Patterns of adult abundance in Chthamalus stellatus (Poli) and C. montagui Southward (Crustacea: Cirripedia) emerge during late recruitment

The aim of this study was to investigate when adult distribution patterns are established in the barnacles Chthamalus stellatus and C. montagui. Adult ‘zones’ were identified by analysing field counts of both species at mid and upper shore heights. Monthly collections of cyprids, < 1 month old metamorphs and recruits (all metamorphosed individuals older than approximately 1 month) were made for C. stellatus and C. montagui in natural barnacle beds at six shores in SW Ireland. This was carried out over one year in 1996/1997, using a hierarchical sampling design. Abundance of total recruits (0-3 months old) was compared between adult zones after the main settlement season had ended. In addition, scales of variability in 0-3 month recruitment into adult zones were compared between the species at two scales: shores (1000s of metres) and sites within shores (10s of metres). Older recruits of each species, up to 11 months of age, were also compared between adult zones.The majority of settlement (measured as attached cyprids) occurred between August and October 1996. In October, there was no effect of adult zone on the abundance of total (0-3 month) recruitment up to that point in either species. Despite this homogeneity in recruitment between adult zones, significant spatial variation was found in 0-3 month recruits of both species at both of the scales examined. In C. stellatus the amount of variation associated with the larger scale (shore) was more than twice that of sites or of the residual variation (replicates within sites). 0-3 month recruitment in C. montagui was also most variable at the scale of shores but the residual variability (between replicates within site) was of similar magnitude to that of shores. Variability in 0-3 month C. montagui recruitment was relatively low at the scale of sites.There was a small but consistent input of recruits to adult zones over 9 months of the year, complicating the assessment of when adult patterns were set-up in these species. By June 1997, characteristic patterns of adult dominance had been established at all shores. Settlement had completely ceased by this time and individual barnacles were potentially 11 months old. Neither settlement nor early recruitment are significant in determining adult zonation patterns in these species. Instead, differential mortality patterns in individuals up to the age of 11 months are implicated in determining patterns of distribution of both species.     

Role of predators in the early post-settlement demography of coral-reef fishes

Populations with dispersive larvae are often demographically open such that local reproduction and subsequent larval settlement are not linked. Thus, understanding whether and how settlement patterns are established and subsequently modified is central to understanding local demography. Settlement is typically not measured directly, but rather it is estimated by recruitment, which is the observation of new individuals sometime after settlement. At Lizard Island on the Great Barrier Reef, I examined how patterns of recruitment of coral-reef fishes were modified across a range of natural recruit densities in the presence and absence of resident predators. Resident predators decreased recruitment and increased mortality for all species, but these effects varied considerably among species. The effects of predators on recruitment were at least partly due to mortality within 2 days after settlement. At their most extreme, predators caused recruitment failure of several species of butterflyfish. For one species of damselfish (Pomacentrus amboinensis), predators both induced weakly density-dependent mortality and obscured any relationship between recruitment and subsequent abundance, while for another damselfish (Neopomacentrus cyanomos), mortality was density-independent and subsequent abundance was a function of recruitment. These contrasting results may reflect differences in prey behavior. P. amboinensis tended to feed near or within the branches of coral inhabited by resident predators, while N. cyanomos tended to feed higher in the water column above the reefs, and thus farther away from resident predators. These results highlight the speed and extent to which patterns of settlement are modified, indicating that caution should be exercised when attributing patterns of recruitment to patterns of settlement. Tremendous between-species variation in how patterns of recruitment, and presumably settlement, were modified by predation indicates that generalizations or between-species extrapolations about the magnitude of these effects may be unwarranted.     

