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| 2. |
- Moksnes, Per-Olav, 1965
(författare)
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Interference competition for space in nursery habitats: density-dependent effects on growth and dispersal in juvenile shore crabs Carcinus maenas
- 2004
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Ingår i: Marine Ecology-Progress Series. - 0171-8630. ; 281, s. 181-191
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Tidskriftsartikel (refereegranskat)abstract
- In marine organisms with dispersing larval stages, current ecological theory suggests that recruitment of benthic juveniles may be limited by juvenile habitats and regulated by early post-settlement processes. However, few studies have attempted to identify the density-dependent mechanisms responsible for these demographic bottlenecks. I conducted a series of experiments to assess if interference competition for space, within nursery habitats, could result in density-dependent juvenile growth and dispersal from refuge habitats, and regulate local populations of juvenile shore crabs Carcinus maenas in shallow nursery areas in Sweden. In laboratory mesocosms, increased natural densities of similarly sized crabs resulted in decreased growth and increased per capita emigration from mussel habitats, even though food was provided in excess, suggesting that competition for space and mutual interference mediated the density-dependent growth and dispersal. This was supported in a field experiment where a density-dependent relation was found between the abundance of juvenile crabs in nursery areas and their migration rates to experimental mussel patches. The results suggest that interference competition for space and density-dependent emigration from refuge habitats, coupled with habitat-specific mortality, is an important regulating mechanism for juvenile shore crab populations, in particular for older juvenile cohorts that have escaped inter-cohort cannibalism through a size refuge. Density-dependent effects on growth were found only at unusually high natural densities and may, therefore, be less important as a regulating mechanism. However, negative effects on feeding rates and growth at high conspecific densities may represent an important selective force in juvenile migration behavior. Density-dependent dispersal was found at juvenile densities that are regularly found in nursery areas, suggesting that nursery habitats become saturated with juveniles during the recruitment season and represent a limiting resource for local populations, consistent with field observations of shore crab populations in the study area.
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| 3. |
- Moksnes, Per-Olav, 1965, et al.
(författare)
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Methods for estimating decapod larval supply and settlement: importance of larval behavior and development stage
- 2001
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Ingår i: Marine Ecology-Progress Series. ; 209, s. 257-273
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Tidskriftsartikel (refereegranskat)abstract
- In marine benthic organisms with a pelagic larval phase, assessment of recruitment regulation necessitates accurate estimates of larval supply and initial settlement densities. We assessed 2 commonly used methods for estimating decapod larval supply, plankton net tows and artificial settlement substrates (ASS), together with a new technique using passive-migration traps. The aim was to evaluate how these methods estimate larval supply and settlement of 3 common decapod species (the shore crab Carcinus maenas L., the brown shrimp Crangon crangon L, and the grass shrimps Palaemon spp.) in a microtidal nursery area on the Swedish west coast. and to assess how these estimates relate to the juvenile recruitment of these species. Discrete plankton net tows outside the shallow nursery area collected a higher proportion of larvae at an early development stage, compared to the other methods. and produced only a snapshot of the variable water-column abundance of shore crab and grass shrimp larvae that correlated poorly with numbers collected with ASS at the same location. Artificial settlement substrates appeared to produce good, integrated relative estimates of shore crab larval supply and settlement. The number of larvae collected from ASS correlated significantly with larval abundance estimated by passive-migration traps in shallow nursery areas, and reflected changes in shore crab settlement densities in caged mussel habitats. We did not detect any effect of predation on ASS placed in nursery areas. However, a large proportion tan estimated 83%) of shore crab postlarvae appeared to emigrate from ASS immersed for periods longer than 12 h, possibly during the night. All grass shrimp larvae appeared to emigrate from the collectors within 24 h, suggesting that ASS immersed longer than 12 h do not produce useful integrated estimates of larval supply for this species. These results demonstrate the importance of assessing postlarval emigration patterns from ASS to optimize immersion and collection time, and to avoid confounded estimates of larval abundance and settlement. PI new method using replicated passive migration traps that fished continuously in opposite directions ton-shore and off-shore) gave promising integrated estimates of net fluxes (total number immigrating minus number emigrating per unit time) of both pelagic postlarvae and early benthic juvenile stages of crabs and shrimp. The majority of the brown shrimp recruits were young juveniles, demonstrating the importance of incorporating juvenile migration in recruitment studies of motile benthic species. High numbers of shore crab and grass shrimp larvae were found to emigrate from the bay, indicating that many decapod postlarvae found in nursery areas may be transitional. The emigrating larvae were on average in an earlier development stage than those migrating to the bay. These short-term experiments demonstrate the importance of assessing larval development stage and migratory behavior when estimating larval supply and settlement for recruitment studies.
