Why are there no insects in the sea?

June 19, 2011 • 3:05 am

by Matthew Cobb

One of my favourite questions relating to evolution is ‘Why are there no insects in the sea?’ Arthropods came onto the land around 380 MY ago, and crustaceans and insects separated soon afterwards, probably because of that ecological shift. More or less, you get crustaceans in the sea, and insects on land. So why didn’t the insects go back into the sea? It’s very hard to be certain of the answer to this – doing an experiment would be pretty tricky, after all. But we can get towards what might be the answer by thinking about some of the possible answers we might give:

– Insects can’t live in water. Although no insect species lives its whole life-cycle in water without access to air, many insect species pass their nymphal stage in freshwater, breathing with gills. Mayflies (more on this in an upcoming post) and dragonflies are two obvious examples. Last year Daniel Rubinoff, an entomologist at the University of Hawaii, discovered a number of moth species that have caterpillars (= larvae) that are equally at home on land or in freshwater. You can see a great video of one of these caterpillars moving between water and land here.

– Insects can’t cope physiologically with salt water. Not true. There are a large number of species of insect that have a larval stage that lives in brackish salt water, so living in the sea is not impossible. (Indeed, this fact shows that the real question should be ‘Why are there no insects that have their full life-cycle in the sea?’)

– The sea is full. I think this is probably the right answer – the niches that insects would occupy in the sea are already full. The insects’ cousins, the crustaceans, are already there. This is what ecologists call ‘competitive exclusion’. Any insect that started going back into the sea would either starve or be eaten, I reckon. Proving this, however, is tricky.

One implication of this is that evolution finds it difficult to go ‘backwards’ because life shapes and changes the ecosystem over time. When environmental conditions changed to make it possible for terrestrial life to evolve, crustacean-like arthropods moved onto the land and rapidly made a series of adaptations that led to their incredible success as insects (reducing the number of appendages, evolving first wings and then highly specialized larval stages, and so on). Retracing their steps back into the sea is no longer possible, for ecological reasons. Were something terrible to happen to some or all of the crustaceans, however, it seems pretty likely that those adaptable insects would be back in the sea in the blink of a geological eye.

58 thoughts on “Why are there no insects in the sea?

  1. Could part of the reason also be insects’ spectacular success out of the water and therefore species have no evolutionary pressure to add new niches?

    1. Surely intraspecies competition is a selection pressure in itself: even in the absence of other species, a successful species will eventually have to compete with other members of the population. Their own success would act as a pressure to exploit other niches.

  2. The “sea is full” idea unfortunately explains too much. For example it explains why there are no mammals in the sea, given that their niche is already taken by fish …

    1. It also doesn’t explain enough, such as why there are so many insects in fresh water but so few crustaceans. Some bivalves live quite happily in fresh water.

        1. Insects are common on land, the question is why crustaceans haven’t colonized freshwater areas. They had plenty of opportunity before insects came along.

            1. People tend to overlook the smaller fresh water crustaceans. Amphipods, copepods, cladocerans. Not sure how well they compare to insect diversity in fresh water, but if you go and pull a bucket of water out of the next pond I can almost guarantee you that you will get more crustacean biomass than insect biomass. They make up a decent part of the zooplankton.

              Regarding “the sea is full”, the whole concept of ecosystem saturation is being discussed pretty critically. It doesn’t actually seem to happen very often.

              That said, remember that the whole going back to the sea thing is not something that happens very often. How often has it happened in mammals/birds? I can only think of 2 cases off the top of my head that actually spend all their life in the water, cetaceans and sea cows. Are there any more cases?

              1. Good point about mammals, but we are taking 380 MY here – that’s a very long time for the insects to dip their tiny jointed feet in the sea, were they to be able to… However, I suspect it’s not anything we’ll actually be able to resolve.

              2. There are also sea otters. They may not look it but they pretty much never venture on land, even giving birth in water.

                If you also include animals that essentially only come on land to breed, then it would also include pinnepeds, many penguins, and several unrelated but behaviorally-similar birds that lived in the northen hemisphere but recently went extinct (such as the great auk).

              3. Reading up on the sea otters, that’s pretty cool. I always assumed they act more or less like “normal” otters.

                But regarding pinnipeds, if you include them you also have to include insects whose larvae develop in brackish water and then things already look different.

                I tend to think that it might be worth evaluating the problem of salinity again. We know that larvae can survive in brackish water. Larvae usually have gills, like crustaceans.
                This is important if you remember that much of the osmoregulation in crustaceans (and I bet also in insect larvae) happens over the gills.

