Bridget A. Holohan

Application of particle image velocimetry to the study of suspension feeding in marine invertebrates

Planar (2 dimensional) particle image velocimetry (PIV) was compared against the established clearance rate method of estimating bulk flow through suspension-feeding organisms to evaluate PIV as a technique to estimate feeding activity. The study of ciliary pumps has relied on advances in technology, as it is difficult to access the pump without excessively disturbing the animal. We used PIV to examine feeding velocities in five species of suspension-feeding, benthic invertebrates including one ascidian (Styela clava) and four bivalves (Mytilus edulis, Crassostrea virginica, Argopecten irradians and Mercenaria mercenaria). In calibration experiments using pipes of varying diameters, we were able to resolve flow with >98.5% accuracy. We extended the pipe model to study water processing in the mussel M. edulis. Both velocity and bulk flow through the mussel changed in predictable ways in relation to clearance rate (P < 0.05). Application of PIV to the study of suspension feeding will further our understanding of the behavior and physiology of economically- and ecologically-important organisms.

The influence of site and season on the gut and pallial fluid microbial communities of the eastern oyster, Crassostrea virginica (Bivalvia, Ostreidae): community-level physiological profiling and genetic structure

The microbial communities and overall health of the eastern oyster, Crassostrea virginica, have long been topics of interest due to the fundamental economic and ecological roles this species maintains. A broad scale characterization of the oyster microbiome over spatial and seasonal scales, however, has never been carried out. The primary goal of this study was to examine the factors mediating microbial communities of the gut and pallial fluid of C. virginica at three sites within the Long Island Sound estuary, with a focus on both genetic structure (T-RFLP) and physiological profiling (EcoPlates) of the microbiome. Results indicated that the genetic structure of microbial communities of oysters was minimally separated across sites, but was influenced by season. Although the microbial community structure was similar, the number of carbon sources utilized by these communities (richness) varied across site, season, and anatomical location within the host. Parameters including oyster condition index, Dermo disease, and ambient water temperature were measured to assess their influence on the oyster microbiome. Only water temperature was found to have a significant relationship with microbial community structure and richness. Results suggest that a core microbiome may exist within the eastern oyster, specifically for those populations that are not genetically distinct.

The diversity and biogeography of abundant and rare intertidal marine microeukaryotes explained by environment and dispersal limitation

Benthic microeukaryotes are key ecosystem drivers in marine sandy beaches, an important and dynamic environment; however, little is known about their diversity and biogeography on a large spatial scale. Here, we investigated the community composition and geographical distributions of benthic microeukaryotes using high-throughput sequencing of the 18S rRNA gene and quantified the contributions of environmental factors and spatial separation on the distribution patterns of both rare and abundant taxa. We collected 36 intertidal samples at 12 sandy beaches from four regions that spanned distances from 0.001 to 12,000 km. We found 12,890 operational taxonomic units (OTUs; 97% sequence identity level) including members of all eukaryotic super-groups and several phyla of uncertain position. Arthropoda and Diatomeae dominated the sequence reads in abundance, but Ciliophora and Discoba were the most diverse groups across all samples. About one-third of the OTUs could not be definitively classified at a similarity level of 80%, supporting the view that a large number of rare and minute marine species may have escaped previous characterization. We found generally similar geographical patterns for abundant and rare microeukaryotic sub-communities, and both showed a significant distance-decay similarity trend. Variation partitioning showed that both rare and abundant sub-communities exhibited a slightly stronger response to environmental factors than spatial (distance) factors. However, the abundant sub-community was strongly correlated with variations in spatial, environmental and sediment grain size factors (66% of variance explained), but the rare assemblage was not (16%). This suggests that different or more complex mechanisms generate and maintain diversity in the rare biosphere in this habitat.

Modulation of pumping rate by two species of marine bivalve molluscs in response to neurotransmitters: Comparison of in vitro and in vivo results.

Most studies regarding the neuroanatomy and neurophysiology of molluscan ctenidia have focused on isolated ctenidial tissue preparations. This study investigated how bivalve molluscs modulate their feeding rates by examining the effects of a variety of neurotransmitters, including serotonin, dopamine, and the dopamine agonist apomorphine on both isolated ctenidial tissue and in intact members of two commercially important bivalve species: the blue mussel, Mytilus edulis; and the bay scallop Argopecten irradians. In particular, we examined the effect of changes in: 1) beat of the lateral cilia (in vitro), 2) distance between ctenidial filaments and/or plicae (in vivo), and 3) diameter of the siphonal openings (in vivo) on alteration of bulk water flow through the mantle cavity. Important differences were found between isolated tissue and whole animals, and between species. Drugs that stimulated ciliary beat in vitro did not increase water processing rate in vivo. None of the treatments increased water flow through the mantle cavity of intact animals. Results suggest that A. irradians was primarily modulating lateral ciliary activity, while M. edulis appeared to have a number of ways to control water processing activity, signifying that the two species may have different compensatory and regulatory mechanisms controlling feeding activity.