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- Reeflex.net — hobbyist/aquarium reference, used for the size measurement only (not peer-reviewed).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