Comparison of spatial overlap between the polychaetes Nereis virens and Nephtys caeca in two intertidal estuarine environments

An intensive study of spatial overlap between the polychaetes Nereis virens (Sars) and Nephtys caeca (Fabricius) was conducted in 1992 on two tidal sand flats, which differ by their exposure to dominant winds and residual currents in the Lower St. Lawrence Estuary (Québec, Canada). Results showed that spatial overlap (Lloyd's index and spatial distribution) was higher among adults of both species at the lower tidal elevation of l'Anse-à-l'Orignal (north-east oriented). Results also suggest weaker interspecific interactions among juveniles of both species because of limited spatial overlap. In Baie-du-Ha!Ha! (south-west oriented), spatial overlap was greater than that observed in l'Anse-à-l'Orignal and appeared important in adults as well as in juveniles. Levins' directional measure of competition indicated an asymmetric spatial overlap between N. virens and N. caeca in Baie-du-Ha!Ha! and a symmetric overlap in l'Anse-à-l'Orignal. The intraspecific encounter values, estimated from Lloyd's mathematical expression, were significantly higher than values of interspecific interactions only in l'Anse-à-l'Orignal. Moreover, no vertical stratification was found inside the sediment, with no effect of the densities and individual body weights of the polychaetes. Juveniles of both species mainly inhabited the organic-rich upper portion of the sediments (0 to 12 cm), while adults colonised greater depths (>25 cm) where organic matter content was lower. A complementary field experiment was conducted in 1993 to investigate interspecific interactions (predation and competition) existing between N. virens and N. caeca. Results from this experiment depend on which species was first-introduced and showed an important variation in mortality rates between allopatric and sympatric conditions. The influence of competition and predation on the structure of these populations is also discussed in relation to their spatial overlap.     

Estimate of removal rate ofNereis virens (Polychaeta: Nereidae) from an intertidal mudflat by gulls (Larus spp.)

The feeding behavior of herring gulls (Larus argentatus), ringed-billed gulls (L. delawarensis) and great blackbacked gulls (L. marinus) on an intertidal mudflat in Maine, USA, was investigated. Remains of fish, mussels, crabs, insects, and the polychaeteNereis virens were recovered from gull feces. Forty-three percent of the fecal samples containedN. virens jaws, setae, or both. A comparison of jaws from fecal material and from worms collected from the natural community demonstrates that gulls preferentially preyed upon larger worms. Feeding was largely confined to 3 h around low tide, and birds fed mostly in the low intertidal and below mean low water where the largest worms were located. Individual birds remained on the flat for a mean of 28.0 min and consumed a mean of 19.2 worms per visit. It was calculated that gulls remove a mean of 808 largeN. virens from the flat per tide from June to October, representing an estimated 0.04% of the standing crop of largeN. virens.     

Hydrolytic enzymes in the coelomic cells of the polychaeteNereis virens during sexual maturation

Nereis virens were collected between April 1989 and April 1990 at Yerseke, Oosterscheldt Bay, The Netherlands. Activities of the hydrolytic enzymes arylsulfatase, acid phosphatase, N-acetylglucosaminidase, aminopeptidase, and carboxylesterase were investigated in coelomic cells (elaeocytes) of individuals at different stages of maturation as determined by the time course of oocyte growth. On a protein-content basis, up to ten-fold higher specific activities were present in the elaeocytes compared to the body-wall tissue. Acid phosphatase, N-acetylglucosaminidase, leucine aminopeptidase, and long- and medium-chain esterase increased continuously from the beginning to the later stage of sexual maturation, with a decline in prespawning individuals. Shortchain esterase and acid aminopeptidase did not show corresponding changes. Activities at different maturation stages were similar for males and females, with the exception of arylsulfatase activity, which was sex-specific and almost lacking in male elaeocytes. The high hydrolytic activities present in the elaeocytes as well as the increase in these activities during sexual maturation are in agreement with the function of these cells in phagocytizing muscle fragments (sarcolytes), which arise as a result of drastic histolysis of the body musculature ofN. virens during the process of sexual maturation.     