Assessing Posidonia oceanica Seedling Substrate Preference: An Experimental Determination of Seedling Anchorage Success in Rocky vs. Sandy Substrates

In the last decades the growing awareness of the ecological importance of seagrass meadows has prompted increasing efforts to protect existing beds and restore degraded habitats. An in-depth knowledge of factors acting as major drivers of propagule settlement and recruitment is required in order to understand patterns of seagrass colonization and recovery and to inform appropriate management and conservation strategies. In this work Posidonia oceanica seedlings were reared for five months in a land-based culture facility under simulated natural hydrodynamic conditions to identify suitable substrates for seedling anchorage. Two main substrate features were investigated: firmness (i.e., sand vs. rock) and complexity (i.e., size of interstitial spaces between rocks). Seedlings were successfully grown in culture tanks, obtaining overall seedling survival of 93%. Anchorage was strongly influenced by substrate firmness and took place only on rocks, where it was as high as 89%. Anchorage occurred through adhesion by sticky root hairs. The minimum force required to dislodge plantlets attached to rocky substrates reached 23.830 N (equivalent to 2.43 kg), which would potentially allow many plantlets to overcome winter storms in the field. The ability of rocky substrates to retain seedlings increased with their complexity. The interstitial spaces between rocks provided appropriate microsites for seedling settlement, as seeds were successfully retained, and a suitable substrate for anchorage was available. In conclusion P. oceanica juveniles showed a clear-cut preference for hard substrates over the sandy one, due to the root system adhesive properties. In particular, firm and complex substrates allowed for propagule early and strong anchorage, enhancing persistence and establishment probabilities. Seedling substrate preference documented here leads to expect a more successful sexual recruitment on hard bottoms compared with soft ones. This feature could have influenced P. oceanica patterns of colonization in past and present time.     

Early life-history processes and their implications for the invasion of the barnacle Balanus glandula

As invasions become increasingly prevalent, it is important to understand how spread may be moderated by environmental conditions. This study considered the effect of location and changing substratum temperature on the early life-history processes of the alien barnacle Balanus glandula and its native comparator, Chthamalus dentatus, along the South African coast. Using settlement plates of different colours, temperature was manipulated to assess settlement, mortality, recruitment and growth of the two species. These variables were tracked over 10 weeks using repeat photography. Unexpectedly, there was no evidence of an effect of temperature on early life-history processes of either species. Settlement by the two barnacles was spatially segregated, with B. glandula occurring only on the West Coast where it is the dominant intertidal barnacle, while C. dentatus settled only on the South Coast, which has only recently been invaded by the alien. Despite this, it was notable that the relative settlement of B. glandula on rock was higher than that of C. dentatus. However, the lack of mortality of the native resulted in comparable levels of recruitment among the species. Nonetheless, the propensity of B. glandula to settle sporadically, coupled with fast growth, suggests that the invader may still possess the ability to become dominant along the newly invaded South Coast. This study highlights that measurements of early life-history parameters may not adequately predict the future range and impacts of alien species unless interpreted within the broader context of the nature of the recipient region and species-specific traits of the invader.     

Biophysical Constraints on Optimal Patch Lengths for Settlement of a Reef-Building Bivalve

Reef-building species form discrete patches atop soft sediments, and reef restoration often involves depositing solid material as a substrate for larval settlement and growth. There have been few theoretical efforts to optimize the physical characteristics of a restored reef patch to achieve high recruitment rates. The delivery of competent larvae to a reef patch is influenced by larval behavior and by physical habitat characteristics such as substrate roughness, patch length, current speed, and water depth. We used a spatial model, the “hitting-distance” model, to identify habitat characteristics that will jointly maximize both the settlement probability and the density of recruits on an oyster reef (Crassostrea virginica). Modeled larval behaviors were based on laboratory observations and included turbulence-induced diving, turbulence-induced passive sinking, and neutral buoyancy. Profiles of currents and turbulence were based on velocity profiles measured in coastal Virginia over four different substrates: natural oyster reefs, mud, and deposited oyster and whelk shell. Settlement probabilities were higher on larger patches, whereas average settler densities were higher on smaller patches. Larvae settled most successfully and had the smallest optimal patch length when diving over rough substrates in shallow water. Water depth was the greatest source of variability, followed by larval behavior, substrate roughness, and tidal current speed. This result suggests that the best way to maximize settlement on restored reefs is to construct patches of optimal length for the water depth, whereas substrate type is less important than expected. Although physical patch characteristics are easy to measure, uncertainty about larval behavior remains an obstacle for predicting settlement patterns. The mechanistic approach presented here could be combined with a spatially explicit metapopulation model to optimize the arrangement of reef patches in an estuary or region for greater sustainability of restored habitats.     

