Abundant Bivalves Research Articles

Brown shrimp ( Crangon crangon, L.) functional response to density of different sized juvenile bivalves Macoma balthica (L.)

Variability in infaunal bivalve abundance in the Wadden Sea is largely determined by recruitment variability. Post-settlement, but pre-recruitment bivalve mortality is high and related to the occurrence of their most abundant predator, the brown shrimp Crangon crangon. To investigate if the mortality patterns of newly settled bivalves can be explained by the foraging behavior of brown shrimp, we carried out experiments on shrimp functional response to three size classes of juveniles of the Baltic Tellin Macoma balthica. The functional response curves for all three prey sizes (0.62 mm, 0.73 mm, and 0.85 mm) were the hyperbolic Holling's type II. The attack rate was highest for the smallest prey size ( a = 0.31, medium and large prey a = 0.22); the handling time was longest for the largest prey size ( T h = 29 s, small and medium prey T h = 15 s). Thus, a large body size is advantageous for the bivalves over the whole density range. Knowledge of individual foraging behavior is needed to model predation mortality of bivalves. The consumption rates in the experiment were theoretically high enough to account for M. balthica mortality in the field.

Spatial variation in the abundance and condition of the bivalve Nuttallia olivacea in relation to environmental factors and sublethal predation

Our aim was to assess the spatial heterogeneity in the abundance and condition of the tellinacean bivalve Nuttallia olivacea in estuarine tidal flats, as well as to reveal the relationships between these variables and abiotic and biotic factors. The intensity of sublethal siphon predation by juvenile stone flounder Platichthys bicoloratus was considered as a possible factor affecting the bivalve condition. A 2-scale sampling of stations at 200 to 800 m intervals and 2 to 5 plots at 5 to 20 m intervals on a transect line at each station was conducted in the Natori River estuary, Japan. Abundance and somatic condition greatly varied at both scales. Densities varied from 0 to 4902 ind. m–2 among stations and from 2480 to 6448 ind. m–2 among plots at the station with the highest density. Body mass index varied from 4.0 to 5.0 between stations and from 4.9 to 5.7 between plots at the station with the highest body mass. It was commonly observed that N. olivacea was abundant and that body condition was high at sites with low silt-clay contents and low elevations, as revealed by generalized linear models. A negative influence of siphon predation was detected only on somatic condition, but it accounted for only 1.2% of the variation, whereas environmental variables accounted for 30.2%. These results indicate that environmental variables are key factors affecting bivalve abundance and condition, while sublethal predation is only a potential factor affecting bivalve condition.

Predation and depth effects on abundance and size distribution of an invasive bivalve, the golden mussel Limnoperna fortunei, in a dam reservoir

We investigated the predation and depth effects on abundance and size distribution of an invasive freshwater bivalve, Limnoperna fortunei, post-larvae using screened (5-, 25-, and 50-mm mesh openings) and unscreened cages at three depths (6, 12, and 1 m above the bottom) in a dam reservoir, Lake Ohshio of Japan. The densities of L. fortunei on the unscreened cages were much lower than those of the 5-mm mesh screened cages at any depth. The predation rates estimated by dividing the density of the unscreened cage by that of the 5-mm mesh screened cage were 97.3% at 6 m depth, 96.6% at 12 m depth, and 95.8% at the bottom. Shell lengths of L. fortunei post-larvae were small on bottom cages, and their size distribution was significantly different from those at other depths. These results suggest that predation and depth affected the abundance and size distribution of L. fortunei in the lentic environment. The impact of L. fortunei invasion on the food web and the possibility of regulating their population by predation pressures are also discussed.