Reproductive energetics,endocrinology and population dynamics of Nephtys caeca and N. hombergi

An investigation of gametogenetic cycles and spawning success in populations of Nephtys caeca and N. hombergi from the River Tyne estuary, North Shields, England, in relation to production of gonadotrophic (GH) and spawning hormones (SH) has been made, based on data collected from 1978–1984. The data show that a consistent pattern of gametogenesis and efficient spawning occurs in populations of N. caeca in north-east England, but that the reproductive cycle of N. hombergi is erratic. Premature oosorption and gametogenic failure in this latter species is associated with a low level of GH production. Following high levels of GH production, gravid females sometimes fail to spawn owing to the non-release of SH. The reproductive energetics and reproductive success of N. caeca and N. hombergi are described in relation to the observed patterns of endocrine activity and the consequent changes in population structure.     

Vertical swimming in wave-induced currents as a control mechanism of intertidal migration by a sand-beach isopod

Both the vertical and horizontal distributions ofEurydice pulchra (Leach) within the water column of the surf zone were recorded throughout complete tidal cycles on a sandy beach in North Wales during the summer of 1989. Upon emerging from the sand, the isopods tended to swim up in the water column, where transport onshore would be facilitated by the wave-induced, onshore currents which laboratory wave-tank experiments have confirmed to occur near the water surface. This combination of active and passive transport to the water's edge results in high numbers of individuals in the narrow swash zone. At and just after the time of high tide, individuals swim to the water/sediment interface where, as again confirmed by wave-tank experiments, the predominant water movement is offshore. Continued swimming near the bottom during the ebb tide before reburrowing in the sand ensures transport downshore and avoidance of stranding above the characteristic level of zonation on the shore. Vertical migrations ofE. pulchra in the water column permit differential exploitation of up and downshore currents to achieve horizontal migration.     

Pollutant effects on the mixed function oxygenase (MFO) and reproductive systems of the marine polychaeteNereis virens

The cytochrome P-450 dependent mixed-function oxygenase (MFO) system ofNereis virens, found in the endoplasmic reticulum of the intestinal epithelium, has similar components and cofactor requirements when compared to other invertebrates. Worms exposed to food containing benzo(a)pyrene or a mixture of polychlorinated biphenyls (Aroclor 1254) had significantly higher MFO activity and cytochrome P-450 content than unexposed ones.N. virens from a site in Porland, Maine, USA, which was heavily contaminated with oil, and nearby clean reference sites were studied. The weight, MFO activity and presence or absence of gametes were determined. The MFO activity in worms from the oil-contaminated site was approximately six times that of worms from the reference sites; worms from the reference areas were six times larger than oil-contaminated worms. Worms from the oil-contaminated area lacked or had undeveloped gametes in spring, summer and fall. In contrast, many of the worms collected from the reference sites at the same times had fully developed gametes.     

Importance of predation by infaunal polychaetes in controlling the structure of a soft-bottom community in Maine,USA

Field manipulations in a Maine, USA intertidal mud flat showed that Nereis virens adults were an important factor in regulating the abundance of Corophium volutator, an amphipod which comprised 63% of the total number of individuals in the benthic community. Removal of N. virens adults resulted in an increase in C. volutator abundance, while addition of N. virens adults led to a decrease. There was a significant negative correlation between the abundance of N. virens adults and that of C. volutator. C. volutator was also negatively correlated with the density of all the other infaunal species combined. These results suggest that there is at least a 3-level interactive system (N. virens —C. volutator — other infaunal species) within the infauna in Maine; N. virens is a factor in controlling the abundance of C. volutator, and C. volutator may play a role in regulating the densities of other infaunal species. Models of generalized cropping in soft-bottom systems may be too simple if they ignore complex trophic relationships within the infauna.     