Environmental constraints on oviposition limit egg supply of a stream insect at multiple scales

Species with complex life cycles pose challenges for understanding what processes regulate population densities, especially if some life stages disperse. Most studies of such animals that are thought to be recruitment limited focus on the idea that juvenile mortality limits the density of recruits (and hence population density), fewer consider the possibility that egg supply may be important. For species that oviposit on specific substrata, environmental constraints on oviposition sites may limit egg supply. Female mayflies in the genus Baetis lay egg masses on the underside of stream rocks that emerge above the water’s surface. We tested the hypothesis that egg mass densities are constrained by emergent rock densities within and between streams, by counting egg masses on emergent rocks. All emergent rocks were counted along 1-km lengths of four streams, revealing significant variation in emergent rock density within streams and a more than three-fold difference between streams. In each stream, egg mass density increased with the density of emergent rocks in 30-m stretches. We used regression equations describing these small-scale relationships, coupled with the large-scale spatial variation of emergent rocks, to estimate egg mass densities for each 1-km stream length, a scale relevant to population processes. Scaled estimates were positively associated with emergent rock density and provided better estimates than methods that ignored environmental variation. Egg mass crowding was inversely related to emergent rock density at the stream scale, a pattern consistent with the idea that oviposition substrata were in short supply in streams with few emergent rocks, but crowding did not compensate entirely for differences in emergent rock densities. The notion that egg supply, not larval mortality, may limit population density is an unusual perspective for stream insects. Environmental constraints on egg supply may be widespread among other species with specialised oviposition behaviours.     

Habitat structure,conspecific presence and spatial variation in the recruitment of a temperate reef fish

Pronounced spatial variation in recruitment occurs in many marine invertebrate and fish populations and is thought to be critical to the demography of these species. In this study I examined the importance of habitat structure and the presence of conspecific residents to spatial variation in larval settlement and recruitment in a temperate fish Tautogolabrus adspersus. I define settlement as the movement of individuals from the water column to the benthic habitat, while I refer to recruitment as numbers of individuals surviving some arbitrary period of time after settlement. Experiments in which standard habitats were stocked with conspecifics showed that resident conspecifics were not an important factor contributing to small-scale variability in recruitment. Further correlative analyses demonstrated that large-scale variation in recruitment could not be explained by variability in older age classes. By contrast, manipulations of macroalgal structure within a kelp bed demonstrated that recruitment was significantly higher in habitats with a dense understory of foliose and filamentous algae than in habitats with only crustose algae. Understory algae varied in their pattern of disperison among sites, and the dispersion of fish matched that of the plants. In order to determine the effects of differences in patterns of algal dispersion on the demography of associated T. adspersus populations, I used experimental habitat units to manipulate patterns of dispersion. Settlement was significantly greater to randomly placed versus clumped habitats; however, no differences in recruitment between random and clumped habitats were detected. Because recruitment is a function of the numbers of settlers minus the subsequent loss of settlers, rates of mortality or migration must have been higher in the randomly placed habitats. These results are counter to the current paradigm for reef fishes which suggests that larval settlement is the crucial demographic process producing variability in population abundance. In this experiment patterns of settlement were modified by varying the patch structure of the habitat.Contribution number 278 from the Center for Marine Biology, University of New Hampshire     

Fine-scale temporal variation in recruitment of a temperate demersal fish: the importance of settlement versus post-settlement loss

