Pseudopolydora Kempi Research Articles

급격한 조석 감소에 의한 새만금 갯벌 대형저서동물 변화

Tidal ranges of inner Saemangeum were largely reduced after the final dyke enclosure, resulting in the former tidal flats becoming either permanently exposed, still influenced by changing tide, or permanently submerged. The effect of reduced tidal range on survival and population stability of tidal flat macrofauna were investigated at three shifted habitats after the dyke completion. At the exposed area, several species survived for 80 days after the dyke enclosure. However, within 120 days, all macrofauna died off due to the elevated temperature during summer. At the intertidal area, some species were maintained until 170 days after the dyke enclosure with a large decrease of the faunal abundance. Species of the submerged area were seen to be more tolerant of the changed environments. The opportunistic species, such as Sinocorophium sinensis, Theora fragilis and Pseudopolydora Kempi, were massively introduced into the submerged area after the dyke enclosure, in which the benthic ecosystem was severely disturbed.

Feeding and growth of juvenile stone flounder in estuaries: generality and the importance of sublethal tissue cropping of benthic invertebrates

We investigated the feeding habits and growth of juvenile stone flounder (Platichthys bicoloratus) in several estuarine nurseries in Sendai Bay, Japan. Prey abundance and composition and juvenile diet varied largely among sites. However, polychaete palps (mostly Pseudopolydora kempi) and bivalve siphons (mostly Nuttallia olivacea) were positively selected and frequently consumed by juveniles, indicating the generality of sublethal feeding of juvenile stone flounder in estuarine nurseries. Recent growth rates determined by otolith microstructure analyses were dependent on juvenile body size and water temperature, but independent of juvenile density and food abundance. Growth rates were nearly maximal from March to May, suggesting that food conditions are nearly optimal in these estuarine nurseries. Sublethal tissue cropping of benthic invertebrates is thought to contribute largely to these high growth rates.

Importance of feeding on regenerable parts of prey for juvenile stone flounder Platichthys bicoloratus in estuarine habitats

Prey availability is one of the most important factors affecting the quality of nursery grounds. Estuaries play an important role as nursery grounds for juvenile stone flounder, but the mechanism behind the consistently high availability of prey has never been examined. This study investigates which prey is mainly selected by juvenile stone flounder (15–55 mm standard length) in the estuary of the Natori River, northern Japan. In a cage experiment, juveniles showed positive selection for the palps of the spionid polychaete Pseudopolydora kempi in March, and for the siphons of the bivalve Nuttallia olivacea in April, May and June in both sandy and muddy-sand habitats. This selective predation showed that sublethal predation on regenerable parts of invertebrates is important for stone flounder. Nuttallia olivacea, the dominant bivalve in the estuary, was more abundant and in better somatic condition in the sandy area in spite of the stronger siphon-cropping pressure by juvenile stone flounder. These results confirm that sublethal predation on highly abundant benthos plays an important role in forming estuarine habitats into areas of high prey availability for juvenile stone flounder, which leads to their high growth rate.

Adhesive-based selection by a tentacle-feeding polychaete for particle size, shape and bacterial coating in silt and sand

