

Foraging strategy of the mudskipper Periophthalmus sobrinus Eggert in a Kenyan mangrove

The feeding behaviour of Periophthalmus sobrinus was studied in a population living along the channels of the Tana river delta. In November 1993 mudskippers were sampled every 2 tidal h for two consecutive spring and neap tides both during the day and night. The fishes were sexed and their stomach contents analysed. The results indicated that the mudskipper has a carnivorous diet and forages during both day and night although feeding greatly decreases after dark. Quantitative and qualitative differences in diet were found during the day and night of the two synodic and tidal phases. These differences were also demonstrable in the two sexes. The study showed that females of P. sobrinus are more selective than males and that differences between the two sexes may be related to reproductive condition. Furthermore, the mudskippers showed a tidal rhythm in feeding behaviour with peaks of activity around low tide (LT) on spring tides and around high tide (HT) on neap tides. Foraging activity seems to be influenced by the different spatial and temporal distributions of the mudskipper's prey caused by synodic, tidal and diel changes in the environment.     

The Influence of Topographic and Dynamic Cyclic Variables on the Distribution of Small Cetaceans in a Shallow Coastal System

The influence of topographic and temporal variables on cetacean distribution at a fine-scale is still poorly understood. To study the spatial and temporal distribution of harbour porpoise Phocoena phocoena and the poorly known Risso’s dolphin Grampus griseus we carried out land-based observations from Bardsey Island (Wales, UK) in summer (2001–2007). Using Kernel analysis and Generalized Additive Models it was shown that porpoises and Risso’s appeared to be linked to topographic and dynamic cyclic variables with both species using different core areas (dolphins to the West and porpoises to the East off Bardsey). Depth, slope and aspect and a low variation in current speed (for Risso’s) were important in explaining the patchy distributions for both species. The prime temporal conditions in these shallow coastal systems were related to the tidal cycle (Low Water Slack and the flood phase), lunar cycle (a few days following the neap tidal phase), diel cycle (afternoons) and seasonal cycle (peaking in August) but differed between species on a temporary but predictable basis. The measure of tidal stratification was shown to be important. Coastal waters generally show a stronger stratification particularly during neap tides upon which the phytoplankton biomass at the surface rises reaching its maximum about 2–3 days after neap tide. It appeared that porpoises occurred in those areas where stratification is maximised and Risso’s preferred more mixed waters. This fine-scale study provided a temporal insight into spatial distribution of two species that single studies conducted over broader scales (tens or hundreds of kilometers) do not achieve. Understanding which topographic and cyclic variables drive the patchy distribution of porpoises and Risso’s in a Headland/Island system may form the initial basis for identifying potentially critical habitats for these species.     

Feeding rhythms of the green-lipped mussel, Perna viridis (Linnaeus, 1758) (Bivalvia: Mytilidae) during spring and neap tidal cycles

Food availability and feeding responses of the green mussel Perna viridis were investigated for two complete tidal cycles during both spring and neap tides. Temporal changes in total particulate matter (TPM), particulate inorganic matter (PIM) and particulate organic matter (POM), were smaller during neap than spring tides. Seston characteristics at different times of a tidal cycle were compared for both spring and neap tides. Only during spring tides were TPM and PIM significantly higher at high tides while POM remained relatively constant (P>0.05). The clearance rate of the mussels underwent temporal variations with tides, and was a negative power function of TPM and a positive linear function of f (organic content), during both spring and neap tides. f was the key factor influencing filtration rate, organic ingestion rate, absorption rate and absorption efficiency. All feeding rates increased linearly with increases in organic content. Pseudofaeces were produced only during spring but not neap tides. Feeding rates and absorption efficiency were highest at low and lowest at high tides (P<0.01). There was no significant temporal change in the wet weight and protein content of the crystalline style with the tidal regime. For the digestive gland, alpha-amylase activity was higher at spring than at neap tides, and higher during high tides in a tidal cycle. The digestive gland cellulase activity did not change significantly with the tides. For the crystalline style, both the activity of cellulase and alpha-amylase were not significantly different (P>0.05) between spring and neap tides. Tidal rhythms in feeding and digestion in this species were likely controlled by temporal variations in food availability in the seawater. By adjusting feeding rates and enzymatic activities, absorption in Perna viridis remained constant, irrespective of the changes in food availability.     

