Sarah A. Woodin

Refuges, Disturbance, and Community Structure: A Marine Soft-Bottom Example

Disturbance is a significant mortality source in many assemblages. The susceptibility of organisms to this mortality source is, in part, a function of the availability of substrate hetero- geneities that act as refuges from the disturbance process. There are at least 5 major categories of temporal and spatial refuges from disturbance: (1) temporal periods outside the activity range of the disturbance process; (2) temporal periods within the activity range of the disturbance process; (3) spatial zones beyond the activity range of the disturbance process; (4) physical heterogeneities within the activity range of the disturbance process; and (5) biologically generated refuges within the activity range of the disturbance process. The last category is particularly interesting because it involves an organisms utilization of a refuge which is the product of another organism or organisms. Data from a marine system are used to demonstrate the effectiveness of several types of refuges, particularly biologically generated refuges. The refuge-forming species is Diopatra cuprea, an onuphid polychaete which inhabits shallow water, medium-grained sand flats from Cape Cod to Florida. The abundance and species richness of other members of the infauna are shown to be positively associated with the presence of the tubes of Diopatra. This effect is confined to the area immediately surrounding the tubes of Diopatra. I dem- onstrated experimentally that a tube-like structure, such as a plastic straw, has the same effect on the infauna as does the tube of Diopatra. Thus, as predicted, the physical and biological refuges affect infaunal abundances similarly. They should not show similar patterns of distribution in space and time however and this is discussed.

An enzymatic globin from a marine worm

Some marine worms, such as Thelepus crispus and Notomastus lobatus, secrete brominated aromatic molecules and other halogenated metabolites as repellants. Other species, such as Amphitrite ornata, do not produce repellants but are adapted to the chemical warfare of N. lobatus and cohabit with them in estuarine mudflats. The halocompounds are tolerated by A. ornata as they are degraded by dehaloperoxidase (DHP). We have determined the amino-acid sequence and crystal structure of DHP and find that its fold is typical of the globin family, indicating that the enzyme evolved from an oxygen carrier protein. Residues at the dimer interface do not correspond to those in tetrameric and dimeric haemoglobins and the spatial arrangement of the dimers is different. The complete amino-acid sequence is most similar to that of myoglobin from the sea hare, with 20.6% identity among 126 overlapping amino acids.

Climate change, species distribution models, and physiological performance metrics: predicting when biogeographic models are likely to fail

Modeling the biogeographic consequences of climate change requires confidence in model predictions under novel conditions. However, models often fail when extended to new locales, and such instances have been used as evidence of a change in physiological tolerance, that is, a fundamental niche shift. We explore an alternative explanation and propose a method for predicting the likelihood of failure based on physiological performance curves and environmental variance in the original and new environments. We define the transient event margin (TEM) as the gap between energetic performance failure, defined as CTmax, and the upper lethal limit, defined as LTmax. If TEM is large relative to environmental fluctuations, models will likely fail in new locales. If TEM is small relative to environmental fluctuations, models are likely to be robust for new locales, even when mechanism is unknown. Using temperature, we predict when biogeographic models are likely to fail and illustrate this with a case study. We suggest that failure is predictable from an understanding of how climate drives nonlethal physiological responses, but for many species such data have not been collected. Successful biogeographic forecasting thus depends on understanding when the mechanisms limiting distribution of a species will differ among geographic regions, or at different times, resulting in realized niche shifts. TEM allows prediction of the likelihood of such model failure.

Occurrence of brominated compounds in soft-bottom benthic organisms☆

Abstract The abundant species of infaunal soft-bodied organisms from two geographically separated sites with very different predatory regimes were examined for the occurrence of volatile brominated compounds. Of the 10 species, three contained substantial amounts of brominated compounds while two others contained trace amounts. A greater proportion of species from the Western Atlantic site with large, sediment-disturbing predators and much lower infaunal densities contained these compounds. The occurrence of the compounds did not follow phylogenetic lineages or food sources. The toxicity of such compounds and potential for predator deterrence may be major determinants of their occurrence.

Allelochemical inhibition of recruitment in a sedimentary assemblage

Chemical signals affect recruitment of organisms in many habitats. Most of the described biogenic chemical moieties in marine environments elicit specific positive responses, for example, of predators to prey or of conspecific larvae to suitable habitats. However, organisms also release noxious chemicals that may elicit negative responses from neighboring members of the assemblage. Herein we measured the effect on recruitment of the release of such compounds (halogenated aromatics) into sediments. The common, sediment-dwelling, terebellid polychaeteThelepus crispus contains brominated aromatic metabolites and contaminates the sediments surrounding its tube with these compounds. Sediments so contaminated are actively rejected by recruitingNereis vexillosa (Nereidae: Polychaeta). Interestingly, many of these noxious biogenic compounds have low solubility in water and, therefore, potentially long residence times in sedimentary environments. The negative response of larvae to sediment contaminated with them is a novel, potentially common, and very important mechanism in which sediment-dwelling organisms release haloaromatic compounds and thus impose a recruitment filter on their community.

Process-specific cues for recruitment in sedimentary environments: Geochemical signals?

