Thorunna furtiva
Reef-crawling nudibranch logged across Luzon and Sabah reefs at 25.6-31.6C, seen year-round but always an uncommon find.
A small, elongated sea slug with a soft, unshelled body typical of chromodorid nudibranchs, usually spotted moving slowly across coral rubble or sponge-covered substrate. Look for the pair of feathery gills near the tail and the rhinophores (the small paired sensory horns on the head) as the two features that confirm it is a nudibranch rather than a flatworm. Exact size and colour pattern for this species are not well documented, so treat close-up photos as essential for a confident ID.
Divers typically find it parked on rubble, sponge or coral surfaces, grazing slowly and paying little attention to a torch or camera hovering close by. Like most nudibranchs it moves at its own unhurried pace across the reef, making it an easy subject for macro photography once spotted. Most sightings come from reef and slope dives rather than open water, reflecting its strictly bottom-dwelling habits.
Nudibranchs in the chromodorididae family, this one included, often absorb chemical defences from the sponges they feed on, which is why so many are boldly patterned despite having no shell to hide behind.
No clear seasonal pattern shows up in the sighting data, and with a comfortable range of 25.6 to 31.6C it has warm water on Philippine, Malaysian and Mariana reefs all year round. Treat it as a year-round find rather than something tied to a particular month.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged across Luzon in the Philippines, which accounts for the bulk of recorded sightings, with a further reliable presence in Sabah, Malaysia. Smaller but consistent numbers turn up around Saipan in the Northern Mariana Islands and in Sulawesi, Indonesia, so it is worth a slow look on muck and reef dives across all four countries.
Mapping where Nudibranch is logged…
Luzon in the Philippines has by far the strongest record of sightings, with Sabah in Malaysia also a reliable spot. Saipan in the Northern Mariana Islands and Sulawesi in Indonesia turn up smaller numbers of sightings too.
There is no detected season for this species, so it is considered a year-round sighting. Warm reef temperatures between roughly 25.6 and 31.6C are present in its range throughout the year.
Look for a small, soft-bodied slug crawling on rubble or sponge, with feathery gills near the tail and a pair of rhinophores on the head. Precise colour pattern is not well documented for this species, so a close macro photo is the best way to confirm identification.
No, this species poses no danger to divers. It has no means of stinging or biting and simply continues grazing when approached.
Exact size data is not recorded for this species, but chromodorid nudibranchs in general are small, typically a few centimetres long, and easy to overlook without a careful reef search.
Many chromodorid nudibranchs, including this species, feed on sponges and can retain the sponge's chemical compounds in their own tissue as a defence against predators, which is part of why they show up in such bright patterns.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Nudibranch
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