Black-margined Nudibranch
Doriprismatica atromarginata
Faithful Sea Slug
Goniobranchus fidelis
About
This tiny sea slug carries three colours in tidy rings, like a warning label pinned to its back. It grows to no more than 4 cm, yet it moves across the reef in full view, unafraid of anything that might want to eat it.
Also known as: creamy goniobranchus, reliable chromodoris
- Length
- 4 cm
- Depth
- up to 30 m
How to tell it apart
Many small reef nudibranchs share a similar ringed pattern of colour bands. On this species, check for the narrow burgundy line marking the border between the creamy-white centre and the orange-red rim, paired with grey rhinophores and gills that are paler only at the tip.
Appearance
The faithful sea slug is a nudibranch, a soft-bodied sea slug without a shell. Its foot is cream-coloured and mostly hidden under a wide mantle, the fleshy sheet that covers its back. The centre of the mantle is creamy-white with scalloped edges. The whole outer margin is orange or red. Where the two colours meet there is a burgundy border. Its width varies from one animal to another. On its head sit a pair of rhinophores, small stalked sensory horns. They are ridged, greyish, and paler at the tip. The gills near the tail end are the same grey shade. They can be pulled in when the animal is disturbed.
Habitat
Look for it on outer reef areas, from the reef top down to around 30 m, especially where there are patches of dead coral. It ranges across the tropical Indo-West Pacific and into the Red Sea.
Did you know?
Most nudibranchs feed at night, but this one grazes by day, feeding almost entirely on a single food source, the purple sponge Aplysilla violacea.
Log it in Into the Blue
Describe what you saw and we name it, then keep every species you find in your Oceandex.