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| 4. |
- Moksnes, Per-Olav, 1965
(författare)
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Self-regulating mechanisms in cannibalistic populations of juvenile shore crabs Carcinus maenas
- 2004
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Ingår i: Ecology. - : Wiley. - 0012-9658. ; 85:5, s. 1343-1354
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Tidskriftsartikel (refereegranskat)abstract
- Juvenile cannibalism may play an important role in population regulation of marine organisms with juvenile stages that aggregate in nursery habitats. I conducted a series of field and laboratory experiments to assess whether three potential density-dependent mechanisms could regulate local recruitment of juvenile shore crabs Carcinus maenas in Swedish nursery areas: (1) intracohort cannibalism, (2) juvenile cannibals' functional response to smaller conspecific prey densities, and (3) juvenile cannibals' numerical effect on prey survival. Juvenile cannibals in mesocosms displayed a type III (sigmoid) density-dependent functional response, resulting in significantly higher proportional mortality of smaller conspecifics at high prey densities. Consistent with the laboratory result, cage-enclosed juvenile cannibals in a field experiment eliminated conspecific settlement peaks in mussel habitats and decoupled the relationship between larval and settlement abundance within three days. Density-dependent interference between cannibals in mesocosms decreased per capita predation rates, but conspecific prey mortality still increased with cannibal densities when the abundance of cannibals passed a critical threshold, suggesting that the recruitment success of consecutive cohorts may be negatively correlated. This was supported in a field experiment where the abundance of juvenile cannibals explained 71% of the variation in settlement loss from uncaged mussel habitats. Density-dependent cannibalism also occurred within juvenile cohorts even though food was provided in excess, but the resulting mortality was too low to eliminate initial density: patterns over a 30-day period. The results demonstrate that cannibalism between juvenile shore crabs can cause high and strongly density-dependent mortality at natural densities, and that both a functional response and a numerical effect of larger cannibals can regulate the recruitment of new cohorts within days of settlement. A coupling of cannibal and settlement densities on a regional scale is expected to decrease recruitment variability within and between local populations. Moreover, the interaction between the type III functional response, mutual interference, and density-dependent dispersal of cannibals is predicted to reduce cannibalistic rates at low prey and high cannibal densities, respectively, and stabilize local population dynamics in relation to the abundance of juvenile habitats, consistent with field observations of shore crab populations in the study area.
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| 5. |
- Moksnes, Per-Olav, 1965, et al.
(författare)
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Settlement behavior in shore crabs Carcinus maenas: why do postlarvae emigrate from nursery habitats?
