# Predation on scyphozoan polyps and selective hydrozoan nematocyst incorporation dynamics in an alien aeolid nudibranch

## TL;DR

A nudibranch species, Caloria militaris, preys on jellyfish polyps but does not use their stinging cells for defense, which is a new finding in nudibranch feeding behavior.

## Contribution

This is the first report of complete selectivity in nematocyst sequestration in a nudibranch with a functional cnidosac.

## Key findings

- Caloria militaris consumes various scyphozoan polyps and can survive on them for up to 258 days.
- Despite intense predation, C. militaris does not incorporate scyphozoan nematocysts into its cnidosac.
- Predation by C. militaris may help regulate jellyfish blooms in the eastern Mediterranean.

## Abstract

Aeolid nudibranchs prey on various cnidarians and incorporate nematocysts from their prey into the cnidosac, located at the tip of their cerata for self-defense. This study examined the predator–prey interactions between two nudibranch species, Caloria militaris and Flabellina affinis, and various cnidarians, focusing on scyphozoan polyps from common eastern Mediterranean medusae, including Aurelia sp. Cassiopea andromeda, Phyllorhiza punctata, and Rhopilema nomadica. An initial short-term experiment assessed predation by both nudibranch species, after which subsequent experiments focused exclusively on C. militaris. Short-term experiments for 24 h and long-term experiments lasting up to 258 days were conducted to determine whether C. militaris consumes a variety of cnidarians, and if it incorporates the prey nematocysts into its cnidosacs.

Laboratory experiments indicate that while F. affinis avoids scyphozoan polyps, C. militaris readily consumes all tested scyphozoan and hydrozoan species and can subsist on them for extended durations of up to 258 days. C. militaris predation rate, reaching 95 polyps day−1 increased with greater prey availability, but did not reach saturation even at very high prey abundances (> 80 polyps day−1), suggesting a higher predation capacity. Surprisingly, despite this intense predation, C. militaris did not incorporate any of the scyphozoan nematocysts in its cnidosac; only nematocysts from its known hydrozoan prey were identified in the cnidosac.

C. militaris is a generalist that preys on hydrozoa and can extensively feed on a variety of scyphozoan polyps, yet it does not incorporate scyphozoan nematocysts into its cnidosacs. This is the first report to demonstrate complete selectivity in nematocyst sequestration over an extended feeding period in a nudibranch possessing a functional cnidosac. These findings contribute to understanding nudibranch feeding ecology and the potential role these predators may play in regulating jellyfish blooms.

The online version contains supplementary material available at 10.1186/s12983-025-00589-9.

The nudibranch Caloria militaris preys on various scyphozoan polyp species and can subsist exclusively on them for extended periods.C. militaris feeds on scyphozoan polyps but does not sequester their nematocysts in its cnidosacs.Predation on scyphozoan polyps by C. militaris may reduce polyp populations and contribute to regulating jellyfish blooms in the eastern Mediterranean.

The nudibranch Caloria militaris preys on various scyphozoan polyp species and can subsist exclusively on them for extended periods.

C. militaris feeds on scyphozoan polyps but does not sequester their nematocysts in its cnidosacs.

Predation on scyphozoan polyps by C. militaris may reduce polyp populations and contribute to regulating jellyfish blooms in the eastern Mediterranean.

The online version contains supplementary material available at 10.1186/s12983-025-00589-9.