Effects of European green crabs (Carcinus maenas) on transplanted Eelgrass (Zostera marina): potential protective measures

The global decline of seagrass causes concern regarding the loss of important ecosystem services, such as increased marine biodiversity. Consequently, restoration projects have gained popularity among scientists and local communities. However, for restoration to be successful, certain challenges must be considered and mitigated. One of the known threats against transplanted eelgrass (Zostera marina) is the European green crab (Carcinus maenas), due to grazing and uprooting of transplanted shoots and seeds. While several studies have shown the damaging effects of green crabs in areas where it is invasive, less is known about their impact on eelgrass within its native range in Europe. Here, we investigated four potential protective measures for eelgrass transplants against the European green crab: cages, biodegradable establishment structure elements (BESE elements), stone anchors, and mussel banks. The efficiency of each protective measure was observed over 11 days in indoor aquariums, in which meristem cuts and shoot uprooting were measured variables. The most efficient protective measure was the cage treatment, with an eelgrass survival of 90% after 11 days, compared to the unprotected control treatment which showed a survival of only 37%. M and BESE elements led to 60% and 57% survival after 11 days, respectively. Additionally, an investigation into the feeding behavior of the green crab revealed that seeds were the preferred part of the Eelgrass plant, with 48% being consumed after 6 days. While our laboratory experiments were small in scale, results highlight the potential for protective measures to mitigate disturbances from biological entities in Eelgrass restoration, thereby enhancing the success rate of Eelgrass transplantation attempts.