No abstract

A unique strain of the red alga Irish moss (Chondrus crispus) is found solely amongst clumps of blue mussels (Mytilus edulis) in a coastal lagoon in Atlantic Canada. Since about 2000, its bed area has shrunk by >99.9%, coinciding with the arrival of the non-indigenous green crab (Carcinus maenas). This study tested two mechanisms by which green crabs may harm the Irish moss. The hypothesis that green crabs directly consume the alga was tested by exposing fronds and clumps to crabs. Crab interaction with the clumps caused limited fragmentation, consumption was very small, and the condition (visible grazing damage) of fronds did not change significantly. A second hypothesis, that during predation and handling of mussels green crabs indirectly displace the seaweed and remove its attachment substrate, was addressed by placing crabs with Irish moss-mussel clumps containing either large or small mussels. Green crabs removed and ate up to 100% of the small mussels but did not consume or displace large mussels. This study concludes that direct consumption is not a plausible mechanism for green crabs to harm this strain. Instead, green crabs harm could be mediated by mussels, whose removal deprives the giant Irish moss of positional stability.

No abstract

In marine sedimentary bottoms, mussels and macroalgae have long been recognized as important autogenic engineers that create habitat and modify abiotic conditions. The structural complexity added by bivalves and macroalgae may also mediate intraguild predation amongst marine decapod crustaceans. While spatial distributions of these ecosystem engineers frequently overlap, there is limited understanding of compounded effects when more than one engineer is present. Here we demonstrate that the coexistence of two ecosystem engineers may create habitat valuable for the survival of a small native species, the Atlantic mud crab (Panopeus herbstii), in the presence of the invasive green crab (Carcinus maenas). Using laboratory and field habitat mimics, we measured mud crab survival rates as a proxy for refuge quality. We compared the refuge provided by a unique association between shells of blue mussels (Mytilus edulis) and the giant strain of Irish moss (Chondrus crispus) to that provided by bare substrate, and by each engineer alone. These experiments revealed that the association of giant Irish moss with blue mussel shells positively and non-additively increased mud crab survival compared to the other less complex habitat mimics. In contrast, parallel experiments revealed that high habitat complexity was less important for young green crabs to survive predation from large conspecifics. These results suggest that the impact of ecosystem engineers on trophic dynamics should be considered in a broader, whole-community context encompassing multiple habitat-forming species present.

In harsh habitats, community structure is believed to be controlled primarily by physical factors while biological interactions are considered to be of little relevance. However, evidence of avoidance behaviors between the life stages of some species suggests otherwise. A long-standing hypothesis from sandy beaches suggests that avoidance between adults and juveniles dictates some aspects of their ecology, but this has not been explicitly tested. Here, we used a field survey to document the diel activity of adults and juveniles of the talitrid amphipod Americorchestia longicornis. We then used laboratory experiments to assess the influence of adults on the activity of juveniles and to measure juvenile mortality rates in the presence and absence of adults, with and without food. In the field, we found clear differences in the diel activity of adults and juveniles. Adults were active throughout the night but not in daylight, whereas juvenile activity peaked at dusk and dawn. Similar patterns of activity were recorded in laboratory tanks that included both stages. However, when adults were excluded, juveniles shifted towards night activity, suggesting that the pattern of activity observed in the field is a mechanism to avoid larger conspecifics. In separate trials, predation by adults was a significant driver of juvenile mortality, regardless of initial juvenile density and food availability. Altogether, these results suggest that segregation between amphipod life stages is driven by the avoidance of direct interactions, specifically, cannibalism. This likely influences the role played by these amphipods, both as prey and as processors of stranded seaweeds.