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Tilburg, C.E., McCartney, M.A., Yund, P.O. (2012) Across-shelf transport of bivalve larvae:  Can the interface between a coastal current and inshore waters act as an ecological barrier to larval dispersal? PLoS One 7(11):e48960.


Using an integrated physical and biological approach, we examined across-shelf advection and exchange and the associated transport of bivalve larvae in the presence of a strong coastal current separated from the coast by a stratified inshore environment.  We tested the hypothesis that the interface of the coastal current and inshore waters can act as an ecological barrier to across-shelf transport of larvae but can be overcome by wind-induced transport or tidal currents. Our coastal study region in the Gulf of Maine encompasses a coastal current (the eastern Maine coastal current, or EMCC) that diverges from the coast as it moves downshelf.  The region inshore of the EMCC is home to several species that exhibit limited recruitment in spite of extensive upshelf larval sources.  Analysis of coastal surface water temperatures and wind velocities revealed episodic decreases in temperature along the coast correlated with alongshelf  (but not upwelling) winds, indicating wind-forced onshore movement of the cold coastal current. Such wind-driven onshore migrations are more common along the northern portion of the study region where the EMCC is near the coast, tidal currents are strong, and wind directions are more conducive to onshore migration but rarer further south where the interface between inshore waters and the EMCC is further offshore and suitable wind events are less common.  The distribution of bivalve larvae was consistent with the physical measurements.  There was little across-shelf variation in larval abundance where the EMCC abuts the coast (and episodic decreases in temperatures were frequent), indicating strong across-shelf exchange of larvae, but strong across-shelf variation in larval density where the stratified inshore waters separate the EMCC from the coast, indicating weak across-shelf transport of larvae. Our results suggest that the interface between the coastal current and inshore waters may constitute a major ecological barrier to larval dispersal in the southern part of the region that may only be overcome by rare, strong wind-forced events.

The eastern Maine coastal current (EMCC) is visible in a satellite image of sea surface temperatures for June 8, 2010, as the tongue of cold water extending southwest from the Grand Manan Channel.  The red ellipse indicates the region in which the EMCC diverges from shore, potentially forming an ecological barrier to the onshore dispersal of invertebrate larvae arriving from the northeast.  Our work focuses on the consequences of this divergence for connectivity among mussel populations.  Image courtesy of the Satellite Oceanography Data Laboratory at the University of Maine’s School of Marine Sciences.


Bloodsworth, K.H., Tilburg, C.E., Yund, P.O. (2015) Influence of a river plume on the distribution of brachyuran crab and mytilid bivalve larvae in Saco Bay, Maine. Estuaries and Coasts 38: 1951-1964.


Larvae of most coastal marine invertebrates develop through a series of pelagic stages in the inner shelf regions where they are subject to strong velocity fields, including
buoyancy-driven flows from river plumes. Taxon-specific larval behavior can interact with flow fields to determine dispersal trajectories. In this study, we examined the hydrodynamic features of the Saco River plume (in the southwestern Gulf of Maine) from July to August and explored how larval behavior may alter the distribution of mytilid (i.e., Mytilus and Modiolus) bivalve larvae and three genera of brachyuran (Carcinus, Hemigrapsus, and Cancer) crab larvae in and around that plume. Hydrographic surveys (via conductivity- temperature-depth casts) and larval sampling (via plankton tows) were conducted to assess temporal and spatial variation in the horizontal and vertical distribution of larvae. The horizontal extent of the Saco River plume varied little during our study and was governed by both inertial and rotational effects. Late stage mytilid larvae were relatively homogeneously distributed in and out of the plume, while the distribution of brachyuran larvae varied among different locations, species, and larval stages. We conclude that mytilid larvae entered the plume through physical entrainment and/or upward swimming processes and could tolerate salinities associated with the plume (<25). By contrast, brachyuran larvae avoided the plume via downward swimming to avoid osmotic stress, or had perished prior to sampling.


University of New England MS student Kylie Bloodsworth prepares to begin a plankton tow to sample mussel and crab larvae in and around the Saco River plume.


Kylie processing a plankton sample after collection.


Yund, P.O., Tilburg, C.E., McCartney, M.A. (2015) Across-shelf distribution of blue mussel larvae in the northern Gulf of Maine: consequences for population connectivity and a species range boundary. Royal Society Open Science 2: 150513.