Population biology and secondary production of the stout razor clam Tagelus plebeius (Bivalvia, Solecurtidae) on a sandflat in southeastern Brazil

The population biology and production of the stout razor clam Tagelus plebeius Lightfoot, 1786 were investigated on an intertidal sandflat on the southeast coast of Brazil (Enseada Beach, Sao Sebastiao, state of Sao Paulo) between April 1997 and April 1998. Two rectangular sites of 50 X 10 m parallel to the waterline were established, site A (upper intertidal level) and site B (middle intertidal level), where the samples were taken in an 0.5 x 0.5 m quadrat. High abundances were recorded in winter and spring, with no significant differences between the sites. The high bivalve abundances were related to the presence of very fine homogeneous sediment with low salinities. Tagelus plebeius had negative allometric growth, characteristic of deep burrowers for the relationships DM/SL and AFDM/SL. Parameters of the modified von Bertalanffy growth function were: L∞ = 67.01 mm, K = 1.73 year-1, t0 = -0.11 year, C = 0.43, WP = 0.96. The instantaneous mortality (Z) was 3.12 year-1, relatively high in comparison to other tropical bivalve populations. Secondary production was 1.53 g AFDM m-2 year-1, with a P/B ratio reaching 1.37 year-1. This high turnover ratio (P/B) was related to a rapid population replacement, connected with the short life span and high mortality of the species.

Spatial and Temporal Variability of Intertidal Rocky Shore Bivalves and Gastropods in Sichang Island, East Coast of Thailand

Spatial and temporal variability of intertidal rocky shore bivalves and gastropods in Sichang Island, east coast of Thailand were studied over 12 months (March 2008 to February 2009). The study area experiences mixed tides with the largest tidal range of 3.6 m above MTL during spring tides. Shores are exposed to the prevailing northeast monsoon in the winter (November to February), resulting in stronger wave action during this season than in summer (March to May) and rainy season (June to October). Transects were positioned at five different heights from mid shore to high shore (2.5 to 3.5m above MTL) dominated by rock oysters in the mid shore. At each height, abundance of bivalves and gastropods were recorded within fifteen replicate quadrats (25 x 25 cm) on a monthly basis. A total of 2 bivalves and 10 species of gastropods were identified. In the high shore, the littorinid Echinolittorina malaccana dominated. Dense beds of the bivalve Isognomon nucleus were abundant in crevices at 2.5 and 2.75 m above MTL, whilst a large number of snails (Planaxis sulcatus) were found near the midshore. Below this, the rock oyster Saccostrea cucculata was dominant, with a variety of mobile gastropods (Cellana grata, C. toreuma, Patelloidea saccharina and Siphonaria japonica). The general sequence of zonation patterns were clear over time, but the vertical extent of some mobile species varied in accordance with the monsoon effect and increasing tidal range in winter, especially on the more exposed of the two sites. The high shore littorinids, E. malaccana moved up shore and were more abundant near the supralittoral line in winter suggesting that the seasonal monsoon effect and tidal range were important factors determining the distribution of some species of mobile gastropods, allowing them to exploit higher tidal levels.

Influence of food on the assimilation of selected metals in tropical bivalves from the New Caledonia lagoon: Qualitative and quantitative aspects

The present study aimed at examining the influence of food quality and quantity on the assimilation efficiency (AE) of metals in two abundant bivalves in the New Caledonia lagoon, the oyster Isognomon isognomon and the clam Gafrarium tumidum. Bivalves were exposed via their food to the radiotracers of three metals of concern in New Caledonia (54Mn, 57Co and 65Zn) under different feeding conditions (phytoplankton species, cell density, and cell-associated metal concentration). When bivalves were fed Heterocapsa triquetra, Emiliania huxleyi and Isochrysis galbana, AE of Mn, Co and Zn was strongly influenced by the phytoplankton species and by the metal considered. In contrast, when fed one given phytoplankton species previously exposed to different concentrations of Co, phytoplankton-associated Co load had no influence on the AE and on the retention time of the metal in both bivalves. Metals ingested with I. galbana displayed generally the highest AE in both bivalve species, except for Mn in clams for which the highest AE was observed for H. triquetra. Influence of food quantity was investigated by exposing bivalves to different cell densities of I. galbana (5×103, 104 or 5×104cellml−1). As for food quality, food quantity was found to influence AE of Mn, Co and Zn, the highest AE being observed when bivalves were fed the lowest cell density. Overall, results indicate that the two bivalve species are able to adjust their feeding strategies according to the food conditions prevailing in their environment.

