Predation on the egg of <i>Coelostoma stultum</i> (Coleoptera: Hydrophilidae) by the alien species <i>Deroceras laeve</i> (Mollusca: Gastropoda: Agriolimacidae)

Slugs are polyphagous herbivores that damage various crops. The genus <i>Deroceras</i> is primarily herbivorous; however, certain species are known to feed on various types of organic matter. In this study, we documented field observations of predation by <i>Deroceras</i> on eggs of the water scavenger beetle <i>Coelostoma stultum</i> at the waterside of paddy fields in central Japan and identified the predator species by DNA barcoding. Molecular phylogenetic analyses revealed that the slugs found sympatrically with <i>C. stultum</i> are <i>Deroceras laeve</i>, whose two haplotypes are widely distributed throughout the Palearctic and eastern margin of Asia. In two of three observed predation events, <i>D. laeve</i> thrust its head into the egg cocoon and preyed on eggs while <i>C. stultum</i> females were spinning. In the other event, no females spinning egg cocoons were observed. Due to the relatively large body size of the slug, it may push the female beetle away and prey on eggs. Considering the native distribution range of these two species, invasion of <i>D. laeve</i> has resulted in a new combination of predator–prey relationships in Japan between <i>D. laeve</i> and native <i>C. stultum</i>. It is a matter of concern that slugs may establish predatory pressures that threaten <i>C. stultum</i>.蛞蝓为多食性植食动物，可对多种农作物造成危害。裸壳蛞蝓属（Deroceras）物种多以植食为主，但部分类群已知可取食多种有机质。本研究记录了日本中部稻田岸带的野外观察结果：裸壳蛞蝓属（Deroceras）物种捕食水食腐甲虫（water scavenger beetle）蠢圆胸水龟甲（Coelostoma stultum）的卵，并通过DNA条形码（DNA barcoding）技术鉴定了捕食者物种。分子系统发育分析显示，与蠢圆胸水龟甲（Coelostoma stultum）同域分布的蛞蝓为光滑裸壳蛞蝓（Deroceras laeve），其两种单倍型广泛分布于古北界（Palearctic）及亚洲东部边缘区域。在3起观测到的捕食事件中，2起发生于蠢圆胸水龟甲雌虫正编织卵茧之时，光滑裸壳蛞蝓会将头部探入卵茧内取食卵粒；剩余1起事件中未观察到正在编织卵茧的雌虫。鉴于该蛞蝓体型相对较大，其可能推开水龟甲雌虫进而取食卵粒。结合二者的原生分布范围来看，光滑裸壳蛞蝓（Deroceras laeve）的入侵已在日本地区形成了其与本土蠢圆胸水龟甲（Coelostoma stultum）之间全新的捕食者-猎物关系组合。蛞蝓可能形成对蠢圆胸水龟甲具有威胁的捕食压力，该现象值得关注。