Reproductive success,relative abundance and population structure of two species of Nephtys in an estuarine beach

During 1978 and 1979 specimens of Nephtys hombergii in the River Tyne estuary (North East England) became gravid during the winter but did not spawn in the spring breeding season; instead their gametes were resorbed. The drastic effects of these spawning failures were apparent in the population structure in 1980, when the 1978 and 1979 year-classes were shown to be virtually absent, by the size of the individuals in the population and the numbers of growth lines in their jaws. The population structure in 1975, by contrast, indicated that recruitment during the preceding 4 to 5 yr had been good. Spawning was also effective in 1980, and an 0 group was detected in August 1980. Comparable data has been obtained for N. caeca, which occurs in the same beach. It shows that recruitment of this species during 1976–1980 was consistently good, whereas analysis of the age-class frequencies suggests that recruitment of N. caeca was poor in 1973 and 1975. The dominance relationship of the two species in the beach has been reversed during the period 1975–1980. It is suggested that the sympatric distribution of the two species is maintained in part by periodic reproductive failure. The causes of this are not yet known; possible reasons, including the extremely cold conditions experienced in 1978–1979 are discussed. The difference in frequency of the 1978, 1979 and 1980 year classes will now permit a rigorous evaluation of the relationship between the number of growth lines in the jaws and age in N. hombergii.     

Reproduction and annual gametogenic cycle in Nephtys hombergii and N. caeca (Polychaeta: Nephtyidae)

The gametogenic cycles of Nephtys hombergii Audouin et M. Edw. and N. caeca Fabricius are described. Both species are polytelic (=discrete, iteroparous) and breed in their second and subsequent years. They spawn in May in the Tyne estuary. After spawning, N. hombergii retains a large number of gametocytes, which are ultimately resorbed. A new generation of gametocytes is proliferated by September and mature gametes accumulate during the winter months. Gametic development is not well synchronised.     

Variations in time budget of the polychaete Nereis virens as a function of density and acclimation after introduction to a new burrow

Behavioural patterns of Nereis virens (Sars) were monitored in the laboratory to determine variations in the time budget (i.e., percentage of time spent in various activities) as a function of density and acclimation period (number of days following the introduction of worms into experimental enclosures). Experiments were carried out from 17 June to 13 August 1989, using worms collected from intertidal sand flats of l'Anse à l'Orignal in the lower Saint Lawrence estuary, Canada. Inactivity, locomotion and irrigation were, in that order, the most frequent behaviours for solitary individuals. Correlations between the time spent in the various activities and time of day indicated a general increase in activity during the night (21.00 to 06.00 hrs). Locomotory activity increased at night, inactivity during the daytime. A general sequence of behaviour of N. virens was revealed. Inactivity, locomotion and irrigation were still the most frequent activities for individuals observed in high-density conditions, but their respective importance differed significantly (locomotion, irrigation and inactivity, in that order). There was also less variability in the time spent for each behaviour in the high-density aquarium. Comparisons between behaviour and time of day suggested fewer correlations among the different components of the time budgets under conditions of high density. Finally, the time budget varied but little over time (number of days alloved for acclimation) in the low-density aquarium as opposed to the high-density aquarium. Our study revealed that population density and acclimation are important in determining behavioural patterns of N. virens.     

Chromium in intertidal sediments of the Clyde,UK: potential for remobilisation and bioaccumulation

Detailed surveys of intertidal sediments have been performed along the north and south shores of the Inner Clyde estuary, UK. Surface sediment data reveal significant spatial variation in Cr content and an association with major sediment characteristics and location within the estuary. Depth variation for Cr and other heavy metals cannot be explained by variation in major geochemical controls such as grain size and organic matter and highlights the impact of historical contamination on sediment quality. These elevated levels at depth may still have environmental impact through redox-reactivity, in association with iron and manganese. Sequential extraction of sediments and pore water analysis of Cr(VI) and Cr(III) provide detailed information on release potential from the sediments. The implication of Cr mobility for biota in the estuary has been assessed by the analysis of a common marine bivalve, Mytilus edulis (Blue Mussel) and a burrowing polychaete, Nereis diversicolor (rag worm) from a number of survey sites. Bioconcentration factors for Mytilus indicate that the weakly held portion of sediment Cr is available for uptake and in the case of Nereis, bioaccumulation appears to be inhibited by sediment organic matter.     