The living, the dead, and the expected dead: variation in life span yields little bias of proportional abundances in bivalve death assemblages

All else being equal, species with short life spans are expected to be overrepresented in time-averaged death assemblages relative to their standing abundance in the living community, but the magnitude of the distortion of proportional abundance and assemblage evenness has received little attention. Here, information from 30 data sets on the living and dead abundances of marine bivalves in local habitats is combined with a global compilation of bivalve life spans to determine whether bias from mortality rate can explain observed differences in species proportional abundances. Although bivalve maximum life spans range from one to 75 years in these data sets, indicating annual mortality rates of 0.97 to 0.09, the “life span bias” (LB) of a species–the difference between its proportional abundance expected dead and that observed alive–is consistently small in magnitude (average change <2%, maximum about 20%) and random in sign relative to observed discordance (OD = difference between that species' proportional abundance observed dead and that observed alive). The aggregate result for 413 living species occurrences is a significantly positive but weak correlation of OD to LB, with only 10% of variation in OD explained. The model performs better among longer-lived species than among shorter-lived species, probably because longer-lived species conform better to the model assumption that species maintain a constant proportional abundance in the living assemblage over time. Among individual data sets, only seven exhibit significant positive correlations between OD and LB. The model also under-predicts the cases where a death assemblage is dominated by a species that is shorter lived than the dominant species in the living assemblage, indicating that some factor(s) other than or in addition to mortality rate is responsible for OD. We can find no evidence of preservational bias linked to life span, for example through body size. This negative outcome reflects a weak biological relationship between life span and living abundance among bivalves in local habitats, contrary to the terrestrial paradigm, and points toward a simpler model of time-averaged death assemblage formation where higher abundances reflect (under-sampled) past populations. Contrary to long-held expectations, variation in population turnover among species is not a major source of taphonomic bias in time-averaged death assemblages among bivalves and perhaps among other marine groups: bias must arise largely from other factors.

Distribution and Abundance of Rudist Bivalves in the Cretaceous Platform Sequences in Egypt: Time and Space

As the rudist bivalves represent important organic buildups in the Cretaceous platform sequences, this study emphasizes vertical and spatial distribution of this group of bivalves in the geographic divisions of Egypt, including Western Desert, Eastern Desert and Sinai. Rudists are encountered in different rock facies ranging from mudstones to carbonates. About sixty eight species belong to twenty one genera are reported from Egypt. They belong to six families: Requieniidae, Monopleuridae, Caprotinidae, Caprinidae, Hippuritidae, and Radiolitidae. The Radiolitidae is the most diverse family, comprising eleven genera and fifty-one species, dominated by species of Radiolites, Eoradiolites and Durania. The elevator morphotype of the Radiolitidae became the dominant species in the Turonian sequences. The diversity (richness) peaks in the Turonian (36 species) Cenomanian (26 species) and Albian (9 species), with few records in Aptian, Coniacian, Campanian and Maastrichtian (totally 5 species). As yet rudists are not recorded from Santonian rocks. Geographically, rudists are highly represented in Sinai (60 species) concentrated in the Cenomanian (23 species) and Turonian (32 species), followed by Western Desert (19 species) and Eastern Desert (8 species). Regarding abundance so far, the relatively highly abundant species in Egypt are Eoradiolites liratus (19 sites), followed by Praeradiolite biskraensis and Eoradiolites sinaiticus (7 sites for each), Praeradiolites ponsianus, Durania humei, Radiolites sauvagesi (6 sites for each), Durania gaensis and Radiolites lusitanicus (5 sites for each). The rare occurrence during the Campanian and Maastrichtian may be attributed to stagnant conditions related to deposition of black shales and phosphatic deposits and the change to deep inner shelf setting respectively. The disappearance of rudists from some segments of the sequence is attributed oceanic anoxia or related to shelf drowning especially at the basal Turonian, which may related to global warming.

Early AlbianMytiloides(Inoceramidae, Bivalvia) from the Northwest Pacific: A Pioneering Species for Late Cretaceous Inoceramid Diversification

The inoceramid bivalve Mytiloides ipuanus (Wellman, 1959) from the Lower Albian is reported for the first time in the Northwest Pacific and described in detail. This is the oldest record of a Cretaceous inoceramid in this region. It is possible that the present study documents the origin of the extreme diversification and abundance of inoceramid bivalves in the Late Cretaceous of the Northwest Pacific. The beginning of inoceramid diversification in the Late Cretaceous is seen during the latest Early Cretaceous, which suggests that M. ipuanus is an important species for inferring the causes of the Late Cretaceous flourishing of inoceramids.

