Recruitment Facilitation and Spatial Pattern Formation in Soft-Bottom Mussel Beds

Mussels (Mytilus edulis) build massive, spatially complex, biogenic structures that alter the biotic and abiotic environment and provide a variety of ecosystem services. Unlike rocky shores, where mussels can attach to the primary substrate, soft sediments are unsuitable for mussel attachment. We used a simple lattice model, field sampling, and field and laboratory experiments to examine facilitation of recruitment (i.e., preferential larval, juvenile, and adult attachment to mussel biogenic structure) and its role in the development of power-law spatial patterns observed in Maine, USA, soft-bottom mussel beds. The model demonstrated that recruitment facilitation produces power-law spatial structure similar to that in natural beds. Field results provided strong evidence for facilitation of recruitment to other mussels—they do not simply map onto a hard-substrate template of gravel and shell hash. Mussels were spatially decoupled from non-mussel hard substrates to which they can potentially recruit. Recent larval recruits were positively correlated with adult mussels, but not with other hard substrates. Mussels made byssal thread attachments to other mussels in much higher proportions than to other hard substrates. In a field experiment, mussel recruitment was highest to live mussels, followed by mussel shell hash and gravel, with almost no recruitment to muddy sand. In a laboratory experiment, evenly dispersed mussels rapidly self-organized into power-law clusters similar to those observed in nature. Collectively, the results indicate that facilitation of recruitment to existing mussels plays a major role in soft-bottom spatial pattern development. The interaction between large-scale resource availability (hard substrate) and local-scale recruitment facilitation may be responsible for creating complex power-law spatial structure in soft-bottom mussel beds

Dispersal Dynamics in a Wind-Driven Benthic System

Bedload and water column traps were used with simultaneous wind and water velocity measurements to study postlarval macrofaunal dispersal dynamics in Manukau Harbour, New Zealand. A 12-fold range in mean wind condition resulted in large differences in water flow (12-fold), sediment flux (285-fold), and trap collection of total number of individuals (95-fold), number of the dominant infaunal organism (84-fold for the bivalve Macomona liliana), and number of species (4-fold). There were very strong, positive relationships among wind condition, water velocity, sediment flux, and postlarval dispersal, especially in the bedload. Local density in the ambient sediment was not a good predictor of dispersal. Results indicate that postlarval dispersal may influence benthic abundance pat- terns over a range of spatial scales

Inhibition of Sunfish Feeding by Defensive Steroids from Aquatic Beetles: Structure-Activity Relationships

The vertebrate hormone deoxycorticosterone is the most commonly occurring component of defensive secretions from aquatic beetles in the family Dytiscidae. Deoxycorticosterone and the structurally related steroids pregn-4-en-20α:-ol-3-one and pregn-4-en-20β-ol-3-one were tested for their ability to inhibit feeding by bluegill sunfish, Lepomis macrochirus, in laboratory assays. Deoxycorticosterone at oral doses of 660μg (2 x 10−6 mol) per pellet caused 94% inhibition in the acceptance of artificial food pellets. At the same molar dosage, pregn-4-en-20α-ol-3-one inhibited food consumption by 58%, while its epimer, pregn-4-en-20β-ol-3-one, did not significantly inhibit feeding. These results indicate that specific stereochemical conditions must be satisfied for the pregnenes to be noxious toL. macrochirus and suggest the existence of a receptor-ligand interaction. The potency of the three steroids in assays of feeding inhibition contradicts earlier results based on toxicity and anesthetic assays in which fish were immersed in solutions of steroids

Mussels Matter: Postlarval Dispersal Dynamics Altered by a Spatially Complex Ecosystem Engineer

