The Most Interesting Things about Mollusks!

Initially, I was attempting to find a kind of snail that looks like Gary from SpongeBob; while I was looking through various mollusk classes, I found an interesting oddity: swimming slugs, or Sea Angels.

Original observation “Naked Sea Butterfly (Clione limacina)” at by @tinekk, February 18, 2017. Accessed 5 February 2021.

This made me wonder: what other interesting things are there about mollusks? Thank you in advance for any interesting and helpful comments.

  1. There are flying squids – that’s pretty cool. They are like flying fish, in that they jump out of the water at speed and glide through the air for a while.

  2. Colosssal Squids are the largest invertebrates on Earth.

  3. Shelled mollusks have one of the best fossil records of any organisms, because the shells preserve so well.

  4. In terms of number of species, the class Gastropoda is secondly only to the class Insecta.

  5. Malacology (the study of mollusks) for most of its history was actually Conchology, the study of shells, and was pioneered and staffed entirely by amateurs.

  6. The most ancient jewelry ever found is strings of beads that consist of seasnail shells.

  7. There are some beaches that are composed entirely of seashells.

  8. On the coast of the northeastern US there are barrier islands. When you find shells on the beaches of those islands, much of what you find is fossil shells that go back as far as the ice age and the last warm interglacial period. But the fossil shells don’t really look much different at all from the recent ones.

  9. Some shells from the Red Sea were found in a shell collection that was uncovered during the excavation of Pompei, which is near Naples, Italy.

I will add more to this list as additional cool things occur to me.


Deep water octopuses are very cool, e.g. blanket octopus was an observation of the day on iNat. Also Spanish dancer nudibranch is must see!

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Reproductive love darts!


Cephalopod brains are wrapped around their esophagus. They have excellent vision and can see not just light but also polarization, and can manipulate both the color and polarization of their skin. I’ve heard rumors that their skin can match background patterns without input from their brains, using unknown mechanisms – but I don’t recall how solid that source is. Their brains are organized very differently from ours (much more decentralized) but they have electrical rhythms that are more like human brains than any other invertebrate species. They’re quite intelligent, but why isn’t known: They don’t live long and aren’t particularly social, despite those being two standard drivers of vertebrate intelligence.

There are also a bunch of cool looks and behaviors. Consider Vampyroteuthis infernalis – the “Vampire Squid from Hell”, the Dumbo octopus, and maybe my personal favorite, the mimic octopus. The legend is (and I’m making up details because my memory fails) a diver was photographing a seahorse, and when her flash went off, all of the sudden she was looking at a shark. Confused, she snapped another photo, and she was looking at a seahorse… Turned out it was an octopus that’s very very good at pretending to be something else.

Oh, man. There’s so much good stuff. There’s an underwater ‘city’ of octopuses. Cuttlefish are adorable. Sometimes male cuttlefish will sneak up to mate with a female cuttlefish being guarded by a mate, patterning their bodies like a courting male on the half facing the female – but like a innocent female on the half facing the rival male!


Oh yeah, speaking of cephalopods the hectocotylus was mistaken for a parasitic worm before they realized it was an amputated mating tentacle.


I always like to tell people about venomous cone snails. Conus geographus is possibly the most venomous animal on earth. Its venom has an LD50 of as low as 0.005 mg/kg (although studies seem to conflict a bit on that number). The most venomous snakes have around 0.02 mg/kg. There is no antivenin. Stings are rare, and there are only about 3 dozen documented deaths. What’s cool is that we can derive painkillers from these conotoxins. We’ve also discovered that insulin is one component of these conotoxins, and it has the potential to treat diabetes.

Banana slugs will also bite the penis off their partner during mating (more about that here).


Some species of female freshwater mussels have a flap of mantle that looks like a fish. They wiggle it around to attract a real fish thinking it’s going to get a meal. Instead it gets grabbed and a load of mussel larvae blown into its mouth or on its body where they attach to its gills or fins to continue their development.
Here’s a great example


Some limpets have iron teeth (denticles made of magnitite) strong enough to grind rock. Strong enough in fact, to be a contender for “nature’s strongest material”


That is an amazing write-up about the much beloved banana slug💕. I grew up with them, so it was really great to learn more about them. I knew they were hermaphrodites and valuable decomposers, but I knew little else. I did not know there were so many species. I thought they just came in different colors (yellow and mottled brown).


Oh, good reminder!

I can watch the cuttlefish, adorable squid, and octopus for hours at the aquarium. I become quite mesmerized by the strangely graceful movements of the squid and flickering colors of the pretty cuttlefish — and their beautiful eyes.
MBA - Tentacles

I do miss visiting aquariums, especially. Maybe, later this year things will open again.

Update: I found a squid video I particularly like. Look at those effortless sails and gorgeous eyes and try not to fall in love:
Be sure to scroll down to the video: “ Bigfin reef squid dazzle their would-be mates with wild colors and rhythmic gestures. It’s literally a performance of a lifetime, since – like many cephalopods – big fin reef squid mate only once.”

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The most ancient jewelry found to date is a 130,000 year old necklace made of eagle claws thought to be made by Neanderthals.

The snail shell necklace is 75,000 years old, but it’s thought that snail shells being used for ornamentation goes back around 100,000 years. At the time it was found this was the oldest jewelry to date, but it’s been superseded by more recent finds, such as the aforementioned eagle claw necklace.


Yes, please check out the Wikipedia article which I wrote a lot of:

And this Wikipedia video starring yours truly, which is especially suitable for upcoming Valentines Day:'s_a_Love_Dart%3F.webm


Some years back, I attended a lecture on early California indigenous artifacts by Mark Hylkema, California State Archeologist. He said, the oldest documented currency, is Ohlone shell money recovered from the Monterey Rd. Highway excavation in San Jose. It was carbon dated 12-15,000 years back. That’s earlier than the Fertile Crescent cultures or Egyptians. :flushed:

This shell money used to be housed in a tiny museum, which was never opened to the public, at a light rail stop, Tamien Station in San Jose. Last I heard, it was still there, sitting in the dark under the passenger platform.

Dang, I used to know the name of the shell the money was made from, but that escapes me at the moment. I will have to see if it percolates up through my memory or if I still have some old notes about it. Alas, I donated Hylkema’s book to a library some years back, so I cannot check the actual source.


I know the Chumash used purple dwarf olive shells (Olivella biplicata) for currency but that looks more recent.


Okinawa is one of my bucket list destinations because of the Starsand beaches (星砂の浜). The whole beach is made of tiny star-shaped shells of a protist called Baclogypsina sphaerulata (so not actually a mollusk, but still interesting. surprisingly, I don’t see any observations on iNat).


Biomineralization is a fascinating subject. Sticking to the topic of mollusks, there’s a deep-sea snail (Chrysomallon squamiferum) that clads itself in iron sulfides it extracts from hydrothermal vents. It incorporates the minerals into it’s shell and the scales covering it foot. It’s shell is actually a layered composite of different materials: There’s a calcium carbonate layer underneath, an organic protein inner layer, and the iron sulfide outer layers. It’s both highly resistant to abrasion and to crushing strain (say from crab claws).


Aren’t they smaller than the colossal squids? I think the giant squids are longer, but the colossal squid has a larger mass when fully grown.


That’s what I was trying to recall! It had the word Olivella, at least, I’m pretty sure.


On the ScubaBoard, there is a thread specifically for Nudibranch Lovers. These shell-less marine gastropods – with a rather Rococo quality to them, I think – seemingly come in as many different combinations of colors and frills as tropical birds and flowers.

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