From flat to round

Last time I said I would move on from worms today, but I thought that I should at least cover the creatures alive today that resemble those which made the fossil burrows I showed last time, and talk a bit about the links between flatworms, and the worms I will cover today, so one more post on worms (for now!)

So, without further ado (cue drum roll), let me introduce you to the subject of my post today, Nemertea (not to be confused with Nematodes, which are something completely different)

This, is a Nemertea (also known as Ribbon Worms):

Basiodiscus mexicanus. Image from UCMP Berkeley

Not all of them are this pretty, a lot of them are dull coloured, and some look like this:

Lineus longissimus, the Giant Ribbon Worm (or Bootlace Worm). Image from seawater.no

The species shown above, Lineus longissimus is one of the longest creatures in the world! It is usually 5-15m long, but has been reported up to 60m long!  In contrast to its length, it is usually 5mm or so wide!

It is extremely common in the UK, and can be found wrapped around the bases of algae, or appearing to have tied itself in knots in shallow pools.

Another Lineus longissimus, this time curled up around itself. Image from MarLin (Marine Life Information Network)

This is a video of one moving around, so you can see how they move differently to the flatworms we looked at before

So, why am I showing you pictures of a slightly gross looking creature? (I cannot decide whether I find them fascinating or gross…maybe they can be both)

Well, this phylum of organisms used to be thought to be directly related to the flatworms we covered previously, but now they are thought to have arisen separately, whereas flatworms and Cnidaria are thought to have evolved from one ancestral species, with one line of descendants becoming the radial corals and jellyfish, and another heading off to become the flatworms, with the bilateral symmetry.

The latest research, looking at both genetics, and how cells develop within an organism (cell lineage) appears to show that they are not related to flatworms, but nevertheless, they have some similar features, as well as some which are more evolved, and one very unique feature.

Lets start with what they have in common, and work out from there.

They have bilateral symmetry, and a triploblast body (the three levels of cell within the body) both of which we first saw in the flatworms. (See here for the diagrams from that post)

They do not have a body cavity as such, so they are usually classified as acoelomate, but they have a special structure, which we will get to shortly, and so there is discussion about whether this is a body cavity or not.

The image below is a diagram of a Nemertea, and hopefully you can see the similarities to the cross section of the flatworms here

Nemertea cross sectional diagram. Image from North Carolina State University: Agriculture & Life Sciences

Here is a cross-section of a flatworm for easier comparison:

Acoelomate cross section. Image from University of Illinois Chicago

Nemertea also have the protonephridia that flatworm have (the flame cells which act as a simple excretory system), and they are able to reproduce both asexually, and sexually, also like the flatworms.

So, they have a few features in common, what do they have different?

Firstly, they have an exit to their digestive system (you can see this at the bottom of the picture above).  This means that solid waste does not exit through the same orifice as food goes in by.  They therefore have a complete digestive system, although still a very simple digestive system, without the liver, kidneys and other organs which we have.  Apart from the obvious yuk factor to only having one way in and out for food and waste, what is the advantage of having two ends to your digestive system?

Well, firstly, I do not think animals are bothered by the yuk factor in the way we are, but there is a very clear advantage to having both a mouth, and an anus.  The advantage is, simply, that you can eat and excrete simultaneously, which means having a complete digestive system is more efficient.

Another important development is that Nemerteans have what is known as a closed loop blood-vascular system.  In flatworms, oxygen and other nutrients are distributed by diffusion (remember how they had a digestive system spread all through their body).  Nemerteans have dedicated blood vessels to transport nutrients around their systems.

Nemertean Circulatory System. Image from North Carolina State University

Finally, I said that Nemerteans have a unique feature, and now I will see if I can explain this, because it is very weird, and very cool, and quite gross at the same time.

Nemerteans have something called an eversible proboscis. Eversible means something that can be turned inside out, and a proboscis is something which sticks out from a head, so an elephant has a proboscis, usually called its trunk.  Butterflies have a proboscis, it is the straw-like organ similar to a tongue which they stick into flowers to suck up nectar.

So, Nemerteans have a body part which is turned inside out, and comes from their head….sounds weird, but what exactly does it mean?  In the image below, the proboscis is shown inside the body, and is the dark line running the length of the body.

Diagram showing the proboscis of a Nemertea, running the length of the body. Image from bumblebee.org

This proboscis lies within a cavity in the body, above the digestive cavity, known as the rhynchocoel.  It is because of this cavity that the discussion arises about whether Nemerteans are acoelomate or coelomate.

