Spatial Patterns Of Recruitment Research Articles

Recruitment of the clam Ruditapes decussatus in the Lagoon of Thau, mediterranean

The spatial and temporal patterns of recruitment of juvenile clams Ruditapes decussatus were investigated in the Mediterranean lagoon of Thau. The periods of release of clam larvae were determined by monitoring the gonad maturity index in samples of adult females. Two massive spawnings were detected. Recruitment, deduced from spat density, was abundant in one part of the lagoon (Etang des Eaux-Blanches) in 1987 but not in the following years. Conversely, it was absent in another part of the lagoon (Grand-Etang) in 1987, although it occurred previously there. The spatial patterns of recruitment in 1987 were highly heterogeneous. They could be related to circulation, modelled for the periods following spawning. In the Grand-Etang, where no recruitment occurred in 1987, the larvae were confined to the vicinity of intensive shellfish culture zones where they presumably were depleted from the water. Episodic local anoxia also likely caused massive mortality among recruits. Recruitment of invertebrates with planktonic larvae in Mediterranean lagoons thus depends on meterological conditions which directly or indirectly determine the timing of spawning, the circulation patterns, and the occurrence of anoxias. In the lagoon of Thau the presence of large areas of intensive shellfish culture is a potential factor of massive mortality for larvae.

Spatial and temporal patterns of Crassostrea virginica (Gmelin) recruitment: relationship to scale and substratum

A series of experiments were performed in a euhaline (North Inlet, South Carolina) and mesohaline estuary (Wando River, South Carolina) to examine recruitment variability across several spatial and temporal scales. Crassostrea virginica (Gmelin) recruitment varied across all temporal and spatial scales with the exception of among reefs within restricted zones of the Wando River Estuary. Both biological and physical factors could be related to the variability observed at different scales. At the finest spatial scales, oyster larvae indicate a preference for crevices and shells or tests of other organisms as attachment sites. Higher recruitment on the shaded sides of plates in the high intertidal zone was consistent with observations documenting the preference of larvae for shaded attachment sites, adverse effects of direct exposure to light and heat, and sedimentation. Observations made at broader spatial scales (100s m) suggested that the availability of potential settlement surfaces may be the underlying factor contributing to the observed distribution of intertidal oyster reefs. Recruitment patterns were affected by choice and position of substrata. Sampling frequency was observed to influence the number of peak recruitment events observed within a season, estimates of total net recruitment, and the variability associated with recruitment observations made at different levels in the intertidal zone. Gregarious settlement behavior, sedimentation, and biophysical modification of the substrata were implicated in the regulation of recruitment patterns at shorter time scales. Competition and predation appeared to be important in regulating recruitment patterns at longer time scales, although the effects varied across the intertidal gradient. The physical and biotic processes which normally operate to structure intrareef spatial recruitment patterns were not observed to be as important in reefs which are physically restricted by tidal factors to very narrow zones within the intertidal gradient.

Seed fate in an ant-dispersed sedge, Carex pilulifera L.: recruitment and seedling survival in tests of models for spatial dispersion.

The spatial pattern of recruitment and seedling survival was studied in an ant-dispersed sedge (Carex pilulifera L.) in a forest clearing in Denmark. Seedlings were generally more aggregated than juvenile and adult plants. Recruitment distances were skewed towards larger values. While 72% plants of presumably ant-dispersed origin survived for 5 years, only 13% auto-dispersed plants were still alive, predominantly as persistent seedlings. The survivorship showed a negative exponential decrease (Deevey type II) with an expected half-life of 17 months for auto-, and 10 years for ant-dispersed plants. Growth-rate and fecundity were significantly larger for ant-dispersed plants than for auto-dispersed plants. Computer simulation was used to test three different dispersal models and two mortality types on the recruitment pattern observed in the field. The simulations confirmed the importance of ant dispersal for recruitment, but gave no conclusive evidence for evaluating recruitment from ant-nests. High mortality levels, comparable to observed predation levels, were needed to simulate field conditions. Density-dependent mortality proved more powerful than distance-dependent mortality in the simulations. Results are discussed in the context of current hypotheses of seed dispersal. While the directed dispersal hypothesis could not be rejected, results were generally more favourable to the escape hypothesis.