This study investigated postlarval dispersal of soft-bottom macrofauna at a spatially complex intertidal mudflat comprising patches of bare sediment and an ecosystem engineer, the mussel Mytilus edulis. At each of four sites in Guard Point Cove, Maine, USA, we took core samples and deployed bedload traps in bare sediment and mussel bed habitats to estimate ambient densities, rates of sediment flux, and several measures of postlarval dispersal. Univariate and multivariate nonmetric multidimensional scaling (nMDS) results showed few significant site effects and no habitat×site interactions. In contrast, there were numerous significant habitat effects. Compared to the bare sediment, the mussel bed habitat had: fewer species; higher ambient density and proportional abundance of the oligochaete Tubificoides benedeni (the dominant species in both habitats); lower ambient densities and proportional abundances of major taxa and the nonoligochaetes as a group; and higher sediment flux and relative (i.e., per capita) dispersal of nonoligochaetes. Macrofauna species dispersed in relative proportions that were different from those in the ambient assemblage. Per capita T. benedeni transport rates were low in mussel beds compared to those for nonoligochaetes, consistent with the view that beds represent favorable habitat for oligochaetes. The number of total macrofauna individuals trap−1 day−1 was negatively correlated with ambient density and positively correlated with sediment flux in both habitats, but these relationships were significant only in the mussel bed. The results indicate that altered transport rates of sediment and postlarvae are important mechanisms by which mussels act as ecosystem engineers to modify soft-bottom habitats. Differential transport rates caused by aggregations of mussels and other foundation species must be considered in explanations of spatial pattern in soft-bottom communities

Separation Anxiety: Mussels Self-Organize into Similar Power-Law Clusters Regardless of Predation Threat Cues

Mussels have myriad effects on population, community, and ecosystem processes. Their aggregation behavior is an inducible defense that links non-consumptive effects of predators to benthic spatial pattern formation. Aggregation increases intraspecific competition but can be beneficial due to lower perimeter-related predation and other risks. Mytilus edulis aggregation responses to predation threats have not been investigated outside of Europe. We studied the effects of chemical cues from heterospecifics (predators Carcinus maenas, Nucella lapillus; herbivore Littorina littorea) and conspecifics (injured and intact M. edulis) on M. edulis aggregation behavior in Maine, USA. Mussels self-organized into fractal power-law spatial patterns like those in the field. Aggregations had lower perimeter:area (P:A) ratios than singletons, despite having more complex, irregular shapes with higher fractal dimensions (D). However, with one exception, no significant differences in aggregation rate, P:A ratio, and D were observed for any chemical cue treatment when compared to no-cue controls. Our experiment revealed higher aggregation rates than reported from similar experiments, leaving little scope for additional aggregation when exposed to chemical cues. We suggest that increased aggregation in response to predation threat is context-dependent: costs outweigh benefits beyond some optimal aggregation size, and mussels in our experiment were at the upper aggregation limit beyond which more aggregation could have negative consequences. Bet-hedging with a power-law distribution of aggregation shapes and sizes may be the optimal spatial strategy, especially if predation and other risks are variable in space and time

Fine-grained spazial genetic structure in the bivalve Gemma gemma from Maine and Virginia (USA), as revealed by Inter-Simple Sequence Repeat markers

Gemma gemma is a small ovoviviparous bivalve distributed in shallow sand flats along the North American Atlantic and Gulf of Mexico coasts. Genetic variation in G. gemma was analysed by means of Inter-Simple Sequence Repeats (ISSRs) at the following levels: (i) between localities (Maine and Virginia), (ii) among 10-mw-diameter patches within localities, and (iii) within patches. Thirty individuals/patch and three patches/locality were analysed. Individuals were genotyped for 67 ISSR polymorphic loci from five primers. The portion of the genetic variation found between localities (2%) was small compared to that found either among patches within localities (37%) or within patches (61%). ISSRs in Q gemma allowed the detection of significant differentiation at individual and patch levels. By contrast. a low degree of genetic variability was found between localities. The small-scale genetic heterogeneity does not follow a simple. consistent pattern. Our results contrast with the generally accepted rule that aplanic species are locally homogeneous and globally heterogeneous and teleplanic species are the inverse

Supplement 1. Recruitment facilitation model with source code and executable file for readers to use.

<h2>File List</h2><div> <p><a href="Mussel_Recruitment_Model_description_and_source_code_June_2014.txt">Mussel_Recruitment_Model_description_and_source_code_June_2014.txt</a> (MD5: e29281a06751245eda695b7b678b2243)</p> <p><a href="mussel_recruitment_model.exe">mussel_recruitment_model.exe</a> (MD5: 0eee8f1d904aca658f2accbe8b13d636)</p> </div><h2>Description</h2><div> <p>The Recruitment Facilitation Model allows the user to choose from four distinctly paired rules called "Edge" (recruitment facilitation at patch edge), "No Edge" (recruitment at any location), "Growth" (recruitment with bed growth), and "No Growth" (recruitment without bed growth), creating four possible scenarios. If desired, each of the four rules may be applied to a mussel bed where the user can spatially structure the probability of recruitment success, creating four additional scenarios.</p> </div