The proboscis itself is a hollow tube of muscle, and the image below shows it being stored in the body, and after being everted (turned inside out)

Proboscis of a Nemertean. Image from Cabrillo College

So, how does the proboscis get from the inside to the outside, and what is the point of it?

There is fluid in the cavity where the proboscis lies, and when the animal runs into prey (quite literally sometimes, as some species are not very good at finding their way around), muscles at the back of the cavity contract, and this causes an increase in pressure in the cavity, which forces the proboscis out of the body.  There is a spike on the end of the proboscis, and this is stabbed into the prey, whilst the proboscis itself coils around the prey.

The proboscis also has toxic slime (mucus) on it, and this goes into the prey.  Some of these toxins are the same as in the puffer fish (tetrodotoxin), so very effective!  The proboscis is then pulled back into the body, bringing the prey with it, this brings it towards the mouth (the opening under the head), and it is then swallowed whole.

I think the description might seem a bit confusing, so this video hopefully makes it clearer (also take a look at the length of the proboscis relative to the length of the animal).

 

I did say I thought they were both gross and fascinating…

Next post will be away from worms, I promise!

References and further reading:

Phylogenetic position of Nemertea derived from Phylogenomic Data -Struck & Fisse 2008: http://mbe.oxfordjournals.org/content/25/4/728.full

Progress in Nemertean Biology: Development and Biology – Turbeville 2002 http://intl-icb.oxfordjournals.org/content/42/3/692.full

Information on Cell Lineage – Chisholm 2001: http://bio.research.ucsc.edu/people/chisholm/lab%20PDFs/Cell%20Lineage%20(EoG).pdf

Backwards before we go forwards

As we move along the evolutionary line, phyla become larger and larger.  As an example of this, the phylum Cnidaria contains jellyfish, corals and sea anemones, and a few other species.  Human on the other hand, are in the phylum Chordata, which includes ALL organisms with a central nerve cord, with or without vertebrae, so this means that all fish, birds, amphibians, reptiles and mammals, as well as a large number of organisms such as sea squirts and lancelets.

Up til now I have been broadly covering one phylum at a time (I skipped some from the previous phylum because they are parasitic, and are later developments), but from now on I will mostly be looking at how the development  of life occurred, as we head along the tree to where we are today. I am skipping over some organisms such as Rotifera, and parasites as whilst these are interesting, they are are not vital to the journey up the tree, as well as terrestrial species of Platyhelminthes etc because at this stage in the story we are still in the marine environment.  This means I may not be directly following through one phylum into another, but rather showing examples of organisms which illustrate specific points along the tree, as well as showing some organisms which, for whatever reason, did not make it through to today.  I find this fascinating, and I hope I can convey that across to you.

To illustrate what I mean about jumping around, the image below is the tree of life.  We have covered so far, the bacteria, the jellyfish (centre top of the diagram), and flatworms (about halfway down the right hand side of the diagram).  We are in the Ediacaran period, just about to move into the Cambrian (the 3rd and 4th rings around the diagram).

Tree of life. Image from the BBC

Alright, so after that slightly boring introduction, where are we going to today?

I was planning on showing you where our journey takes us after flatworms, but in researching this, I decided it was better to lay out a little background first, so today is about fossils, and what life was like in the early seas.

The image below is one of the oldest known animal fossils, Charnia, which is similar in its body shape to the sea pens I covered in an earlier post, although it is not thought to be related to it.

Charnia fossil, image from Wikipedia

This image is of a living sea pen, so you can see the physical similarity between them. Discussion as to whether they are related is centered around where the growth of new polyps occurs.  In living sea pens, new growth happens at the base of the fronds (near the “stem”), and so pushes the branches outwards as it grows, whereas in Charnia, the new growth appears to have occurred at the end of the branches (See references for link to journal paper, Antcliffe & Brasier 2007)

Modern day sea pen. Similar in shape to a Charnia, but not thought to be related. Image from UCMP

Other common fossils from the Edicarian (or Vendian) period resemble other Cnidarians, but again, discussion continues as to whether they are indeed like these, and related to them, or members of extinct groups of organisms.

Dickinsonia fossil. Thought to be similar to some species of Cnidaria today, although discussion is still ongoing.  Image from wikipedia

The final image shows one of the Cnidaria thought to be similar to Dickinsonia.