- 2003
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Ingår i: Marine Ecology-Progress Series. ; 250, s. 215-230
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Tidskriftsartikel (refereegranskat)abstract
- In contrast to general settlement models of invertebrate larvae, recent Swedish studies of settlement of the shore crab Carcinus maenas indicate that many postlarvae close to metamorphosis emigrate from their preferred settlement habitats. In the present study, we assessed physical and biological factors affecting shore crab settlement and emigration from nursery habitats, and explored possible ultimate factors in this behavior. Postlarval abundance in the field was not affected by the tidal phase, but varied significantly according to the light conditions. Settlement densities on artificial substrates were significantly higher during the day compared to at night. In contrast, densities of planktonic postlarvae were highest at night. This result suggests that the postlarvae cling to shallow benthic habitats during the day and swim in the plankton at night. Mesocosm experiments demonstrated that most postlarvae remained in the benthos during the day, even in suboptimal habitats, but that a large proportion (on average 58%) actively emigrated at dusk, irrespective of settlement conditions. Emigration rates were significantly higher and metamorphosis rates lower in open sand compared to mussel habitats, whereas food levels and settlement densities had only small effects on the settlement process. Emigrated postlarvae were at a significantly earlier developmental stage and metamorphosed on average 1 d later than the megalopae that remained in the benthic habitat. Post-larvae >2 d from metamorphosis emigrated even from optimal settlement conditions. These results suggest that settlement in shore crabs is not an irreversible process once a late-stage postlarva has found a settlement habitat, but involves 2 separate behavioral components: (1) the selection of a benthic habitat during the day, which is mainly affected by the availability of refuges from predation; and (2) the choice to stay or emigrate the following night, where emigration is induced by a decrease in light and determined mainly by the development stage of the postlarvae. We propose a conceptual model of settlement in the shore crab where the dynamic behavior of the postlarvae is an adaptation to diel differences in predation risk between the settlement habitat and the plankton.
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| 6. |
- Moksnes, Per-Olav, 1965
(författare)
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The relative importance of habitat-specific settlement, predation and juvenile dispersal for distribution and abundance of young juvenile shore crabs Carcinus maenas L
- 2002
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Ingår i: Journal of Experimental Marine Biology and Ecology. ; 271:1, s. 41-73
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Tidskriftsartikel (refereegranskat)abstract
- 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. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 7. |
- Queiroga, Henrique, et al.
(författare)
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Vertical migration behaviour in the larvae of the shore crab Carcinus maenas from a microtidal system (Gullmarsfjord,Sweden)
- 2002
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Ingår i: Marine Ecology-Progress Series. ; 237, s. 195-207
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Tidskriftsartikel (refereegranskat)abstract
- Tide-related vertical migration behaviours which assist in horizontal transport of marine larvae have been documented in many species from meso-tidal systems; however, little is known about larval behaviour in areas where the tides do not produce predictable changes in water currents. We assessed the vertical migration behaviour in larvae of the shore crab Carcinus maenas (L.) from the Swedish west coast, to explore possible transport mechanisms in a micro-tidal system. Replicated plankton samples were collected at a fixed station from 2 depths during flood and ebb tides, and analysed for effects of tides and light on temporal and vertical patterns in abundance. In a complementary experiment, first-stage zoeae were hatched in the laboratory from field-collected females and studied in constant darkness for endogenous circa-tidal or circa-diel rhythms. The abundance of different larval stages decreased from the first to the fourth zoeal stage by a factor 10, whereas megalopal densities were similar to those of the first zoea. This pattern in abundance indicates an offshore/onshore transportation of larvae during ontogeny, suggesting a behavioural component in larval dispersal. However, contrary to what has been demonstrated in larvae from meso-tidal areas, none of the larval stages displayed a vertical or temporal pattern in abundance related to the tide, although a clear semi-diurnal tidal change of approximately 0.3 m was registered throughout the sampling periods. Instead, the highest densities of all larval stages were consistently sampled during low light conditions, indicating a diel pattern in swimming behaviour. This behaviour appears to be controlled by exogenous cues, since the laboratory experiment with first zoeae in constant darkness did not indicate any consistent tidal or diel rhythm in vertical migration. The lack of an endogenous circa-tidal vertical migration rhythm suggests that Swedish shore crabs are reproductively isolated from the British Isles populations, where this behaviour has been demonstrated to be inherited. The possibility that: C. maenas larvae in micro-tidal systems may use predictable sea and land breezes as a mechanism for cross-shelf transport is discussed.
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