Effects of microhabitat on recruitment variation in a Gulf of Maine reef fish

Variation in recruitment of larvae can be important in determining temporal and spatial variat~on in adult demersal fish populations. Thls study examines how microhabitat use by newly settled cunner Tautogolabrus adspersus (Pisces. Labrldae) influences small-scale recruitment variation. The microhabitat occupied by cunner recruits was characterized by quantifying 14 attributes of the microhab~tat at 2 sites in Southern Malne, USA. Attributes at randomly selected sites were also recorded. Cunner recruits were distributed nonrandomly anlong microhabitats wlthin sltes. Recruit presence was positively associated with the presence of macrophytes and negabvely associated with microhabitats of low structural complexity. Association with specific microhabitats differed among sites; however, microhabitats available also differed among sites. While presence or absence could be prebcted by attributes of microhabitats, small-scale densities of fish could not. These data suggest that microhabitat use is nonrandom, and that the distribution of microhabitat types may influence small scale spatial patterns of recruitment in this species.

Population size-structure, juvenile growth, and breeding periodicity of the sea star Asterina miniata in Barkley Sound, British Columbia

Populations of Astenna rniniata (Brandt, 1835) exhibited a heterogeneous spatial pattern of recruitment in 2 adjacent estuarine inlets in Barkley Sound, British Colun~bia. Two distinct cohorts of juvenlles occurred in Grappler Inlet, while juveniles were absent nearby in Bamfield Inlet. The breedmg period~city of adults, field observations of spawning, and rates of juvenile growth suggest that larvae settled in Grappler Inlet during summer 1983 and 1984. Surveys of populations In subsequent years lndlcate that no recruitment occurred in Bamfleld or Grappler Inlet in 1985 to 1987. Survivorship of juvenlles was hlgh in laboratory experiments that assessed predation by 7 species of macroinvertebrates and 1 carnivorous fish Low densities of A. rniniata outslde Grappler Inlet, coupled with slow crawling speeds, suggest that the appearance of juvenlles in Grappler Inlet was probably due to autochthonous settlement. Patterns of recruitment of A. rniniata are affected primarily by complex Interactions between pre-settlement and early post-settlement events rather than migratory movements or juvenile mortality.

Recruitment of Coral Reef Fishes: Effects of Distribution of Predators and Shelter

Factors affecting recruitment and early survivorship of juvenile coral reef fishes were studied on St. Croix, United States Virgin Islands. The faunal assemblage studied included diurnally active fishes found in the rubble/sand habitat. The most abundant members were: beaugregory (Stegastes leucostictus), foureye butterfish (Chaetodon capistratus), mahongany snapper (Lutjanus mahogoni), surgeonfishes (Acanthurus bahianus and A. chirurgus) and French grunt (Haemulon flavolineatum). During the period 1978—1981, set of experimental reefs constructed from Strombus gigas shells were built in various locations relative to a major reef. Recruitment of juveniles of almost all species in the rubble/sand fish assemblage occurred more heavily on reefs that were built 20—40 m away from the main reef (into the lagoon) than on those built at the edge of the backreef. A series of experiments revealed that this is probably due to two factors: differences in available shelter and differences in encounter rates with predators. Seagrass and algae, which provide shelter to very small juvenile fishes, are absent in a "halo" zone close to the reef due to the feeding activities of reef—associated grazers (fishes and urchins). Experiments with artificially produced halos and artificial seagrass and algae showed that part of the explanation for the observed spatial pattern of fish recruitment is the spatial pattern of the shelter provided by seagrass and algae. Experiments in which potential prey fish were tethered demonstrated that the risk of predation for small juvenile fishes was considerably higher close to the reef than it was 20 m away. The results of these experiments suggest that patterns of settlement and juvenile survivorship of coral reef fishes are affected by complex interactions with other reef organisms (in this case, he activities of reef—associated grazers that modify available shelter) and by the frequency of encounters with predators. If they survive, all of the juveniles, regardless of where they initially settle, eventually migrate to a nearby reef. The species studies here differed in the degree to which juveniles use the sea—grass/algal habitat as a refuge from predation; these differences suggest that spatial patterns of recruitment, interacting with predation, may influence both population abundances and species composition of older juveniles on the reef.

