Peak Recruitment Research Articles

Spatial and temporal characteristics of benthic invertebrate communities at Culbin Sands lagoon, Moray Firth, NE Scotland, and impacts of the disturbance of cockle harvesting

In the present study, Culbin Sands lagoon, a protected site in NE Scotland, was surveyed every 2 to 4 weeks during a three-year period (1994-1996) to study benthic invertebrate communities. Beds of Mytilus edulis covered 18000 m 2 . 53 macroinfaunal species were identified outside these areas. The most conspicuous were: the lugworm Arenicola marina (mean up to 55 casts m -2 ); and bivalves Cerastoderma edule (mean up to 158 ind. m -2 ) and Macoma balthica (mean up to 79 ind. m -2 ) after settlement. The standing stock ranged from 20 to 32 g AFDW m -2 yr -1 respectively from more exposed to more sheltered areas. Most species showed a clear recruitment peak in autumn, but others (e.g. Capitella capitata , and Spionidae) displayed several peaks in a year. Communities were also compared between the sampling sites before and after an incidental disturbance caused by cockle Cerastoderma edule harvesting, which took place in June 1995. One site showed –0.7% variation in the total standing stock, but +22% for smaller-cockles, as larger filter-feeding cockles were removed therefore enhancing their own larval settlement. Polychaete Spionidae populations also increased after larger cockles were removed. The polychaete Arenicola marina population returned to its normal activities just after the dramatic disturbance of the sediment.

Temporal variability of benthic macrofauna on Cassino beach, southernmost Brazil

The temporal variability of benthic macrofauna on Cassino beach, southernmost Brazil, was studied for a period of one year (June 2004 to May 2005) based on monthly sampling. Three sites were selected distant 50m from each other. At each site, 3 transects were established, 2m equidistant from one another. Each transect extended from the base of the primary dunes to the inner surf zone at approximately 1m in depth, with 7 or 8 sampling levels. Within transects, the distance between the levels was 20m until the upper swash zone, from which distance was 10m until the 1-meter isobath. The temporal variation in the abundance of benthic macrofauna observed in the present study can be attributed to (1) the positive effects of the recruitment peaks and migration of particular species to the swash zone and (2) negative effects of the migration of some species to deeper waters, as well (3) as mortality through natural causes (stranding and action of predators) and (4) human causes (harvesting and vehicle transit). We attribute the expressive abundance increase of benthic macrofauna to recruitment. The stranding, that is, the trapping of the organisms on the upper parts of the beach, is likely the main cause of abrupt drops in benthic macrofauna abundance.

Fire season effects on the recruitment of non‐sprouting serotinous Proteaceae in the eastern (bimodal rainfall) fynbos biome, South Africa

Abstract Research in Mediterranean‐climate shrublands in both South Africa and Australia shows that recruitment of proteoid shrubs (non‐sprouting, serotinous Proteaceae) is best after warm‐season (summer and autumn) fires and worst after cool‐season (winter and spring) ones. This pattern has been attributed to post‐dispersal seed attrition as well as size of pre‐dispersal seed reserves. Here we investigate patterns of post‐fire recruitment for four proteoid species in the eastern part of South Africa's fynbos biome, which has a bimodal (spring and autumn) rainfall regime. Despite the lack of significant differences in recruitment between cool‐ and warm‐season burns, we find some evidence for favourable recruitment periods following fires in spring and autumn, immediately before, and coinciding with, the bimodal rainfall peaks. This suggests that enhanced recruitment is associated with conditions of high soil moisture immediately after the fire, and that rapid germination may minimize post‐dispersal seed attrition. In two of the species, we also find a shift from peak flowering in winter and spring in the Mediterranean‐climate part of the fynbos biome, to summer and autumn flowering in the eastern part. Because these two species are only weakly serotinous, warm‐season flowering would result in maximal seed banks in spring, which could explain the spring recruitment peak, but not the autumn one. We conclude that eastern recruitment patterns differ significantly from those observed in the western and central parts of the biome, and that fire management protocols for the east, which are currently based on data and experience from the winter‐rainfall fynbos biome, need to be adjusted accordingly. Fire managers in the eastern fynbos biome should be less constrained by requirements to burn within a narrow seasonal range, and should therefore be in a better position to apply the required management burns.

