Settling For less: How Dasysiphonia japonica, an invasive alga, impacts the early life stages of Mytilus edulis

Background/Question/Methods

The blue mussel, Mytilus edulis L., is an ecologically and economically important species that enhances biodiversity through both the modification and stabilization of its habitat. Attributed to warming and predation, M. edulis is disappearing from their native range in the Gulf of Maine (GOM). However, a documented 16% annual decline in recruitment indicates a notable decrease during early life stages. Interactions with invasive algal species may be an impetus for larval M. edulis loss, but such interactions are poorly characterized. M. edulis recruits initially on filamentous macroalgae (primary settlement phase), before recruiting to mussel beds (secondary). M. edulis are known to occur in the interstitial space provided among branches of Dasysiphonia japonica, an invasive alga that has spread throughout the GOM since 2010. We conducted an in situ settlement choice experiment and a growth experiment in Nahant, MA to quantify the relationship and effect that D. japonica has on M. edulis growth, settlement, and survivorship using Vertebrata lanosa and furnace filter pads as a reference. We hypothesized that D. japonica has a direct negative effect on the growth and survivorship of M. edulis that settle within it. This study contributes to understanding invasive- native species interactions in changing ecosystems.

Results/Conclusions

Unexpectedly, M. edulis did not settle within D. japonica more so than other filamentous algal species, nor was survivorship decreased when settled within these spaces. As expected, growth was slower in M. edulis settled within D. japonica when compared to those settled within V. lanosa and furnace filter pads over the five-week period. M.edulis recruits on D. japonica grew slowly over 5 weeks increasing 0.193 (±0.004) to 0.196 (±0.004) mm. M. edulis recruits seeded within furnace filter pads increased 0.232 (±0.007) to 0.246 (±0.01), recruits seeded within V. lanosa increased 0.212 (±0.006) to 0.216 (±0.0098). As D. japonica is able to form dense aggregations and can be extremely abundant on subtidal rocky reefs, cohorts of M. edulis larvae may increasingly use it as a settlement substrate. This results in slowed growth rates among larger fractions of cohorts of mussel recruits leaving them vulnerable for longer periods to sources of size-dependent mortality, such as predation. Smaller sizes within early recruitment periods of mussel beds may have a lasting effect on the population sizes of established mussel beds within the native region of M.edulis.