Fortnightly periodicity in the abundance of diatom and dinoflagellate taxa at a coastal study site

From 3 July to 15 September 2000, plankton samples were collected roughly every 2 days at three stations within 1.5 km from shore at Sunset Bay, OR. In these samples we enumerated Coscinodiscus-like diatoms, Protoperidinium spp., and the potentially toxic phytoplankter, Pseudo-nitzschia spp. Using time series analysis, the abundance of these phytoplankters was compared to wind stress, tidal range, and sea surface temperature (SST). SST was significantly cross-correlated with wind stress (lower SST occurred during upwelling favorable winds) and maximum daily tidal range (cold and warm anomalies in SST occurred around the neap and spring tides, respectively). We found no significant cross-correlations between wind stress (upwelling vs. downwelling winds) and the abundance of any of the taxa. Significant cross-correlations were found between phytoplankton abundances and the maximum daily tidal range (peak concentrations occurred between the neap and spring tides) and SST (peak concentrations occurred during periods of warmer SST). We hypothesize that this pattern of abundance may be caused by shoreward transport of offshore phytoplankton populations by internal tidal waves.     

Lunar synchronization of the monthly reproductive rhythm in the sea urchin Centrostephanus coronatus Verrill

The monthly reproductive rhythm in the diadematid sea urchin Centrostephanus coronatus Verrill at Santa Catalina Island, California, was studied in the summer of 1973 and the results are compared with data for the summer of 1969. In the summer of 1973 the more extreme spring tides coincided with the new moon, while in the summer of 1969 the more extreme spring tides coincided with the full moon. The reproductive rhythm in both years was closely synchronized with lunar phases and not with the monthly tidal cycles; spawning occurred near the third lunar quarter in both years. These observations suggest that this monthly reproductive rhythm is synchronized by monthly changes in moonlight, and not by monthly tidal changes.     

The relative importance of habitat-specific settlement, predation and juvenile dispersal for distribution and abundance of young juvenile shore crabs Carcinus maenas L.

Young juveniles of many motile benthic species are concentrated in structurally complex habitats, but the proximate causes of this distribution are usually not clear. In the present study, I assessed three potentially important processes affecting distribution and abundance of early benthic stages in the shore crab (Carcinus maenas): (1) selection of habitat by megalopae (postlarvae); (2) habitat-specific predation; and (3) post-settlement movements by juveniles. These processes were assessed concurrently over 3-9 days at two spatial scales: at the scale of square meters using cage techniques within nursery areas, and at the scale of hectares using isolated populations of juvenile shore crabs in small nursery areas as mesocosms. The results were compared to habitat-specific distribution in the field.Shore crab megalopae and first instar juveniles (settlers) were distributed non-randomly among micro-habitats in the assessed nursery areas, with great densities in both mussel beds, eelgrass and filamentous algal patches (on average 114-232 settlers m⁻²), and significantly smaller densities on open sand habitats at all times (on average 4 settlers m⁻²). The same habitat-specific settlement pattern was found in cages where predators were excluded, suggesting that active habitat selection at settlement was responsible for the initial distribution. Older juveniles (second to ninth instar crabs) were also sparse on sand, but in contrast to settlers, were concentrated in mussel beds, which showed significantly greater densities than eelgrass and algal habitats. The cage experiment demonstrated a dynamic distribution of juvenile crabs. Young juveniles constantly migrated over open sand habitats (20 m or further) and colonized the experimental plots in a habitat-specific pattern that reflected the distribution in the field. This pattern was also found for very small crabs colonizing predator-exclusion cages, suggesting that selection of habitat by migrating juveniles caused the ontogenetic change in habitat use. Although post-settlement movements were great within nursery areas, juvenile dispersal at a regional scale appeared to be small, and the recruitment of juvenile shore crabs to the shallow bays occurred mainly through pelagic megalopae.Conservative estimates at the scale of whole nursery areas, based on migration trap data and field samples, indicated great mortality of settlers and early benthic stages of shore crabs. Results from the cage experiment suggest that predation by crabs and shrimp were responsible for the high settlement mortality. Both enclosed cannibalistic juvenile crabs and local predators on uncaged habitat plots caused significant losses of settlers in all habitats (on average 22% and 64% 3 day⁻¹, respectively). The effect of predators was highly variable between trials, but differed little between habitat types, and predation had no detectable proximate effect on juvenile distribution, despite the great losses. Small settlement densities on sand habitats in combination with a refuge at low prey numbers, and an aggregation of cannibalistic juvenile crabs in nursery habitats appear to decrease the effect of habitat-specific predation rates on the distribution of juvenile shore crabs. This study demonstrates that active habitat selection at settlement followed by a dynamic redistribution of young juveniles can be the proximate processes responsible for habitat-specific distribution of epibenthic juveniles, and indicate that predation represents a major evolutionary process reinforcing this behavior.     

