Before I start this post, I would like to make it clear that I am not an entomologist, so I apologize if I say anything very wrong; I hope not. Although I am sure many of you, indeed connoisseurs of the subject, are already thinking, “there are insects in the sea”. Certainly, several species of Halobates live on the surface of the ocean, and some other insects in interstitial areas of beaches, but they are quite rare and do not go into the sea depths. The fact I want to point out here is that compared to the richness and abundance of insects on land it is surprising that there are virtually none in the sea. The reasons can be diverse, and theories are not lacking:
To start with, the insect’s respiratory system is aerial and does not allow the exchange of gases in the water. This, however, has been solved by some beetles, or larval stages of dragonflies and mosquitoes, to name a few examples, which live in lakes and rivers. Then, what happens at sea? It has been speculated that insects, being aerial, would not have the ability to migrate to deep areas of the ocean during the day to avoid predation, as do groups of similar size to the sea.
We also have evolutionary reasons for the absence of major numbers of insects at sea; It is believed that insects evolved from crustaceans more than 400 million years ago and that their evolution was closely linked to that of plants. For example, winged groups such as butterflies, beetles, and bees have a parallel evolution to the appearance of flowers. There are very few flowering plants in the sea, which would explain the reason for the lack of many groups of insects there.
Finally, it is important to consider not only the group itself but its functions in the ecosystem. Insects include herbivores, parasites, decomposers, etc. These functions at sea are conducted, among others, by a group of small crustaceans that is an old friend of those who follow my blog, the copepods. Copepods are large herbivores, acting together with worms and other organisms as decomposers, and there are many parasites of fish, mollusks, and other marine animals. They perform their functions with exquisite meticulousness and efficiency and have no rival in abundance or biomass within the world of metazoans.
The evolutionary origin of copepods is widely debated, as there are very few copepod fossils, but recent evidence indicates that its origin was found in the Cambrian, about 500 million years ago. It is very difficult, then, for a group like insects that spread ashore to return to the sea millennia later and take the place of the already well-settled copepods in the ecosystem, although rarer things are they have seen (whales, seals, turtles, etc.). Looking closely, also seeing the evolutionary success of copepods, perhaps we should rethink the question that entitled this post to “Why are there no copepods on land?”