A research group from Okayama University has discovered that American crayfish frequently prey on the eggs and larvae of Japanese brown frogs, which are listed as endangered species in the Red Data Books of several prefectures.
Because the American crayfish is omnivorous and feeds on both animals and plants, it is known that if this species invades, establishes, and proliferates, it can have a serious negative impact on native organisms. There have also been confirmed cases of frogs and other amphibians experiencing population declines or local extinctions after the American crayfish invades and establishes, but the details of this mechanism were unknown.
Therefore, the research group conducted a laboratory experiment in which eggs and larvae of Japanese brown frogs were kept in the same tank as American crayfish for 24 hours. As a result, it was confirmed that the American crayfish exerted strong predatory pressure on the eggs and larvae of Japanese brown frogs.
In an experiment in which crayfish and eggs were placed together, 88.9% of the eggs were eaten, and in an experiment in which crayfish and larvae were placed together, 86.7% of the larvae were eaten. Even in an experiment in which aquatic plants were placed as hiding places for the larvae, 77.8% of the eggs were eaten by the American crayfish, even though the larvae were observed to use the plants as hiding places.
On the other hand, when comparing predation pressure due to differences in the body sizes of American crayfish, it was found that the presence of hiding places (aquatic plants) significantly increased the survival rate of larvae only for small-sized American crayfish. However, for medium- and large-sized crayfish, the presence of hiding places did not result in a statistically significant difference in survival rate, and it was confirmed that the larvae were easily found and preyed upon.
These results indicate that predation of eggs and larvae by crayfish is a threat to native frog species. To conserve rare frog species, it is urgent to prevent the invasion of crayfish and to take rapid extermination measures.