We tested particle selection by a surface deposit-feeding, tentaculate spionid polychaete, Pseudopolydora kempi japonica Imajima and Hartman. In experiments with peroxide-cleaned, sizegraded but otherwise natural silts and sands, individual worms showed peak preference for particles 80‐99 mm in diameter, compared with previous (Self and Jumars, 1988) documentation of peak preference for particles of 7 mm in experiments conducted with plastic and glass beads. These results imply that microtektites will not in general be good tracers of mixing of mineral grains of comparable size. Animals exhibited statistically signie cant but not marked differences in size selectivity for subrounded versus subangular grain shapes; the size preference peak was broader in subangular grains, for which orientation of the grain can alter probabilities of both contact and retention. When one size class of grains was coated with the bacterium Halomonas halodurans (ATCC 29686), animals in general showed enhanced selection (relative to controls with no food value on any size class) of that size and smaller grains, even though these smaller grains lacked food value. Greatest selection, however, generally occurred for the coated size class. Results frominclusion of glass beads in some of the experiments and from separate experiments with tentacle analogs imply that this selective capability may be largely passive and mechanical. Natural grains, due to surface texture, have more surface area for adhesive contact than do smooth glass beads so that larger grains than beads are retained. Bacteria-coated grains, in turn, appear to be picked up preferentially due to adhesion with the bacterial coating; petroleum jelly-coated microscope slides also succeed in selective retention of the size class that is bacterially coated. Substantial selection by adhesiveutilizing deposit feeders apparently can be achieved without investment in complex, time- and energy-consuming sensory systems and behaviors. Experiments with natural grains showed notably more scatter than prior experiments with glass beads, but this difference is consistent with the mechanism. For nonspherical particles, both contact and retention depend on orientation as well as size.

Responses of deposit-feeding spionid polychaetes to dissolved chemical cues

Few studies have investigated capabilities or ecological implications of chemical sensing in deposit feeders. We tested three species of spionid polychaetes ( Boccardia proboscidea Hartman, Polydora cornuta Bosc, and Pseudopolydora kempi japonica Imajima and Hartman) for responses to a variety of compounds: amino acids, carbohydrates, organic acids, nucleotides, and plant and animal extracts that might have been used as general indicators of food quality or of the presence of specific foods. Five individuals of each species were exposed to 44 different dissolved chemical cues introduced in slow, laminar flow to control location and dose (1 mM for most cues) of delivery. Cues that altered feeding behaviors of one or more species were then tested against ten additional individuals of each species, in part to overcome dangers of multiple testing in the original screening. We observed differences among the three species both in their background behavioral patterns and in their responsiveness to dissolved cues applied at palp level just above the sediment–water interface, although significant changes in feeding behavior occurred only in Boccardia and Pseudopolydora. Taurine, threonine and valine acted as strong phagodepressants, whereas proline was significantly phagostimulatory to Boccardia. No hexoses elicited significant responses. Natural extracts were more stimulatory than the low-molecular-weight monomers tested.

INTRASPECIFIC VARIATIONS IN δ13C INDICATE ONTOGENETIC DIET CHANGES IN DEPOSIT-FEEDING POLYCHAETES

Many species change diets during development. Often, these ontogenetic changes are discrete and coincide with metamorphosis (e.g., amphibians), but more gradual niche changes can occur during growth. Identifying nondiscrete changes in diet and understanding their implications at the population and community levels are especially difficult for ecologists who study detritivores and other species that have poorly characterized diets. Theory and several lines of evidence suggest, however, that benthic juveniles of species that deposit feed as adults may be unable to meet their nutritional demands by deposit feeding. To reject the null hypothesis that both juveniles and adults of deposit-feeding species assimilate the same diet and to infer ontogenetic changes in diet, I used stable carbon isotopes as a natural diet tracer. I quantified body-size-dependent variations in the δ13C of four species of tentaculate, surface-deposit-feeding polychaetes: the ampharetid Hobsonia florida and the spionids Pseudopolydora kempi japonica, Polydora cornuta, and Pygospio elegans. In addition to worm tissues, I measured the isotopic compositions of the most likely primary producers at each field site (benthic diatoms, macroalgae, and saltmarsh grasses) to predict the worms’ carbon sources. All species showed significant size-dependent variations in δ13C. Furthermore, populations of P. kempi japonica at two different sandflats had similar ontogenetic trends despite isotopic differences in available foods at each site. Individuals fed a fixed diet in the laboratory, however, did not show significant size-dependent variation in δ13C, leaving ontogenetic changes in diet as the most parsimonious explanation of the field data. Regression analyses indicated that the gradual change in δ13C with body size was nonlinear, with most of the change in δ13C occurring before individuals reach sexual maturity. The complex life cycle of these species, therefore, includes both a radical change in niche when larvae metamorphose to juveniles and a gradual niche shift as benthic juveniles grow. The isotopic data indicate that the smallest juveniles assimilated much of their carbon from benthic diatoms (δ13C ≈ −20‰), while adults assimilated most of their carbon from detritus derived from macroalgae (δ13C ≈ −9‰) or saltmarsh grasses (δ13C ≈ −14‰). Because abundances of benthic diatoms or other high-quality components of sediment are more variable and more likely to be in limiting supply than detritus or bulk sedimentary organics, populations of species that deposit feed as adults may experience food-related recruitment bottlenecks during the juvenile stage.

