More Sea Anemones

Yesterdays post was about sea anemones, and today is going to cover briefly how these creatures feed, and the relationships they have with other species of animal (symbiosis, from Greek sym together, and biosis living, so simply meaning things that live together…see, those scary sounding terms which put people off are fairly straight forward usually)

So, yesterday we covered the structure of the sea anemone, where the tentacles at the top of the animal direct food into the top of the pharynx (like we do with our hands), but, anemones do not have tongues, or a digestive system as advanced as ours, so how do they actually eat, and digest their food?  And, having a simple gut as mentioned yesterday, it only has one entrance/exit, so how exactly does that work, and is it as gross as it sounds?

As with us, when food enters the throat (pharynx is from the Greek for throat), the tissues within the throat expand by a wave of contractions, which work up from the base to the top, so the part of your throat at the back of your mouth expands to allow the food to enter, then the wave reverses, and the food is pushed downwards, and out of the throat into the stomach.  (Try putting your hand on your throat as you swallow to feel this).

Sea anemone feeding, or at least trying to. From BBC (very awesome pictures)

Once dinner has been digested, contractions within the tissues of the stomach (peristaltic contractions, from Greek peri and stallien meaning to wrap around), push the waste back up so they can be excreted. This type of movement of tissue is the same as we have in our small intestine, and our throat after swallowing.

Peristaltic movement (image from Wikipedia)

The diagram below shows the different stages in the feeding cycle of sea anemones, and is followed by some other pictures from the same site, mainly because they are awesome pictures, but also because they show anemones in different stages of contraction and expansion.

Feeding cycle for anemones, from asnailodessy.com

Different stages of contraction and expansion within a sea anemone, also from asnailodyssey.com

Sea Anemone releasing matter from its pharynx (White item in centre of photo). From asnailodyssey.com

So, is the feeding as gross as it seems? I do not think so, although when watching it, it can make your stomach feel a bit queasy, but this is purely because we have two exits from our stomach, and so associate exiting of material through our mouth as a sign that something is wrong.

Now, onto the symbiosis I mentioned at the start of the post.  There are several forms of symbiosis, with varying degrees of benefit to the host.  Parasitic symbiosis is the one most people are aware of, where one species lives in or on another, and is harmful to the host, examples of this in humans are malaria and tape-worms.

Mutualism is when both species gain a benefit from the symbiosis. The bacteria in our stomachs, bees and flowers are both examples of mutualism.  In sea anemones, they have mutualistic relationships with clown fish, as shown in the picture below.  The clown fish is protected from predators by the tentacles of the sea anemone, and in return, the clown fish fights off fish which would otherwise feed on parts of the anemone.  Also, the clownfish also excretes ammonia rich waste, which is used by the bacteria in the stomach of the anemone.

Clownfish on an anemone. From wikipedia

Commensalism is when one species benefits without harming the other. Anemones are often used as examples of this, and with good reason!  The picture below shows a Boxer Crab.  This animal carries anemones in its claws for protection! They are the white objects in the picture below.   If anything threatens the crab, it waves around the anemones, with the tentacles towards the attacker… if the attacker gets too close, the nematocysts will fire from the tentacles.  This relationship may be more mutualistic than commensalist, as the crab excretes nitrogen rich waste in the same way the clownfish does, and so may provide nutrients for the anemone.

Boxer crab carrying two anemones for protection. Image from MS-Starship.com

Another really good example from the anemone is of the anemone crab (a porcelain crab species), which lives in the tentacles of the anemone, and filter feeds particles passing through the tentacles.  As with the other species which have symbiosis with anemones, the crab has had to evolve an immunity to the toxins of the tentacles.

Porcelain Crab in the tentacles of an anemone. They filter feed on particles passing through the current in the tentacles. Image from MS-Starship

I find it fascinating trying to work out reasons for how these relationships evolved, and why they arose.  For a crab or fish to begin living on what is an aggressive toxic animal means that the benefit of the protection gained must outweigh the danger of being accidentally eaten, or stung by the host.  In the case of the boxer crab, I think that originally, anemones may have settled onto a crabs claws, and then over time, the crab began to utilise the anemones in the local area.  In the case of clownfish, or porcelain crabs, which came first, the immunity, or the behaviour, is something which keeps me busy for hours when I start thinking about it.  I am sure there are evolutionary biologists out there who know the answer, but, I prefer for now to try and work it out for myself, maybe over time I will study more ethology (the study of animal behaviour), and be able to better understand it for myself.  Presently, I think that it was a mix of immunity and behaviour.  Some ancestral fish would have had a slight immunity, which made it able to utilise the anemone for a short period of time, and over time, this was selected for because of the protection gained from the behaviour….being able to hop into an anemone when a predator came past, even for a few moments, is an advantage, and if while there, food is available, that is a double advantage.