Prey base shifts in red rock lobster Jasus edwardsii in response to habitat conversion in Fiordland marine reserves: implications for effective spatial management

Persistence of subpopulations dispersed among spatial management units, such as marine reserves, relies heavily upon the quality of local habitat, as well as size and connectivity of habitat patches. We examined the relative abundance of the red rock lobster Jasus edwardsii and its primary bivalve food resources within 3 marine reserves in the Doubtful–Bradshaw Sound complex in Fiordland, New Zealand. We used stable isotope analysis (δ13C, δ15N and δ34S) to resolve differences in the carbon sources to food webs supporting J. edwardsii inhabiting these regions. Furthermore, we examined patterns in the relative concentration and δ13C signature of specific fatty acid biomarkers as an independent test of the carbon sources to each population. We found distinctive patterns in the relative abundance of red rock lobsters, with significantly more animals in the marine reserves at Te Awaatu Channel and Kutu Parera than in the surrounding open fishing areas and within the reserve at Taipari Roa. Taipari Roa Reserve is distinctive in that bivalve abundance is extremely low due to freshwater input from the Manapouri hydroelectric power plant. Analysis of δ13C, δ15N and δ34S of red rock lobster muscle tissue as well as δ13C of 16:1ω7 and 18:1ω7 indicated that, in those areas where heterotrophic bivalves are rare, red rock lobsters rely more on recycled carbon made available by chemoautotrophs. These findings suggest that efficacy of the new marine reserves is influenced by habitat quality in terms of the availability and abundance of food resources for red rock lobsters. We highlight the importance of considering habitat quality for effective implementation of marine reserves.

Small-scale distribution of Solea solea and Solea senegalensis juveniles in the Tagus estuary (Portugal)

The distribution of Solea solea and Solea senegalensis in the Tagus estuary was studied following a small-scale approach. Preliminary sampling revealed that sole concentrated in two areas within their nursery grounds, the main subtidal channel and a large intertidal mudflat. Beam trawls were conducted intensively in the two areas in July 2006. Depth, salinity and water temperature were measured. Substrate samples were collected for sediment type determination and macrobenthos identification and quantification. Generalized linear models were applied in order to explain the occurrence and variability of soles' densities, using depth, salinity, water temperature and abundance of polychaetes, oligochaetes, amphipods, isopods and bivalves as explanatory variables. While S. solea was more abundant in the main subtidal channel, a deeper, warmer and lower salinity area, S. senegalensis abundance was highest at the intertidal mudflat area. Presence of both species in the two areas was associated with abundance of polychaetes (generally with another variable associated), and for S. senegalensis in the subtidal channel it was associated with amphipods and depth. Abundance of S. solea in the main subtidal channel was associated mainly with polychaetes abundance, while that of S. senegalensis was associated with amphipods density. In the intertidal mudflat, bivalves and polychaetes presented significant relationships with both species densities. Some of the factors that had been reported to be important for the distribution of these species in previous studies also do so at a finer scale; however, this small-scale approach provided an in-depth knowledge on habitat selection and spatial segregation of these species within this nursery area.

