In terms of fish anatomy, what aids significantly in their movement through water by reducing drag?

The effective movement of fish through water is significantly aided by mucus. Mucus covers the skin of many fish and serves multiple purposes, one of which is to reduce drag as the fish swims. This slippery, viscous layer allows water to glide more smoothly over the fish's body, minimizing resistance and enabling more efficient movement.

In contrast, fins are primarily responsible for propulsion and steering within the water. While they play an essential role in movement, their contribution to reducing drag is secondary to that of mucus. Scales, which protect the fish's body, can somewhat contribute to hydrodynamics but may not be as effective in drag reduction as the lubricating properties of mucus. Gills are vital for respiration and do not directly influence the fish's movement through water.

Together, these anatomical features allow fish to exhibit streamlined shapes and efficient swimming techniques, but mucus specifically enhances hydrodynamic efficiency by decreasing frictional forces with the surrounding water.