Friday 5: Five Reasons There Are Few Marine Insects

Point Reyes

That water out there… Quite inhospitable to insects! (Point Reyes, California)

Last week I mentioned some of the few truly marine insects, the water striders in the genus Halobates. I realized that I haven’t ever talked about marine insects and I think it’s time to rectify that situation! Aquatic insects make up only about 3% of the insects on the planet, but there are only a few hundred species that can be considered at all marine. The vast majority of those are found in estuaries, salt marshes, and other locations where freshwater and marine habitats come together rather than living out in the open ocean. There have been several ideas proposed for why insects have not successfully colonized the ocean, so for Friday 5 this week I give you 5 of those reasons. But first, let me talk about aquatic insects in general.

I know I’ve mentioned it in other posts, but I think it bears repeating: insects evolved on land. There are certainly other arthropods that have lived in marine habitats all along such as the crustaceans, but several million years ago some marine animals (probably crustaceans) crawled onto land and their offspring eventually evolved into insects. Only after they’d established themselves on land did insects begin to colonize water. Insects are thus built for life on land and living in water has presented several challenges that today’s aquatic insects have overcome. But very, very few insects ever colonized the ocean. There’s still a lot of debate about why, but there have been several great ideas put forth, including the following five.

1. Insects can’t survive their whole lives in water

Some insects spend their whole lives in water, including several beetles and true bugs. However, those that do are almost always insects that rely on surface oxygen to breathe and they can’t remain underwater indefinitely. That’s a problem.  It’s hard to be an insect living in the open ocean when you have to stay near the surface. There are lots of things that like to eat tasty little floating protein nuggets, so any insects that tried to colonize the open ocean were likely eaten or faced other problems (lack of places to lay eggs, finding mates in a gigantic ocean, etc) that prevented them from surviving in the ocean. The vast majority of marine insects live in shallow areas along shores, in mangrove swamps, in estuaries, etc, because the habitat is a lot better for them there.


This pelican would likely be happy to eat an insect hanging out near the surface!

2. The sea is too salty

This idea has been largely rejected at this point, but it’s been around so long I feel I have to mention it. The idea here is that insects can’t survive in salty water because they have problems regulating salt and water in their bodies in salty waters. However, there are several aquatic insects (especially dragonflies and damselflies) that live in brackish habitats that are far saltier than the ocean. Early insects may have had problems adjusting to the salt levels, but considering current insects have figured out ways to deal with it, it’s likely that the salt didn’t form a barrier for colonization of ocean habitats for insects.


The salty Pacific Ocean at Lanakai on Oahu, Hawaii

3. The respiratory system will collapse at depths where insects can avoid Predation

As I mentioned above, fish and other predators are a problem for insects living out in the open water. In fact, there are hardly any insects that live in open water even in ponds and lakes, just one family of flies call the phantom midges (Family: Chaoboridae). Being out in the open near the light is dangerous – you might as well have a big, blinking “Eat Me!” sign attached! So, diving deep is important if you want to survive. Unfortunately, diving deep enough to put a marine insect out of reach of fish also means that the pressure is likely high enough to collapse the air-filled respiratory tract and the insect will suffocate. This is one of the issues all aquatic insects face: getting enough oxygen in aquatic habitats with a respiratory system that evolved on land. But, if getting enough air means you’re 99% certain to be eaten by a fish… Well, you might need to find somewhere else to live.

San Carlos

Not many places for an insect to hide if it’s floating near the surface in the ocean at San Carlos, Mexico.

4. Insects are more buoyant in salt water than in fresh

Have you ever noticed that you float more easily in the ocean or salt lakes than in freshwater? Well, the same thing happens to air-filled insects. They float very well in marine environments, which means it’s hard for them to dive into deeper waters and, once again, are very vulnerable to predation if they float up near the surface. Better for them to stay along the shores where there are more things to hold on to at the bottom!


Mangroves are nice, hospitable places for marine insects compared to the open ocean

5. crustaceans fill up all the niches

Even if the insects could make it to the bottom where they’re safe from predators, they might still have a hard time finding a place to hide. Crustaceans are close relatives of insects and have lived in the ocean since before insects evolved, so crustaceans may already live in the places the insects might attempt to colonize.  But, the crustaceans have been there longer and can out-compete their land-evolved brethren. This is my favorite of the ideas about why insects haven’t colonized the ocean, but it remains mostly speculation so far.


A crustacean filling a niche an insect might use?

Though there aren’t very many of them on our planet, marine insects are super cool organisms! If you want to learn more, I recently discovered that the book on marine insects (Marine Insects, edited by the great marine entomologist Lanna Cheng) is available online and as a PDF. I encourage you to check it out! And, the next time you’re by the ocean, looking out over the water, I hope you’ll take a moment to ponder the fact that there is a massive part of our planet where the insects, undeniably one the most successful groups of organisms living on Earth, just can’t survive.  Amazing.  Really amazing.