Summary of the session

Recruitment of sessile marine invertebrates onto small panels at two subtidal sites in southern Australia varied considerably on all spatial and temporal scales. Recruitment at West Lakes was consistently higher than at Edithburgh, usually by an order of magnitude or more. There were strong seasonal trends in the number of recruits and the number of colonizing species per panel at West Lakes, but these trends were unclear at Edithburgh. Small panels received fewer recruits than did larger panels, although the number of recruits per cm2 did not change.Individual species showed similar patterns of recruitment. The temporal pattern varied from year-round settlement, with seasonal peaks (most serpulids), to regular recruitment at the same time each year (Didemnum sp., Botrylloides leachii Savigny), to irregular, intense peaks of recruitment (Electroma georgiana Quoy and Gaimard, Ciona intestinalis (Linnaeus)). Even those species that showed regular peaks in recruitment showed great variation between years in the magnitude of the peaks.The spatial pattern of recruitment of individual species varied from random (Spirorbis spp.) to strongly aggregated (Elminius modestus Darwin, Mania ione Gray). Aggregated patterns of recruitment were probably due to settlement near adult conspecifics (Elminius modestus, Balanus amphitrite Darwin), or to “swarms” of larvae settling together (Hydroides norvegica (Gunnerus)).There was little synchrony between the two sites. Major peaks in recruitment at one site were not accompanied by peaks at the other site, and even a good year for a particular species at one site was not necessarily matched for the species at the other site, providing little support for the concept of generally “good” or “bad” years for a coastal region as a whole.Replicated panels that were immersed at the same time varied greatly in their recruitment. This was partly explained by the distance between panels: the similarity between a pair of panels sometimes decreased as the distance between them increased. This relation rarely explained >20% of the variation in similarities, however. Small panels were generally less similar to each other than were large panels. The increase in similarity as panels became closer together could not always be explained by gregarious behaviour of larvae, and was probably due to small-scale patchiness in the distribution of larvae in the plankton. There remained a large amount of variation in recruitment that could not be explained by the behaviour of individual species or by patchiness in the plankton.

Patterns of recruitment of sessile invertebrates in two subtidal habitats

Recruitment of sessile marine invertebrates onto small panels at two subtidal sites in southern Australia varied considerably on all spatial and temporal scales. Recruitment at West Lakes was consistently higher than at Edithburgh, usually by an order of magnitude or more. There were strong seasonal trends in the number of recruits and the number of colonizing species per panel at West Lakes, but these trends were unclear at Edithburgh. Small panels received fewer recruits than did larger panels, although the number of recruits per cm 2 did not change. Individual species showed similar patterns of recruitment. The temporal pattern varied from year-round settlement, with seasonal peaks (most serpulids), to regular recruitment at the same time each year ( Didemnum sp., Botrylloides leachii Savigny), to irregular, intense peaks of recruitment ( Electroma georgiana Quoy and Gaimard, Ciona intestinalis (Linnaeus)). Even those species that showed regular peaks in recruitment showed great variation between years in the magnitude of the peaks. The spatial pattern of recruitment of individual species varied from random ( Spirorbis spp.) to strongly aggregated ( Elminius modestus Darwin, Mania ione Gray). Aggregated patterns of recruitment were probably due to settlement near adult conspecifics ( Elminius modestus, Balanus amphitrite Darwin), or to “swarms” of larvae settling together ( Hydroides norvegica (Gunnerus)). There was little synchrony between the two sites. Major peaks in recruitment at one site were not accompanied by peaks at the other site, and even a good year for a particular species at one site was not necessarily matched for the species at the other site, providing little support for the concept of generally “good” or “bad” years for a coastal region as a whole. Replicated panels that were immersed at the same time varied greatly in their recruitment. This was partly explained by the distance between panels: the similarity between a pair of panels sometimes decreased as the distance between them increased. This relation rarely explained >20% of the variation in similarities, however. Small panels were generally less similar to each other than were large panels. The increase in similarity as panels became closer together could not always be explained by gregarious behaviour of larvae, and was probably due to small-scale patchiness in the distribution of larvae in the plankton. There remained a large amount of variation in recruitment that could not be explained by the behaviour of individual species or by patchiness in the plankton.