Spectrofluorometric quantification of neutral and polar lipids suggests a food‐related recruitment bottleneck for juveniles of a deposit‐feeding polychaete population

A growing body of evidence suggests that juveniles of species that deposit feed as adults are more susceptible to food limitation than are conspecific adults. To quantify ontogenetic variations in the nutritional condition of the depositfeeding polychaete Pseudopolydora kempi japonica, I modified existing spectrofluorometric assays of neutral and polar lipids. The hydrophobic fluorochrome Nile red fluoresces yellow‐orange when bound to neutral lipids and red‐orange when bound to polar lipids. The ratio of neutral lipids (energy storage) to polar lipids (cell membranes) provides an index of nutritional condition that is scaled to body size. For individual P. kempi japonica ranging in length from 2 to 12 mm, I quantified the florescence from each lipid class and analyzed sizedependent variations in the neutral/polar lipid ratio. P. kempi japonica showed a size‐dependent increase in the neutral/polar lipid ratio up to a body length of −6–8 mm; little size‐dependent change in the ratio occurred beyond this size. Ontogenetic changes in both particle‐size selection and the assimilated diet of P. kempi japonica display size‐dependent patterns that are similar to the lipid data. Together, the three lines of evidence suggest that P. kempi japonica juveniles experience a food‐related recruitment bottleneck until they grow to −6–8 mm. Once juveniles grow beyond this size, they appear to forage like adults and achieve adult levels of neutral lipid. Because deposit‐feeding benthos ingest a low‐quality and nutritionally dilute diet that will be difficult for small juveniles to digest, ontogenetic niche shifts and food‐related recruitment bottlenecks during the juvenile stage may be common in the population ecology of species that deposit feed as adults.

Mechanisms of particle selection by tentaculate suspension feeders during encounter, retention, and handling

Selection among food particles can occur during any of the successive steps in suspension feeding: particle encounter, retention, and handling. We made predictions for mechanical particle-size selection in encounter and retention by tentaculate suspension feeders (e.g., polychaetes, cnidarians, and echinoderms), and we measured concurrent selection during each step in feeding for two species of spionid polychaetes ( Pseudopolydora paucibranchiata Okuda and Pseudopolydora kempi japonica Imajima and Hartman). In flume experiments we measured selection between two sizes of plastic beads was measured in flume experiments using video analyses of encounter and retention, and we determined handling selection by subsequent examination of gut contents. Encounter was strongly biased toward large particles, as predicted for the physical mechanism of direct interception. In contrast, retention was often biased toward small particles, as predicted by a model of the balance of forces on an encountered particle (i.e., an adhesive force which promotes retention vs. drag and lift forces which may cause particle loss). Handling was also biased toward small particles, apparently by active rejection of large particles. Flow speed and palp width affected selection only during particle retention. As predicted by the retention model, the retention bias toward smaller particles was stronger at higher flow speed and for worms with narrower palps. Retention mechanics alone thereby resulted in small worms ingesting relatively fewer large particles (and more small particles) in fast flow than they did in slow flow. Furthermore, in fast flow small worms ingested relatively fewer large particles than did larger worms. Given the wide range of particle sizes and types available in the field, retention mechanics can directly influence feeding ecology by placing constraints of flow speed and appendage size on the diet obtainable by tentaculate suspension feeders.