In order to understand variability in recruitment to populations of benthic and demersal marine species, it is critical to distinguish between the contributions due to variations in larval settlement versus those caused by post-settlement mortality. In this study, fine-scale (1–2 days) temporal changes in recruit abundance were followed through an entire settlement season in a temperate demersal fish in order to determine 1) how dynamic the process of recruitment is on a daily scale, 2) whether settlement and post-settlement mortality are influenced by habitat structure and conspecific density, and 3) how the relationship between settlement and recruitment changes over time. Settlement is considered to be the arrival of new individuals from the pelagic habitat, and recruitment is defined as the number of individuals surviving arbitrary periods of time after settlement. Replicate standardized habitat units were placed in 2 spatial configurations (clumped and randomly dispersed) and monitored visually for cunner (Tautogolabrus adspersus) settlement and recruitment every 1–2 days throughout the settlement season. The process of recruitment in T. adspersus was highly variable at a fine temporal scale. Changes in the numbers of recruits present on habitat units were due to both settlement of new individuals and mortality of animals previously recruited. The relative importance of these two processes appeared to change from day to day. The magnitude of the change in recruit number did not differ between the clumped and random habitats. However, post-settlement loss was significantly greater on randomly dispersed than clumped habitats. During several sampling dates, the extent of the change in recruit abundance was correlated with the density of resident conspecifics; however, on other dates no such relationship appeared to exist. Despite the presence of significant relationships between the change in recruit number and density, there was no evidence of either density-dependent mortality or settlement. Initially, there was a strong relationship between settlement and recruitment; however, this relationship weakened over time. Within 2 months after the cessation of settlement, post-settlement loss was greater than 99%, and no correlation remained between recruitment and the initial pattern of settlement. The results of this study demonstrate that the spatial arrangement of the habitat affects the rate and intensity of post-settlement loss. Counter to much current thinking, this study suggests that in order to understand the population ecology of reef fishes, knowledge of what habitats new recruits use and how mortality varies with structural aspects of the habitats is essential.     

Temporal scales of variation in settlement and recruitment of the mussel Perna perna (Linnaeus, 1758)

Population dynamics of many intertidal organisms are strongly affected by the abundance and distribution of larvae arriving on the shore. In particular, not only absolute numbers of settlers but also the degree of synchronisation of settlement can have a strong influence on whether density-dependent or density-independent processes shape adult shape populations. Temporal variation in rates of settlement and recruitment of the mussel Perna perna on the south coast of South Africa was investigated using a nested spatial design at different temporal scales. Variability in settlement at spring tides was examined at two temporal scales: lunar (to investigate the effect of state of the moon on settlement) and tidal (to investigate the influence of state of the tide on mussel settlement). Recruitment over neap tides was examined at one temporal scale, fortnight (to investigate the effect of date on mussel recruitment).Strong temporal variation was evident for both settlement and recruitment, but not at all time scales. Distinct peaks of settler/recruit abundance were observed during the lunar and neap tide studies. Recruitment intensity differed over the course of the year, and pulsing of recruitment was generally synchronised among locations. However, the strength of pulsing differed dramatically among locations, giving a significant interaction between fortnight and location. The finest temporal scale, investigated in the tidal study, did not reveal a significant effect of the state of the tide on settlement. The state of the moon (new or full) was not significant as a main factor (p = 0.052), although generally more settlers arrived on the shore during new moon. Phase of the moon appeared to have an effect on settler abundances, but only when and where densities were high.     