Predicting the impact of an invasive seaweed on the fitness of native fauna

Summary 1. Understanding the impacts of invasive species on natural ecosystems is an important component of developing management strategies. Habitat-forming invasive plants and sessile invertebrates often support a high diversity and abundance of native fauna, suggesting some benefits of invasion. However, the fitness responses of these native fauna, and thus the net benefit from their association with habitat-forming invasive species, are not well understood. 2. We determined how fitness-related life-history traits, patterns of resource allocation among life-history traits, and survivorship of an abundant bivalve, Anadara trapezia , responded to invasion by the habitat-forming seaweed, Caulerpa taxifolia , by transplanting A. trapezia into invaded and uninvaded habitats over a 12-month period. 3. Although A. trapezia recruits into C. taxifolia in high numbers, adult growth, body condition, shell condition, female reproduction and survivorship were all significantly lower in C. taxifolia compared to unvegetated sediment. Notably, we observed high mortality in C. taxifolia after heavy rainfall events, highlighting a potential link between sublethal effects on condition, stochastic environmental perturbation and survivorship. 4. In addition to the overall reduction in fitness, there were changes in scaling relationships between reproduction and body size following invasion. Female A. trapezia in C. taxifolia habitat allocated proportionally more resources to reproduction (including reproductive tissue and number of eggs per follicle) than those in unvegetated sediment despite their poor condition. Maximizing reproduction following invasion may impose a further cost to already stressed A. trapezia and contribute to the higher mortality observed when living in C. taxifolia . 5. Synthesis and applications . The full impact of habitat-forming invasive species is complex and understanding it cannot be based solely on descriptions of native species diversity or abundance. Our study has identified how the presence of long-lived species within habitat-forming invasive species may simply indicate an extinction debt. A decline in the fitness of A. trapezia in C. taxifolia appears to increase its probability of mortality in the long-term. We recommend that management approaches for C. taxifolia and other habitat-forming invasive species combine an understanding of impacts on species diversity, abundance and the fitness of associated fauna to provide a more pluralistic understanding of their effects.

Spatial intertidal distribution of bivalves and polychaetes in relation to environmental conditions in the Natori River estuary, Japan

This paper aims to reveal spatial variation in the abundance of infaunal bivalves and polychaetes at different spatial scales (station: 200–800 m intervals; plot: 5–20 m), and to reveal environmental variables affecting the spatial distribution of animals in the Natori River estuary, Japan. We found six bivalve species and eight polychaete species from 52 plots at 12 stations. Nuttallia olivacea and Heteromastus sp. were found to be the most abundant species of bivalves and polychaetes, respectively. Assemblage patterns of bivalves and polychaetes were classified into five distinct groups. Substrata (silt-clay contents), salinity, and relative elevation were the variables found to affect the infaunal assemblage patterns. Chlorophyll a was not a significant variable, but benthic animals were absent at sites with extremely low chlorophyll a conditions. Macrobenthic assemblage patterns were different not only between stations but often differed between plots at the same station, reflecting the complex assemblage structure of benthic invertebrates. Detailing such animal–environment relationship is essential in understanding the potential food supply for estuarine fishes.

Staggered nested designs to assess scales of variability: the advantages of a spatially explicit analysis

A staggered nested sampling design was used to identify spatial scales of variation in the abundance of an intertidal clam Austrovenus stutchburyi. A georeferenced sampling design permitted assessment of abundance at spatial lags between 0.1 and 87 m. An analysis of variance approach produced imprecise estimates of variability, whereas spatially explicit analyses improved the resolution greatly. A geostatistical model identified the spatial scale of residual variance as 13 m and that of the asymptote of spatial dependence as 17 m. It also permitted mapping of bivalve abundance. Staggered nested designs are highly efficient for comparing hierarchies of scale, but in this study analysis of detailed positional information was required to tease out useful spatial information.

Growth and production of Pisidium amnicum in the freshwater tidal area of the River Minho estuary

Pisidium amnicum is an important indigenous species in the freshwater tidal portion of the River Minho estuary and until the introduction of the non-indigenous invasive species Corbicula fluminea was the most abundant bivalve in this ecosystem. Between January 2005 and August 2006 monthly samples were collected in three sites to study the abundance, biomass, growth and production of P. amnicum. Results indicate that P. amnicum has continuous growth throughout its life span and growth rates were higher during the early phases of development, coincident with the spring and summer months. Life span estimated was 24 months. The annual 2005 growth production of P. amnicum estimated was 2.339 g AFDW m −2 year −1, and the mean annual biomass was 1.594 g AFDW m −2, resulting in a P / B ¯ ratio of 1.47 year −1 and a turnover time of 248.7 days. The annual 2005 elimination production estimated was 7.541 g AFDW m −2 year −1, resulting in an E / B ¯ ratio of 4.73 year −1. It is concluded that P. amnicum continues to play an important role in the food web in areas where it coexists with C. fluminea. Previous studies showed declines in the indigenous population after the introduction of C. fluminea and currently, areas of high abundance are restricted to small patches in the upper limit of the tidal influence. Consequently, P. amnicum is now under considerable pressure and this situation requires ecological and conservational attention.

