#indonesiascubadiving — Public Fediverse posts

Goniobranchus rufomaculatus: Nudibranch of Lembeh Strait

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Goniobranchus rufomaculatus: Nudibranch of Lembeh Strait

Marine Biology

Out on the open sand in Lembeh Strait, Indonesia, this nudibranch immediately stood out—not for its color, but for how fast it was moving. While nudibranchs are famous for being slow, this individual was covering the seabed at a noticeably brisk clip. It is Goniobranchus rufomaculatus: a small white nudibranch scattered with yellow dots, rimmed along its scalloped mantle edge with blue-violet spots.

Species Identification

Scientific Name Goniobranchus rufomaculatus (Pease, 1871) [1] Classification Dorid nudibranch; family Chromodorididae [1] Synonyms Chromodoris rufomaculata Pease, 1871 (basionym); Chromodoris histrio Bergh, 1877; Glossodoris rufomaculata (Pease, 1871) [1] Appearance Smooth white-to-cream mantle stippled with small yellow-orange dots; scalloped mantle margin ringed with discrete oblong blue-violet spots (not a continuous band); rhinophores and gill plume pale translucent white [2] Size Reported up to ~2.8 cm (1.1″) [6] Diet Sponges — a feeding pattern well documented across the wider Chromodorididae family [3]; direct dietary records specific to this species are limited Range Documented from the Society Islands (type locality), the Marshall Islands, Queensland (Australia), and the Philippines [1]. This post’s Lembeh Strait, Indonesia sighting is a firsthand field observation, not a formal range record. Conservation Status Not evaluated by the IUCN — typical for nudibranchs generally

Biology & Ecological Role

Across the wider Chromodorididae family, sponges are the primary food source, and several species are highly specific about which sponge they’ll eat.[3] Direct feeding records for G. rufomaculatus specifically weren’t found in the sources checked for this post, but the pattern is well established at the family level.

Many chromodorid species turn that sponge diet into a defense: they absorb chemical compounds from their prey and store them, largely unchanged, in their own tissue. In studies of related white-bodied Goniobranchus species, these defensive compounds tend to concentrate most heavily along the mantle rim.[4] Whether G. rufomaculatus follows this exact pattern hasn’t been directly tested — but if it does, the placement lines up neatly with the blue-violet-spotted edge that makes this species so recognizable, a predator’s first bite landing right where a toxin might be waiting.

Like other nudibranchs, this species is expected to be a simultaneous hermaphrodite — carrying both male and female reproductive organs. In mating nudibranchs generally, two individuals line up right side to right side and exchange sperm, so both partners go on to lay eggs, typically as a spiral ribbon of gel attached to the reef.

Field Notes & Observations

Watch the margin, not the body. The blue-violet spotting described in the original 1871 species description[2] runs along the scalloped mantle edge — the body itself is white with yellow dots, not blue.

Similar species exist, and they’re hard to separate from a single photo. White-spotted Goniobranchus is a genuinely unresolved group taxonomically — a 2024 molecular study found that G. rufomaculatus is not closely related to the similarly colored G. aureopurpureus despite the resemblance, and identified open questions about where G. rufomaculatus ends and G. albopustulosus begins.[5] If you’re trying to confirm an ID on a similar-looking animal, the full margin pattern, mantle texture, size, and location all matter — and a comparison photo or a second opinion goes a long way.

References

1. WoRMS / MolluscaBase — Goniobranchus rufomaculatus (Pease, 1871). marinespecies.org ↩
2. Pease, W.H. (1871). Original species description. American Journal of Conchology 7(1): 11–19. Biodiversity Heritage Library ↩
3. Rudman, W.B. & Bergquist, P.R. (2007). Feeding specificity in the sponge-feeding Chromodorididae. Molluscan Research 27(2): 60–88. ↩
4. Distribution of Defensive Metabolites in Nudibranch Molluscs. Journal of Chemical Ecology. link.springer.com ↩
5. Testing the Species Status of the Chromodorid Nudibranch Goniobranchus aureopurpureus (Mollusca: Gastropoda). researchgate.net ↩
6. Reeflex.net — hobbyist/aquarium reference, used for the size measurement only (not peer-reviewed). reeflex.net ↩

Indonesia Scuba Diving, Lembeh Strait, Marine Life – Creature Feature, Nudibranch, sea slug

Ceratosoma trilobatum Laying Its Egg Ribbon in Lembeh Strait

The Tri-lobed Ceratosoma, or Ribbon Nudibranch, is a visually captivating marine species found in the Indo-Pacific. This salmon-pink slug exhibits unique egg-laying behavior, depositing a coiled ribbon while moving across the substrate. Filming these behaviors in Lembeh Strait requires patience, as natural movement is essential to capture these rare moments.

https://scubahanknyc.com/2026/03/04/ceratosoma-trilobatum-laying-its-egg-ribbon-in-lembeh-strait/

The Mighty Crocodile Snake Eel

The crocodile snake eel burrows into the sand with only its eyes and snout showing. It looks more like a lurking myth than a real animal. I first met this unusual eel while diving near Siladen Island, Indonesia. It was on a sandy slope where the reef faded into open water. What seemed at first like a random bump in the sand slowly turned into a pair of watchful eyes and a crocodile-like head.

Crocodile snake eels (Brachysomophis crocodilinus) are classic ambush predators. They spend much of their time buried, waiting for fish or cephalopods to wander just a bit too close. For macro-loving divers and underwater photographers, finally spotting one feels like cracking a secret code in the sand.

Click on the photograph to enlarge it.

The crocodile snake eel belongs to the family Ophichthidae. This family includes worm and snake eels known for their elongated bodies. They also exhibit burrowing behaviour. Adults can reach impressive lengths, with males documented up to around 120 centimetres. Their common name comes from the crocodile-like shape of the head. The head has a long snout and strong jaws. Its eyes are positioned high on the head. The body is typically mottled or patterned to blend in with sand, rubble, or coral debris.

Rather than actively chasing prey, the eel waits in place until a small fish or octopus comes within striking distance. Then, with a rapid lunge, it grabs its meal and retreats back into the sand. For divers, the best approach is to move slowly over sandy patches. Divers should scan for eyes or a snout protruding from the seabed rather than looking for a fully swimming eel.

Thanks to the dive guides at Siladen Resort and Spa for pointing out the bright orange eel in the sand.

Related Posts

Are crocodile snake eels dangerous to scuba divers?

Crocodile snake eels are generally considered harmless to humans. They are ambush predators that focus on small fish and cephalopods rather than large animals. Most of the time they remain buried in the sand with only the head showing, and they prefer to stay hidden rather than confront divers. As with any wild animal, giving them space and avoiding physical contact is the best approach.

At what depths do crocodile snake eels usually live?

Scientific references place crocodile snake eels at depths from the intertidal zone down to around 30 metres, with many observations in very shallow water, roughly 0–2 metres. They tend to favour sandy or rubble areas near coral reefs where they can dig burrows and wait for passing prey. For divers, this means they can be encountered on shallow lagoon dives, sand slopes, and reef outskirts rather than only on deep drop-offs.

Where can you see crocodile snake eels in Indonesia?

Crocodile snake eels are part of a wider Indo-Pacific distribution that includes Indonesia, East Africa, the Society Islands, Japan, and Australia. In Indonesia, divers often report them on sandy slopes and lagoon areas near islands such as those around North Sulawesi, where sand and rubble zones sit beside healthy coral reefs. Slow, careful exploration of these habitats gives you the best chance of spotting their distinctive eyes and snout protruding from the sand.

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