The effects of larval abundance, settlement and juvenile mortality on the depth distribution of a colonial ascidian

The colonial ascidian Didemnum candidum (Savigny) is more abundant at shallow depths on floating docks than at greater depths along pilings in Pearl Harbor, Oahu, Hawaii. To compare the effects of selective settlement and postsettlement mortality on adult distribution, I determined if settlement was nonrandom relative to depth, if differences in adult abundance were responsible for the depth distribution of settlement, and if juvenile mortality varied with depth. A plankton pump was used to measure variation in larval abundance with depth. Acrylic settling plates were suspended at different depths and sampled nondestructively to measure settlement intensity and juvenile mortality. Settlement and mortality of the didemnid ascidians Diplosoma listerianum Milne-Edwards and Diplosoma sp. were also measured in the field and compared to that of D. candidum. Settlement of D. candidum was indeed nonrandom over depth. Both planktonic larvae and settled juvenils were more numerous at 0.5 than 3 or m. Settlement intensity on vertical plates was greatest within 1–2 cm of the water surface in the laboratory and within 2–3 cm in the field. Directly adjacent to the floating dock, where adults were equally abundant, settlement on horizontal plates was greater at shallow depths, suggesting directed movement of larvae upward. However, the ratio of settled juveniles to planktonic larvae (number of settlers: number of larvae) did not significantly differ with depth, suggesting that larvae were not more likely to settle at a particular depth. Settlement of the Diplosoma species was also heaviest near the surface. Juvenile mortality was greater at 0.5 than at 3 or 6 m for both D. candidum and the Diplosoma species. Experimental settlement showed that mortality, per se, of D. candidum was independent of depth. Rather, mortality was density-dependent, and the higher mortality near the surface was due to the greater number of larvae settling there. At this location, nonrandom settlement appears to determine the adult distribution of D. candidum, despite greater juvenile mortality at shallow depths. The pattern of settlement over depth is largely determined by adult proximity, rather than active larval behavior.     

Induction of metamorphosis in the Asian shore crab Hemigrapsus sanguineus: Characterization of the cue associated with biofilm from adult habitat

Recruitment of crabs to nursery habitat requires settlement of the megalopal stage on suitable substratum followed by metamorphosis into the first juvenile stage. Reducing the time to metamorphosis may result in higher recruitment and survival. Previous work has shown that metamorphosis of the Asian shore crab is accelerated by cues from three different sources: (a) water-soluble exudate produced by conspecific adult crabs; (b) biofilm covering rocks in natural habitat for this species; and (c) abiotic rock from natural habitat. The objective of the present investigation was to characterize the metamorphic cue associated with biofilm from rocky intertidal habitat and to compare the three metamorphic cues (exudate from conspecific adults, biofilm from rocky intertidal, and texture of substratum) that have been identified for H. sanguineus. Results of our study show that megalopae of the Asian shore crab respond strongly to biofilm associated with rocky intertidal habitat that has developed for at least 8 days. We also found that megalopae respond to textured rock surfaces from natural habitat, even when those surfaces had been rendered abiotic. The cue remains active after the biofilm has been exposed to − 20 ºC for 12 h, but is de-activated by a few minutes exposure to 100 °C. Moreover, the biofilm cue appears to work in synergy with cues from other sources, but requires actual contact with the biofilm. Our findings show that addition of biofilm to an abiotic textured rock surface significantly decreases mean time to metamorphosis, and simultaneous exposure of megalopae to biofilm-covered rock and to exudate from adult H. sanguineus decreases mean time to metamorphosis even further. The response of this species to multiple cues—and particularly to biofilm in the absence of adult conspecifics—provides a clear advantage in the colonization of virgin habitat and helps explain the very rapid spread of this invasive species along the majority of the east coast of the United States in only two decades.     

