Lionfish have been called the “Pirates of the Caribbean,” but “Ninjas of the Caribbean” might be a better nickname.

Lionfish are predatory fish, native to the Indian and Pacific regions, that have invaded the Atlantic and Caribbean.  One of the main problems with invasive lionfish is that they are eating so many native reef fish.

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Are lionfish such successful predators because they are invisible to prey?

A recent study published in PlosOne by O.M. Lonnstedt and M. I. McCormick (2013) investigated this very question.

Set-Up

Fish use both chemical and visual cues to sense their surroundings, similar to the way we use sight and smell. An important way fish use these cues is to identify a threat (like a predator) so that they can respond appropriately (hide, swim away, etc). Researchers wanted to see if prey fish responded differently to the chemical and visual cues of two common predators, compared to their responses to lionfish. They also tested if fish could “learn” to identify threats.

The prey fish were juveniles of a type of damselfish called the blue green chromis (Chromis viridis). When a damselfish is injured, a chemical is released that signals to other damselfish to display “anti-predator behaviors” such as reducing their activity or finding shelter. In this study, there were four groups of prey. Three groups were “taught” that each of the three predators (one group per predator) are threats by being exposed to the cues from the predators (visual and chemical) combined with the alarm-signaling chemical. These groups are referred to as the “experienced” prey. The remaining group was “inexperienced” and was not taught to identify any of the three predators as a threat.

The three predators used in the study were :

• the freckled hind, a common predator in the Indo-Pacific
• the zebra lionfish, a relative of the lionfish
• the lionfish, specifically P. volitans that has invaded the Caribbean.

Researchers observed how the prey (both experienced and inexperienced) behaved towards these three different predators, and also if the prey survived the encounter.