                Adults, however, “breathe” via trachea. How would you handle osmoregulation if your body is covered by a chitinous exoskeleton that won’t allow a whole lot of ion transport, and you don’t have any other large surfaces in direct contact with the surrounding water?
                The excretion system might be an obvious answer. Not sure, my basic zoology lectures were so horribly long ago…but I seem do remember that in insects, it is specifically adapted to use little water (weight reduction for flight). Maybe there are physiological barriers.

  3. Very interesting – thank you

    I read it in conjunction with this WEIT posting from March 2010: Insects are crustaceans!

    I wonder if there are any undiscovered microscopic insects with a solely marine life cycle ? I’m imagining an insect that has adopted a mite-like lifestyle. Moving to say a saline reef environment from a fresh water environment along with its host. The host most probably being some kind of Arthropod.

    Perhaps some assumed ‘mites’ are really insects & we don’t realise because they have come to closely resemble mites (convergent lifestyle = convergent appearance). There are about 50,000 described species of mite – what if a few of them are insects ?

  4. Geerat Vermeij in ‘the Evolutionary World. How Adaptatation Explains Everything From Seashells to Civilisation’ has a chapter entitled ‘Invaders, Incumbents and a Changing of the Guard’ which starts off precisely with this question as to the extreme scarcity of insects making the transition from land to see.

    I’ve just reread it, and it still doesn’t quite make much sense to me. His argument is that insects at the border between land and sea if they happen to move slightly into the ocean become inactive and are outcompeted by species already there and which are highly active.

    He doesn’t think that other lines move from land to sea at times of mass extinction opening up niches in the sea. He thinks that for example the prof whales entered the ocean at warmer periods with more production of resources so less competitive species were able to survive. At extinction events, open niches are more likely to be filled by surviving marine species rather than ones from the land.

    Some land species are capable of outcompeting marine species. Sea grasses which only entered the oceans after the evolution of flowering plants because they have roots capable of drawing nutrients from the mud and sand can outcompete seaweed which only has holdfasts and which have to draw nutrients directly from the water.

    1. Interesting about seagrasses – have you an article/chapter on that you can put a link in for or is it in the Vermeij book?

  5. Regarding nices – surely sometimes a creature can exploit a new niche if it comes in from another place. Perhaps the problem is the universal nature of the oceans – they all connect so it is a lot easier for existing species to spread perhaps in larval form? Wouldn’t we need salt seas that would be nurseries for brackish water loving insects to evolve in that were then later connected back to the oceans? Water ‘islands’…

  6. Some 10yrs ago my first scientific mentor was determining ESTs from lobster. The closest hits he got were typically to Drosophila – the phylogenetically closest species represented to any extent in the databases.

  7. Some mammals returned to the sea, but they didn’t do this by regressing to adopt their ancestors’ amphibian or fishy ways of life.

    Marine mammals move their horizontal tails up and down. Fish move their vertical tails side to side. Marine mammals didn’t regress to or re-invent gills.

    Mammals returned to the sea by developing new ways of living there, and they did this even though there were plenty of other life forms there already.

    Should insects return to the sea, they would not regress to some crustacean or pre-crustacean form. They would adopt new forms better than both insects and crustaceans.

    An interesting post but I don’t think you’ve found the answer. I don’t know it either.

    1. Precisely. This has happened at least once – there are fully marine waterstriders, Order Hemiptera, Family Gerridae. Genus Halobates is the pioneer, perhaps leading the way for more marine insects, exploiting niches not yet discovered by other groups?

      See also James C. Trager’s point, below.

      Would you consider an insect to be marine if the air-breathing adults spend their lives on/near the surface of the sea, rather than entirely submerged at all times?

      Personally, I think the question is poorly founded – there certainly are insects in the sea, properly out on the open ocean, with their full life cycle conducted in intimate contact with salt water. So the question could be re-worded “why are there so few insects in the sea”.

      1. I read somewhere sometime that there are wingless flies living on the Sargassum of the Sargasso Sea. Also salt flies in Utah at the Great Salt Lake.

  8. Marine mites that feed on the fluids of marine plants and other animals are known (http://www.springerlink.com/content/l615447n4821357q/), but most other modern arachnid groups, like insects, have not invaded the seas. No mite-like insects have been discovered.

    I was thinking of whale lice when as an exception to the rule of no insects spending their whole life in sea water, but was reminded when I looked them up that they are crustaceans (of another sort, if you will), family Cyamidae. (More – http://species-identification.org/species.php?species_group=crustacea&id=249)

    Pinnipeds (seals, etc.) have sucking lice (true insects) as external parasites, but pinnipeds spend a significant portion of their lives out of water.