Next time will be starting out on corals…I have no idea at the moment how many posts that will be, as the topic is huge, and amazingly interesting!

Oxygenation of the atmosphere – or why we love bacteria, part I

One of the critical steps in the development of life as we know it today was the evolution of cyanobacteria (also called blue-green algae).  These single-celled organisms are responsible for adding oxygen to the atmosphere, and paving the way for the higher life forms that we have today.

They were not the first autotrophs (organisms that generate their food directly from sunlight, or inorganic chemicals, as opposed to heterotrophs (like us), who get their carbohydrates second-hand by consuming other organisms),  one view is that organisms similar to purple sulfur bacteria existed first, and it is the transition from these to cyanobacteria that today’s post is about.

Purple sulfur bacteria are only able to grow under anaerobic conditions (that is, conditions without oxygen present), because the synthesis of pigments in the cell is suppressed if oxygen is present. The early atmosphere of the earth was not oxygenated as it is today (as evidenced by sulfur isotopes in rocks over 2.45 billion years old, among other indicators), although there is some debate about the exact timing of the first atmospheric oxygen (NB: This does not mean I am saying “Science does not know what happened”….the time period in discussion is a few million years, and the discussions are about whether any atmospheric oxygen was immediately reacted with iron or other chemicals in the oceans).

Ok, so about these cute little purple sulfur bacteria, they are photosynthetic, so this means they use sunlight to obtain their carbohydrates.  Unlike plants today which use chlorophyll (the reason they have a green colour, but more on that later), these purple bacteria use a compound called bacteriochlorophyll, which uses different wavelengths of light, and does not produce oxygen as a waste product – This is kind of handy, since, as we saw in the previous paragraph, sulfur bacteria do not function so well in the presence of oxygen.

So, I have just mentioned photosynthesis, which is the method that plants use to obtain sugars from sunlight.  The equation for this in sulfur bacteria is as follows (Simplified version).

CO₂ + 2H₂S → CH₂O + H₂O + 2S, (or carbon dioxide + 2 lots of hydrogen sulfide is converted using light to carbohydrate + water + 2 sulfur molecules.)

Hydrogen sulfide is the rotten egg smell, and is emitted by volcanos.  The early earth was very geologically active, and volcanic activity was much higher than it is today, providing plenty of hydrogen sulfur for these bacteria to use.

This equation is important, because it was the discovery of this method of photosynthesis which demonstrated that it is water, and not carbon dioxide which is used for producing oxygen.  Prior to that, it was thought that the carbon dioxide was split to produce the waste product of oxygen.

So, back on the early earth, there were these bacteria pumping out sulfur, and using up the CO2 and H2S from volcanos.  Because they were purple, they used light in a different wavelength to the  plants of today, which reflect back the green portion of the incoming light, and thus we see them as various shades of green.  These purple sulfur bacteria reflected light back at both the higher end of the spectrum (red) and the lower end (blue), giving them their colour.

Absorption of light for purple and green chlorophyll pigments.

(Image from http://www.sidthomas.net/SenEssence/Genes/chlinsen.htm )

So, now we have oceans filled with reddish purple bacteria! I always try imagining how our planet would have looked back then, not so much a pale blue dot with green patches, but more likely a vivid colour like we see today when we get blooms of purple sulfur bacteria in places like Yellowstone park and other oxygen low, sulfur rich hot springs.

The next development was for some bacteria to evolve to use the light that was not being used by these organisms.

As you can see from the image, the purple bacteria would have used the light at in the centre of the spectrum, leaving light at the ends available for use, so any organisms which evolved to utilise this would have had a competitive advantage. If you want to know more about the evolution of cyanobacteria, without getting overly heavy on technical stuff, there are two systems within plants which do the conversion of light, these are called Photosystem I and Photosystem II. Green sulfur bacteria have Photosystem I only, and purple bacteria have Photosystem II only, so it is currently thought that the evolution of an organism which had both, either by endosymbiosis, or by sexual reproduction (yes, bacteria get it on too!) led to the cyanobacteria. (This is not my main area of interest, so please feel free to correct me if I got that wrong).

Ok, so now we have bacteria that reflect light in the green region, and use it from the ends of the spectrum, and have two photosystems…so what?

Well, it so happens that chlorophyll (the pigment involved), when using electrons to generate energy, gets electrons by splitting water, instead of hydrogen sulfide.  (This happens in photosystem II, in case you were wondering) So, now we have a new equation for photosynthesis, which is more familiar:

CO₂ + 2H₂O → CH₂O + H₂O + O₂

By using water (which was in abundance), and producing oxygen (which the sulfur bacteria could not tolerate), the cyanobacteria gained a large evolutionary advantage.  In addition, due to the release of oxygen into the atmosphere, ozone was formed by the reaction of sunlight with oxygen to form O₃

Ozone is what is primarily responsible for filtering out the UV from the sun, so once atmospheric oxygen, and ozone were in place, the stage was set for the long evolutionary road which would eventually lead to me sitting here typing, and you sitting somewhere else in the world reading these words.