Ontogenetic changes in particle-size selection by deposit-feeding spionid polychaetes: the influence of palp size on particle contact

Scaling arguments suggest that juveniles of species that deposit feed as adults must ingest relatively higher quality diets to compensate for a digestive constraint imposed by small body (and gut) size. Understanding how and why particle-size selection varies ontogenetically can provide clues to the relative quality of forage juveniles obtain. Particle selection by a tentacle or palp feeder can be viewed as the net result of particle contact, collection, and post-collection rejection. A geometrical model of particle contact that includes effects due to palp size predicts that thinner palps and hence smaller worms are biased toward contacting larger particles. The model's prediction was tested using adhesive-coated palp mimics (monofilament line) of varying diameters spanning the range of actual palps to contact and collect particles from a mixture of two sizes of glass beads (16 and 88 μm dia). Results were consistent with the contact model's prediction: thinner palp mimics collected a lower proportion of small beads (higher proportion of large beads). Intraspecific variations in particle-size selection due to body, and palp size were determined for Pseudopolydora kempi japonica Imajima and Hartman, Polydora ligni Webster, Boccardia proboscidea Hattman, and Pygospio elegans Claparéde by conducting laboratory feeding experiments with the same mixture of glass beads used with mimics. P. kempi japonica results were consistent with the contact model and the palp-mimic experiment. B. proboscidea and P. elegans showed no significant variation with body size. Polydora ligni showed a negative relationship between particle size and body (and palp) size; insufficient mouth size of small juveniles is a likely explanation. Assuming that food value (per unit volume of ingested material of sedimentary detritus is negatively correlated with particle size, results suggest that deposit-feeding juveniles cannot increase diet quality without post-contact sorting mechanisms that probably increase handling costs-perhaps leading to macrophagous evaluation of individual particles based more on nutritional content than on mechanical properties like size. Increasing the time spent suspension feeding relative to deposit feeding is another likely means for juvenile spionids to increase diet quality and perhaps overcome the combined digestive and deposit-feeding constraints imposed by small gut and palp size.

Quantifying sediment disturbance by browsed spionid polychaetes: implications for competitive and adult-larval interactions

Sediment-dwelling animals (infauna) in marine soft-bottom habitats often lose body parts exposed above the sediment surface to browsing predators. Such tissue loss can cause reduced activity, feeding, growth, and reproduction. Browsing (or sublethal) predation may also influence competitive interactions via changes in sediment disturbance activity. Increased browsing rates should result in decreased sediment disturbance. We quantified the area of sediment disturbed by groups of spionid polychaetes subject to different regimes of feeding palp loss. For Pseudopolydora kempi (Southern), cores with higher numbers of regenerating worms had less area of surface sediment disturbed (reductions ranged from 30% to nearly 100%), and this effect persisted for at least 1 wk. Fecal production was similarly reduced. Fecal production by another spionid, Rhynchospio glutaeus (Ehlers), was reduced only when all worms in a core were regenerating both feeding palps. Tube-building was not affected in either species. By quantifying the zone of influence of browsed infauna, we can make better predictions of the level of browsing necessary to reduce competitive and adult-larval interactions.

Tissue loss induces switching of feeding mode in spionid polychaetes

For many animals, foraging is a complex activity involving decisions of when, where, and how to feed, as well as what to feed upon. Flexibihty in foraging (i.e. switches among activities, habitats, or food items) is presumed to contribute to overall fitness, and is an important component of theoretical models of animal feeding behavior. The importance of switching among feeding methods is less well described, but may be very important for manne sediment dwellers (infauna). For example, many infauna switch between suspension and surface-deposit feeding depending on water flow. However, infauna often lose feeding appendages to browsing predators, and the ability to switch to alternative feeding methods which are not dependent on those appendages should be advantageous after such tissue losses. Laboratory experiments examined the effect of feeding appendage loss on 2 species of spionid polychaetes which have different alternative feeding modes when intact. Behavior of individual worms with 0, 1 or 2 palps removed was videotaped for 2 h within 3 to 4 d of palp removal. Loss of both feeding palps induced switches to alternative feeding modes involving mouth-feeding: Rhynchospio glutaeus fed on the surface, Pseudopolydora kempi japonica fed below the surface. As measured by time spent feeding and by fecal production, the alternative was effective for R. glutaeus but not for P. k. japonica. The results emphasize the potential importance of injury and subsequent switching as factors determining feeding behavior.

