Abstract
The Asian brush-clawed shore crab Hemigrapsus takanoi was introduced to the northern Wadden Sea (southeastern North Sea) in 2009 and now represents one of the most abundant brachyuran crab species. Abundance studies revealed an increase of mean crab densities on mixed reefs of native blue mussels (Mytilus edulis) and Pacific oysters (Magallana gigas) from 18 individuals m−2 in 2011 to 216 individuals m−2 in 2020. Despite its current high densities only little is known about the feeding habits of H. takanoi, its effects on prey populations and on the associated community in the newly invaded habitat. We summarize results of individual field and laboratory experiments that were conducted to assess feeding habits and consumption effects caused by Asian brush-clawed shore crabs and, additionally, compare the feeding ecology of H. takanoi with the one of the native shore crab Carcinus maenas. Field experiments manipulating crab densities revealed that both crab species affected the recruitment success of blue mussels, Pacific oysters and Australian barnacles (Austrominius modestus) with highest number of recruits at crab exclusion. However, endobenthic polychaetes within the reefs were differently affected. Only the native C. maenas caused a significant reduction in polychaete densities, whereas the introduced H. takanoi had no effect. Additional comparative laboratory studies revealed that single C. maenas consume more juvenile blue mussels than Asian brush-clawed shore crabs of the same size class. When offering amphipods as a mobile prey species, we found the same pattern with higher consumption rates by C. maenas than by H. takanoi. For Asian but not for native shore crabs, we detected a sex-dependent feeding behavior with male H. takanoi preferring blue mussels, while females consumed more amphipods. Considering mean crab densities and feeding behavior, our results suggest that despite lower consumption rates of single crabs, Asian brush-clawed shore crabs can cause stronger impacts on prey organisms than the native C. maenas, because H. takanoi exceeds their densities manifold. A strong impact of the invader on prey populations is supported by low amphipod occurrence at sites where H. takanoi density is high in the study area. Thus, the introduced Asian brush-clawed shore crab is an additional consumer with significant effects on the associated community of mixed reefs of mussels and oysters in the Wadden Sea.
Highlights
Worldwide, the introduction and spread of non-native species cause changes in diversity and species interactions in recipient ecosystems (Pyšek and Richardson 2010; Lowry et al 2013), and this is well known for coastal habitats (Ruiz et al 1997; Cohen and Carlton 1998; Gollasch 2006; Williams and Grosholz 2008)
We found a negative correlation between densities of H. takanoi and Gammarus spp. in the pooled samples of both study sites (n = 12, Spearman’s rank correlation: r = -0.828, p < 0.001)
The results of this study reveal a strong density increase of introduced Asian brush-clawed crabs in the northern Wadden Sea during the last decade, whereas densities of native C. maenas have decreased
Summary
The introduction and spread of non-native species cause changes in diversity and species interactions in recipient ecosystems (Pyšek and Richardson 2010; Lowry et al 2013), and this is well known for coastal habitats (Ruiz et al 1997; Cohen and Carlton 1998; Gollasch 2006; Williams and Grosholz 2008). Further examples are the Asian brush-clawed shore crab Hemigrapsus takanoi Asakura & Watanabe, 2005 and the Asian shore crab Hemigrapsus sanguineus (de Haan, 1835), which have been introduced to coastal areas of northern Europe in the 1990s, presumably by shipping (Gollasch 1999; Obert et al 2007; Landschoff et al 2013; Geburzi et al 2018) Both species originate from the northwestern Pacific Ocean, and in the southeastern North Sea, crabs have first been detected in 2004 in the southern and in 2009 in the northern part (Gollasch 1999; Gittenberger et al 2010; Obert et al 2007; Landschoff et al 2013). H. takanoi achieves highest abundances in natural epibenthic mixed reefs of native blue mussel (Mytilus edulis Linnaeus, 1758) and introduced Pacific oysters (Magallana gigas Thunberg, 1793), which is a preferred habitat for native shore crabs C. maenas (Kochmann et al 2008; Van den Brink et al 2012; Markert et al 2014)
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