Field Assessment of the Predation Risk - Food Availability Trade-Off in Crab Megalopae Settlement

Settlement is a key process for meroplanktonic organisms as it determines distribution of adult populations. Starvation and predation are two of the main mortality causes during this period; therefore, settlement tends to be optimized in microhabitats with high food availability and low predator density. Furthermore, brachyuran megalopae actively select favorable habitats for settlement, via chemical, visual and/or tactile cues. The main objective in this study was to assess the settlement of Metacarcinus edwardsii and Cancer plebejus under different combinations of food availability levels and predator presence. We determined, in the field, which factor is of greater relative importance when choosing a suitable microhabitat for settling. Passive larval collectors were deployed, crossing different scenarios of food availability and predator presence. We also explore if megalopae actively choose predator-free substrates in response to visual and/or chemical cues. We tested the response to combined visual and chemical cues and to each individually. Data was tested using a two-way factorial design ANOVA. In both species, food did not cause significant effect on settlement success, but predator presence did, therefore there was not trade-off in this case and megalopae respond strongly to predation risk by active aversion. Larvae of M. edwardsii responded to chemical and visual cues simultaneously, but there was no response to either cue by itself. Statistically, C. plebejus did not exhibit a differential response to cues, but reacted with a strong similar tendency as M. edwardsii. We concluded that crab megalopae actively select predator-free microhabitat, independently of food availability, using chemical and visual cues combined. The findings in this study highlight the great relevance of predation on the settlement process and recruitment of marine invertebrates with complex life cycles.     

The role of temperature and maternal ration in embryo survival: using the dumpling squid Euprymna tasmanica as a model

Using a ‘model’ sepiolid, Euprymna tasmanica, this study investigated the role of maternal nutritional and thermal history on egg quality and subsequent embryo survival. E. tasmanica is a multiple spawner, therefore it was possible to track egg quality and hatching success over successive spawning episodes. A two-factor orthogonal experimental design, involving two feeding levels (high and low rations) and two temperatures (summer and winter), was implemented with half of the replicates used to explore embryonic development and the remaining half examining egg-yolk quality via fatty acid analysis. Differences in reproductive output and embryo mortality were largely attributed to maternal ration and not temperature. Females maintained on low ration produced smaller clutches, consisting of smaller eggs and exhibiting higher embryo mortality rates than high ration females. Both batch fecundity and relative hatching success declined over successive clutches. Lipid content was also significantly lower in low ration females, however, the relative quality in terms of lipid and fatty acid constituents was maintained regardless of treatment and spawning frequency. It is suggested that elevated embryo mortality rate in eggs spawned by low-fed females was a function of insufficient maternally derived yolk resources to fuel embryogenesis. Results indicate that maternal nutritional and reproductive history are important determinates for offspring survival, potentially having significant effects on the magnitude of subsequent recruitment events in squid populations.     

Vulnerability of newly settled plaice (Pleuronectes platessa L.) to predation: effects of habitat structure and predator functional response

Plaice (Pleuronectes platessa) nursery grounds on the Swedish west coast have been subject to increasing cover of annual green macroalgae during recent years, with growth of algae starting at the time of plaice settlement in April to May. A laboratory experiment was performed to investigate how the vulnerability to predation of metamorphosing plaice was affected by the presence of filamentous algae. Predation by shrimps (Crangon crangon) on settling plaice larvae was higher on sand than among algae, whereas predation by crabs (Carcinus maenas) was unaffected by habitat type, suggesting a lower overall mortality of plaice in the vegetated habitat. When predators and prey were presented with a combination of the two habitats, predation by shrimps was as high as that in the sand treatment alone, whereas predation by crabs was lower than that in the two treatments with one habitat. Based on these results, an additional experiment was performed, investigating the functional response of shrimps to six densities of juvenile plaice in a sand habitat with alternative prey present. The proportional mortality of juvenile plaice (12-16 mm total length (TL)) was density-dependent and was best described by a type III (sigmoid) functional response of the predatory shrimps. The results suggested that the combined predation pressure from shrimps and crabs was lower among algae than on sand, but settling plaice and predatory shrimps chose the sand habitat. Plaice densities in the sigmoid part of the obtained functional response curve represented normal to high field densities of plaice on the Swedish west coast, suggesting that shrimp predation could have a stabilising effect on plaice recruitment. The formation of macroalgae mats could therefore lead to a concentration of plaice juveniles in the remaining sand habitat and increased mortality through density-dependent predation by shrimps.     

