Dec 062013

The Last Eukaryote Common Ancestor–affectionately known to cellular evolution geeks as “LECA“– was almost certainly a predator.  Later in the history of the lineage, certain eukaryotes would form permanent symbiotic alliances with photosynthetic bacteria, and once that occurred, some lucky cells could just bask in the sunshine, drawing energy from light with the help of their chloroplasts. Until that happy day, however, all nucleated cells survived by using a bizarre trick that LECA must have known: the ability to ingest other cells by phagocytosis (literally, “cell-devouring”).

Another thing about LECA and its descendents is that, thanks largely to the mitochondria which give them a fairly generous energy budget, eukaryotes of all kinds have a real genius for evolving new and outlandish bodily structures. Unlike the prokaryotes, bacteria and archaea, which tend to have rather unimaginative body shapes, energy-rich eukaryotic cells have developed a huge (and hugely entertaining) diversity of organs, appendages and specialized equipment.

So, in the eukaryotes we have a population of ravenous cell-eating cells that also have a talent for morphological innovation. Clearly, this has the makings of an arms race, as prey and predator (and most eukaryotes are both!) hurry to develop new instruments for attack and self-defense. This race  is one of the likely drivers of their exceptional somatic diversity. There are other evolutionary drivers of eukaryote complexity–including some not directly tied to adaptive pressures–but the need to avoid being devoured  is certainly an important one.

There are many ways to avoid being eaten. One of the simplest strategies is to be too big to swallow. However, there is a cost to that.  Big bodies are expensive: they require a lot of energy to keep going, which means the organism has to gather more food.  But what if you can be big without having to maintain a large mass of living tissue?

One way to accomplish that is to congregate with others of your kind in close-knit (even clonal) colonies, as many flagellates do.  Take, for example, the planktonic heterokont Synura, which usually lives in big, rolling spherical clusters made up of many cells. If you want to eat a single Synura, you will need to be big enough to eat the whole sphere.

Obviously, there are plenty of creatures big enough to consume a whole colony (I’ve seen Stentor pyriformis gobbling Synura like popcorn balls); but the strategy of colonial aggregation does take some of the smaller predators out of the game.

There are a few downsides to colonial living (decreased freedom of movement, for one); but the tradeoffs can be acceptable, especially for those protists whose way of life doesn’t require much personal mobility. Not surprisingly, photosynthetic organisms, which don’t have to chase their dinner around, often live in colonies.  In the microworld, familiar examples are the filamentous green algae, or the great spherical death stars we call Volvox (which are not only colonial, but actually have some cellular specialization, like plants and animals).

Volvox by R. Wagner

Volvox aureus. Image by R. Wagner. Click on the picture to see more Volvox on Dr. Wagner’s site.

Another good survival strategy is to borrow “bigness” from your surroundings.  Mix yourself in with a pile of debris, or crawl into a hole in the mud, and your enemies will be unable to get their mouths around you.  In short: take shelter.  It is the same plan, here, whether you are a protist, a beetle, or a juicy human surrounded by bearsharks: position yourself inside something bigger, pricklier and tougher than your own delicious body, and you might survive.

One major disadvantage of opportunistic shelter-seeking, as practised by cautious organisms of all sizes, is that your fortress isn’t always located close to your preferred food source; and if you leave it to go foraging, you risk becoming a food source yourself. One good solution is to build (or secrete) your own shelter in an optimal location. This approach, often combined with the strategy of colonial living, has given rise to an amazing array of ingenious shells, houses, tube dwellings and domestic mucous piles which protists have created for their own protection. In my next couple of posts, I will look at some examples of protistan architectural achievements.

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