Reproductive periodicity of the sparid,Acanthopagrus pacificus,on a hierarchy of temporal scales

The reproductive periodicity of the sparid, Acanthopagrus pacificus, over four temporal scales is described. Acanthopagrus pacificus had a short spawning season between June and September, and within this, a peak reproductive period from July to early September. During the peak period there were several spawning peaks corresponding to a lunar periodicity, with intense reproductive activity on new and full moons that peaked during the period of the full moon when the tidal range was greatest. At the smallest temporal scale, spawning occurred at night on ebb tides. Because this study draws on data collected in 1991 and 1995, it provides a useful baseline against which to judge future changes in reproductive periodicity.     

The surf zone: a semi-permeable barrier to onshore recruitment of invertebrate larvae?

The supply of larvae to the shore is important for population replenishment and intertidal community dynamics but its variability at most scales is not well understood. We tested the relationship between nearshore mussel larval abundance and intertidal settlement rates over several years at multiple spatiotemporal scales in Oregon and New Zealand. Abundance of competent larvae nearshore and intertidal recruitment rates were simultaneously quantified using collectors mounted at different depths on moorings 50-1100 m from shore, and at adjacent rocky intertidal sites. Total mussel larval abundance and oceanographic conditions were also measured in some locations. At all scales, abundance of nearshore mussel larvae was unrelated to intertidal recruitment rates. In the intertidal, patterns of mussel recruitment were persistent in space, with sites of consistently high or low recruitment. In contrast, nearshore competent larval abundance showed generally similar abundances among sites except for a high, and spatially-inconsistent, variability in Oregon during 1998 only. On moorings, recruitment tended to be greater on midwater collectors than shallower or deeper. Finally, on moorings larval abundance in traps and recruitment on collectors was unrelated. These results suggest that (1) among sites, the size of the nearshore larval pool is relatively uniform while onshore recruitment varies and is unrelated to larval abundance, (2) temporal variability in nearshore larval availability is not strongly expressed onshore, (3) vertical stratification of competent larvae nearshore is strong and may influence transport and recruitment, and (4) within-coast variability in onshore recruitment is strongly driven by processes occurring locally within the surf zone that need to be studied to understand coastal recruitment dynamics.     

Flux of decapod larvae and juveniles at a station in the lower Canal de Mira (Ria de Aveiro,Portugal) during one lunar month