The interplay of substrate nature and biofilm formation in regulating Balanus amphitrite Darwin, 1854 larval settlement

The settlement of marine larvae is influenced by a wide range of physical and biological factors. It is still poorly known how the nature of substrate and the biofilm can interact in regulating settlement patterns of invertebrate larvae. Here we use laboratory experiments focused on settlement behaviour of the barnacle Balanus amphitrite. The aim of this work is to understand whether: (i) the nature of substratum can affect biofilm formation and its structure, (ii) the nature of substratum can affect B. amphitrite larval settlement, (iii) the age of the biofilms and the nature of substrate can interact in influencing larval settlement.Four kinds of substrata (marble, quartz, glass, and cembonit) were biofilmed under laboratory conditions for 5, 10 and 20 days at the temperature of 28 °C. Settlement response was investigated with 5-day-old cyprids. Biofilms were quantitatively and qualitatively analysed by scanning electron microscopy. The settlement of B. amphitrite larvae significantly differed among substrata; also, the patterns of development of biofilm assemblages changed with substrate. In addition, the larval attractiveness of different substrates tends to disappear with biofilm age.     

The influence of resident adults on recruitment: a comparison to settlement

For species recruiting into established sessile communities, the adult colonies and individuals already present form a significant part of the environment and have the potential to alter both larval settlement rates and post-settlement mortality. Settlement rates can be reduced by predation on larvae, by the removal or addition of substratum space, or by stimulation or prohibition of larvae from settling on adjacent substratum. Once attached, the recruiting individual can still be influenced by predation or overgrowth by residents, by the added physical structure for firmer attachment, or by being camouflaged from motile predators. To examine those processes by which residents affect recruitment we exposed experimental substrata with three densities of adults of a single species at a site in eastern Long Island Sound, USA for a 1-wk period. Seven different species of common invertebrates were used in nine separate experiments. The major effect of most resident species was the usurpation of space and the restricting of recruitment to adjacent unoccupied areas. This was particularly true for resident ascidians and bryozoans, but less so for barnacles and oysters. In fact several species recruited in higher densities on or next to oysters and barnacles. Comparison to 1-day settlement experiments indicated that the encrusting ascidian species Diplosoma and possibly Botryllus reduced recruitment relative to settlement, probably by overgrowing newly-settled individuals. However, in the presence of most resident species, recruitment patterns were not greatly different from settlement patterns, indicating that the effects of the attached community on recruitment may result from influences on settlement.     

Recruitment density can determine adult morphology and fecundity in the barnacle, Semibalanus balanoides

Although consequences of the settlement preference of larvae have been well documented, the consequence of these settlement choices on subsequent mortality, morphology and fecundity has been little studied. The aim of this study was to determine the relationship between recruit and adult density and to determine the effect of recruitment on adult morphology and egg tissue mass. This study follows 48,718 barnacles (Semibalanus balanoides) from recruitment at the end of the settling season to reproductively mature adults at a field site in the Clyde Sea (UK). Overall survivorship of the recruits to adulthood was 8.5%, although survivorship was up to 42% on low density settlement panels. In low density colonies (< 10 recruits cm-2), recruitment density was related to adult density (P < 0.001), whereas no relationship was found for higher density colonies. A shell morphology index measured at adulthood was related to recruitment density for low density recruited colonies (P < 0.001) but not high density colonies. Using ANCOVA, variations between the colonies in shell and egg tissue mass were not explained by mass of somatic tissue. However, egg mass was explained by recruitment density (P < 0.01). These results show that adult density is not a reliable indicator of the previous population density of the colony. Moreover, there are marked differences in population development between colonies with high and low recruit densities in terms of impact upon shell morphology and egg production. The dynamics that operate between recruits at the end of the settlement season and sexually mature adults to create the patterns elucidated in this paper, and other literature, remain unclear.     

