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Solar-Powered Nudibranch

Phyllodesmium longicirrum Bergh, 1905

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The Largest Solar-Powered Nudibranch

Phyllodesmium longicirrum (Bergh, 1905) is the largest aeolid nudibranch in the Indo-Pacific. It grows up to 140 mm. Divers call it the Solar-Powered Nudibranch. It belongs to the order Nudibranchia, suborder Aeolidina, family Myrrhinidae. Furthermore, zooxanthellae live inside its cerata. These algae photosynthesise sunlight and produce energy directly for the animal. As a result, it can go long periods without feeding on its prey coral. Find observations on iNaturalist.

Giant Solar Paddles

The cerata are large and flat — like solar panels. They maximise surface area for photosynthesis. In contrast to most aeolid nudibranchs, this species carries no cnidosacs. Instead, it stores diterpene chemicals from its prey as defence. It feeds on Sarcophyton trocheliophorum soft coral. Its close relatives Phyllodesmium colemani, Phyllodesmium jakobsenae and Phyllodesmium koehleri all appear in our Marine Life Encyclopedia. Together, these four species show the remarkable diversity of solar-powered nudibranchs at Romblon.

Finding It at Sunset Cove

At Sunset Cove Beach & Dive Resort, our guides search Sarcophyton soft coral colonies on sandy rubble at 5 to 30 metres depth. The animal is large and visible from a distance. However, it blends well against the coral. Therefore, check the base and underside of colonies carefully. For underwater macro photographers in Romblon, the enormous flat cerata create outstanding images. Consequently, this species ranks among the most impressive nudibranch photography subjects at our dive sites.

Taxonomy & Classification

Order	Nudibranchs (Nudibranchia)
Suborder	Aeolidina (Aeolid Nudibranchs)
Family	Myrrhinidae
Subfamily	Phyllodesmiinae
Genus	Phyllodesmium
WoRMS AphiaID	548881
iNaturalist	358202

Originally described as Myrrhine longicirra by Bergh (1905) from the Siboga Expedition. Family currently listed as Myrrhinidae (WoRMS) — also placed in Facelinidae in older literature. The genus Phyllodesmium is the most advanced solar-powered nudibranch group, with P. longicirrum being the most extreme example of zooxanthellae symbiosis — capable of surviving extended periods without feeding by relying on photosynthetic energy production by its internal algae.

Morphology

Size	60–140 mm
Colour Variations	Body translucent white to pale cream. Cerata large, flattened, leaf-like — evolved into "solar paddles" to maximise surface area for photosynthesis. Cerata typically translucent to brownish due to zooxanthellae colonies living inside. Rhinophores smooth, moderately long. Oral tentacles slender. Juveniles appear more translucent and less developed in cerata structure.
Identifying Features	The largest aeolid nudibranch in the Indo-Pacific — up to 140mm. Instantly recognisable by its enormous flattened leaf-like cerata which function as solar panels. Translucent body with brownish zooxanthellae visible through cerata walls. Always found on or near its prey Sarcophyton soft coral. Unlike most aeolids, lacks cnidosacs — defence relies on diterpene chemicals sequestered from its coral prey. The sheer size and distinctive cerata shape make misidentification very unlikely.

Largest known aeolid nudibranch. Body elongate, flattened. Cerata arranged in distinct rows, highly modified into large flattened paddle-like structures that house zooxanthellae. Rhinophores smooth (not lamellate). Oral tentacles long and slender. Lacks cnidosacs — the defensive nematocyst sacs found in most aeolids — instead storing diterpene chemicals from Sarcophyton coral as chemical defence.

Habitat & Ecology

Substrate	coral, rubble
Depth Range	5–30 m (most common: 12 m)
Temperature	24–29°C
Primary Prey / Host	Sarcophyton trocheliophorum (soft coral, family Alcyoniidae)

Activity & Behaviour

Active Time	day, night
Seasonal Presence	Year-round — most reliably found near Sarcophyton soft coral colonies
Egg Ribbon	White, coiled ribbon
Spawning Season	Not yet documented at SSC
Chemical Defence	Present
Aposematic Colouring	Absent
Camouflage Strategy	Zooxanthellae housed in cerata provide brownish colouration that blends with Sarcophyton soft coral colonies. Body transparency also aids concealment.
Escape Response	When threatened, drops cerata which continue to writhe (autotomy). Relies primarily on chemical deterrents — diterpene compounds sequestered from Sarcophyton coral prey.

Scientific Literature

Further studies on the taxonomy and biology of the octocoral-feeding genus Phyllodesmium Ehrenberg, 1831 (Nudibranchia: Aeolidoidea)
Rudman, W.B. (1991). Journal of Molluscan Studies 57(2): 167–203

Literature for divers

Romblon Field Data

One of the most rewarding macro subjects in Romblon. Look for Sarcophyton soft coral and search the base and underside of colonies carefully. Can be large enough to be visible from distance but surprisingly easy to miss when still. Photograph cerata in detail to show zooxanthellae colouration. Wide angle shots showing the full animal beside coral are highly effective. Active during both day and night.

Frequency	rare
First Sighting	2019-01-01
Max Individuals	3