Interlinked temporal changes in environmental conditions, chemical characteristics of sediments and macrofaunal assemblages in an estuarine intertidal sandflat (Seto Inland Sea, Japan)

Five field surveys were conducted in an estuarine intertidal sandflat of the Seto Inland Sea (Japan) between April 1994 and April 1995. Chlorophyll a, pheopigments, total organic carbon and acid-volatile sulphides (AVS) of surface and subsurface sediments, and macrofaunal assemblages were investigated in parallel at 15 stations. Monthly hydrological data of low-tide creek water adjacent to the flat were used as a complementary environmental characterisation of the study area. Strong temporal changes were found among sampling dates, most remarkably in autumn with a major increase of algal detritus and AVS, a sharp reduction in macrofaunal abundances and species richness, and a massive mortality of the clam Ruditapes philippinarum. This dystrophic event was preceded by a photoautotrophic and hypertrophic spring–summer characterized by abundant fresh (i.e., living) algal material, including microphytobenthos and macroalgae (Ulva sp.). In summer, abundant macrofaunal assemblages reached the highest biomass values (455 g wet weight m⁻² or 60.6 g ash free dry weight m⁻²), with a major contribution of filter-feeding bivalves Musculista senhousia and R. philippinarum. These are among the highest values reported in the literature for sedimentary shores. From autumn, there was a progressive recolonisation of macrofauna, initiated by few opportunistic polychaetes (e.g., Cirriformia tentaculata and Polydora sp.), apparently promoting a fast sediment recovery in winter, and followed by new bivalve recruits in the next spring. This study provides the first evidence of significant and interlinked within-year changes in chemical characteristics of sediments and macrofaunal assemblages in an estuarine intertidal flat at a small spatial scale (i.e., tens of meters). This demonstrates the high temporal variability of species–environment relations in these systems and a close relationship in seasonally driven trophodynamic processes among primary producers and benthic consumers. We conclude that a thorough parallel evaluation of the temporal changes in chemical characteristics of sediments should be taken into account in assessing the year-round distribution and changes of intertidal macrofauna, particularly in eutrophic, estuarine intertidal flats.An erratum to this article can be found at     

Population dynamics of the tanaid Hargeria rapax (Crustacea: Peracarida) in a tidal marsh

The tanaidacean Hargeria rapax (Harger, 1879) was sampled along intertidal transects semi-monthly at one site and quarterly at two other sites in salt marshes on Sapelo Island, Georgia, USA, from July 1985 to July 1986. Tanaids were most abundant near the mean highwater line and became progressively less abundant at lower intertidal elevations. Population density was greatest in the winter (December to February) when there were >29 000 individuals/m² at one high intertidal station. Although reproductive individuals were present most of the year, peaks in reproductive activity occurred in autumn (late August to early November) and spring (early March to mid June). An increase in population density coincided with increased reproductive activity only in autumn. Tanaid cohorts produced in the spring and summer rarely survived beyond 6 to 8 wk, but those produced in the autumn overwintered and lived 22 to 26 wk. The sex ratio among mature individuals was 2.8:1 (females: males). Mature females ranged in size from 2.2 to 3.9 mm total length (TL) and mature males were 2.3 to 4.1 mm TL; there was no significant sexual difference (Student's t-test, P>0.05) in the mean TL of mature individuals. The mean (±SD) size of brooding females was 2.9±0.32 mm TL and the mean (±SD) nunber of offspring/brood was 8.3±4.99 young/female. The timing of tanaid reproduction together with the effects of predation by juvenile fish and crustaceans may account for most of the spatial and temporal patterns of tanaid abundance observed in this study. There was a significant linear relationship (P<0.001, r ²=0.54) between the growth rate (GR, mm/d) of individuals and average daily air temperature (°C) described by the equation: GR=0.00178 (°C)-0.00971. The potential annual contribution of tanaid production to higher trophic levels, estimated from knowledge of standing stocks, growth rates and fecundity, was 5.71, 0.91 and 0.46 g dry wt/m² for high, mid and low intertidal areas, respectively. The high intertidal marsh, which supports the largest and most persistent standing stock of H. rapax, provides a rich foraging area for aquatic predators at high tide and an important source of recruits from which tanaid populations at lower intertidal elevations are recolonized after periods of intense predation pressure.     