Demographics and disturbance history of a boreal old‐growth Picea abies forest

AbstractQuestion:To what extent do tree growth, mortality, and long‐term disturbance patterns affect stand structure and composition of an old‐growth Picea abies forest?Location:Boreal Sweden.Methods:We linked data from three 50 m × 50 m permanent plots established in 1986 with dendrochronology data to evaluate tree growth and mortality over an 18‐year period and to describe a several‐hundred‐year disturbance history for this forest type.Results:Averaged over all diameters, P. abies trees had an annual mortality rate of 0.60%; however, diameter had a striking effect on both growth and mortality, with trees of intermediate diameters (ca. 20–30 cm) showing faster growth and lower mortality. Their increased vigor gave rise to a diameter distribution resembling the ‘rotated sigmoid’(not reverse‐J) proposed for such conditions, and it led to a deficit of snags of intermediate diameters. Slow‐growing trees had an increased likelihood of dying. Although recruitment occurred in most decades over the past 400 years, two prominent recruitment peaks occurred (mid 1700s and 1800s), neither of which appeared to cause a shift in tree species composition. The lack of fire evidence suggests that fire was not responsible for these recruitment peaks.Conclusions:Taken together, these results depict a rather impassive system, where canopy trees die slowly over decades. Field observations suggest that fungal infections, mediated by wind, account for much of the mortality during these periods of relative quiescence. However, these periods are at times punctuated by moderate‐severity disturbances that foster abundant recruitment.

Seasonal Patterns of Juvenile Fish Abundance in Seagrass Meadows in Teague Bay Bank Barrier Reef Lagoon, St. Croix, U.S. Virgin Islands

Considerable knowledge has been gained regarding fish use of nearshore habitats such as seagrass meadows or mangrove lagoons in the Caribbean (e.g., evaluation of nursery value, trophic linkages). However, few studies have been conducted on fish recruitment to seagrass habitat around the Caribbean. Juvenile reef fish in seagrass meadows at Teague Bay, St Croix, U.S. Virgin Islands were surveyed from October 1998 through September 1999 using a visual census technique. Grunts (Haemulidae) were the most abundant juveniles observed (60% of all fish), followed by wrasses (Labridae, 20%) and parrotfishes (Scaridae, 13%). French grunt, Haemulon flavolineatum, were the most numerous species (59.5% of all fish), followed by slippery dick, Halichoeres bivittatus (18.5%), and bucktooth parrotfish, Sparisoma radians (10.4%). Most numerically abundant fish species demonstrated peaks in recruitment during late summer and fall. Our results imply that the functioning of seagrass beds incorporates strong seasonal patterns of small-fish abundance that need to be accommodated in any study wishing to understand their importance to fisheries.

Reproductive biology of the rock shrimp Sicyonia dorsalis (Decapoda: Penaeoidea) from the southeastern coast of Brazil

Summary Sicyonia dorsalis is not commercially exploited in Brazil, but it forms a critical link in marine food chains by converting detritus food sources into microorganism biomass that is available for higher trophic levels. We examined the reproductive biology of this species, sampled for five and a half years in a tropical locality, the southeastern coast of São Paulo (23° S), Brazil. Monthly samples were taken from January 1998 to June 2003 at depths of 5 to 45 m. Degree of ovarian development was used to examine breeding in adult females. Recruitment was estimated by changes in proportion of individuals in juvenile size classes of carapace length (CL #8.2 mm) in relationship to the total number of individuals. The largest immature female measured 9.2 mm CL, but 75% of females were smaller than 7.3 mm CL. The largest immature male measured 6.8 mm CL. The relative frequency of reproductive females was negatively correlated with the relative frequency of recruitment (p <0.001). Reproductive females were found in every season, with peaks in December-February following the recruitment peak in March-June, and a second peak of spawning in May-July and of recruitment in September-November. The continuous reproductive pattern observed for S. dorsalis, combined with the hypothesis that during spring and summer the phytoplankton production is higher, when the South Atlantic central water mass intrudes into the region during upwelling events, suggests that food availability for protozoeal and mysis larvae may be an important selective factor shaping the seasonal breeding pattern for S. dorsalis. The classical paradigm of continuous reproduction at lower latitudes, with increased seasonality of the breeding period at higher latitudes, seems to be valid for this species.