    1. Forgot to mention marine water striders, of the bug family Halobatidae. Though the modern ones apprently spend their entire lives at sea – on the surface, like fresh water Gerridae, fossil halobatids are known from the Dominican tropical forest amber deposits. How they got there is anyone’s guess.

  9. The major adaption of insects is wings, which work quite well in air, but not so well in water. Even aquatic insects, like diving beetles, don’t fly underwater. That is not a complete answer, because some insects have given up flight in adapting to an unusual lifestyle.

  10. Is it possible that some did return to the sea and they just don’t fossilize well enough to leave evidence?

    1. Erm..I meant fossilize well enough in saltwater. I know we have plenty of land insect fossils….doh!

  11. One of my favourite questions relating to evolution is ‘Why are there no insects in the sea?’ Arthropods came onto the land around 380 MY ago, and crustaceans and insects separated soon afterwards, probably because of that ecological shift. More or less, you get crustaceans in the sea, and insects on land.

    I’m not even sure if this is a meaningful question, since it looks like insects are a subgroup of Crustaceans, which are paraphyletic. Quoth wikipedia:

    “The exact relationships of the Crustacea to other taxa are not yet entirely clear. Under the Pancrustacea hypothesis,[34] Crustacea and Hexapoda (insects and allies) are sister groups. Studies using DNA sequences tend to show a paraphyletic Crustacea, with the insects (but not necessarily other hexapods) nested within that clade.[35]”

    1. Yes you’re right Nick. But I don’t see how the fact that insects are crustaceans (probably) affects the matter, or renders the question meaningless. The group of arthropods we call insects are absent from the sea (as outlined in the post). Why? That enigma remains whatever their position with regard to crustaceans (whatever they are).

    2. As long as the insects remain monophyletic, where’s the problem?

      If the crustacea are paraphyletic then either the insects need to be included in c. as a subclade (and the question of why that large group is all non-marine remains) or c. needs to be dismembered into 2 or more clades “surrounding” the insect clade, and the issue of why (just?) the one clade (insecta) among multiple allied ones has not entered the sea remains.

      Now if insecta is polyphyletic, things would get sticky and the question would have to change substantially. Any evidence for that?

  12. “- Insects can’t cope physiologically with salt water. Not true. There are a large number of species of insect that have a larval stage that lives in brackish salt water, so living in the sea is not impossible. (Indeed, this fact shows that the real question should be ‘Why are there no insects that have their full life-cycle in the sea?’)”

    One might still think that there is some inherited upper physiological limit on environmental salt concentrations for insects in general. Brackish OK, full sea water not. But, it occurs to me that there are “brine flies” that spend their larval stage in bodies of water saltier than the sea, which suggests the issue is not some genetic barrier in insects that evolution has not yet overcome. Those flies occupy habitats that also have few competitors, which might be a clue.

  13. I think the lack of a niche to fill is the best explanation.

    I don’t think the existence of aquatic mammals argues against that, either, because of the size difference – a largish mammal can make a good living eating fish by the shore, then spending ever more time in the water. All without being eaten by anything.

    A small insect has no such path. Anything suitable as a food source is already exploited by something already in the water, well-adapted to that purpose.

    Now, following extinctions, it’s feasible for new niches to open up that insects might exploit. But perhaps they were filled by already-aquatic critters faster than insects could evolve.

  14. Interesting that there are flies and mosquitoes that have larvae with very high salt tolerance, but no adult insects that tolerate saline water. I also think there are no adult insects with gills–all, even the aquatic diving beetles–breathe air.
    Ineluctable physiological constraints are not ruled out.

    1. Good point. But I’m back with my scientist’s beef: how could we test the constraint hypothesis?

        1. Thanks for the article! Puts an intreating extra spin on the question. I shall use this in next year’s lecture!

    2. At least one adult insect will tolerate hypersaline water:

      Trichocorixa reticulata (Corixidae, Hemiptera)

      “At the Salton Sea it occurs mainly in protected embayments or isolated pools at the fringe of the lake, where salinity is usually higher than the main body of the lake but where fish are scarce or absent.”


      The Salton Sea is more saline than the ocean, but more dilute than Mono Lake or the Great Salt Lake — some fish species are able to tolerate the conditions, and can even be abundant. If this true bug can survive the higher salinity of marginal pools, presumably it could survive in the sea itself, if it weren’t for the fish.

      It looks like fish predation rather than physiological problems with salt, or competition from crustacians, keeps insects out of the sea.

      1. But corixids are common in freshwater habitats where fish are present and abundant. Why doesn’t fish predation keep them out of freshwater too?

