The structure responsible for locomotion in diatoms is the raphe (Option B).

Diatoms are unicellular algae with silica-based cell walls called frustules, and certain pennate species glide across substrates for movement.

Option Breakdown

• (A) Coste: Likely a misspelling of “costa,” which refers to thickened ridges or costae in the diatom frustule that reinforce structure but play no role in locomotion.​

• (B) Raphe: Correct answer. This elongated slit in the valve face secretes mucilage (adhesive polysaccharides), enabling gliding motility via actin-myosin interactions and capillary forces on solid surfaces.

• (C) Valve: One half of the frustule (epivalve or hypovalve) forming the box-like cell wall; provides protection and shape but not motility.​

• (D) Girdle: The connecting band (cingulum) between valves allowing slight expansion for cytokinesis; aids cell division but not active locomotion.​

The structure responsible for locomotion in diatoms is the raphe, a key feature driving their gliding motility across surfaces. Diatoms, vital phytoplankton contributing 20-50% of oceanic primary production, rely on this slit-like structure in pennate species for movement without flagella.

Diatom Frustule Anatomy

Diatoms have a unique two-part silica frustule: valves (flat faces) and girdle bands (sides). Pennate diatoms feature a central raphe canal, while centric types lack it and remain non-motile. The raphe secretes mucilage trails, adhering to substrates for propulsion via myosin-actin “highways.”

Raphe Mechanism in Locomotion

Raphe acts as a capillary slit where fluid ejection or mucilage adhesion generates thrust, achieving speeds up to 10-20 μm/s. Studies confirm it produces forces of 1-50 millidynes, essential for benthic diatoms navigating sediments.