Choice chamber experiments to test the attraction of postflexion Rhabdosargus holubi larvae to water of estuarine and riverine origin

Although the recruitment of larvae and juveniles of marine fishes into estuaries has been well documented, little is known about the factors governing the immigration of estuary-associated marine fishes into estuaries. Fishes have a well-developed sense of smell and it has been suggested by several workers that olfactory cues of freshwater or estuarine origin serve as stimuli, attracting larvae and juveniles of estuary-associated species into estuaries. Attraction of postflexion Rhabdosargus holubi larvae to estuary and river water from the Kowie estuarine system, South Africa, was measured using a rectangular choice chamber. In experiments, conducted during peak recruitment periods, larvae selected estuary and river water with a significantly higher frequency than sea water. This study, the first to assess the possible role of olfaction in the recruitment process of an estuary-associated marine fish species, demonstrates that larvae are able to recognise water from different origins, probably based on odour.

Juvenile Penaeid Shrimp Density, Spatial Distribution and Size Composition in four adjacent habitats within a Mangrove-Fringed Bay on Inhaca Island, Mozambique

The effects of habitat characteristics (mangrove creek, sandflat, mudflat and seagrass meadow) water salinity, temperature, and depth on the density, spatial distribution and size distribution of juveniles of five commercially important penaied shrimp species (Metapenaus monoceros, M. stebbingi, Fenneropenaeus indicus, Penaeus japonicus and P. semisulcatus) were investigated during a high shrimp recruitment peak lasting from January to June 2002, in four contiguous habitats within a non-estuarine mangrove bay at Saco da Inhaca, Inhaca Island, Southern Mozambique. A total of 14,976 specimens representing the five species were collected by means of a 1m beam-trawl fitted with a 2 mm mesh net with a cod-end. Every two weeks corresponding with spring tides on three consecutive nights, three trawls of 50 meters each were carried out each night in each habitat at 1.40–2.15 a.m. after the daily spring tide high water peak. Species distributions among the four habitats during the six months sampled showed significant differences in habitat preference (p>0.001). Fenneropenaeus indicus, M. stebbingi and P. japonicus dominated the catches in sand flats with densities of 27 ± 0.94, 18 ± 0.98 and 7 ± 0.76 shrimp /50 m2, respectively. Metapenaeus monoceros dominated catches in mudflats at a density of 21 ± 0.78 /50 m2, followed by M. stebbingi with 13±1.2 /50 m2. Penaeus semisulcatus was almost exclusively captured in seagrass meadows at densities of 8± 0.57 /50 m2. Only the Metapeneaus species showed a wide range of habitat utilization. The differences in habitat preference emphasises spatial partitioning between shrimps, reducing competition. Multiple linear regressions of juvenile shrimp densities for each species with water depth, salinity and temperature showed some significant effects. Fenneropenaeus indicus density showed a significant negative relationship with salinity and water depth and a positive relationship with temperature and turbidity. Metapenaeus stebbingi numbers showed a significant positive correlation with increasing salinity and temperature, but a negative one with depth. Penaeus japonicus numbers were significantly related to decreasing salinity and depth. Penaeus semisulcatus abundance was significantly related to decreasing temperature, and increasing turbidity and depth. More than 50% of the total shrimps captured had carapace length of &#8804; 3 mm, substantiating the nursery function of the different habitats sampled in the study. Keywords: penaeid, shrimps, juveniles, distribution, habitats, mudflats, sandflats, mangrove creek, seagrasses, Inhaca Island, Mozambique West Indian Ocean Journal of Marine Science Vol. 3 (2) 2004: pp. 163-178

Diffuse Axonal Damage, Myelin Impairment, Astrocytosis and Inflammatory Response Following Microinjections of NMDA into The Rat Striatum

