The Maldivian sponge snail is another fascinating marine creature from the Maldives. I have seen dozens of them, and now I have spend hours on research for them, and yet I feel I have no idea what they are. These snails are for sure of of the most mysterious things that I wrote about. I’m keen to find out more.
Snails and slugs are one class of molluscs, and our sponge snail is one of approximately 70.000 known species of snails and slugs. Estimations are saying in the end there might be 4 times more species. Bivalves, the clams are with 20.000 species the second largest class of total 7 or 8 classes. Famous, but small is the class cephalopoda with 900 species of octopus, squids and such.
The other classes of molluscs are very interesting, and probably less popular. Chitons, tusk shells, worm-like molluscs, and molluscs with cap-like shells form the other four classes, with total 2000 species. They seem to preferably live in the deep sea, except for the chitons.
Snails and slugs in the class Gastropoda have plenty of species, marine, terrestrial and aquatic. A huge effort to sort the systematics out was done in ‘Taxonomy of the Gastropoda’ (Bouchet & Rocroi, 2005). Molecular phylogenetics is challenging a lot of the old school work done since the 17th century.
While I did my research on Coriocella hibyae I found a group of Belgian doctors being involved in snails…
Let’s begin with Wikipedia, which is always a good starting point for my research. There’s an article about the velvet snail, or Hiby’s Coriocella. It says the species was described by Wellens in 1991. This is the original description: Coriocella hibyae sp. n. a new Lamellaria species (Gastropoda: Prosobranchia) from the Republic of the Maldives. Journal of Conchology. 34: 73-80 1991
Wellens described in total 3 of the seven Coriocella species, and redescribed one more. He is an expert in Coriocella I want to say. In 1995 Wellens described C. jayi from Reunionand Mauritius, and in 1999 C. safagae from Egypt.
It’s always interesting to read about the person who discovered, and, or described the animal I’m writing about. But this time it was very hard. Species are often described decades or centuries ago, and you can find a lot of stuff about the scientist of these days. Now you would think someone working in 1991 would be even easier to find..
The Maldivian sponge snail was described after Wellens ‘wife and dive buddy’ Mrs. Hiby found them in December 1984 in the Maldives, Embudu island, in the South Malé Atoll. He named it after her, after examination of the female they collected, plus five more snails collected in 1976 around Baros island, North Malé Atoll. These two places are approximately 20km apart. In 1976 it wasn’t Wellens or Hiby who found the snails, it was M. Goldstein and J. van Goethem. I guess they stored the conserved sponge snails in the Royal Belgian Institute of Natural Sciences. I don’t know anything about Mrs. Hiby, but the other three Belgian gentlemen are not only active in Conchology, but also in human medicines. Conchology.be lists a stunning number of other ‘shellers’.
Mr. W. Wellens and I. Hiby are a bit of a mystery. I was able to find a photo of him, but even his first name was a secret to me for the longest time. His last publication about snails was in 2000, so maybe he was never very active in the world wide web. He worked in Belgium, and from the little I read I think he’s a diver. The fact that he is a doctor helped me to find his given name, but since he keeps it so secretive, I’ll keep it to myself here. I was hoping to find more information about the snails. In 1998 he redescribed C. nigra. In fact this publication has more illustrations, and some comparison of the four species ‘with five bosses’. They visited Egypt in 1992, and had 4 snorklers and 2 divers searching. It seems C. hibyae has the most prominent fingers.
Interesting is the distribution of the four mentioned species. You can draw a line from Egypt to Tonga. C. safagae is from the Red Sea, C. hibyae from the Indian Ocean, C. semperi from the Indo Pacific, and C. tonganus from the Pacific.
They found the Maldivian sponge snail, or velvet snail, at 1m depth. But he moved in troubled waters with his studies.
Snails belong to the second largest phylum of all, the molluscs. To me the largest phylum, the Arthropoda (insects, arachnids, crustaceans and so on) is less confusing than the molluscs. There are more than one million species of arthropods, and only 100 thousand (give or take) molluscs. The better known Cephalopods make only 900 of the 100.000 species.
Let me show you how difficult it is to navigate in a phylum with so many unknown members. C. hibyae is already the 4th gastropod on picmybug. Let’s see how closely related they are.
Nudibranches like Doto greenamari are in the subclass Heterobranchia, order Nudibranchia.
The Giant African land snail is in the same subclass, but order Stylommatophora.
The Monorail is also Heterobranchia, order Systellommatophora.
The Maldivian sponge snail however is in the subclass Caenogastropoda (like 60% of all Gastropoda), order Littorinimorpha. I couldn’t name any members of those groups. It’s all Greek to me.
Velutinidae is the family of our sponge snail called, and here are 293 species listed on WoRMS, the World registerof marine species. I’m afraid there’s no popular snail in the family which would help us to know where we are. Cowries and other owners of nice shells we might find at beaches are at least in the same order. You might only have heard of periwinkles or pelican’s foot shells when you happen to live near their habitat I guess.
But the genus Coriocella is not new. Coriocella nigra, the type species, was described in 1824 already, C. tongana in 1832. After that, 138 years no new species was described. Currently seven species are listed.
About the snail itself not much is known. I can only repeat the little other people wrote.
Wellens calls the coloration green in his papers, what makes me wonder if we saw the same thing. I would have said it’s black, with a faded dark grey center. The black areas have a blue shimmer when you shine your torch on it. But you could also see a grayish green I guess. Read ‘The science of color perception‘ in the Knowable Magazine for more information…
C. hibyae looks like a slug, but it isn’t. There is a thin shell under the surface, which looks more or less like a typical shell. There are five bosses, or fingers on the snails back, and I have no explanation why. Probably it’s for defense or camouflage? The snails do look like sponges (Porifera) in the water, a bit like the frog fishes. It’s hard to tell where the head is, because the tentacles are small and hard to spot. Four bosses are facing in a more or less 45 degree angle away from the body, and one is in the center facing straight up. The body is narrowing down from head to tail. That means the snail in my picture is climbing straight up.
It is mentioned in several sources that Coriocella feed on tunicates, and lays their eggs in the remains of it. I’m keen to see that in the water! I only saw them sitting doing nothing obvious so far. There is a photo of eggs ‘on black sponge’ from C. nigra on underwaterkwaj.com, so it’s maybe not only tunicates they use? Definitely more to find out about the ‘Belgian’ sponge snail…
According to iNaturalist you can find this species in the entire Indian Ocean, from Mozambique to Thailand. So it is not exclusively Maldivian. The other species are distributed over the rest of the world, and some show striking green or blue colors.
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