To grow and survive or reproduce and die?: life-history strategies and ecological interactions between the mussels Mytilus edulis (Linnaeus, 1758) and Mytilus trossulus (Gould, 1850) in the Northwest Atlantic

Lowen, John Benjamin (2008) To grow and survive or reproduce and die?: life-history strategies and ecological interactions between the mussels Mytilus edulis (Linnaeus, 1758) and Mytilus trossulus (Gould, 1850) in the Northwest Atlantic. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The spatial and temporal distributions of two closely related blue mussel species (Mytilus edulis and M. trossulus) which co-exist within a mosaic hybrid zone in Newfoundland are not well understood. Where closely related species do not clearly differentiate along resource or physiological gradients, resource allocation to traits that enhance their ecological differentiation in fluctuating environments could stabilize their co-existence. Accordingly, a key objective of this study was to relate the plasticity of growth (which influences future survival), reproductive effort (which influences dispersal and colonization), and survival (linked to maintenance), among M. edulis, M. trossulus and their reciprocal F1 hybrids to variation in site productivity. The findings determined the design of a subsequent series of experiments, which focussed on predator inducible defenses and density dependent intra-specific and inter-specific interactions between M. edulis and M. trossulus. Reproductive cycles were generally similar among the parental species and F1 hybrids. Gamete output was significantly higher in M. trossulus than in M. edulis and the F1 hybrids. M. edulis prioritized allocation of resources to somatic traits (growth, shell thickness and adductor mass) to a greater extent than M trossulus, while M. trossulus prioritized allocation to reproduction (reproductive effort) to a greater degree than M. edulis. As in M. trossulus, but not in M. edulis, growth among sites in both hybrid groups did not change. Flexible allocation to reproduction among sites was evident in the F1 hybrids, but relative to M. trossulus overall reproductive effort was considerably lower. Hybrid edulis (M edulis female parent) survival was also lower than in both parental species, while hybrid trossulus (M. trossulus female parent) survival was not significantly different, although variation was greater in hybrid trossulus. These attributes likely reduce the establishment success of the F1 hybrid groups. In an experiment on predator induced defensive allocation, M. edulis also developed a thicker shell and heavier adductor muscle than M. trossulus in the presence of sea stars, and a thicker shell than M. trossulus in the presence of crabs. Sea stars and crabs also induced stronger byssal attachment in both species, albeit to a greater extent in M. edulis than M. trossulus. Compared with M trossulus, M. edulis growth decreased at a much greater rate with increasing density. Given that food availability (if limited) and/or space may increase with decreasing density, these findings provide additional evidence that M. edulis prioritizes allocation to somatic traits to a greater extent than M. trossulus. M. edulis also grew faster in the presence of M. trossulus than when grown with an equal number of M edulis only. Survival of M. edulis and M. trossulus was not affected by density, regardless of whether the mussels were maintained in the presence or absence of the other species. In conclusion, faster growth resulting from increased space availability or site productivity, or from the presence of M trossulus, together with enhanced defenses in the presence of predators, increases the resistance of M. edulis to biotic disturbance and therefore its likelihood of displacing M. trossulus. Conversely, M. trossulus invests more resources to reproduction relative to M. edulis, which likely increases its ability to re-colonize disturbed environments. Inter-specific differences in colonization or displacement abilities in disturbed environments reinforce stabilizing and equalizing mechanisms maintaining species co-existence. Resource allocation trade-offs and associated ecological differentiation in disturbed environments likely play an important and overlooked role in maintaining species diversity among assemblages of closely related interbreeding sessile species.

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