Summary

Emigration and immigration of decapod larvae from estuaries depend on timing of larvae occurrence in the water column relative to the tidal, tidal amplitude and day cycles. The phase relation of these natural cycles varies with tidal regime and geographically, resulting in different time-patterns of hatching of first stage larvae and of presence of late stage larvae in the water column. Vertical migration behaviour according to phase of tide also controls transport inside estuaries. These mechanisms were investigated in a field study conducted on the northwest coast of Portugal where neap ebb tides occur during the night around the quarters of the moon. Flux of decapod larvae through one sampling station was measured during one lunar month at the Canal de Mira (Ria de Aveiro) in the spring of 1990. The sampling programme was comprised of a set of 25-h fixed station studies, separated by 25-h intervals during which no sampling took place. Plankton samples were collected with a pump every hour at three depths. Current velocity and direction at the standard depths, as well as height of the water column, were also measured every hour. Hourly instantaneous flux of larvae through a 1-m-wide vertical section of the Canal de Mira was calculated for the most abundant forms. A total number of 13 combinations of species and larval stages were analyzed, belonging to the families Atelecyclidae, Pirimelidae, Portunidae, Pilumnidae, Grapsidae, Palaemonidae, Crangonidae and Thalassinidae. Patterns of net larval flux along the lunar month could be grouped into three types. Type 1 includes first zoeas that were consistently exported to the sea. Type 2 comprises late zoeas, megalops and juveniles that were consistently imported into the estuary. First zoeas that were imported during some of the 25-h studies but were exported during the others were included in Type 3; in species of this type import periods appeared to alternate with export periods according to lunar phase. Flux of Type 1 larvae followed a semi-lunar pattern. Release activity of Type 1 zoeas took place during the night and started during neap tides around the quarters of the moon, but maximum releases occurred 3–4 h after high tide of average amplitude tides, 3–4 days after the quadratures. These observations agree with the hypothesis that hatching is timed to occur on ebb tides of the largest possible amplitude so that larvae are easily dispersed from areas with a high density of predator fishes. However, based on other observations on the Portuguese coast, it cannot be ruled out that hatching might depend on a minimum number of hours of darkness experienced by the females. Larvae included in Type 2 comprise forms that may have been retained inside the estuary for the entire larval phase, as well as one form that was imported from shelf waters. No semi-lunar pattern of import was detected in this last form. Fluctuations of net flux observed in Type 3 larvae, as well in other forms that were not included in any of the types, were more difficult to explain. These larvae were first zoeas of species belonging to different taxonomic, morphological and ecological groups and may show a diversity of adaptations to the way of life of the adults. Imports and exports of larvae depended not only on time-patterns of abundance, but also on time-patterns of larval vertical distribution. As a general rule, larval stages showed patterns of depth distribution that were consistent with vertical migration rhythmic behaviours synchronized with the tidal cycle. Though the effect was not always statistically significant, first-stage larvae were closer to the surface during ebb, especially during the night, enhancing seaward transport. On the contrary, later zoeal stages, megalops and juveniles were usually closer to the surface during flood, suggesting migration to the water column during this phase of the tide and landward transport.     

Lunar influence on prey availability,diet shifts and niche overlap between Engraulidae larvae in tropical mangrove creeks

The influence of the lunar cycle on prey availability, diet shifts and overlap between larval Anchovia clupeoides and Cetengraulis edentulus was evaluated in mangrove creeks of the Goiana Estuary. Copepod eggs were highly abundant in the first and last quarter, at the full moon and zoea of Ucides cordatus (Ocypodidae) in the new moon. The Engraulidae larvae fed on microcrustaceans, algae and early planktonic stages of benthic organisms. The relative importance of prey varied according to prey availability in all moon phases. Larval diets were more even in the full and new moons, when the relative importance of calanoid copepods and zoeae of U. cordatus as food items increased (index of relative importance, >80% I_RI). Mangrove creeks were very important feeding grounds for engraulid larvae during spring tides. Larval diets were more diverse in the first and last‐quarter moon and included protozoeae of Caridean shrimp, larvae of Anomalocardia brasiliana (Veneridae), Isopoda, Gastropoda, ephippium of Daphnia sp. and nauplii of Cirripedia, Harpacticoidia and cyclopoid Copepoda. The last five items were not found in the creeks, suggesting feeding in the main channel. During neap tides, mangrove creeks were probably also used as refugia. These larvae are opportunistic and feed on highly available prey and both species feed on the same items, leading to high dietary overlap in all moon phases. The lunar cycle, which is related to the spring‐neap tidal cycle, was the major driver of quantitative and qualitative changes in feeding of engraulid larvae on a short time scale.     

Temporal and spatial patterns in the abundance of wintering Red-breasted Mergansers Mergus serrator in an estuary

Tidal, temporal and spatial aspects in the abundance of wintering Red-breasted Mergansers in the Ythan estuary, northeast Scotland, were investigated. The number and location of birds were examined in relation to four variables: tidal cycle and tidal range, and month and time of day. A significant increase in the numbers of this piscivorous bird occurred as the winter progressed. Numbers were highest in early morning and then decreased significantly during the day. Spring tides were favoured over neap tides and a significant increase in numbers was recorded between low tide and high tide. The lower part of the estuary was preferred over the upper part with no detectable shift in the preference occurring in relation to any of the four variables.     

