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Which taxonomic rank comes directly below genus?

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A nautilus mollusk with a spiral, brown-and-white striped shell and numerous white tentacles extending from its soft body, set against a dark background. — species A nautilus (*Nautilus*) floating in the water column of the ocean.

The taxonomic rank that comes directly below genus is species.12

While some classification systems include a subgenus between the genus and species levels, it is considered an intermediate or optional rank rather than one of the primary, obligatory ranks in the biological hierarchy. In the standard hierarchy used by zoologists and botanists, the sequence proceeds directly from genus to species.231