 

 

More Darwin says Hi!

Ok, so to round the week off, one more post on wierdness in nature.  I spent a while deciding whether to blog about some skeptical stuff, but decided to finish the week as I started it, with more animal WTFness.

Today, I present the tasty (or so I hear) dinner from the Crustacean family known as the lobster.  Image is from www.visualdictionaryonline.com

 

Pay attention to the location of the "Green Gland"

The green gland (or antenna gland) located between the brain and mouth.  I have been reliably informed that if you “accidentally” eat this while eating a lobster, it is extremely bitter, and liable to make you throw up…not quite sure what you are doing poking your fork that far into the lobsters head, when I dissected one of these this week, it was fairly far back in the head, relative to the meaty parts.

Anyway, back to the matter at hand.  The green gland is defined as “Organ producing a secretion that allows toxic substances to be eliminated from the body; its opening is located at the base of the antennae. “  This is a euphemistic way of saying it is the equivalent of the bladder and kidneys on humans.  It exits just under the antenna.

If you have problems visualising this, then imagine your nasal cavity having your bladder placed just behind it.  Unlike the snail we looked at earlier this week, at least the lobster has its anus in the correct location.

So, as with the snail and slug, this could either be a result of developing an exoskeleton and having your internal organs shifted around, or…intelligent design.

Once again, I will let you decide…

Eyespots (Also known as stigma, not to be confused with stigmata, which is something completely different!)

Time for eyespots, not just because of the irreducible complexity silliness, but because I actually think these are awesome. The first time I saw one under a microscope, I got even more excitable than usual (Trust me, when it comes to nature, my standard state is very excitable!) This is a euglenoid (picture from http://www.okc.cc.ok.us/biologylabs/documents/Zoo/Euglenozoa.htm) The arrow points to a photosensitive spot, also known as a stigma. This is FAR from being what we would consider an eye, but, it gives the organism an ability to sense light, and therefore sense when something (a predator for example) is in the way of the light. The ones I have seen are not as clear as this, and can be at either end of the organism, but is always present in euglenoids. I have used Euglenoid rather than the phyla name, to avoid the “euglenoza/euglenophyta” discussion. I usually use Euglenophyta, but that is purely because that is the term I heard for them, and it kind of stuck. If you want to look up more about the phyla in general, either term works. Euglena is the term for the genus. I try to avoid getting caught up in the ever changing taxonomy of organisms in general though, so will mostly use generic terms.

 

 

Slugs and snails and puppy dog tails (Ok, no puppy dog tails)

So, this is the first in a series of posts otherwise known as “Darwin says Hi!”

These will cover the awesome diversity of the natural world, mostly things which have made me do an O.O face when I first saw them.

Without further ado, lets jump into the subject for today: Gastropods, specifically the ones better known to you and me as slugs and snails.  Slugs are effectively snails without shells.  They do have a mantle (the shell of a snail), but it is vestigial.  Their anatomy is very similar, so I will use the example of a snail.  Below is the generalised anatomy of a Pulmonate snail (This is a general term for a land snail)

Note location of the anus, vagina and penis relative to the head

Yes, a snail quite literally poops on its head, yes it has a penis on the back of its head, yes the vagina is also on the back of the head.  It might be assumed that this is from the evolution of the shell over time, and this was assumed for a while, but it is now thought that torsion (the twisting of the organs from the left to right side of the snail, and the movement of the anus from the back to the front) is a seperate evolutionary event to the development of the shell (coiling).  In addition to pooping on its head, the entrance to the lungs (called a pneumostome) is in the vicinity of the anus, and the other excretory pore.  The picture below shows the torsion which occurs during the development of slugs and snails.  This occurs in the second phase of development, known as the veliger laval stage.

Torsion in a developing snail

Whilst I previously said that the torsion and coiling evolved seperately, there is an argument that the torsion is a result of ancestral coiling. The fossil record appears to show that coiling occured prior to torsion.  This could then mean that the shell evolved as a defense mechanism, and the further that a snail is able to withdraw its head into its shell, the more effective the shell is as a defense. Without torsion, the snail is unable to to withdraw its head fully. The evolutionary benefits of being able to pull completely into the shell outweigh the disadvantages of excreting in the region of the head.  Usually air is expelled along with the excrement, allowing for it to clear the head area, especially the pneumostome, and, even allowing for ocassional fouling of the head region, I think I would prefer pooping near my head to having my head bitten off by a predator.