Studies of population connectivity have largely focused on along-shelf, as opposed to across-shelf, processes. We hypothesized that a discontinuity in across-shelf mixing caused
by the divergence of the Eastern Maine Coastal Current (EMCC) from shore acts as an ecological barrier to the supply of mussel larvae to the coast. Existing data on the relative
abundance of two congeneric blue mussels, Mytilus edulis and M. trossulus, were analyzed to quantify the association of M. trossulus with the colder temperature signal of the EMCC and generate larval distribution predictions. We then sampled the across-shelf distribution of larvae along two transects during 2011. Larvae were identified using restriction digests of PCR amplicons from the mitochondrial 16S rDNA. Mytilus edulis larvae were consistently abundant on either the inshore and offshore transect ends, but not homogeneously distributed across the shelf, while M. trossulus larvae were less common throughout the study area. The divergence of the EMCC from shore appears to create a break in the connectivity of M. edulis populations by isolating those inshore of the EMCC from upstream larval sources. Across-shelf transport processes can thus produce connectivity patterns that would not be predicted solely on the basis of along-shelf processes.

Larval mussel sampling.  Goes with Yund et al., 2015b.  Undergraduate students pumping water through a plankton net to sample mussel larvae in Machias Bay.


The mussel crew processing a plankton sample on board the R/V Llyr.


Yund, P.O., McCartney, M.A. (2016) Family effects on the growth and survival of congeneric blue mussel larvae (Mytilus edulis and M. trossulus). Marine Biology 163:76. DOI 10.1007/s00227-016-2851-9.


Larvae of closely related bivalve species may grow or survive better under different environmental conditions, and such environmental effects may determine how species are distributed, the location of spat collection sites for aquaculture, and optimal hatchery rearing conditions. Although genetic variation within a species affects larval response to environmental conditions, intraspecific variation has been largely ignored when larval performance is compared between closely related species.  To examine this problem, we reared replicates of five different families of larval blue mussels (Mytilus edulis and its more northerly relative, M. trossulus) under three different temperature (10°, 13°, and 17°C) and two food conditions in all possible combinations.  Growth and survival were strongly affected by temperature and family, but also how family identity interacted with temperature, and the three-way interaction of family, temperature, and food.  The average number of days to 20% survival ranged from 10.2 to 20.0 among families and did not cluster by species.  The two M. trossulus families were intermediate in survival and ranked first and third for growth.  The temperature by family interaction effect for survival was very strong, and the two M. trossulus families responded very differently to temperature manipulations.  One M. trossulus family exhibited highest survival at 13° C, while the other M. trossulus family and all three M. edulis families exhibited highest survival at 10° C.  By contrast, greatest growth consistently occurred at 17° C in all families, indicating that higher mortality in warmer water may be at least partially offset by more rapid growth in body size.  Our results highlight the need to evaluate both growth and survival when assessing environmental effects on larvae and the need to ensure a high level of genetic diversity when pools of larvae are used to compare the performance of closely related species.


Morello, S.L., Yund, P.O. (2016) Response of competent blue mussel (Mytilus edulis) larvae to positive and negative settlement cues. Journal of Experimental Marine Biology and Ecology 480:8-16. DOI 10.1016/j.jembe.2016.03.019.


In nature, bivalve larvae are exposed to a wide variety of cues that can be used to select a settlement site.  These cues include mixtures of potentially positive and negative signals.  First, we used a simple test chamber to assess whether competent blue mussel (Mytilus edulis) larvae responded negatively or positively to scent cues from a variety of other intertidal species.  Next, we used choice experiments to test the responses of larvae offered a mixture of conflicting (positive and negative) cues from the same direction and conflicting cues from different directions.  Larval responses to individual cues were predictable based on known ecological interactions that occur after settlement.  Larvae were attracted to odors from adult mussels, tended to move towards odors from a filamentous alga, avoided odors from two predators of juvenile and adult mussels (a gastropod and a crab), and exhibited little response to odors from an herbivorous gastropod. Negative and positive cues offered from the same direction produced movement both toward and away from the mixture, while offering the combined cues from different directions resulted in net movement toward the positive cue.  Larvae presented with a choice between two negative cues mainly remained where they started, in the middle of the test chamber, and exhibited no choice.  These results highlight the need to consider the combined effects of a broad range of cues when evaluating the net effect of cues on settlement in the field.


VIDEO of mussel larval behavior (click here)

This video shows the outcome of one experimental trial.  A mussel larva starts in a central well that is exposed to two different cues from different directions (the two side channels).  It orients toward the positive cue in the right channel and starts swimming towards it.  As the larva gets closer to the source and encounters a higher concentration of cue, it speeds up and swims rapidly into the source well.


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