Critically endangered dragonflies, Libellula angelina inhabit in Okegaya marsh in Iwata, west side of Shizuoka. Their population had decreased sharply since 1990s because of the explosive increase in population of red swamp crayfish (Procambarus clarkii, hereafter referred to as crayfish). Crayfish not only prey on dragonfly larvae but also cut and feed macrophytes that serve as larvae’s habitats and hiding places. We investigated the effects of crayfish on macrophytes and tested whether plants less affected by crayfish could provide safe habitats for dragonflies.
  
[Experiment 1] Three types of macrophytes—arrowhead (Sagittaria trifolia), threeleaf arrowhead (Sagittaria trifolia ‘Caerulea’), and Vallisneria natans —were each grown at different coverages, and crayfish were released. In the control groups, only macrophytes were planted. A total of 12 experimental plots with different species and coverages ofmacrophytes, and body size categories of female crayfish were set up. The number of intact stems and leaves, cut fragments, eaten stems and leaves were recorded.
[Experiment 2] In all experimental plots, larvae of the genus Sympetrum were tested. A plot with threeleaf arrowhead and one without macrophytes were established, and male crayfish were released according to body length in two size categories. A total of five different experimental plots were set up. In addition to survey items from Experiment 1, the survival days of the larvae were recorded.

[Experiment 1] Vallisneria natans were the most affected by crayfish, with entire leaves being cut and consumed in some plots. As a submerged plant, whole shoots were submerged in water, making them more susceptible to being cut or preyed by crayfish. Threeleaf arrowhead were the least affected among the macrophytes tested. They are emergent plants with thick stems, and only a part of its shoots is submerged in water, making it more resistant against crayfish damage.  
[Experiment 2] In the control group, the emergences of Sympetrum larvae were confirmed, indicating threeleaf arrowhead can function as suitable emergence sites. In the presence of crayfish, the survival days of the larvae decreased regardless of whether threeleaf arrowhead were present, suggesting they do not provide safe habitats for dragonfly larvae. A positive correlation was found between the body size of crayfish and the number of cuts damaged plants, indicating larger crayfish with bigger claws affect the plants easily. It will be necessary to identify other macrophytes that are less susceptible to crayfish and can provide safe habitats for dragonfly larvae.