Variation in barnacle recruitment over small scales: larval predation by adults and maintenance of community pattern

Over small spatial scales, variation in the density of settlers of benthic sessile species is the result of interactions among larval behavior, local hydrodynamic conditions, and the physical, chemical and biological characteristics of the benthic habitat. It has been shown repeatedly that adult benthic filter-feeders can consume larvae of their own and other species, but their effects on the distribution and abundance of recruits have rarely been demonstrated under natural conditions in the field, particularly on hard substrata. Here we experimentally quantified the effect of the large intertidal barnacle, Semibalanus cariosus (Pallas), on the density of recruits of three common barnacle species. The experiments were conducted at the peak of the barnacle recruitment season over three successive years, on the west coast of San Juan Island, Washington. A persistent and well documented community pattern in the mid intertidal zone of the study site is a sparse bed of adult S. cariosus with bare rock spaces essentially devoid of small barnacles among the large individuals. Field experiments consisted of small areas from which either all adult S. cariosus were killed leaving the shells attached to the rock, or live adult barnacles were left intact. Our results showed that over small spatial scales of a few to tens of centimeters, the large barnacle S. cariosus can interfere and significantly reduce net settlement and recruitment of conspecific as well as other barnacle species. Between 65 and 100% reduction in settlement could be attributed to larval predation by adults, as implied by barnacle settlement patterns on different treatments and by the presence of nauplius larvae in cirri and stomach contents of S. cariosus. The negative effect on barnacle settlement was consistent between years of relatively low barnacle recruitment, which appears to be the most common situation at the study site, but it disappeared on a year of unusually high recruitment, when settling larvae seem to have swamped the filtration ability of adult S. cariosus. The different barnacle species displayed contrasting settlement patterns on bare rock and on the lateral shells of the large barnacles, which appear to be a result of differences in larval behavior. Comparisons against the relative availability of these substrata in the experimental plots suggested that larvae of different species sample the benthic microhabitat in very different ways.     

DISTRIBUTION AND RECRUITMENT OF SUBTIDAL GENICULATE CORALLINE ALAGE1

Geniculate coralline algae are common members of kelp forest communities. The structure provided by their stiff branches greatly influences the abundance and species composition of benthic animals and can affect associated algae by inhibiting recruitment, but the branches are themselves substrate for a large number of other taxa. However, other than qualitative observations, little is known about the within-site distribution, recruitment, and growth of these algae. We examined the distribution of the dominant corallines at a subtidal site in central California. Abundances of Calliarthron tuberculosum (Post. & Rupr.) Dawson, Bossiella californica ssp. schmittii (Manza) Johans., Calliarthron cheilosporioides Manza, Corallina vancouveriensis Yendo, and unidentifiable juveniles were determined at depths of 10, 15, and 20 m and on horizontal rock, vertical rock, and cobble. Calliarthron tuberculosum was most abundant (≤39% cover) at all depths, growing primarily on horizontal surfaces. Vertical surfaces and cobbles were dominated by B. californica ssp. schmittii (40 and 15% cover, respectively). These two most abundant species had the highest cover at 15 m. Calliarthron cheilosporioides and C. vancouveriensis were relatively rare (<1% cover) and generally grew on horizontal rocks and at shallower depths. Unidentified juveniles were also rare and occurred mainly on horizontal rocks and cobbles at 20 m. The settlement and growth rates of coralline crusts and the initiation and growth rates of young erect fronds from these crusts were determined in clearings made in the spring and fall at the three depths. Crust densities and diameters were highest at 10 m and in spring clearings. Settlement and growth tended to decrease with increasing depth. Trends were similar in fall clearings, but initial settlement was lower. Initiation and growth of fronds decreased with depth and were also higher in fall clearings. These variations in depth and substrata distribution, as well as settlement and growth, suggest there is considerable variation in the population biology between species in this group of subtidal plants.     

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).