Fungia coral, image from wikipedia

And one more of the Fungia coral

Fungia coral, image from UCMP Berkeley

There are other fossils from the Precambrian period which have physical similarities to jellyfish or sea anemones today, but their exact nature, and what their body shape was exactly, is a subject of ongoing discussion among paleobiologists (People who study fossil biology). In any event, it appears that few of the Precambrian fossils which we find have living relatives today, and the majority of organisms alive today arose during the Cambrian period, and so it is this period I will be focusing on.  If you are interested in Precambrian fossils, there is an amazing location in Newfoundland called Mistaken Point, and several location in Russia which have fossils from this period.  Sites I have been browsing around while reading up on this have been UCMP (University of California Museum of Paleontology)  and their site on the Vendian period and also Fossil Museum.net

Flatworms are thought to have arisen when there was not a great deal of sedimentation on the sea floor, and predators were not very mobile, especially on the bottom of the sea.  Jellyfish mostly move up and down the water column, and corals are not mobile.  However, as time progressed, sediment accumulated on the sea floor, and as smaller flatworms may have been prey for larger ones.  Flatworms are a good shape for moving along the sea floor, but not so great for burrowing to hide from predators, or find food in the sediment.

There were places to hide in the early seas, there were microbial mats (the mats were likely from the blue-green algae, or cyanobacteria, which I talked about here), and today they are found in many extreme environments as well.  The image below shows a mat formed by organisms which use methane as a food source (methanotrophs)

Microbial mat on the sea floor (white and orange sections on the photo). Image from NOAA Ocean Explorer

It is thought that organisms could hide under these mats, or feed on the microbes which make them up, and burrowing fossils have been found from the Precambrian period, which seem to indicate that there were organisms which could burrow horizontally, that is, dig a little into the sediment and move across the floor just below the surface.

Ediacaran trace fossil showing horizontal burrowing (much like traces you see of worms on the beach) Image from wikipedia

These burrowing fossils would seem to indicate that the organisms were round rather than flat, and the image below shows more clearly the comparison I made with the traces you see of worms on beaches.

Helminthoidichnites fossils (Remember how Platyhelminthes we covered earlier means flat worm? the Helminthes part means worms, so this is worm-like fossils) Image from Amherst College

So, to recap the rambling post above, the history of life in the Precambrian is a bit messy, and whilst there is evidence of life forms which are similar to those today, it seems that they are not direct ancestors, so as we move forwards, whilst I will cover some Precambrian stuff, most of the organisms I cover will be from the Cambrian period onwards.

I hope I managed to give a bit of an explanation of why flatworms may have evolved towards rounded shapes, and the next post will attempt to cover the movement from these rounded worms to the next group.  The explanations I have written today are by no means set in stone, and are among many different explanations for life in the Precambrian.  One of the things I love most about science is that ideas are always open to challenges, and what is considered as a possibility, even written in journals, may be discounted if sufficient evidence arises against it.

For the more sciencey people reading, I have had some difficulties within the Precambrian, as the phyla present there are not directly related to those around today, yet seem to have morphological similarities (annelids and cnidaria etc). I felt that it was important to take a detour into this period, as many people are unaware of the existence of life in the Precambrian due to the amount of times that “the Cambrian explosion” is used for the beginning of complex life.  I have tried to give a possible reason for the change in morphology from flatworms onwards, and next post will move into brachiopoda as I think these came along next, then onto the basic mollusca.

I will try my best to make the next post less technical and heavy, however, I think that the readers of this blog do not want the information watered down too much, and you are more than welcome to ask if there is anything you do not understand, and I will do my best to answer it, or try to point you in the direction of information on the subject.

References:

Charnia growth patterns: http://oxford.academia.edu/MartinBrasier/Papers/416927/Charnia_and_Sea_Pens_Are_Poles_Apart

Friday Documentary 2

Today I began writing a post which turned out to need a lot more research than I originally planned, and to need a lot of re-wording so it does not sound too boring, so, instead of a Friday post, I am going to link to another awesome documentary.

This one is David Attenboroughs First Life, and it is a bit of an update on Life on Earth, which is the series I am working through for these posts.   It is well worth watching, and I hope you enjoy it as much as I do (I have watched most of these documentaries at least 5 or 6 times, as each time I notice something I did not see last time I watched it)

One final thought before I link the video.  When I was getting these DVDs, I noticed that an entire DVD set of say, Life on Earth, costs about the same as a movie. Now, maybe I am strange, but I think that a documentary series is far better value than a single movie.  I usually get my DVDs from the Amazon BBC site, so maybe prices are different elsewhere, but for me, they are great value.