White matter damage and inflammatory response are important secondary outcomes after acute neural disorders. Nevertheless, a few studies addressed the temporal outcomes of these pathological events using non-traumatic models of acute brain injury. In the present study, we describe acute inflammatory response and white matter neuropathology between 1 and 7 days after acute excitotoxic striatal damage. Twenty micrometer sections were stained by hematoxylin and eosin technique for gross histopathological analysis and immunolabed for neutrophils (anti-mbs-1), activated macrophages/microglia (anti-ed1), astrocytes (anti-gfap), damaged axons (anti-betaapp) and myelin basic protein (MBP). Recruitment peak of neutrophils and macrophages occurred at 1 and 7 days post-nmda injection, respectively. Diffuse damaged axons (beta-app + end-bulbs) were apparent at 7 days, concomitant with progressive myelin impairment and astrocytosis. Further studies using electron microscopy and blockers of inflammatory response and glutamatergic receptors should be performed to confirm and address the mechanisms of white matter damage following an excitotoxic lesion.

Experimentally Valid Predictions of Muscle Force and EMG in Models of Motor-Unit Function Are Most Sensitive to Neural Properties

Computational models of motor-unit populations are the objective implementations of the hypothesized mechanisms by which neural and muscle properties give rise to electromyograms (EMGs) and force. However, the variability/uncertainty of the parameters used in these models--and how they affect predictions--confounds assessing these hypothesized mechanisms. We perform a large-scale computational sensitivity analysis on the state-of-the-art computational model of surface EMG, force, and force variability by combining a comprehensive review of published experimental data with Monte Carlo simulations. To exhaustively explore model performance and robustness, we ran numerous iterative simulations each using a random set of values for nine commonly measured motor neuron and muscle parameters. Parameter values were sampled across their reported experimental ranges. Convergence after 439 simulations found that only 3 simulations met our two fitness criteria: approximating the well-established experimental relations for the scaling of EMG amplitude and force variability with mean force. An additional 424 simulations preferentially sampling the neighborhood of those 3 valid simulations converged to reveal 65 additional sets of parameter values for which the model predictions approximate the experimentally known relations. We find the model is not sensitive to muscle properties but very sensitive to several motor neuron properties--especially peak discharge rates and recruitment ranges. Therefore to advance our understanding of EMG and muscle force, it is critical to evaluate the hypothesized neural mechanisms as implemented in today's state-of-the-art models of motor unit function. We discuss experimental and analytical avenues to do so as well as new features that may be added in future implementations of motor-unit models to improve their experimental validity.

Temporal trends of littoral fishes at deep Posidonia oceanica seagrass meadows in a temperate coastal zone

Seasonal abundance and biomass of littoral fish at Posidonia oceanica seagrass meadows were characterized throughout an annual cycle in Mediterranean waters. Bimonthly beam trawl hauls were performed between 18 and 38 m depth at 4 sites sampling from 2 locations. Approximately 8230 littoral fish were collected belonging to 25 families and 51 species with Labridae and Sparidae families being predominant in terms of abundance. Mean fish abundance was 92 ± 7.5 individuals · 1000 m − 2 peaking in March, April and September and total abundance showed significant statistical differences between May and October. Densities of Diplodus annularis, Gobius sp., Mullus surmuletus, Parablennius tentacularis, Sarpa salpa, Sciaena umbra, Scorpaena porcus, Serranus cabrilla, Symphodus ocellatus, Symphodus rostratus, Symphodus tinca and Synodus saurus differed significantly between seasons. Total community biomass varied significantly along the year with maximum values observed in July (1500 g · 1000 m − 2 ) and minimum biomass recorded in October. The number of species ranged between 5 in January and 21 in July while the total number of individuals was 293 in July and 21 in September, and Margalef diversity index differed between 4.11 in July and 1.3 in January. Biomass peaked in summer linked mainly to the increase of S. porcus, Serranus scriba and some Symphodus species. Diversity was maximal in July and the larger mean size for most of the fish species was achieved from May to July corresponding to the recruitment peaks of some of the fish species. The SIMPER analysis showed that there are seasonal differences in the trophic roles of the fish communities at seagrass meadows. The temporal patterns observed highlight the multifunctional role of deep seagrass meadows during the summer months when all the measured parameters are maximal. Those observations point out the need for conservation measures to be intensified in the warm season.