The Effect of Palp Loss on Feeding Behavior of Two Spionid Polychaetes: Changes in Exposure.

The effects of sublethal predation on foraging behavior are potentially important. Such predation changes both the condition of the individual and the cost or risk associated with further predation. Tissue loss to predators is a very common phenomenon in marine sedimentary environments; often the tissue lost is the organism's feeding structure. We asked how the loss of feeding structures would affect the foraging behavior of two species of spionid polychaetes, and whether the responses to palp loss are predictable. Rhyncospio glutaeus and Pseudopolydora kempi japonica represent the ends of a spectrum of risk associated with tissue exposure during feeding. In both species, the loss of one or both palps significantly increased the amount of tissue exposed, and the frequency of exposure, but not the duration of each exposure. All changes were consistent with the normal foraging behaviors of these species. Estimates of relative energy gain suggest that these responses may partially mitigate the effect of palp loss, although potentially increasing the risk of predation.

Effects of animal density and supply of deposited and suspended food particles on feeding, growth and small-scale distributions of two spionid polychaetes

Many soft-sediment taxa can switch between deposit feeding and suspension feeding, depending on near-bottom flow conditions. Because the foraging spaces of individuals are typically greater when deposit feeding than when suspension feeding, density-dependent interactions should be more pronounced when deposit feeding. To test this hypothesis, laboratory experiments were conducted with two species of spionid polychaetes, Boccardia pugettensis (Blake) and Pseudopolydora kempi japonica (Imajima & Hartman). Densities of the two species were manipulated singly and together to examine intraspecific and interspecific effects. Food particles were supplied by episodic bedload transport, under which conditions both species deposit feed, or by continuous suspended load transport, under which conditions both species facultatively suspension feed. Contrary to the hypothesis, feeding and growth rates in the intraspecific experiments showed no density dependence when deposit feeding or suspension feeding, even when densities of conspecifics were as high as 220% (for Boccardia) and 940% (for Pseudopolydora), the average densities in the field. Supporting the hypothesis, there was weak evidence from the interspecific experiments that a high density of Boccardia exerted a negative effect on Pseudopolydora's growth rate when animals were deposit feeding but not when suspension feeding. Other, more clear, density-dependent effects were detected. Emigration of Boccardia from high-density treatments was pronounced when deposit feeding, but no emigration occurred when suspension feeding. Pseudopolydora emigrated at high density whether deposit feeding or suspension feeding. When deposit feeding, individuals of both species maintained a constant distance to their nearest neighbor, regardless of density of animals. When suspension feeding, however, distances between neighbors significantly decreased at high densities.

Analysis of the benthic infauna from Bahia de San Quintin, Baja California, with emphasis on its use in impact assessment studies

An ecological analysis of benthic polychaete assemblages is presented. Samples were collected at ll stations in July, September and December 1981, and February and May 1982. Thirty-nine species in 21 families of polychaetes were identified. Spatial variations may be due to microhabitats and/or the intrinsic biology of the species. The abundance increased in spring because of reproduction and recruitment. The values of the Shannon diversity index were as high as 3.1 and 0.9 for evenness. Clusters among stations could not be identified which may have been because of the dominance of five species (Exogone occidentalis, Pseudopolydora kempi, Scoloplos acmeceps, Prionospio heterobranchia and Neanthes arenaceodentata) and the relative abundance of secondary species. The abundance, number of species and evenness were not significant (p > > 0.05) when correlated with the content of organic matter and sediment particle size. The log-normal distributions of individuals among species did not detect pollution-induced changes in the community. The benthic community in Bahía de San Quintín was found to be at or near equilibrium which may be the result of a relatively high resilience to disturbance. A qualitative assessment of the possible impact of domestic sewage on the infauna and an outline for the ecological management of the bay are presented.

