What offers protection and an outer boundary for fish while swimming in aquatic environments?

The outer boundary and protection for fish while swimming in aquatic environments is provided primarily by the skin. Fish skin is specifically adapted to their aquatic habitat, serving not only as a barrier against physical abrasion and pathogens but also playing a crucial role in the regulation of osmosis and gas exchange.

The outer layer of fish skin is covered by a specialized structure called scales, which further enhances its protective function. Beneath the scales, the skin contains mucous cells, which produce mucus. This mucus layer reduces friction as the fish swims and provides an additional barrier against microorganisms and parasites. Furthermore, the mucus can aid in visibility within the water by refracting light.

Other options, while important to the fish's structure and function, do not primarily serve as the outer boundary. Bone and cartilage contribute to the internal skeleton and structural support, while mucus, although helpful, is not strong enough on its own to serve as the primary outer protective layer. Thus, the correct choice is skin, as it encompasses the essential protective features needed for the fish's survival in aquatic environments.