Of the million or so named species of insect, only about 3% are aquatic in at least part of their life cycle, and the majority are freshwater. The small proportion of freshwater to terrestrial insects could be explained by the fact that the proportion of freshwater to terrestrial habitats on earth is a similar figure. This reason doesn’t explain the distinct lack of richness of marine insect species on the other hand.
There’s only several hundred or so insects that have colonised marine habitats in some way, and most of these are limited to the so called ‘bridges’ to the open oceans. These ‘bridges’ include habitats like estuaries, intertidal shores and mangrove swamps. In fact very few insect species are known to inhabit the open oceans. Any claims of sub-surface dwellers has been disproven and so, to date there are no known submarine insects. For such a successful group of organisms, it is unusual that they have not fully exploited every available niche. What are the factors that have therefore limited their colonisation of the seas?
Insects are commonly fed upon by pelagic feeders such as fish in freshwater systems. As such, many make use of shelter provided by benthic plants or substrate. Equivalent shelter in marine environments would only exist at great depths. Could it therefore be depth that is the limiting factor to insect colonisation? Insect’s sea dwelling relatives, the crustaceans deal with this by residing close to shores, the bottom of the sea, or by migrating up and down the water column throughout the day. Many respiring insects would struggle implementing this because of their air-filled respiratory system. Air bubbles would require repeated surface replenishment, and there would be problems with buoyancy at great depths. A notable exception to this is the larval stage of the midge Chaoborus. This insect uses air sacs as buoyancy organs which allow it to dive to around 70 metres in lakes, allowing it to avoid a fishy death. Depth can’t be the only limiting factor because of this exception, since the depths it reaches are comparable to many vertically-migrating crustaceans.
Insects that live in the ‘bridges’ to open oceans are exposed to lower salinity gradients and are often able to spend only part of their lifecycle in high salinity areas. Perhaps it is an intolerance of high salinities that limits their spread? Considering many insects colonise aquatic environments with salinities 10 times higher than sea water (e.g. Brine flies from the Great Salt Lake in Utah), this probably isn’t the reason. Insects have a different osmoregulatory system which allows them to cope with a wide variety of salinities, suitable for freshwater and hypersaline environments. But the sea doesn’t vary much in salinity, making this adaptation fairly redundant. Crustaceans take advantage of this by avoiding complex osmoregulation, and simply conform to the same salinity as the sea internally.
Perhaps the path of insect evolution has tended primarily towards adaptations for terrestrial life only? Abilities like flight and internal fertilisation that have helped insects become the most successful order of animals on earth are at best a hindrance in completely marine environments. As a result, the adaptations that have allowed insects to become so successful on our planet could be the exact reasons they are unable to truly establish in the seas.
No single factor has led to exclusion of insects from colonising the seas. But at every hurdle insect’s older cousins, crustaceans, have achieved more simple and elegant solutions. These have probably allowed them to competitively exclude insects from almost every marine niche. After all, they had colonised most of them before insects had even evolved. Competitive exclusion is likely the biggest contributing factor to why there aren’t plenty more insects in the sea.
One of the few remaining niches insects seem to have discovered is the sea’s surface, striding on water thousands of kilometres from the nearest land, well suited to the needs of several species of Halobates, close relatives of water striders. They have long lost their ability to fly and cannot swim underwater without risk of being trapped by the surface tension. As a result, they probably live the most two-dimensional existence of any animal (Abbott E. A., 1884).
Author – Christopher Lambert (@)
Abbott, E.A. 1884. Flatland; A Romance of Many Dimensions, with Illustrations by the Author, A Square.
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