They are shells of marine snails of the family Trochidae (subclass Prosobranchia, class Gastropoda), having a spiral, conical shell. They are found from the intertidal zone to the deep open seas, with greatest diversity in shallow waters along rocky shores of the equator to high latitudes. Tropical top shells tend to be larger and more colourful than from other regions such as the Atlantic shores or the Pacific. All species are herbivorous, feeding on algae or films of spores on rock surfaces. Males and females are in separate individuals, and fertilization is external, with most species having free-swimming larva.
The interiors of all top shells are nacreous (Definition of Nacreous: consisting of or resembling mother-of-pearl). This has made them popular among humans usage. For example, the largest species, Trochus niloticus, was once extensively fished for its lustrous mother-of-pearl layer, which was used in the manufacture of pearl buttons.
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