  15. The ‘no insects in the sea’ problem has interested me for a while. Every other major group of animals (that I can think of at least) sports representatives in the sea. So why not insects?

    Does anyone know the evolutionary origins of sea spiders? They are chelicerates (i.e. more closely related to actual spiders than crustaceans and insects), but did they evolve on land and return to the water or evolve in the sea? Are there any other marine chelicerates?

    I was just thinking that maybe the ‘book lungs’ that chelicerates seem to mostly have might give them greater ability to return to water than the spiracle breathing system of insects (based on the assumption that diffusion is greater in air than in water, which I could have got wrong). So insects might have to first develop paedomorphosis (i.e. retain larval gills) and then move into the sea. Or move the larval period into the sea then develop paedomorphosis. Are there any paedomorphic insects?

    If you’ve read this far I hope all that babble wasn’t too confusing!

    1. Gas diffusion in air compared to water at ~25C is typically greater by a factor of 50 or more, plus there is much more gas available in air than there is dissolved in the water.

    2. Sea “spiders” evolved in the sea and never came out. Unlike other chelicerates, they don’t have book lungs/gills, but absorb oxygen directly (they have a very large surface area – volume ratio). Other marine chelicerates are the sea scorpions (defunct) and the horsehoe ‘crabs’.

      The position of sea spiders on the arthropod tree is still disputed, but they are very basal (ie they branched off very early on).

      Not sure if it’s open access, but here’s an article I wrote about sea spiders: http://www.sciencedirect.com/science/article/pii/S0960982210006020

  16. There is not necessarily any reason other than insects happened to not evolve that way. Just as we can’t expect humans to evolve again from an ancestral species, why should we expect insects to evolve into creatures which spend 100% of their life in the water? A more interesting question would be “is there anything preventing insects from developing an aquatic life in the future?”

    [OT: What happened to the page background and stuff?]

    1. You’re right that we have no reason to expect that insects should evolve for life in the sea. It’s just curious that a return to the sea has evolved independently many times in several less diverse groups (plants and animals) that haven’t been around for as long as insects. What’s more, there are many examples of insects that have evolved to make use of freshwater habitats.

  17. No insects in the sea – this depends on the definition of what is ‘in the sea’. Also, a usual view of insects is that the adult stage is the ‘real’ insect and larval or nymphal stages somehow don’t count, even though the greater part of the life cycle may be invested in these phases. Here in New Zealand, at least, rocky shore pools well below mean sea level, in full salinity, may host quite a few larvae of chironomid and other dipteran larvae (tabanids, empids) as well as trichopterans (caddis) – all of several species. Oceanic insects – well, there’s a real absence.

  18. I guess its something to do with the salinity. There are plenty of insects (larvae at least) in the baltic sea which has noticeably lower salinity than most other seas. This sea also contains many ‘freshwater’ fish species (pike, roach, perch etc).

    1. Animals with jointed exoskeletons are called Arthropods. Lobsters and other crustaceans are arthropods with antennae or mouthparts or legs on each body segment. Insects are simplified arthropods with only three segments and only six legs, all in the thorax. “Bugs” are members of the order Hemiptera. They all have sucking mouthparts like the nasty bedbug. Another example is the fresh water Giant Water Bug (4 inches) which is large enough to suck the fluids out of small fish (minnows).

  19. I feel that the question is analogous to asking why there are no humans that still live their entire lives in trees. While we still have some “cousins” that have varying degrees of arboreal lives, we are the product of direct ancestors whose environmental pressures pushed them towards living on the ground. At this point there hasn’t been sufficient evolutionary advantage for us to make the change back to an arboreal life. But if global warming melts enough of the glaciers, who knows? To my understanding turtles are an interesting counter example of a species that has moved between land and sea several times over the course of their evolution.

  20. A related ecological riddle seems to be the question why the biomass is stored primarily in higher trophic levels at sea, but in lower trophic levels at land. That is, most biomass is found in plants at land but in fish and above at sea.

    Maybe, it would take the reproductive rate of a mussle, if you wanted to enter the sea at the trophic level of an insect, but could not dive deep enough to escape predation.

  21. Inexperienced comment-er here.. perhaps it’s because humans never made it in the sea? Insects seem to me to follow humans around quite a bit, I am thinking of flies and mosquitoes and dung beetles .. sorry, I’m an artist and anthropologist and probably shouldn’t be exposing my myopia here..

  22. we don’t see them going back at this point in time doesn’t mean they never will.
    Insects are thriving on land and crustaceans are thriving in the sea. When things aren’t too terrible, there is no need for change. Maybe if the Earth’s climate changed drastically and many of them struggled to survive in their present habitat, perhaps they will devolve.

Leave a Reply