Sexual reproduction of the photosymbiotic ascidian <Emphasis Type="Italic">Diplosoma virens</Emphasis> in the Ryukyu Archipelago,Japan: vertical transmission,seasonal change,and possible impact of parasitic copepods

Diplosoma virens is a colonial ascidian hosting prokaryotic algae Prochloron sp. in the common cloacal cavity of the colonies and is sometimes parasitized by notodelphyid copepods. In ascidian–Prochloron symbiosis, it is generally known that the host larvae acquire the algal symbionts from their mother colonies to maintain the symbiosis. A histological study of the sexually mature colonies of D. virens showed that the algal symbionts attach to pre-hatching larvae on the rastrum (plant rake) projected from the postero-dorsal part of the larval trunk, and then the rastrum is packed in the posterior half of the larval trunk that will become a cloacal cavity after metamorphosis. This process is the same as that of D. simile. Monthly sampling of D. virens colonies showed that they have embryos in summer in Ryukyus, situated near the northern-most limit of the coral reefs in the West Pacific. While the frequencies of copepod parasitism were variable among the populations, the colonies from a highly parasitized population had a significantly smaller number of eggs/embryos per zooid than the colonies from the less parasitized populations. The parasites probably have an inhibitory impact on the sexual reproduction of the host colonies.Communicated by T. Ikeda, Hakodate     

Photosynthetic responses and daily carbon balance ofColpomenia peregrina: Seasonal variations and differences between intertidal and subtidal populations

The seasonal photosynthetic responses and daily carbon gain of upper intertidal, low intertidal and subtidal (3 to 4 m depth) populations ofColpomenia peregrina were examined over a 2 yr period (1986–1988) in Santa Catalina Island, California, USA. The populations showed significant differences in their photosynthetic responses, daily carbon balance and carbon-specific growth rates when normalized to tissue area or to chlorophyll content. The substantial plasticity with respect to photosynthetic responses shown byC. peregrina is considered to be an important factor in facilitating the colonization of both intertidal and subtidal habitats. This species appears to have a cellular carbon metabolism influenced by responses to season and tidal elevation. Highest net daily carbon balance, predicted carbonspecific growth rates and net growth efficiency were achieved in upper intertidal habitats during summer. These parameters decreased in winter and progressively declined with increasing depth as plants become increasingly exposed to low-light regimes. The diminishing net daily carbon balance and predicted carbon-specific field growth rates found during winter suggest that standing stock and lower subtidal limits of distribution ofC. peregrina are at least partly controlled by these two factors.     

Seasonal variation in utilization of biogenic microhabitats by littorinid snails on tropical rocky shores

Mobile species may actively seek refuge from stressful conditions in biogenic habitats on rocky shores. In Hong Kong, the upper intertidal zone is extremely stressful, especially in summer when organisms are emersed for long periods in hot desiccating conditions. As a result, many species migrate downshore between winter and summer to reduce these stressful conditions. The littorinids Echinolittorina malaccana and E. vidua, for example, are found on open rock surfaces high on the shore in winter but the majority migrate downshore in summer to the same tidal height as a common barnacle, Tetraclita japonica. In the laboratory, where environmental conditions could be controlled to approximate those occurring on the shore, we tested whether the downshore migration allowed littorinids to select barnacles as biogenic habitats to reduce stress and if this behaviour varied between seasons. In summer, littorinids demonstrated a strong active preference for the barnacles, which was not observed in the cool winter conditions, when animals were found on open rock surfaces even when barnacles were present. Littorinids, therefore, only actively select biogenic habitats during the summer in Hong Kong when they migrate downshore, suggesting that such habitats may play an important, temporal, role in mitigating environmental stress on tropical shores.     

Algal “gardening” behavior by nereid polychaetes: Effects on soft-bottom community structure

Nereid polychaetes (Nereis vexillosa and Platynereis bicanaliculata) attach pieces of drift algae to their tube surfaces. The presence of permanent algal cover increases the predictability of the food supply for at least the herbivores (including the nereids) and the deposit feeders, and modulates the temperature and salinity stresses of the marine intertidal soft-bottom environment. However, it may affect the access of organisms to the oxygenated water layers above the sediment surface. This is true for polychaetes that live head downwards in vertical tubes. It is suggested that the attachment behavior of the nereid polychaetes increases the ability of the plant to colonize habitats both temporally and spatially.     

