Polypodium vulgare L.

原著和名: オホエゾデンダ科名: ウラボシ科 = Polypodiaceae採集地: 島根県 隠岐 島後 (隠岐 島後)採集日: 1975/11/22採集者: 萩庭丈壽整理番号: JH014154国立科学博物館整理番号: TNS-VS-96415

Impact of the invasive parasitic copepod <i>Mytilicola orientalis</i> on native blue mussels <i>Mytilus edulis</i> in the western European Wadden Sea

<p>Invasive species can indirectly affect native species by modifying parasite–host dynamics and disease occurrence. This scenario applies to European coastal waters where the invasive Pacific oyster (<i>Magallana gigas</i>) co-introduced the parasitic copepod <i>Mytilicola orientalis</i> that spills over to native blue mussels (<i>Mytilus edulis</i>) and other native bivalves. In this study, we investigated the impact of <i>M. orientalis</i> infections on blue mussels by conducting laboratory experiments using controlled infections with larval stages of the parasitic copepod. As the impact of infections is likely to depend on the mussels’ food availability, we also tested whether potential adverse effects of infection on mussels intensify under low food conditions. Blue mussels that were experimentally infected with juvenile <i>M. orientalis</i> had a significantly lower body condition (11–13%) compared with uninfected mussels after nine weeks of infection. However, naturally infected mussels from a mixed mussel and oyster bed did not significantly differ in body condition compared with uninfected mussels. Contrary to effects on mussel condition, we did not find an effect of experimental infections on clearance rates, shell growth or survival of blue mussels and no clear sign of exacerbating effects of food limitation. Our study illustrates that invasive species can indirectly affect native species via parasite co-introductions and parasite spillover. The results of this study call for the integration of such parasite-mediated indirect effects of invasions in impact assessments of invasive species.</p

Parasite body length (corrected for host size using linear regression) for both introduced <i>Mytilicola</i> species.

<p><b>Females (left) and males (right) of</b><i>M</i>. <i>intestinalis</i> (grey) and <i>M</i>. <i>orientalis</i> (white) in each surveyed region. The boxes represent the interquartile range, the whiskers denote the lowest and highest values within the 1.5 interquartile range, the black line in each box denotes the median, the large black dots represent the mean and the smaller dots outside the boxes are outliers.</p

Sampling locations of blue mussel (<i>Mytilus edulis</i>) and Pacific oyster (<i>Magallana</i> (previously <i>Crassostrea</i>) <i>gigas</i>) hosts.

<p>Left: The sampled regions in the Dutch Delta and the Wadden Sea (shaded area), with the islands Sylt (north) and Texel (south). Above right: Sampling locations around the islands of Sylt and Texel in the Wadden Sea. Below right: Sampling locations in the Dutch Delta. For exact coordinates see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193354#pone.0193354.s003" target="_blank">S1 Table</a>.</p

Discriminant distributions of <i>Mytilicola</i> for each sex separately.

<p>(A) Females (n = 92) and (B) males (n = 90) of the parasitic copepods <i>M</i>. <i>intestinalis</i> (grey) and <i>M</i>. <i>orientalis</i> (white). The x-axis is the discriminant score of the first discriminant function and the y-axis is the relative frequency of the observations. The light grey area indicates “mistaken” assignments in comparison with molecular identifications.</p

Sample sizes of introduced <i>Mytilicola</i> species in different sub-categories.

<p>Sample sizes of introduced <i>Mytilicola</i> species in different sub-categories.</p

Relationship between host shell length (mm) and parasite body length (μm) per sex for each <i>Mytilicola</i>-host species combination.

<p>(A) <i>M</i>. <i>orientalis</i> in blue mussels (<i>Mytilus edulis</i>). (B) <i>M</i>. <i>intestinalis</i> in blue mussels. (C) <i>M</i>. <i>orientalis</i> in Pacific oysters (<i>Magallana gigas</i>). Fitted lines are significant regressions.</p

Schematic representation (not to scale) of both sexes of both introduced <i>Mytilicola</i> species.

<p>On the left both sexes of <i>M</i>. <i>intestinalis</i> and on the right both sexes of <i>M</i>. <i>orientalis</i>, all viewed from the ventral side, with indications of the body size measurements that were taken. Note the dorsolateral thoracic protuberances which are folded inwards in <i>M</i>. <i>intestinalis</i> and extended outwards in <i>M</i>. <i>orientalis</i>. For detailed drawings see original species descriptions, including views from other sides and close-ups [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193354#pone.0193354.ref015" target="_blank">15</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193354#pone.0193354.ref023" target="_blank">23</a>]. Drawings with courtesy of Felipe Ribas.</p

Boxplots of <i>Mytilicola orientalis</i> body length (μm) in both host species.

<p>Female (left) and male (right) copepods originating from oysters (<i>Magallana gigas</i>) in grey and from mussel (<i>Mytilus edulis</i>) hosts in white, from the Dutch Delta and Wadden Sea. The boxes represent the interquartile range, the whiskers denote the lowest and highest values within the 1.5 interquartile range, the black line in each box denotes the median, the large black dots represent the mean and the smaller dots outside the boxes are outliers.</p

Principal component analysis of <i>Mytilicola</i> for each sex separately.

<p>(A) Females (n = 92; seven morphological variables). (B) Males (n = 90, six variables). In grey, individual parasites that were molecularly identified as <i>M</i>. <i>intestinalis</i>; in black, similarly identified <i>M</i>. <i>orientalis</i> (gds = length of the genital double-somite; acr = angle between caudal rami and anteroposterior axis).</p