Trafficking of dendritic cell subsets to the lung controls viral replication and pathology in human paramyxovirus infections (44.6)

Abstract Three subsets of pulmonary dendritic cells (DC) have been described in mice: conventional DC (cDC, CD11c+MHCII+), plasmacytoid DC (pDC, CD11cintB220+Ly6C+), and the recently characterized IFN-producing killer cells (IKDC, CD11cintB220+CD49b+). Here, we examined the recruitment of DC to the lung in the context of two human paramyxovirus infections: respiratory syncytial virus (RSV) and human metapneumovirus (hMPV). Recruitment of DC to the lung in infected mice was determined by flow cytometry. Both infections induced a peak of cDC recruitment (&amp;gt;3×106) by day 8 for RSV and by day 10 for hMPV. cDC remained elevated throughout the inflammatory and the resolution phases of infection (1×106) compared to control (1×105). RSV induced a peak of 8×105 pDC recruited by day 3, while hMPV induced similar numbers but at day 8. pDC returned to basal levels (1×105) by day 6 (RSV) and by day 12 (hMPV). IKDC mirrored the pattern of migration of pDC but overall numbers of IKDC were smaller: peak for RSV was 5×105 IKDC and &amp;lt;4×105 IKDC for hMPV. Furthermore, depletion of pDC with anti-mPDCA1 antibody resulted in significant increases in peak titers and pulmonary inflammation in RSV and hMPV infected mice. In conclusion, this study demonstrates that all three subsets of DC are attracted to the site of infection in the course of paramyxovirus infections and pDC play a critical role in viral replication and pathogenesis. NIH/NIAID P01 AI062885

Temporal patterns in the upstream migration of European glass eels ( Anguilla anguilla) at the Couesnon estuarine dam

The temporal migration patterns of European glass eel Anguilla anguilla at the Couesnon estuarine dam (Mont-Saint-Michel Bay, France) were examined in winter and spring 2004 and 2005. The dam which is located close to the river mouth constitutes a major obstacle for upstream-migrating glass eels. The migration was observed at different temporal scales, from within individual tides to complete tidal cycles between successive spring tides. The maximum number of glass eels arrived downstream of the dam at the beginning and in the middle of the flood tide. Glass eels migrated through the dam openings preferentially from the middle of the flood to the beginning of the ebb tide. Eel densities were highest during the second tide of each tide cycle that arrived at the estuarine dam and when the difference in water level between upstream and downstream of the dam was greatest, particularly at the end of the flood. Analysis of the influence of each environmental factor provided a good prediction of the glass eel recruitment peaks and, therefore, of the most favourable temporal windows for their migration. The water level and temperature were the most important environmental factors. These results provide the information needed for a dam-management program that is compatible with glass eel migration.

Seasonal Cycle of Jassa Marmorata Holmes, 1903 (Amphipoda) in the Ligurian Sea (Mediterranean, Italy)

This paper describes the life cycle of the amphipod Jassa marmorata with data recorded over two years (from May 2002 to April 2004) in the Marine Protected Area of Portofino (Ligurian Sea, Italy). The population was sampled monthly at |$5\;{\rm{m}}$| depth on artificial substrata. For each sample, the number of males, females, ovigerous females, and juveniles was recorded and related to water temperature. Two kinds of males, minor and major, were found, distinguishable by the presence of a thumb on the second gnathopod. The sex-ratio was significantly biased towards females (F:M = 5.6:1). The cycle revealed a female increase in mean body size during the cold season (late winter and early spring). This resulted in a peak in recruitment during the spring; females are fertile all through the year, but egg production is positively correlated with female size. During the summer of 2003, an anomalous increase in sea-water temperature was recorded. In this period the population completely disappeared, but the density during the next winter recovered again to usual values.