More Turbellaria: Moving, eating, mating

After being a bit technical last post, today will be more visual I hope!  I am hoping that I can show you some very interesting things today about Turbellaria, and hopefully something you had no idea about, but will find fascinating.  Apologies for not posting this yesterday, I was trying to track down some good videos and images.

So, lets start out with moving around.  As I mentioned in my last post, Turbellaria are mobile organisms.  Although jellyfish and medusa stages of the Cnidaria are not sessile (stationary), they do not really move, they are more carried around by currents within the water (This is called passive locomotion).  So, another step forward that Turbellaria make is being more in control of their movements (Active locomotion).

Being more mobile may have led to the cephalization that I mentioned last time, as in order to move, you need to see, or sense, where you are going, and so sensory organs became located at the front end of the organisms. Many species of Turbellaria have light sensitive spots on their front end (these are called ocelli), and some have small flaps sticking out of the side of their heads, which look like little ears, but are usually chemo-receptors (in us, we have our taste buds and our sense of smell as chemo-receptor)

A Turbellarian (Sabussowia ronaldi) with the eyespots visible (the dark spots). Image from wikipedia

So, now they can see, at least a little bit, where they are going, how do they move where they want to go?

Many Turbellarians move by moving their muscles (and in smaller species, cilia) in an undulating motion (like ripples moving along their body), and sliding over mucus which they secrete (A bit like snails, but more wiggly!).  There are some species however, which swim, and they do this in the same manner as the ones which wriggle along the sea floor or ground, by waves of muscle contraction.  I think this looks really fun, and beautiful, so here is a video of a marine flatworm swimming.

Now, to get a little bit stranger, once they get where they want to go, how do they eat, and what do they do with their food?

As I mentioned last post, Turbellaria have blind guts, that is, there is only one entrance and exit.  This does not mean that absolutely everything which is taken in and not used comes back out the same way, although the majority of it does, as we shall see shortly.  Turbellaria, and their phylum Platyhelminthes, are the first organisms we encounter which have excretory organs.  In humans, these would be things like kidneys, which filter out waste products from our blood.

Turbellaria have nothing as complex as our kidneys, but they do have what are known as protonephridia.  These are effectively tubes which are closed at one end, and have small holes which draw fluids in from the body. They have special cells called flame cells, which have flagella attached to them (flagella are structures extending from a cell, which often move in a whip-like motion, which is where they get their name.  Flagellum is Latin for whip. These are often used for movement in bacteria) These beat, and move fluids down into the open end of the tube, where they are moved along with the help of cilia, and the tube opens out at the edge of the body.  These protonephridia are used for something called osmoregulation which is mostly removing excess fluids from the organism, although small amounts of ammonia are also removed.

Protonephrida. The blue arrows indicate where fluids enter near the flame cells, which beat and move the fluids down the tubes so they can leave the body at the surface.  Image from Biology 8th Edition: Campbell & Reece via College of DuPage

Turbellaria are carnivorous, and feed on things such as small crustaceans (crabs are crustaceans), insects, and very small organisms called rotifers.  The cool thing about Turbellarian feeding is not what they eat, but how they eat it.

The image below is the general anatomy of a Turbellarian, it shows some of the structures I have already mentioned, including the eyespot, and the brain-like collection of nerves at the front end, and the protonephridia, and some parts that we will come to in a bit, however, the part we are interested in now is the tube sticking out on the underside, labelled as “pharynx”.

General anatomy of a Turbellarian. Image from Geochembio.com

The image below more clearly illustrates the location of the pharynx.  In humans, our pharynx is the tube which runs from our mouth down towards our stomach, behind our larynx (the tube which we breath through).

Diagram of a Turbellarian, showing the pharynx, and gastrovascular cavity. In humans, the gastrovascular cavity is our stomach and intestines. Image from DuPage college, and Biology 8th Edition: Campbell and Reece

So, the pharynx in turbellaria is a tube which can be pushed out of a hole in the underside.  As these do not have teeth, or a mouth for holding food and starting digestion as we do (food begins to be digested within our mouths, before we swallow it), turbellaria instead have digestive juices come directly out of their pharynx onto their food, which begins to digest it, and it is then sucked back up into the pharynx, where it goes to the cavities within the organism for digestion to be finished.  Undigested food is excreted back through the mouth.

The video below shows a pharynx extended from a turbellarian ready for feeding (at around 0.27 seconds), as well as some of the other features I have mentioned in this post.

