What does sea shells feed on ? This a question that many shell lovers have think about it at least once ! Even if we now have a lot of informations, we cannot clearly know all species feeding habits. Every year our knowledge increase on that question. Also, shells (just like us) may feed on something which is not usual for them because they do not have choice, and it's not their regular food…
Bivalvia shells have filter-feeding kind of feeding. Gasteropoda is a much more complex group. Some do feed on fixed animals (sponges), on algae, on grass, on other marine gasteropods, on worms, on fishes, on dead animals (necrophagous shells).
We can consider that Cowries feeding habits are mostly sponges (animals) and algae. Though some experiences have shown that some species (stercoraria cowrie) can feed on Bivalvia shells ( mussels). I personnally work on the feeding habits of the Mediterranean lurida cowrie. Even if they from time to time feed on algae fixed on the aquarium glasses, they clearly prefer the sponge named Verongia aerophoba. [READ THIS]
Example of feeding on animals : Cypraea Luria lurida feeding on the sponge Verongia aerophoba
Cone Shells are predators and their feeding habits are complex. We can separate them in 3 categories : Cone shell that feeds on other molluscs (molluscivorous), Cone shells feeding on worms (vermivorous), Cone shells feeding on fishes (piscivorous). It is easy to understand that Cone shells need venom to kill their prey. That is the reason why they are potentially dangerous for humans. And of course species that feeds on fishes have much more powerful venom that the others. Here is the reason why some species could kill a human with a single shot ! Lethal species like the well known Conus geographus have to be handled with care when live !
Example of a vermivorous cone shell : Conus regius from Martinique attacking Hermodice carunculata
On this video, you can see a Cone Shell attacking a clown fish. It's really fast and the fish is instantly immobilzed by the venomous sting. Just imagine what big species like Conus geographus or Conus striatus can do to human if not handled with care !
More videos can be seen [there]
Some shells have necrophagous feeding habits. We can name Olividae shells for example. You can catch this kind of shell by making a trap. Fill a container with dead fish, make some holes in order to let the smell difuse in the sea. Then burrow you trap in a sandy area (where Oliva shells occur) for several days (use weights in order to be sure that you trap will not be swapt avaw by current or wave action). Then come back and check around your container…
Muricidae feed on other molluscs (including Muricidae), they dig a hole in the calcareous protection of shells.