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

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

The influence of flood cycle and fish predation on invertebrate production on a restored California floodplain

Although floodplains are known to be tightly controlled by the flood cycle, we know comparatively little about how flooding influences predators and their consumption of secondary production, particularly in highly seasonal floodplains typical of Mediterranean climates. In this study, we investigate how the seasonal dynamics of a central California floodplain influence the timing and magnitude of fish predation and the abundance and composition of invertebrates. For 3 years (2000–2002), we compared changes in abundances and size distributions of invertebrates through the flood season (January–June) with seasonal changes in the abundance of larval and juvenile fishes. Using diet analysis of fishes and manipulative feeding experiments with fishes in field enclosures, we link specific changes in invertebrate populations directly to feeding preferences of seasonally abundant fish. Early in the flood season prior to March, we found little influence of fish predation, consistent with the near absence of larval and juvenile fishes during this period. Coinciding with the midseason increase in the abundance of larval and juvenile fishes in April, we found significant declines in zooplankton abundance as well as declines in the size of zooplankton consistent with fish feeding preferences. Our results were consistent with results from feeding enclosure experiments that showed that fish rapidly depressed populations of larger cladocerans with much less effect on smaller cladocerans and calanoid copepods. At the end of the flood season, zooplankton abundances rapidly increased, consistent with a switch in the feeding of juvenile fish to aquatic insects and subsequent fish mortality. We also found that zooplankton biomass on the floodplain reached a maximum 2–3 weeks after disconnection with the river. We suggest that floodplain restoration in this region should consider management strategies that would ensure repeated flooding every 2–3 weeks during periods that would best match the peaks in abundance of native fishes. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Modelling processes that generate and maintain coral community diversity

This study addresses why adjacent sites on coral reefs are different, why some places support so few coral species, and how human impacts are affecting coral community dynamics and ultimately diversity. The paper includes a brief review outlining what theory tells us and how that theory relates to empirical data. A metapopulation and a metacommunity model are introduced. The metapopulation model can be used to predict a species' probability of site occupancy and the metacommunity model can be used to predict the number of species supported at any one site. The models identify the primary mechanisms that generate and maintain local (10s m²–10s km²) coral diversity based on a regional (100s km²) perspective. Local diversity appears regulated by differential post-settlement mortality that in turn leads to local extinction of some species. Indeed, harsh local environments cause high levels of local extinction and these environments support few coral species.     

MACROALGAL RECRUITMENT IN A HIGH MARSH CREEK OF NORTH INLET ESTUARY,SOUTH CAROLINA1

The temporal and spatial variability in recruitment patterns of macroalgae were stuided by evaluating the appearace of propagules on marble tiles set over a depth gradient in a high marsh tidal creek in North Inlet Estuary, South Carolina. Ulvoids and Porphyra occurred over a wide range of depths in the intertidal zone unlike Ectocarpus and members of the Florideophycease, which grew over a more restricted zone. Ulvoid propagules were the most abundant, attaining maximum densities at 0 and -15 cm mean low water (MLW) at all seasons except winter. In summer, coincident with high tubbidity, ulvoid densities decreased at -15 cm MLW but not in other seasons. Ulvoid densities at 0 MLW were depressed in January, February was preceded with the lowest recoreded temperatures and salinities during the study period, while the decrease in July was preceded by the highest recorded temperature. The number of taxa was highest in April and September, representing winter-spring and summer-fall transition periods, respecitively. The biomass of adult forms was higher on the tiles than on naturally occurring substrala, perhaps due to lack of stable suitable substrata in the field. Inhibition of subsequent recruitment by initial recreuitment was evident only on tiles submerged in October. The initial recruitment pattern of Porphyra rosengurtii coll et Cox and Ectocarpus siliculosus (Dillwyn) Lyngbye seem to determine the biomass distribution of adult forms, while that of ulvoids may be altered later by other factors. The considerable decrease in the number of adult forms compared to the initial denisties of propagules indicales high juvenile mortalities.     

Trophic ecology of the Lake Superior wave zone: a stable isotope approach

Stable carbon and nitrogen isotope ratio analyses were used to characterize the primary energy sources and trophic positions of 16 common Lake Superior wave zone invertebrate species. Isotope data from six tributary species that were taxonomically and ecologically matched with common wave zone species revealed broad energetic separation between these similarly structured benthic food webs. Previously published stable isotope data for Lake Superior wetland and pelagic food webs were used to assess the relative importance of inter-habitat energy flow within the Lake Superior ecosystem. The results of these comparisons indicate that the Lake Superior wave zone is energetically distinct from its tributaries, wetlands, and to a lesser extent from its vast pelagic realm. This information and approach should prove useful in future studies on the bioenergetics of inter-zonal migrants and other species that forage in multiple habitats within the lake and also in revealing energetic connections among terrestrial, riverine, littoral, and pelagic food webs in the coastal ecosystems of Lake Superior.     

Management strategies for sustainable invertebrate fisheries in coastal ecosystems of Galicia (NW Spain)

Artisanal coastal invertebrate fisheries in Galicia are socio-economically important and ecologically relevant. Their management, however, has been based on models of fish population dynamics appropriate for highly mobile demersal or pelagic resources and for industrial fisheries. These management systems focus on regulating fishing effort, but in coastal ecosystems activities that change or destruct key habitats may have a greater effect on population abundance than does fishing mortality. The Golfo Artabro was analysed as a representative example of a coastal ecosystem in Galicia, and the spider crab Maja squinado used as a model of an exploited coastal invertebrate, for which shallow coastal areas are key habitats for juvenile stages. The commercial legal gillnet fishery for the spider crab harvests adults during their reproductive migrations to deep waters and in their wintering habitats. Illegal fisheries operate in shallow waters. The annual rate of exploitation is >90%, and <10% of the primiparous females reproduce effectively at least once. A simple spatially-explicit cohort model was constructed to simulate the population dynamics of spider crab females. Yield- and egg-per-recruit analyses corresponding to different exploitation regimes were performed to compare management policies directed to control the fishing effort or to protect key habitats. It was found that the protection of juvenile habitats could allow increases in yield and reproductive effort higher than in the present system, with such protection based in the control of the fishing effort of the legal fishery. Additionally, there is an urgent need for alternative research and management strategies in artisanal coastal fisheries based on the implementation of a system of territorial use rights for fishers, the integration of the fishers into assessment and management processes, and the protection of key habitats (marine reserves) as a basic tool for the regulation of the fisheries.     

Immediate Effects of Experimental Human Trampling on Mid-Upper Intertidal benthic Invertebrates at the Asinara Island MPA (NW Mediterranean)

To assess potential risks of human visitation to ecological communities, the immediate effects of human trampling were investigated experimentally on small invertebrates inhabiting mid-upper intertidal hard bottoms covered by algae. Two different experimental intensities of trampling (60 and 120 footsteps) and controls (with no trampling) were applied to quadrats 20×20 cm in size (experimental area), within the two ‘no-entry, no-take’ zones of the Asinara Island MPA (Italy, Mediterranean Sea). One day after trampling ended, samples of benthic fauna were collected and the animals attributed to macrofaunal and meiofaunal components. Analyses of variance on the nine most common taxa of macrofauna identified significant higher abundance of bivalves, gammarid amphipods, polychaetes, isopods, oligochaetes in controls than in trampled plots. For nematodes, polychaetes, ostracods, oligochaetes, bivalves, acari, caprellid amphipods and tanaids a significant higher abundance of meiofaunal animals was found in controls than in trampled areas. Although no information on recovery is available, these results suggest that macrofaunal and meiofaunal taxa are vulnerable to this type of disturbance.