Grand Phyllodesmium

Overview

Phyllodesmium magnum is a large, specialized aeolid nudibranch known for its symbiotic relationship with photosynthetic zooxanthellae. It is widely distributed throughout the tropical and subtropical Indo-West Pacific, where it is typically found in association with its soft coral prey.

Appearance

This species is characterized by its elongate body and distinctive, flattened, curved cerata that often obscure the dorsum. The cerata contain branched digestive glands that house zooxanthellae, which can give the animal a mottled brown or sometimes lilac appearance depending on the concentration of these symbionts. While adults are generally translucent white or cream, juveniles may exhibit purple pigmentation.

Biology & behavior

Unlike many other aeolid nudibranchs, P. magnum lacks defensive cnidosacs for storing stinging cells. Instead, it employs a defensive strategy known as autotomy, where it can voluntarily cast off its cerata when threatened. These detached cerata produce a sticky secretion and wriggle vigorously, serving as a distraction to potential predators. The species has also been observed engaging in "trailing" behavior, where individuals follow one another in a head-to-tail fashion.

Feeding

Phyllodesmium magnum feeds exclusively on soft corals, particularly species within the family Alcyoniidae such as Sinularia and Sarcophyton. By consuming these corals, the nudibranch acquires zooxanthellae, which it retains in its tissues to derive secondary nutritional benefits from the byproducts of photosynthesis.