Sellafield waste radionuclides in Irish sea intertidal and salt marsh sediments

Low level liquid radioactive waste discharges from the Sellafield nuclear fuel reprocessing plant in north west England had generated environmental inventories of about 3 × 10¹⁶ Bq of¹³⁷Cs, 6.8 × 10¹⁴ Bq of^239,240Pu and 8.9 × 10¹⁴ Bq of²⁴¹Am by 1990. Most of the^239,240Pu and²⁴¹Am and about 10% of the¹³⁷Cs has been retained in a deposit of fine marine sediment close to the discharge point. The quantities of radionuclides discharged annually from Sellafield decreased by two orders of magnitude from the mid-1970s to 1990 but estimated critical group internal and external exposure decreased by less than one order of magnitude over this period. This indicates that during the period of reduced discharges, radionuclides already in the environment from previous releases continued to contribute to the critical group exposure and highlights the need to understand processes controlling the environmental distribution of the radionuclides. Redistribution of the contaminated marine sediment is potentially of major significance in this context, in particular if it results in transport of radionuclides to intertidal areas, where contact with the human population is relatively likely. A review is presented of published work relating to Sellafield waste radionuclides in Irish Sea sediments. Data on temporal and spatial trends in radionuclide concentrations and activity ratios are collated from a number of sources to show that the dominant mechanism of radionuclide supply to intertidal areas is by redistribution of the contaminated marine sediment. The implications of this mechanism of supply for trends in critical group radiation exposure are considered.     

Spatial variability of length-specific production in shell,somatic tissue and sexual products of Macoma balthica in the Lower St. Lawrence Estuary

A field study on the spatial variability of production and some demographic parameters was conducted in 1988 in ten populations of Macoma balthica located on the north and south shores along the entire length (230 km) of the Lower St. Lawrence Estuary (LSLE, Canada) at the same intertidal level. Standard-length estimates (10 mm) of shell and somatic tissue production for the period of May to November were highly variable between stations and greater on the north shore than on the south shore. Standard-length estimates of the gamete production for the period of July to November were also highly variable between stations but there was no variation between shores. The inter-population variability of the standard-length estimates of production in shell, somatic tissue and sexual products was as large as the intrapopulation variability between both the upper and lower tidal levels measured in previous studies. There were no significant linear relationships between standard-length estimates of production and biotic (density) or abiotic (temperature, chlorophyll-a in top sediment, mean phi, water salinity) factors, but we observed some significant quadratic relationships between standard-length estimates of production and mean sediment-surface temperature during the growing season. The standard-length estimates of production were lower at the coldest and warmest stations than at the more temperature stations. There was also a significant negative linear relationship between mean sediment-surface temperature during the growing season and the grain size structure of sediment, indicating that the sediment texture, indirectly, largely influenced the inter-station and the inter-shore variability of production in shell, somatic tissue and sexual products of M. balthica in the LSLE.     

Photoperiodic control of growth and segment proliferation by Nereis (Neanthes) virens in relation to state of maturity and season

The effects of two fixed photoperiodic regimes on the rate of somatic growth and segment proliferation in the semelparous polychaete Nereis (Neanthes) virens Sars has been investigated. The two photoperiods (16 h light:8 h dark and 8 h light:16 h dark, hereafter LD 16:8 and LD 8:16) were close to the extremes experienced naturally by N. (N.) virens in the course of a year. The experiments were carried out during the ambient winter and all worms (including four-setiger larvae raised out of season) showed elevated feeding, growth and enhanced segment-proliferation rates when reared under LD 16:8, compared to worms reared under LD 8:16. The rate of replacement of lost segments after caudal ablation was also higher under LD 16:8. The number of segments present when the segment proliferation rate had fallen to zero was higher in individuals grown from birth under LD 16:8 compared to those grown under LD 8:16. Feeding activity showed a degree of spontaneous recovery under LD 8:16 from January onwards, but remained lower than under LD 16:8. This difference continued into the following spring, and was not attributable to differences in sexual maturity. We conclude that the rate of feeding in N. (N.) virens and other indicators of somatic growth rate such as rates of caudal regeneration and segment proliferation are directly influenced by the photoperiod, but that the seasonal cycle of growth also includes a circa-annual component modifying the response to static photoperiods according to the time of year. Received: 20 October 1998 / Accepted: 4 February 1999