Black seaslugs in reef tank

January 9, 2003
From: Brett Lynch

Dear Dr. Rudman,
I have a reef tank for almost ten years now and as long as i can remember I've had these black seaslugs. They grow to about an inch and have a white stripe down their back and can cover it up with their frilly black cape?. They have two prominent anttenae. They don't feed on my corals except a pink Pocillopora (sps) which they devoured in one night. Most of my corals are LPS'S and are from Bali, Indonesia area. The black slugs readily spawn. All 15 spawn at once right after the white lights goes out. These slugs have a cartilige hardness I can feel when I remove some due to their proliferation, weird, like squid bones?? They only come out at night and wait under rocks till the brightest lights go out. When I ask my dealer he explained that they where in the snail family, but I can't find any information on them. Also I have no algae problems since my guests have had a strong hold?
Please help,
Brett Lynch

Dear Brett,
From your excellent description I am certain your animals are a snail called Scutus If you have a look at the Fact Sheet you will see the white line you describe is in fact part of its flattened shell. I'm glad to hear that since you've had these beasts you have no algal problems, because if you look at the earlier messages on the page from other aquarium keepers, you will see I suggest that Scutus might help with unwanted algal film on the glass and rocks. I am a bit surprised by your comment about them eating a coral colony. Is it possible the colony was already dead and they were eating the algal film which would grow quite quickly on the skeleton?

I am interested in your comment on spawning. Does that mean you have seen them releasing eggs and sperm into the water? Is it a milky cloud? You'll see I have just recently explained how they spawn in answer to another message.
Best wishes,
Bill Rudman

Rudman, W.B., 2003 (Jan 9). Comment on Black seaslugs in reef tank by Brett Lynch. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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