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.

Recent work on larval settlement cues has emphasized mechanisms by which larvae exploit individual, positive cues (cues that larvae move toward), often in complex flow fields. Yet in natural systems, larvae of habitat generalists probably respond to multiple settlement cues, including a mixture of positive and negative cues. First, a simple test chamber in which cue dispersal was dominated by diffusion was used to assess whether competent blue mussel (Mytilus edulis) larvae responded negatively or positively to cues from a variety of intertidal species. Second, choice experiments tested the responses of larvae offered a mixture of conflicting (positive and negative) cues from the same direction and conflicting cues from different directions. Responses to individual cues were predictable from established ecological interactions. Larvae were attracted to odors from conspecifics, tended to move toward odors from a filamentous alga, avoided odors from two predators of post-settlement mussels, 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 that was largely consistent with predictions from the individual cue responses. Larvae presented with a choice between two negative cues mainly remained where they started, in the middle of the test apparatus, 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.

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): 1-12.

Intraspecific genetic variation is widely recognized to affect how bivalve larvae respond to variation in environmental conditions, but has been largely ignored in comparisons of larval performance between closely related sibling species. Replicates of five different full- sib families of larval blue mussels (Mytilus edulis and its more northerly congener, M. trossulus) were reared under different temperature (10, 13, and 17 °C) and food conditions in a full factorial design. Growth and survival were strongly affected by temperature, family, the interaction of family and temperature, and the three-way interaction of family, temperature, and food. Family means for days to 20% survival ranged from 10.2 to 20.0 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. tros- sulus 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, indicating that higher mortality in warmer water may be at least partially offset (at a population level) by more rapid growth in body size. The results illustrate the importance of assaying both growth and survival when evaluating environmental effects on larvae and the need to ensure a high level of genetic diversity when pools of larvae are used to study genetically similar species.

Protopopescu, G.C., Beal, B.F.

2015.  Settlement response to various rope substrates in blue mussels (Mytilus edulis Linnaeus) in a hatchery setting.  Journal of Shellfish Research 34(2): 383-391.

A critical aspect of blue mussel (Mytilus edulis L.) aquaculture is industry dependence on a highly variable supply of wild seed. The objective of this study was to investigate responses of cultured pediveligers of blue mussels to different types of rope collectors. The study consisted of two trials in which competent larvae (approximately 5,000 per experimental tank—400 L) were exposed to rope collectors (polyethylene and polypropylene) exhibiting diverse structural features such as long loops, short loops, long filaments, short filaments, and smooth (Trial I—four rope types; Trial II—five rope types). In Trial I, rope segments (3 cm long) were placed at two different levels in four culture tanks (top and bottom of water column). In Trial II, segments were placed in the middle of the water column in six culture tanks. After 5 days, rope collectors were removed from experimental tanks and the number of settled larvae on each segment was counted. Rope collectors with the highest structural complexity/greatest surface area (long loops) elicited the strongest settlement response (highest densities) of mussel larvae, whereas those with the lowest complexity/least surface area (smooth) elicited the weakest response. Position within tank (top versus bottom; Trial I) had no significant effect on settlement density. Hatchery-reared mussel seed could be a reliable alternative to wild seed, and ropes with complex features should be used as larval collectors as they enhance settlement density which, in turn, could reduce production costs.

Yund, P. O., Tilburg, C. E., and Michael A. 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): 3-16.

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 analysed 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.

Bloodsworth, K.H., Tilburg, C.E. and 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: 1961-1964.

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.

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