Palaeosalinity variations in the Early Cretaceous of the Neuquén Basin, Argentina: Evidence from oxygen isotopes and palaeoecological analysis

The Neuquén Basin is a stratigraphically- and economically-important Early Cretaceous depocentre located in west-central Argentina. The Early Valanginian to Early Barremian succession (upper Vaca Muerta, Mulichinco and Agrio Formations) contains a rich fossil record, with abundant bivalves and ammonoids. Palaeosalinities are determined systematically throughout the succession, based on an oxygen isotope analysis of unaltered oyster shells from two localities in Neuquén province. A total of 188 oyster samples from 52 stratigraphic levels were processed. A total of 52 mean values of δ 18O isotope are used to calculate palaeosalinities, assuming an estimated water temperature of 25 °C. The observed range of palaeosalinities, between the maximum and the minimum, is 19, which is in conflict with published accounts of this being a fully marine succession. The isotopic data are combined with independent faunal evidence to evaluate palaeosalinity variation both laterally and temporally. Significant fluctuations in water salinity are indicated, with a clear tendency to increase from brachyhaline to euhaline and near hyperhaline conditions through time. The fluctuations were probably due to dilution from normal marine water, caused primarily by variations in rainfall and continental runoff. In addition, the presence of a volcanic island arc along the western margin of the basin may have at least partially isolated the basin from the marine waters of the Pacific Ocean. Also, a gulf-shaped basin may have inhibited hydrodynamic exchange with the ocean while enhancing retention of continental waters.

Feeding ecology of Xenoturbella bocki (phylum Xenoturbellida) revealed by genetic barcoding

The benthic marine worm Xenoturbella is frequently contaminated with molluscan DNA, which had earlier caused confusion resulting in a suggested bivalve relationship. In order to find the source of the contaminant, we have used molluscan sequences derived from Xenoturbella and compared them to barcodes obtained from several individuals of the nonmicroscopic molluscs sharing the same environment as Xenoturbella. Using cytochrome oxidase 1, we found the contaminating sequences to be 98% similar to the bivalve Ennucula tenuis. Using the highly variable D1-D2 region of the large ribosomal subunit in Xenoturbella, we found three distinct species of contaminating molluscs, one of which is 99% similar to the bivalve Abra nitida, one of the most abundant bivalves in the Gullmarsfjord where Xenoturbella was found, and another 99% similar to the bivalve Nucula sulcata. These data clearly show that Xenoturbella only contains molluscan DNA originating from bivalves living in the same environment, refuting former hypotheses of a bivalve relationship. In addition, these data suggest that Xenoturbella feeds specifically on bivalve prey from multiple species, possibly in the form of eggs and larvae.

Benthic community associated with coral reefs in the coastal area of Gulf of Aqaba, Red Scs, Egypt

Benthic community associated with coral reefs was investigated at four sites along the coastline of Gulf of Aqaba, Red Sea Egypt. The four sites (Taba, Nuweiba, Dahab and Shann El-Sheikh) are important popular spots for recreational activities and the reefs of these sites are of fringing type. Drupella cornus and Trochus maculatus were the most important gastropods associated with coral reefs. Taba and Sharm El-Sheikh showed high occurrence of worm shells (Gastropoda) Dendropdma maximum (up to 30 individuals / colony). Concerning the abundance of bivalves (Mollusca), Taba showed greater abundance of Spondylus varium and Pteria penguin ^ whereas Sharm El-Sheikh had the greatest abundance of Tridacna maxima (up to 40 individuals/lOm). The studied sites exhibited high density of Echinoderms, particularly the sea urchins Diadema setosvm, Tripneitstes gratilla and Heterocentrotus mammillatus. The study revealed selective associations between mollusc species and coral growth forms. The bivalve Pteriapenguin was found to be attached with plate-like coral Mellipora platyphylla and the branching coral Millepora dichotoma. The vennetid gastropod Dendropoma maximum was also found to be inserted into the Melliporid coral colonies. Aggregates of Drupella cornus were observed attached with branching corals, especially Acropora spp. and Stylophora spp. In contrast to the Drupella cornus, the corallivore gastropod CoralUphila neritoidea and the bivalve Tridacna maxima were predominantly preying on the massive corals of Poriies spp. The distribution of coral growth forms was compared. In general, massive, branched and plate-like colonies were the dominant growth forms of corals in the area of study. Massive colonies were dominant in the reef flat area, reef margin and the deep parts of the outer reef slope. Branching coral colonies were flourishing on the fringing reef flat, the barrier reef flat and the outer reef slope until 10m depth (upper layers of outer reef slope), their importance declined with depth. All plate-like coral colonies in the area of study belong to Hydrocorals (Milleporidae); Millepora platyphyla. They are abundant in the reef edge and upper zone of outer reef slope (the reef zone exposed to high wave action). The considerable abundance of macroalgae in the coral communities of the sampling sites were probably resulting from increased levels of nutrients caused by enhanced terrestrial runoff of sewage and sediments concomitant with increased human pressure in the area and coastal