Spatio-temporal heterogeneity in coral recruitment around Moorea, French Polynesia: Implications for population maintenance

The spatio-temporal variability of scleractinian coral recruitment was investigated from December 2000 to December 2003 around Moorea Island (French Polynesia). Nine stations, each with 20 terracotta tiles, were placed on the outer reef slope, at 3 sites (Vaipahu, Tiahura, Haapiti), and at 3 depths (6, 12 and 18 m). The relative contribution of the different families of recruits (Pocilloporidae: 60.4%, Poritidae: 18.8%, Acroporidae: 11.2%) and the very low recruitment rates (maximum: 35 recruits per tile, higher mean densities of 10.8 recruits per tile, average 40.77 recruits m − 2 year − 1 ) recorded at Moorea are similar to recruitment patterns recorded on sub-tropical reefs. Over the duration of the study period, we detected a marked seasonal variability in recruitment rates, with the peak recruitment for all families recorded in December–March periods, corresponding to periods of warmest SSTs. However, recruits of Acroporidae were also relatively abundant in the September–December period in some years, which coincides with the known spawning periods of some Acropora species. Total recruitment rate decreased after the first year of the survey, and was probably the result of the bleaching event that occurred in early 2002, which may have reduced fecundity of some coral populations. A lower proportion of recruits were found on the upper surface of the tiles (14.5%), compared to the lower surfaces (57.1%) and sides (28.4%), which is likely the result of intense grazing by herbivorous fish and urchins. We detected a patchy distribution at the station scale, and a significant variation in recruitment patterns among depths and sites. Pocilloporidae recruited more abundantly at 6 and 18 m, whereas Poritidae were generally more abundant at 12 m depth. In contrast, Acroporidae showed no clear depth pattern during the study period. Recruitment was lowest at the most exposed site (Haapiti), especially for Acroporidae, and probably reflects lower settlement rates and/or higher early post-settlement mortality caused by frequent high swells and their associated strong currents. These distinctive characteristics in recruitment patterns of the 3 dominant families of recruits underline the important role of life history strategies in understanding the spatial patterns, dynamics and maintenance processes of coral populations. The relatively low coral recruitment rates recorded from this study indicate that recovery from severe or frequent perturbations will be slow.

How to validate a length-based model of single-species fish stock dynamics

This paper validates a new length-based model of the dynamics of fish stocks or crustaceans by hierarchically testing statistical hypotheses and thereby investigating model complexity. The approach is based entirely on scientific survey data and on determination of the statistical distributions of the number of fish caught per haul in each length class. In our example, the negative binomial distribution is statistically accepted and linked to the population level through the new length-based model. The model is derived from the characteristics of continuous recruitment, individually based growth, and continuous, length-dependent mortality rates. Continuous recruitment with annually varying recruitment peaks and individually based growth was crucial for obtaining a model that could be statistically accepted. Natural mortality was estimated as well by the model. The model was applied to survey data for Atlantic cod (Gadus morhua) in the Baltic. Its simple generic nature, as well as the validation procedure, is useful in studying and understanding life history and stock dynamics.

Extended breeding and recruitment periods of fishes on a low latitude coral reef

The temporal dynamics of fish recruitment to equatorial Indo-Pacific coral reefs are not well known. This paper documents fish recruitment over a 2.5-year period in Kimbe Bay (PNG) and shows that it is much less seasonal than is typically described for higher latitude coral reefs. Two families, wrasses (Labridae) and damselfishes (Pomacentridae), which accounted for 90% of all non-cryptic reef fish settlers, exhibited contrasting patterns. Most wrasse species had year-round recruitment with irregular peaks in abundance between November and May. Damselfish species showed a wider range of recruitment patterns, but most had negligible recruitment during the wet season (December–February), followed by one or two recruitment peaks between May and November. Species with longer seasonal recruitment periods exhibited higher cumulative levels of recruitment. For three focal damselfish species, reproductive output was reduced during the wet season, but this alone did not account for the low recruitment at this time. The lack of damselfish recruitment during the wet season is hypothesised to be due to a combination of reduced reproductive output and increased larval mortality associated with monsoonal conditions. The results indicate that there are consistent family-wide recruitment strategies that may play a significant role in the dynamics of populations in equatorial waters.