The most biologically and geochemically active marine sediments are characterized by steep chemical gradients within the top centimeters of sediment (Berner, 1980).Acommon feature of these environments is disruptions of surface sediments by both physical and biotic forces. Growth and mortality rates for new recruits are affected by many of these surface perturbations.At the same time, these disturbances also impose a discontinuity in concentration across the sediment-water interface, and accordingly, a change in surface chemistry. In this paper we present evidence that the cue used by juveniles to distinguish between recently disturbed and undisturbed surfaces may be disruption of geochemical gradients that are typical of nearshore benthic systems. New juveniles exposed to ammonium concentrations typical of disturbed surface sediments exhibit behaviors consistent with rejection of the habitat. Conversely, new juveniles placed onto sediments containing ammonium levels typical of undisturbed sure cial sediments rapidly initiate burrowing activity, a sign of ‘ ‘ acceptability.’ ’ We also present a numerical model, which assesses the dynamics of small-scale chemical shifts that accompany sediment disruption, to determine (a) what is the magnitude of surface chemistry changes associated with disturbance (i.e. what is the signal strength)? and (b) what are the spatial and temporal scales associated with the return to the undisturbed condition (‘ ‘ recovery’ ’ )? Model results show that the signal strength, and the return to ‘ ‘ acceptable’ ’ conditions, are strongly ine uenced by the initial gradient. Model predictions of the time required to ‘ ‘ recover’ ’ indicate that times to recovery are longer than the interval between disturbance events, but are of the same temporal scale (minutes to hours). Thus, our results suggest that the dynamics of sure cial gradients provide a strong signal over appropriate time scales that may reveal the intensity of disturbance and the likelihood of mortality for juveniles.As such, transport-reaction processes which govern porewater concentrations in sure cial sediments may also play a role in recruitment processes.

Process-Specific Recruitment Cues in Marine Sedimentary Systems

In marine sediments, many of the processes associated with high post-settlement mortality of infauna have similar effects on the sediment surface. In most cases the original sediment surface is either removed, buried, or mixed with subsurface sediment. The experiments reported here tested the ability of new juvenile infauna to discriminate between undisturbed and recently disturbed sediment surfaces (i.e., subsurface sediment exposed). Recently settled juveniles of two polychaete species (Nereis vexillosa and Arenicola cristata) and one bivalve species (Mercenaria mercenaria) were exposed to simulated erosional and mixing events as well as to fresh feces, burrow tailings, and feeding tracks. Where the disturbance buried or removed several millimeters of the sediment surface, the time to initiate burrowing or the percentage of individuals failing to burrow increased significantly over times and percentages for juveniles on undisturbed surfaces. In all cases the results were consistent with the hypothesis that new juveniles reject (or are significantly slower to burrow into) disturbed sediment surfaces, if the disturbance is less than several hours old. For example, 51% of nereid juveniles did not burrow when placed on subsurface sediments, whereas 100% burrowed into surface sediments; their average burrowing time on surface sediments was 29.3 s compared with 109.7 s on fecal mounds of arenicolid polychaetes or 106.1 s on burrow tailings of thalassinid crustaceans. Individuals that did not indicate acceptance of a sediment surface by burrowing were all rapidly eroded from the surface in the presence of flow. Erosion of nonburrowing individuals occurred within 90 s of initiation of flow. Burrowing individuals were not eroded. The decision as to the acceptability of a sediment was made within 30 s. These data imply that the new juveniles are utilizing cues associated with a process, the disturbance of surface sediments, in addition to the species-specific cues described elsewhere.

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

Effects of browsing predators activity changes in infauna following tissue loss

Effects of tissue loss on defecation and/or tube building are documented for three infaunal species of polychaete annelids, Abarenicola pacifica, Axiothella rubrocincta, and Spiophanes bombyx. Abarenicola and Axiothella feed head down and expose their tails while defecating; their tail tips were experimentally ablated. Spiophanes feeds on the sediment surface with its pair of tentacles and its head. One or both of its tentacles were experimentally removed. The tissues removed in the experiments are those often lost to browsing predators in field populations. Defecation frequency and amount were significantly reduced in the experimental individuals relative to controls in all three species. In Spiophanes tube building was also significantly reduced; in Axiothella it was not. These results indicate that rates of biogenic sediment modification can be strongly affected by tissue losses of infauna to browsing predators.

Amphitrite ornata, a marine worm, contains two dehaloperoxidase genes.

Abstract:Amphitrite ornata, a terebellid polychaete, inhabits marine environments that are contaminated by biogenically produced halometabolites. These halogenated organic compounds are toxic and quite diverse. To survive in this environment, A. ornata produces a novel dehaloperoxidase (DHP I) that detoxifies haloaromatic compounds. In this study we identified and characterized two dehaloperoxidase genes, designated dhpA and dhpB, from an A. ornata complementary DNA library. The deduced amino acid sequences (DHP A and DHP B) of the two dhp genes both contain 137 amino acid residues, but they differ at 5 amino acid positions. Allelic variation was observed for both genes as well. Polymerase chain reaction–restriction fragment length polymorphism assays of genomic DNA from 19 in individuals showed that each individual contains both the dhpA and the dhpB genes. Therefore, the two types of DHP are encoded by separate genes and are not alleles of a single gene. Furthermore, DHP A and DHP B may have different substrate specificities since they have amino acid differences in the active site.