Point Reyes

The open ocean, as seen on a foggy day from Point Reyes, California


Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth

24 thoughts on “Friday 5: Five Reasons There Are Few Marine Insects

  1. Thanks for the clear explanation about this. And yes, mangroves are more hospitable for marine insects. There are many mangroves where I live and the biodiversity that can be found there is great!

  2. A wonderful Friday 5! Just re-read your posts on aquatic insect respiration. Are there really no adults that retain gills they had as a nymph? Curious why that doesn’t happen..

    • There aren’t many insects that live in water as adults period, and those that do are mostly beetles and bugs. They typically either sit near the surface to breathe or carry air bubbles with them underwater that they use like scuba tanks. The air bubbles are often used as “physical gills,” gas films that share properties with true gills in they they absorb oxygen from the water, but they aren’t a part of the actual bug and merely something that sits on the bug. No adult aquatic insects I know of have true gills and the vast majority of them either carry air under their wings or carry gas films to act as physical gills instead. Pretty interesting that they don’t have true gills though, right? But then, most things that have gills are things that evolved in water. Insects did not, so they’re dealing with a very different evolutionary background.

  3. It’s interesting to me that the one group of marine insects (at least the one group known to me) are exploiting an environment not really used by crustaceans. Why have crustaceans never developed into anything like water striders (except, of course, the ones that did develop into water striders, the long way around. : – )

    • I’d bet it’s because they’re adapted for life in water whereas insects were originally land dwellers. Aquatic things, things that have been aquatic throughout evolutionary history, tend to have a hard time breathing and maintaining their water levels on out of water. Of course, there are lots of other reasons why crustaceans never became striders too! Perhaps something as simple as having their claws at the tip of their feet rather than further up their legs like water striders do is enough to break through the surface film? And now I have a great image of a lobster running around on the surface of the ocean. How completely cool would that be??!!

      • Now that you mention the breathing issue, it occurs to me that there are a fair number of crustaceans in the water strider habitat, they just do it from the other side of the surface, hanging below the layer of surface tension instead of walking on top of it. Mirror images. : – )

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  5. Wow. The normal chain of thought goes straight from marine insects to evolution, land or water; your post takes care of the less trodden trail.
    This was educating and fun to read :)
    PS. “…you might as well have a big, blinking “Eat Me!” sign attached!”
    I like :D

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  7. I think tracheae fit into several of your hypotheses and perhaps this may be what limits insect colonization of marine habitats. Especially in adult insects (and most arachnids), their gas exchange system seems to limit them to habitats where they have access to the atmosphere (tracheae also seem to be one of the primary constraints on size). I have heard of a beetle known only from an interstitial freshwater system in Western Australia, but nothing on how it manages to breathe.

    I think competitive exclusion by more ancestral crustaceans is falsified by several lines of evidence. First, mites evolved on land too, but I can think of at least three successful reinvasions of marine systems and oodles of intertidal mites. The Halacaridae, for example, are pretty much everywhere from the intertidal to the bottom of marine trenches to the interstices – and are in competition with lots of similarly sized crustaceans. I don’t know for sure, but I think these mites probably respire through their cuticle and lack tracheae. Also, lots of relatively recent lineages of crustaceans have managed to move into freshwater systems in competition with insects (and having gills instead of tracheae could explain why crustaceans tend to dominate the freshwater column) and some amphipods, isopods, and ostracods are fully terrestrial. Finally, several lineages of mammals recolonized the oceans in spite of presumed competition from pre-existing marine animals. So, I think a reason other than competition is needed to explain why insects have done so poorly in comparison to these far less diverse groups.

    • Ooh, thanks for the long response! I always read about how competitive exclusion by crustaceans is the most popular idea for the lack of marine insects, but there’s rarely a lot of supporting information. It’s actually really nice to read about another arthropod that invaded marine habitats from land and succeeded. Really appreciate the refutation! As an insect respiration fanatic, I’d be thrilled it it turned out that respiration, in one way or another, was the limiting factor preventing marine invasions by insects. :)

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  10. I am getting approx 15 water bug bites each time I swim approx 30mins out into Pacific Ocean, in mazatlan. Each feels like a slight sting and seems to happen all at one time ( not spread out during my swim). I see the red mark as soon I get out of bathing suit. I don’t speak Spanish but the local swimmers talk about yoo yoos ( I am not sure my spelling is even close to what they are telling me).
    I have reduced bites down to 1 per swim by wearing tight collared tshirt over my bathing suit.
    Do you know what these water bugs are? I am interested in knowing proper name to look up more on internet and see if same is present in other parts/ all oceans. I am thinking this is happening when I swam in other side of Mexico, Dominican Republic, and Jamaica.

    • Are you sure you’re not getting into jellyfish? There are next to no marine insects in general, so it is highly unlikely that you’re getting bitten by insects while you swim in the ocean. You’d have to be really close to shore to even SEE insects that live in the ocean, and most of those are harmless flies anyway. I’m quite sure you’re encountering something other than insects. I suggest jellyfish because I’ve gotten stung by jellies several times in the Pacific on the Mexican coast and that’s what I thought of immediately after I read your comment. Sounds similar to my experience there!

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