Madrella ferruginosa from the Marshall Islands

August 24, 2007
From: Scott Johnson

Hi Bill,

While reading Binyamin Koretz' recent message (#20270) on Madrella ferruginosa, I noticed that there are not yet too many records of it on the forum. The species occurs in the Marshall Islands, but for a long time I considered it quite rare. Until a few months ago, we'd found only two specimens in over 22 years of nudibranch hunting here. Recently, however, we started concentrating on the Halimeda algae habitat on our lagoon sandy slopes, and a fair number of rare species or new records for this area have been popping out. One of these is Madrella. For a couple of months, we could see as many as 10 specimens in an hour or so searching in the right places. Lately, though, they seem to have vanished. We have found none on our last few trips into the Halimeda. Curiously, we first saw egg masses just before most of the animals disappeared. (Egg mass photos are still being processed.)

Madrella is pretty easy to find when it is there. Running fingers through Halimeda clumps often causes them to fall out (although occasionally you end up with spines from small urchins in your fingers). Even if the nudibranchs do not dislodge from a Halimeda clump, disturbing them gives off quite an unmistakable cloud of yellow-orange fluid, as you note on the fact sheet. When you see that, a careful look through the algae always reveals one or more Madrella specimens.

Even though the animals have pretty much disappeared for now, it is probably temporary. Their food source, the dark-colored bryozoan under the animals in Jeanette's photos, grows abundantly within and around the bases of the Halimeda plants. The bryozoan, when damaged, gives off the same yellow-orange color as the nudibranchs.

Locality: Kwajalein Atoll, 8-12m, Marshall Islands, Pacific, 6 May 2007, Halimeda patch on sandy lagoon slope. Length: 14-18 mm. Photographer: Jeanette Johnson.

Scott Johnson

Johnson, S., 2007 (Aug 24) Madrella ferruginosa from the Marshall Islands. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Thanks Scott,

I must check on the name of this bryozoan. I used to know it but at the moment it escapes me. The small bright orange spheres on the bryozoan colony are small embryos or larvae, which float away to settle elsewhere. I woyld be interested to see the egg photos when they are processed as it would give us an idea of its development type. 

Best wishes,
Bill Rudman

Rudman, W.B., 2007 (Aug 24). Comment on Madrella ferruginosa from the Marshall Islands by Scott Johnson. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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