Pellet accumulation, sediment supply, and crowding as determinants of surface deposit-feeding rate in Pseudopolydora kempi japonica Imajima & Hartman (Polychaeta: Spionidae)

Pseudopolydora kempi japonica Imajima & Hartman was exposed in the laboratory to varying frequencies of sediment supply and fecal pellet removal and to varying degrees of intraspecific crowding. When pellets were not removed, feeding rates were approximately halved after only four pellets had accumulated in the feeding areas of individual worms. Crowding and increasing temperature accelerated feeding-rate depression by decreasing the time required for such accrual. Pellet removal returned feeding rate to the initial level, even under crowded conditions. Over the short term of these experiments (16 h), the only strong effect of variation in sediment supply was seen at the highest supply rate, which depressed feeding. Pellet accrual thus was clearly dominant over both sediment supply and intraspecific crowding in determining feeding rates. These results suggest the need to determine both the frequency of fecal removal by sediment transport in the field and to match this frequency in future laboratory experiments when simulation of field conditions is attempted.

Gut-marker and gut-fullness methods for estimating field and laboratory effects of sediment transport on ingestion rates of deposit-feeders

By developing a method for quantifying feeding rate in the presence of sediment transport, we determined whether sediment transport or one of its correlates influenced feeding rate of deposit-feeders in the field. The method entailed application and subsequent removal of magnet-susceptible marker particles, so that no marker was present in the ambient sediments during the period over which feeding rate actually was measured. Error variance was reduced by a paired statistical design in which the technique was used twice in the same individuals, to assess feeding rate immediately prior to the sediment transport event as well as during it. Low levels of sediment transport significantly affected feeding rates of Pseudopolydora kempi japonica Imajima & Hartman (a spionid polychaete) both in a small, imprecise, laboratory flume and in an intertidal environment characterized from late spring through early fall by a short pulse of sediment transport coincident with the incoming tide. Deposit-feeding rates immediately after the latter pulse were enhanced by a (geometric) mean of 50% in field trials from May through October. Both in the laboratory and in the field, however, the magnitude of the (albeit statistically significant) sediment transport effect was overshadowed by variation due to nutritional history of individuals and to other unidentified factors. The form of the field data for P. kempi japonica (feeding rates estimated for two successive intervals in the same individuals) allowed us, in addition, to analyze the response of gut fullness to mean feeding rate (averaged over the two feeding bouts), previous feeding rate (first bout), most recent feeding rate (second bout) or recent change in feeding rate (difference between first and second bouts). The strongest correlation was found with the last of these variables, suggesting that gut fullness may provide a tracer-free method of gaining some kinetic information on continuously feeding animals. Contrary to prevailing impressions, gut fullness in such continous feeders may provide more information on recent change in feeding rate than on feeding rate itself. We gathered additional data on P. kempi japonica that differed in emersion history (in tide-pools versus outside them prior to the pulse of sediment transport with the incoming tide) but were not exposed to markers. Animals both inside and outside tide-pools showed fuller guts immediately after the sediment transport pulse, consistent with an interpretation of accelerated feeding caused by some aspect of the sediment transport event. Incidentally collected individuals of Hobsonia florida (Hartman) (an intertidal ampharetid polychaete), on the other hand, showed no obvious response to the same conditions that triggered accelerated feeding and fuller guts in Pseudopolydora kempi japonica.