I said at the beginning of the post that I would show you some things that you probably didn’t know, and this is where I do that, I hope you find this next section as interesting and weird as I do!

Turbellaria are able to reproduce both sexually, and asexually.  When they reproduce asexually, they usually divide themselves in two behind their pharynx, then each new animal regrows the parts that it is missing, so one part grows a new head end, and one grows a new tail end, some divide themselves lengthwise to reproduce asexually.  This in itself is fairly cool, as asexual reproduction is usually confined to much smaller organisms.

However, the really cool part is when they reproduce sexually.  If you look at the detailed diagram of the anatomy above, you can see that it has various genital organs.  The image below shows this in clearer detail

General body plan of a Turbellarian.  Image from Southwest Tennessee Community College

In the image above, you can see that the animal has both a penis, and a genital pore, as well as testes, oviducts and ovaries.  This means that Turbellarians are hermaphrodites, having both male and female genitals. This is useful from an evolutionary perspective, as it means that if you encounter another of your species, you are able to reproduce.  In humans, in order to reproduce, we need someone of the opposite sex, whereas Turbellarians just need to bump into another of their own species.

So, when they bump into each other, do they both fertilise each other, or how do they decide which one is going to be receiving the sperm?

Well…this is where it gets weird.  Flatworms do something called penis fencing, yes…you read that right.. (This is part of the reason I was delayed making this post, I had to find a video which clearly showed this occuring, and internet searching “penis fencing” can lead to some…strange results!)

Basically, both flatworm extend their penises, and they have a fight with them, and try to insert theirs into their opponents genital pore.  Unfortunately, they are not always very good with their aim, and tend to stab each other quite a bit in the process!  I think the video below illustrates this much better than I can with words.

I hope you found this as interesting as I did researching this!

Next post will be moving a bit further up the evolutionary tree.

Platyhelminthes: Flatworms and Body Plans

As promised, today is back onto moving through the history of life on Earth.

These next few posts are about a phylum known as Platyhelminthes (from the Greek platys meaning flat, and helmins meaning worm), and specifically about a class called Turbellaria.  For anyone reading this who already knows about the order in which the various phyla and classes are thought to have originated, yes, I know I missed Ctenophores, but I will cover them in future posts.  Also, I know that both Platyhelminthes and Turbellaria,  are debated within the taxonomic community, but, as far as I know, they are the names commonly used at present, and so that is why I have chosen to use them.

First of all, what are Platyhelminthes, and why am I writing about them just after Cnidaria?

Well, Cnidarians are radially symmetrical, this means that they are a bit like a pie, in that if you divided them up into slices, each piece would look almost the same.  They do not have a left or right side, as we do, they have a top (dorsal, like a sharks dorsal fin is its top fin), and a bottom (ventral) side only. The image below illustrates this type of symmetry.

Radial Symmetry and Bilateral Symmetry. Image from Glasgow University Animal Diversity Wiki

The image above shows a second type of symmetry, bilateral symmetry where there is a clear left and right side.  This type of symmetry is common to most animals, as well as plant leaves.  Plathyminthes are the simplest organisms with this symmetry, they are also the first truly mobile organisms I have covered, as Cnidaria are either sessile, or move mainly with currents within the oceans.

Another difference between Cnidaria and this group of organisms is something called cephalization. Cephalization is when sense organs, and nerve control becomes centralised at one end of a body. In more complex organisms, such as ourselves, this is our head, but with Turbellaria, it is not immediately obvious that there is a head.

Before I dive into more technical stuff, I had better show you what on earth a Turbellarian is.

A flatworm (Turbellarian) Pseudoceros dimidiatus. Image from Wikipedia

See? They are very weird creatures!  Here are some more pictures (because I think they are extremely beautiful, and the name flatworm makes them sound a bit boring and gross)

Also Pseudoceros cf dimidiatus. A subspecies of the previous image. Image from starfish.ch

And one more, because they look so cool!

Prostheceraeus giesbrechtii Image from wikimedia

As you can see, apart from looking really amazing, these are externally very simple organisms, but a little closer to us than Cnidaria.  As mentioned above, these have bilateral symmetry, and cephalization. I think some of them look quite cute, but then, as you will discover as we move through these posts, I have a strange idea of what is cute! The cephalization is not clear in the Turbellaria shown above, but I think the image below makes it more obvious, as well as looking really cute.

