Small aquatic insects survive even when eaten — Resistance inside catfish mouths using legs

A research group led by Professor Shinji Sugiura, Graduate School of Agricultural Science at Kobe University, has revealed that small aquatic insects living in reservoirs can survive an attack by a predator catfish by resisting in their mouths and being spat out alive. This finding overturns the conventional belief that small, non-toxic insects are easily preyed upon once they enter a fish's mouth. The results were published in Scientific Reports.

It is generally believed that fish can easily prey on any target that fits in their mouth. On the other hand, small prey is less nutritious and therefore less likely to be actively targeted when large prey is abundant, and their small size makes them less likely to be detected. However, little attention has been paid to how small prey defend themselves once caught by fish.

The research group previously discovered an insect that can escape alive through a frog's anus after being swallowed, proving that some insects having been swallowed can survive by coming out from the predator's body.

In particular, the adult water scavenger beetle (Regimbartia attenuata), which inhabits rice paddies, is known to survive passage through the digestive tract of frogs, such as the pond frog (Pelophylax nigromaculatus), escaping alive through the anus. However, there has been little research on whether these insects can similarly survive predation by fish or what specific defense strategies Japanese aquatic insects use against fish.

In this study, adults of two species of whirligig beetles (Gyrinidae: Dineutus orientalis, Gyrinus japonicus), three species of diving beetles (Dytiscidae: Hydaticus bowringii, Hydaticus grammicus, Hyphydrus japonicus), and three species of water scavenger beetles (Hydrophilidae: Hydrochara affinis, Sternolophus rufipes, Regimbartia attenuata) were offered to catfish in laboratory aquarium tanks, and post-predation behavior was observed.

Observations were made by placing one catfish and one insect in an aquarium. Twenty individuals of each insect species and 17 catfish were used. Each insect individual was used for the experiment only once, while the same catfish individuals were used multiple times.

The results showed that all of the insects fed were taken into the mouth, of which 49% were digested and 51% were spat out of the mouth alive. The proportion of those digested (predation success rate) varied greatly from 20% to 90% depending on the insect species. It tended to be lower in Gyrinidae and Dytiscidae than in Hydrophilidae.

Adults of Gyrinidae and Dytiscidae are known to secrete chemical defense substances, and it is thought that they are more likely to be spat out by catfish than Hydrophilidae, which do not possess such secretory glands.

However, in all families, it was found that the smaller the species (such as G. japonicus, H. japonicus, and R. attenuata), the higher the proportion of those spat out alive. For catfish, which do not have teeth to grind prey, it is possible that small prey was difficult to control within the mouth and difficult to swallow. Particularly in the smallest species, R. attenuata, 6 out of 20 individuals (30%) were digested, and 14 individuals (70%) were spat out alive.

The time until being spat out was less than one second at the shortest, 76 minutes at the longest, and the median was 3.5 seconds.

Furthermore, to investigate the mechanism of being spat out, R. attenuata with their middle and hind legs removed were provided to catfish. The catfish digested 17 out of 20 individuals (85%), and those spat out were limited to only 3 individuals (15%).

Normally, R. attenuata swim skillfully using their middle and hind legs, but it is thought that when preyed upon by catfish, they resist by clinging to the inner walls of the mouth or returning to the inside of the mouth even when about to be swallowed and are ultimately spat out. On the other hand, in individuals with their legs removed, such resistance became difficult, making them easier to digest.

Note that all digested R. attenuata were discharged as carcasses as feces 26 to 166 hours later, and no individuals were confirmed to have escaped alive from the anus. In other words, it was shown that "escaping from the anus" for R. attenuata is a strategy specialized for frogs, and they use a different defense strategy against catfish: "resisting inside the mouth to force them to spit it out."

Journal Information
Publication: Scientific Reports
Title: Small prey fight back: post-capture defences shape prey-predator size relationships
DOI: 10.1038/s41598-026-39251-7