Effects of defecation by arenicolid polychaete adults on spionid polychaete juveniles in field experiments: Selective settlement or differential mortality

Cores of frozen and then thawed sediment were implanted into the intertidal zone during the period of larval settlement of a spionid polychaete Pseudopolydora kempi (Southern). The cores were of four treatment types: sediment blanks to which no macrofauna were added, worm-smell cores to which individuals of an arenicolid polychaete Abarenicola pacifica Healy and Wells were added for 1 day and then removed, regeneration cores to which individuals of Abarenicola with damaged tails were added, and worm control cores to which intact Abarenicola were added. Defecation activity of Abarenicola was significantly lower in the regeneration cores than in the worm control cores. Densities of Pseudopolydora juveniles at termination (9 days) were not consistent with a hypothesis of differential mortality as a function of Abarenicola activity. Densities were consistent with a hypothesis of the avoidance of cores with any evidence of Abarenicola, past or present

Non-overlapping distributions of spionid polychaetes: The relative importance of habitat and competition

The spionid polychaetes, Pygospio elegans Claparède, Pseudopolydora kempi (Southern), and Rhynchospio arenincola Hartman are found in False Bay, Washington. Two species, Pygospio elegans and Pseudopolydora kempi, co-occur in the high intertidal zone. The third species Rhynchospio arenincola occurs only in low intertidal areas. Reciprocal transplant experiments were used to test the importance of intraspecific density, interspecific density, and habitat on the survivorship of experimental animals. For all three species, only habitat had a significant effect. Individuals of each species survived better in experimental containers in their native habitat, regardless of the heterospecific and conspecific densities used in the experiments. The physical stresses associated with the prolonged exposure of the high intertidal site are experimentally shown to result in Rhynchospio mortality. From these experiments, habitat type is the only significant factor tested which can explain the observed distributions; the presence of confamilials has no detected effect on the survivorship of any species, suggesting that competition does not serve to maintain the patterns of distribution.

Facilitation of Soft‐Bottom Benthic Succession By Tube Builders

Controlled field experiments were used to test the effects of surface—deposit feeders on succession at the Skagit flats, an intertidal sandflat in northern Puget Sound. The tube builders Hobsonia florida (Polychaeta, Ampharetidae), Pseudopolydora kempi japonica (Polychaeta, Spionidae), and Lanais sp. (Crustacea, Peracarida) facilitate the recruitment of other taxa on 10—cm2 azoic patches. Simulated animal tubes facilitated the immigration of Tanais sp. and oligochaetes. Macoma balthica, a tellinid bivalve, facilitated the immigration of H. florida, while inhibiting that of Tanais sp. These experiments clearly documented that facilitation rather than inhibition is the dominant process governing succession in the skatole community. The facilitation model of succession offers a viable alternate explanation for many soft—bottom benthic processes previously explained by the inhibition model.

The Role of Density Dependence in a Marine Infaunal Community

Abundance data from a marine sand flat in the Pacific Northwest reveal a strong negative correlation between abundances of the spionids Pygospio elegans and Pseudopolydora kempi, and hence suggest that competition is occurring between these two deposit—feeding polychaetes. The data also suggest that the specific winner of a competitive encounter is determined by the initial densities of the two species. A density—dependence experiment was performed in the field to test this hypothesis, in which the density of each species was independently varied over four densities in experimental containers. Immigration, emigration, and mortality combined to cause Pygospio densities in all experimental treatments to converge to ambient density after 8 wk. After 8 wk, Pseudopolydora densities were converging on the ambient density; the densities of some experimental treatments were significantly different from each other, but in no case did an experimental density differ significantly from ambient. The rate of convergence was strongly density dependent in both species. In four different experiments, conducted for 1, 2, 4, and 8 wk, Pygospio survivorship was strongly affected by its own density but was weakly affected, if at all, Pseudopolydora density. Pseudopolydora survivorship was affected only by its own density except for the 1—wk experiment, in which Pygospio density also had a significant effect. The field results conflict with the results from a laboratory experiment where Pygospio rapidly migrated away from Pseudopolydora but not from conspecifics. I propose a spacing hypothesis which predicts that Pygospio relocates as often as necessary to minimize contact with Pseudopolydora. Static spatial data from the field provide some support for the hypothesis. In the laboratory, Pygospio underwent asexual reproduction only at low densities and in the absence of Pseudopolydora. This response is interpreted as an adaptation for the rapid utilization of resources.