We’ve all dealt with toxic roommates. For palmate newts (Lissotriton helveticus) living in small ponds with invasive goldfish(Carassius auratus), it’s not that different from a toxic human roommate. Except, this toxic roommate may bite at you, influence your behaviors, or even eat your eggs, rather than just your labeled ham sandwich. 

 When dealing with invasive species, there are many factors that can impact just how dangerous they may be to an environment. This is why researchers wanted to better understand how invasive goldfish personalities may impact the behaviors of the palmate newts they live with. Specifically, They wanted to study how much foraging newts will do when living with different goldfish personalities.

Two Types of Newts by Harrison Solomon licensed under CC BY-NC-ND 4.0

An interesting quirk of palmate newts is that they have two life cycle options. They may mature in water(paedomorphic), or they may mature on land (metamorphic). Researchers wanted to see if there were any significant differences between how these two would react to goldfish, adding another layer to their study.

To study the goldfish-newt relationship, researchers set up 24 different tanks, each with 4 newts (two pedamorphic and two metamorphic) and one goldfish. They kept these animals together for several weeks. Every day, they put in frozen food, and surrounded it with a feeding cage that would prevent the fish from eating it. Goldfish differed in their behavior; some would continuously try to swim around the cage, and some bite at newts, while were more mellow. Goldfish did not change in terms of personalities, and fish that simply swam away from the  newts didn’t affect the newts’ behaviors much. However, aggressive fish greatly impacted the personalities of the newts.

A Table of Newts and Goldfish by Harrison Solomon licensed under CC BY-NC-ND 4.0

Like how pecking and trailing the cage were measures of goldfish aggression, researchers could measure the “boldness” of newts by measuring how likely they were to engage in the risky behavior of foraging while an aggressive fish is nearby. Researchers could estimate a newt’s boldness by the amount of time they chose to find food in parts of the cage where they would be exposed to the goldfish. 

Interestingly, researchers found that the paedomorphic newts forage much less in risky areas than the metamorphic newts, suggesting that the metamorphic newts’ average boldness may be higher, and perhaps be related to the “gap year” they spend on land. It could also be due to the fact that paedomorphic newts tend to be smaller, and so being bit may be a more traumatic experience. Boldness wasn’t just determined by morph ; researchers found that overall, when looking at individual newts, those that expressed bold behaviors were more likely to follow a continued pattern of boldness. While certain newts may be more willing to forage in risky areas, both types of newts reacted the same way to increased fish aggression – they both foraged considerably less as fish were more aggressive. Overtime, the newts did eventually get used to the fish, and slowly showed increases in the amount of foraging they did the longer that the goldfish were present. 

The study concluded that there is a risk of invasives not usually considered-their personality. If a particularly aggressive fish is less likely to get competition from newts, since it’s scaring them away, then highly aggressive fish may be more disruptive to ecosystems. It’s also important to know the difference in reaction to fish from the two types of newts; if an area has a large amount of metamorphic newts, it may be less at risk of being affected by invasive goldfish than paedomorphic newts. 

Overall, newts can be negatively affected by a bad roommate, same as humans, but the personality of both roommates determines how negative it becomes. One thing’s for sure though; these newts would definitely request a transfer.

Sources Refrenced

Laurane Winandy, Mathieu Denoël, The aggressive personality of an introduced fish affects foraging behavior in a polymorphic newt, Behavioral Ecology, Volume 26, Issue 6, November-December 2015, Pages 1528–1536, https://doi-org.libproxy.kenyon.edu/10.1093/beheco/arv101