Biology concepts – bilateral symmetry, radial symmetry, planulozoa hypothesis, cephalization, last animal common ancestor, porifera, platyhelminth, cnidarian, echinodermata
Halloween was a classic slasher film. Jamie Lee
Curtis looks so young, decades before Freaky
Friday or yogurt commercials. Michael Myers
could cut a man in half with his machete, but
could he produce two mirror image halves?
But how many ways can you be cut in half? Top to bottom is one way, leaving you with your head attached to one half and your feet attached to the other. Or you could be cleaved through your ears and down through your body. Then you would have your nose attached to one half and your bum attached to the other.
However, there’s only one way to slice you that will give two mirror images, each with the same components. If Chucky happens to catch you through the top of your head, down through your nose and straight down to where your legs split, each half will have one eye, one arm, one leg, one ear. This can only occur because you are bilaterally symmetric. Most animals (about 99%) have bilaterally symmetric bodies, so we have to at least consider the possibility that this provides some sort of advantage.
If bilateral symmetry is advantageous, why are jellyfish still radial? Because it works for them; no pressure/ random mutation combination sent them on that path. Remember, evolution doesn’t have a plan, it is neither reactive nor proactive. Random mutations are always occurring, and sometimes a change in environment makes renders a random mutation advantageous. It’s simply hit or miss. If the mutation or the pressure occurred at some other time, they would miss each other.
Radially symmetric animals tend to be sessile (non-moving), free-floating, or very slow movers. They don’t chase prey down, so they don’t need to be fast. This is the advantage of bilateral symmetry; it coordinates movements so that an animal can move in a particular direction faster. In fact, one 2102 paper puts forth the idea that maneuverability is the main reason for the maintenance of bilateral symmetry in animals.
However, fast movement wouldn’t be much use if you didn’t know where you were going. This is why bilateral animals also have a head. A head is a place to store your sensory apparatus and your neural tissue to process those sensory inputs. You think its an accident that our brain is located the same place as our eyes, ears, nose, and mouth?
Slow or sessile animals (like cnidarians) that filter feed or catch what runs into them have no head. They have few sensory neurons, and only loosely associated ganglia of neural tissues spread throughout their bodies.
Bilateral animals have a head, and radial animals don’t have a head. This sounds like a fairly plain story – as animals diverged and evolved, some developed a head and became bilateral. Or..... did they become bilateral and then develop a head? Maybe the animals can tell us which way it was.
The flatworms (platyhelminthes) were the first divergence of animals to have their neural ganglia clustered in their anterior end. Going along with this, they have sensory systems located at that end too. They have eyespots, although they are really just patches that detect light or dark.
Platyhelminthes have a define head ganglion of
nerves and have started develop more senses at
the anterior end; see the sensors that stick up.
And they move faster and in a straight line. They
are headed, and headed in a particular direction.
So we have gone from animals with no head and radial symmetry to animals with a head and bilateral symmetry. This doesn’t help answer the question of which came first. Aren't there any animals in between?
Yes, there are, and they give us a little bit of a clue as to which came first. The ctenophora (pronounced "ten", cteno = comb and phora = bearing) is a phylum of animals that lie between the cnidarians and the platyhelminthes. Ctenophoran animals are the comb jellies. Both cnidarians and comb jellies have been around for over 500 million years, so they’ve had time to settle in to a niche.
The comb jellies look round at first glance, but their architecture is a bit more complex than the cnidarian jellyfish. They have internal and external features that allow only for two planes of symmetry that give mirror images (see picture). These especially include the combs, rows of fused cilia that line their sides, and the fact that they don’t have stinging cells (cnidocytes). Remember that ONLY cnidarians have cnidocytes.
A 2004 study investigated the relationships between biradial and bilateral animals in evolution. If biradial is the link between radial and bilateral, then would seem to suggest that bilateralism occurred before cephalization. Called the Planulozoa Hypothesis, the authors suggests that ctenophora are the sister clade of bilateralians, and that all three of the groups – cnidarians, ctenophora and bilaterals – are the descendents of a single bilateral ancestor.
Ctenophora larvae have bilateral features, so this supports the planulozoa hypothesis (the free swimming larvae of all three phyla are called planulae). This would then suggest that cnidarians were once bilateral and then returned to radial symmetry.
Additionally, the if the planulozoa hypothesis holds, then bilateralism would seem to predate cephalization (development of a head). The larvae of ctenophores and some ctenophore features show that a move to true bilateral symmetry came before platyhelminthes and the emergence of a head. The conclusion – the streamlined body came before the head. But that confuses me, one isn’t much good without the other.
Ctenophores – the comb jellies, often show
bioluminescence. They only have two perpendicular
planes of mirror image symmetry. You can see the
fused cilia that form the combs on each ridge.
Wait a minute, there’s a fly in the bouillabaisse. Ctenophores have a nervous system that is more complex than many other animals – it’s just not centralized to a head. Centralizing the nervous system, with the sensory processing and muscular control, is a crucial part of cephalization. They seem to developed a strong neural system without adding the head itself.
A 2014 study of the genome of several ctenophores showed that they do not have the same neuron-building gene regulation pathways as any other phylum of animals, and they only use one of the most common neurotransmitters; all their other neuron signalling molecules are unique to ctenophores alone. This suggests that they evolved radically differently than the phylums around them, cnidarians and flatworms. This does not support the planulozoa hypothesis at all. Ctenophores may have developed all on their own and therefore can't help us answer the question of which cam first the bilateral body or the head.
Other things about symmetry development make you say, “Huh?” as well. Look at that same cladogram of animals above – see the right side where the sea star is located? What’s a radially symmetric animal doing way over there after everyone else switched to bilateral symmetry?
The echinodermata (sea stars, brittle stars, sea cucumbers; echino = spiny, and derm = skin) also support the planulozoa hypothesis, since they seem to have undergone the same regression as the cnidarians. Echinoderms include the brittle stars, sea stars, sea cucumbers, barnacles and sea urchins. They have bilateral symmetry as larvae, but many of them become radial (pentaradial or such, depending on the number of arms) when they become adults.
Secondary radial symmetry is term for when a bilateral larva becomes a radial adult; but it is more interesting than that. The easy way for that transformation to occur would be for the arms to grow out of the larva, with the top (aboral) and mouth (oral) sides remaining the same. But that’s not how it happens.
But like I say, it works for them. The adult sea stars and other echinoderms are fairly slow. Their lifestyle doesn’t require a head or a bilateral body, so they went biologically simpler and energetically cheaper and returned to radial symmetry. All the mechanics were still in their genomes - it was really pretty smart.
However much they have tried to regress as adults, the brittle stars seemed to have retained at least a little bilateral activity. The way they move is a lot like a bilateral animal, according to a 2012 study. One arm points forward, the direction they are traveling. The arms on either side then push the animal along, like a crawling bilateral animal. I guess you can’t completely go home again.
Next week – Bilateral animals are simple - just two mirror images, right? Well no. You won’t believe the number of complex animals that break symmetry in order to give them a unique shape or function.
For more information or classroom activities, see:
Biologic symmetry –
Ctenophora vs .cnidarians –