Population dynamics of the green-lipped mussel, Perna canaliculus, at various spatial and temporal scales in northern New Zealand

Ecological processes that differentiate and maintain intertidal populations of mussels, Perna canaliculus, were studied within three sites at Ninety Mile Beach, northern New Zealand. At these three sites (Scott Point, The Bluff and Tonatona Beach), the dynamics of larval availability, primary and secondary settlement, recruitment and mortality rates were investigated at various spatial and temporal scales. (1) Mussel concentrations in seawater were variable with respect to study site and time of year, with highest abundances at the northernmost population (Scott Point) and lowest concentrations at the middle population (The Bluff). In seawater at all three sites, small mussels (< 0.25 mm in shell length) were more abundant in August 2000, while larger mussels (> 0.5 mm in shell length) were more abundant in March 2001. (2) Primary and secondary settlement patterns were investigated during short-term (daily) and long-term (monthly) settlement experiments, within quadrats that were cleared of all mussels in both the mussel bed and in adjacent algal band habitats. At all sites, primary settlement (< 0.5 mm in shell length) was high within the algal band habitat in August 1999, 2000 and 2001. Conversely, secondary settlement (> 2.0 mm in shell length) was high within cleared areas in the mussel beds in November-March 1999-2000 and 2000-2001. Abundance of mussels settling on artificial substrates placed in the intertidal did not differ greatly from comparable areas of natural substrates (bare rock or algae within cleared quadrats). (3) Recruitment and mortality rates were recorded during monthly surveys of the adult populations. Within three mussel size classes (< 24, 25-74 and > 75 mm in shell length), peak recruitment coincided with high mortality in August of the 2 years studied. However, the most dramatic turnover of the population was observed at Scott Point in both years, following a spawning event. In adjacent waters at Scott Point, large accumulations of drift algae covered (up to 100% cover) with juvenile mussels may deplete food supplies usually delivered to intertidal adult mussels, causing their demise. Mats of adult mussels were observed “peeling-off” from the rocky shore at this time of the year, making space available to the new recruits. Where nearshore algal accumulations were moderate to low, only moderate to low mussel turnovers were observed (e.g. Tonatona Beach and The Bluff).     

FORAGING ACTIVITY OF HELCION PRUINOSUS (PATELLOGASTROPODA) ON A SOUTH AFRICAN BOULDER SHORE

Foraging activity of the mid-shore limpet Helcion pruinosuswas monitored on an exposed boulder beach in the Eastern Capeof South Africa. Numbers of limpets active in 10 fixed quadratswere monitored and compared in different seasons and during springand neap tides. Foraging excursions were mainly associated with nocturnallow tides, limpets only emerging from beneath rocks when theywere completely uncovered by the receding tide, and they hadall retreated to beneath rocks before being splashed by theadvancing tide. The number of animals active was greater onfull moon spring tides. Significantly more limpets were activeon an autumn spring tide than on similar tides in other seasons. ^* To whom correspondence should be addressed. (Received 27 March 2000; accepted 5 July 2000)     

Will the invasive mussel Mytilus galloprovincialis Lamarck replace the indigenous Perna perna L. on the south coast of South Africa?

The mussel Mytilus galloprovincialis is invasive worldwide, has displaced indigenous species on the west coast of South Africa and now threatens Perna perna on the south coast. We tested the hypothesis that Mytilus will replace Perna by examining changes in their distribution on shores where they co-exist. Total cover, adult density, recruit density, recruit/adult correlations and mean maximum lengths of both species were measured in 2001 at two contrasting sites (Plettenberg Bay and Tsitsikamma) 70 km apart, each including two locations 100 m apart. Cover and density were measured again in 2004. Total mussel abundance was significantly lower in Tsitsikamma, and recruit density was only 17% that of Plettenberg Bay. Abundance and cover increased upshore for Mytilus, but decreased for Perna, giving Mytilus higher adult and recruit density and total cover than Perna in the upper zones. Low shore densities of recruits and adults were similar between species but cover was lower for Mytilus, reflecting its smaller size, and presumably slower growth or higher mortality there. Thus, mechanisms excluding species differed among zones. Recruitment limitation delays invasion at Tsitsikamma and excludes Perna from the high shore, while Mytilus is excluded from the low shore by post-recruitment effects. Recruitment limitation also shapes population structure. Recruit/adult correlations were significant only where adult densities were low, and this effect was species-specific. Thus, at low densities, larvae settle or survive better near adult conspecifics. After 3 years, these patterns remained strongly evident, suggesting Mytilus will not eliminate Perna and that co-existence is possible through partial habitat segregation driven by recruitment limitation of Perna on the high shore and post-settlement effects on Mytilus on the low shore.