Is reproductive failure responsible for reduced recruitment of intertidalMytilus edulis L. in the western Dutch Wadden Sea?

The musselMytilus edulis is an abundant bivalve in the Dutch Wadden Sea, both in intertidal areas as well as in subtidal culture plots. From mid 1980’s to late 1990’s, strong declines in mussel intertidal populations have been observed, both in terms of occupied area and in biomass. Despite the efforts since 1999 to preserve intertidal mussel beds, abundance and biomass of mussels in the Dutch Wadden Sea are at present still much lower than in the 1970’1s. In the present paper, we tested whether reproductive failure could have been the cause for the low recruitment ofM. edulis in the western Dutch Wadden Sea. Water temperature is an important factor influencing growth and reproduction in bivalves. The observed increase in mean temperature in the Dutch Wadden Sea during the last decades suggests that an effect of temperature on the population dynamics of mussels might be expected. Nevertheless, no significant relationship was found between body condition and mean winter temperature. Therefore, a decrease in body condition and, consequently, in reproductive output, due to increasing temperature, will not have been a cause for the observed low recruitment in recent years. The seasonal patterns of individual growth supported this view: most adults developed gonads during the spawning season suggesting that reproductive (gametogenic) failure was not a cause for the low recruitment. Nevertheless, reproductive investment by the mussel population studied was not optimal since complete spawning hardly occurred. In addition, the high gonadosomatic ratio at the end of the summer did not result in heavy spawning, suggesting resorption of gonadal tissue due to unfavourable environmental conditions (too high temperature and/or food limitation during summer). Reduced recruitment could be due to poor spawning but other factors that exert their influence after spawning (predation on larvae or postlarvae, lack of settlement substrate) or on the adult population (decrease in number of spawners) are also likely to be involved in the observed low recruitment during the last years.

Effects of an area closed to fisheries on the composition of the benthic fauna in the southern North Sea

Abstract Duineveld, G. C. A., Bergman, M. J. N., and Lavaleye, M. S. S. 2007. Effects of an area closed to fisheries on the composition of the benthic fauna in the southern North Sea. – ICES Journal of Marine Science, 64: 899–908. The effects of fishery exclusion on the composition of the macrofauna were determined by comparing the fishery-exclusion zone around a gas production platform in the southern North Sea (Frisian Front) with nearby regularly fished areas. A Triple-D dredge was used, in addition to a standard box corer, to collect the relatively rare and larger species. Multivariate analysis showed greater species richness, evenness, and abundance of mud shrimps (Callianassa subterranea, Upogebia deltaura) and fragile bivalves (Arctica islandica, Thracia convexa, Dosinia lupinus, Abra nitida, Cultellus pellucidus) in the Triple-D samples from the exclusion area. Although box cores did confirm the higher abundance of both mud shrimps in the exclusion zone and demonstrated greater densities of the brittlestar Amphiura filiformis, they did not clearly reveal the distinctness of the exclusion zone. This is attributed to the large proportion of small, short-living species in the samples and the relative scarcity of vulnerable larger species common to all the box core samples. There was no evidence of greater recruitment in the relative small exclusion zone, despite its positive effect on adult survival. The observation that the fishery affects deep-living mud shrimps may point to consequences for the functioning of the benthic ecosystem other than simple loss of biodiversity.