Patchiness of eggs, larvae and juveniles of European hake Merluccius merluccius from the NW Mediterranean

AbstractWe estimated patchiness for different life stages (egg, larvae and juveniles) of European hake (Merluccius merluccius) from two ichthyoplankton surveys (September and November 1999) and four bottom‐trawl surveys (November 1998, February, June and September 1999). Lloyd's patchiness P = 1 + 1/k, where k is the dispersion parameter of the negative binomial distribution, was calculated. Juveniles showed lower indices of patchiness than early ontogenetic stages. In juveniles, patchiness was highest for smaller size classes (3.5–5 cm TL) settling to their near‐bottom habitat and was very low for those larger than 5 cm up to the 30–35 cm size class (approximately 3 yr of age, 50% mature individuals). Patchiness of settling juveniles was especially high in the June and September surveys, coinciding with peak recruitment to the bottom. Both eggs and yolk sac larvae showed patchiness values &lt;10, increasing in flexion stage larvae from 12 to 20. This pattern is different to that observed in other studies for pelagic species, where patchiness of early‐stage eggs is very high, but coincides with results obtained for other demersal species with pelagic eggs. Factors that may explain these patterns are discussed in light of oceanographic conditions, adult distribution and larval ontogenesis.

The Role of Seed and Vegetative Reproduction in Plant Recruitment and Demography in Tallgrass Prairie

Recruitment, establishment and survivorship of seed- and vegetatively-derived shoots were quantified biweekly in annually burned and infrequently burned tallgrass prairie to investigate the contributions of seed and vegetative reproduction to the maintenance and dynamics of tallgrass prairie plant populations, the demography of seedlings and ramets, and the influence of fire on the demography of grasses and forbs. Clonally produced grass and forb ramets comprised >99%of all established shoots present at the end of the growing season, whereas established seedlings accounted for <1%,emphasizing the rarity of successful seedling establishment and the importance of vegetative reproduction in driving the annual regeneration and dynamics of aboveground plant populations in tallgrass prairie. Most recruitment from vegetative reproduction occurred early in the growing season and was higher in annually burned than infrequently burned sites, although low levels of new stem recruitment occurred continuously throughout the growing season. Peak recruitment on annually burned prairie coincided with peak recruitment of the dominant C4 grasses Andropogon gerardii and Sorghastrum nutans prior to prescribed spring fire, with a second peak in recruitment occurring following fire. On infrequently burned prairie, grass and forb recruitment was highest in early April and declined steadily through May. The naturalized C3 grass, Poa pratensis, was responsible for most of the early recruitment on unburned sites, whereas A. gerardii contributed most to recruitment later in May. Infrequently burned prairie was dominated by these two grasses and contained a larger forb component than annually burned prairie. The principal demographic effect of fire was on ramet natality rather than mortality. Fire regime, plant functional group, or timing of cohort emergence before or after fire did not affect ramet survivorship. C4 grass shoots that emerged early and were damaged by fire showed similar survivorship patterns to tillers that emerged after fire. Differences in species composition between annually burned and infrequently burned prairie are driven by fire effects on vegetative reproduction and appear to be related principally to the effect of fire and detritus accumulation on the development of belowground vegetative meristems of C4 grasses and their emergence dynamics.

Temporal changes in the genetic structure of intertidal populations of Hediste diversicolor (Polychaeta: Nereididae)

In order to assess the occurrence of temporal variations in the genetic structure of populations of Hediste diversicolor, four intertidal sites (10 m 2 wide) located in two estuaries of the Adriatic and Tyrrhenian coasts of Italy (Pialassa and Serchio) were repeatedly sampled from 2001 to 2003. Temporal genetic changes were investigated at six allozyme loci ( ALD, FH, HBDH, LDH, PGI and SDH). Allelic frequencies, percentages of polymorphic loci, mean observed and expected heterozygosities varied both between sampling times and among sites. At each site, F ST values revealed significant temporal changes between sampling times. The observed temporal genetic variability along with patterns of genetic fragmentation previously described suggests the occurrence of ‘chaotic genetic patchiness’ within estuaries. Results support the model proposed by Authors who, analysing population dynamics and secondary production of H. diversicolor, hypothesised that densities of H. diversicolor are primarily affected by periodical mortality events followed by recruitment peaks. Genetic drift, related to mortality events, and/or sweepstake reproductive success, with a small number of individuals responsible for the recruitment within patches, appear to be major processes promoting the genetic structuring of H. diversicolor within estuaries.