A tiny fly larva living in one of Africa's deepest lakes has evolved air-filled organs strong enough to withstand pressures found more than 200 meters underwater, according to a new study. The findings challenge a long-standing explanation for why insects have never colonized the open ocean and suggest that insect physiology may not prevent the eventual colonization of pelagic marine environments. Aquatic insects have successfully colonized nearly all freshwater environments and account for more than 60% of freshwater animal species. However, no aquatic insects are known to inhabit the deep open ocean. It's thought that this is because their air-filled tracheal breathing systems would implode under the hydrostatic pressures encountered during the deep daily dives required to avoid predatory fish. However, the larvae of the lake fly Chaoborus edulis, which thrive in one of the deepest lakes in the world, Lake Malawi, challenge this assumption. Here, Evan McKenzie and colleagues show that C. edulis has evolved an adaptation that allows it to survive in deep freshwater environments. Instead of using their modified tracheal air sacs for breathing, these larvae use them as buoyancy-control organs, enabling precise movement through the water column while breathing through their skin. These specialized sacs contain flexible layers of the protein resilin that expand and contract in response to chemical changes, allowing the larvae to withstand intense underwater pressure without collapsing.
To better understand C. edulis's physiological abilities, McKenzie et al. used sonar to track the larvae as they migrated throughout the water column in Lake Malawi. They also compared Chaoborus species from shallower lakes to examine how air-sac properties vary with habitat depth. They found that C. edulis regularly decends to more than 200 meters in depth into the lake's permanently oxygen-free hypolimnion during the day to avoid fish before returning to surface waters to feed at night. What's more, McKenzie et al. discovered that the larvae use a two-part buoyancy strategy: their air sacs actively adjust depth in shallower waters, then become rigid at greater depths to resist crushing pressure while reducing energy costs of remaining submerged. The air sacs of the largest larvae resisted implosion at depths exceeding 500 meters. "Surviving the depth of this water column raises the possibility that other insects exploit deep water habitats, contrary to widespread belief," write Jon Harrison and H. Arthur Woods in a related Perspective.
