Why are insects uncommon in the open ocean?

Insects are primarily terrestrial and freshwater organisms, and their absence from the open ocean is largely due to their evolutionary specialization for life on land. While insects have successfully colonized diverse environments, the sea has remained the domain of crustaceans, which occupy the ecological niches that insects fill in terrestrial and freshwater habitats.12

A primary factor limiting insects is their respiratory system, which relies on a network of tracheae that deliver oxygen directly to tissues. This system is fundamentally designed for air-breathing. Although some insects have evolved specialized adaptations—such as bubble breathing or the use of a plastron—to survive in freshwater, these methods are generally insufficient for the challenges of the open ocean. Furthermore, the tracheal system imposes a physical size limit on insects, as gas diffusion becomes ineffective over long distances.3

While a few insects, such as those of the genus Halobates, are found on the surface of the sea, insects are notably absent from the vast majority of marine environments.124