Dugesia subtentaculata. Image from wikipedia

So, apart from having symmetry similar to us, and a primitive form of head (These still have no brain as we would recognise one), what else do these have that separates them from Cnidaria?   This is where it can get a bit technical, but I will try to keep it clear, it is one of those cases where I know what I want to write, but am not sure if I can get across what I mean simply enough.

There are two further important differences between Cnidaria and Platyhelminthes:

First, they are what is known as triploblasts.  This means that when they are developing, very shortly after fertilisation, the group of cells forms what is known as a blastula. This is essentially a ball of cells, with a hole in the middle, and they look a bit like this:

Blastula. Image from University of Wisconsin

Now, this is where it gets a bit cool….The cells of the developing embryo then folds inwards at one end, and continues until the folding in of the cells effectively splits the cavity in two.

Diagram showing Gastrulation. Image from University of Wisconsin

Pictures of gastrulation occurring. Image from University of Wisconsin

This happens in Cnidaria as well as with Platyhelminthes, right the way up to us.  In us, the opening which you see here becomes the exit of our digestive passage, and another hole is formed at the opposite end which becomes the mouth (We are deuterostomes, meaning second mouth). Some organisms form the mouth where the original opening is, and the exit at the opposite end, and they are called protostomes (first mouth).  Turbellaria have only the one opening, in common with Cnidaria.  This is called a “blind gut”.

So, after a bit of a detour, what does this have to do with triploblasts?  Well, in organisms such as Cnidarians, there are 2 layers of cells, the ectoderm which is the outside cells, like skin, and the endoderm, which forms the lining of the gut. Flatworms, like us, have these two layers, and an additional layer, the mesoderm.  In mammals, the ectoderm forms the skin, the endoderm forms the lungs, the digestive system, and liver, and the mesoderm forms muscles, cartilage and blood vessels, among other things.

Comparison of diploblasts and triploblasts. Image from Kenyon College

Finally, Turbellaria are acoelomate. This means that they have no body cavity,as we do, containing their organs.  This cavity, often fluid filled, serves to protect the organs from crushing. This sounds more complicated than it is, as these images illustrate.  The first shows an acoelomate body, where the organs are in contact with the epidermis (skin layer) through the mesoderm.

Acoelomate body structure. Image from University of Illinois Chicago

 

The second image shows a coelomate body diagram, and it can be seen that the organs are suspended in a cavity, which allows for compression without affecting the organs.  If an acoelomate has pressure exerted on its epidermis, this squashes the entire area underneath, including any organs which are there, whereas in a coelomate, the cavity compresses before the organs are compressed, adding an additional layer of protection.

Coelomate body structure. Image from University of Illinois Chicago

 

The next few posts will be on how Turbellaria feed, how they move, and how they reproduce.

 

Jellyfish…or is it?

This will be the final post on Cnidaria for now, I will be returning to them later, but I got a bit side-tracked within this phylum, and want to get back to working through evolution.

So, without further ado, let me introduce you to the topics of today’s post.

Portuguese Man O’ War. Image from wikipedia

This, quite frankly, beautiful creature is in the class Hydrozoa (from Greek  ’hudōr ’ meaning water, and ‘zoia‘ meaning animals).  The image above is the Portuguese Man O’War (Physalia physalis), and although these are in the same class as jellyfish, and are thought of as jellyfish by many people, due to their similar appearance, I personally would say they have much more in common with the other Cnidaria we have covered, such as the corals and sea pens.

The images below show various jellyfish, and illustrate why Portuguese Man O’War are often thought to be jellyfish.

Lion’s mane Jellyfish (Cyanea capillata). One of the largest jellyfish. The longest tentacale tail recorded was 37m!  Image from wikipedia

Box jellyfish (Chironx fleckeri). One of the most poisonous jellyfish. Image from Factzoo.com

Freshwater jellyfish (Craspedacusta sowerbii).One of the smallest jellyfish (20-25mm in diameter)  Image from biolib.cz

Another interesting fact is that only one of the three images above is a Hydrozoan.  The first one (Lion’s Mane Jellyfish) is a Scyphozoan, this class is the “true” jellyfish, with a dome shaped jelly above the tentacles.  The second, (the box jellyfish) is a Cubozoan, with a square jelly, and the final one is the hydrozoan.  So, as you can see, the common names for organisms can be a bit mis-leading if trying to find relationships between organisms.  This is one of the reasons those annoying Latin names come in handy! (That, and there is one Latin name for an organism, which is the same globally, whereas each country, or even region, has its own name for each organism)

Jellyfish, of one kind or another, have been around a VERY long time, which is why I am covering Cnidaria at the beginning of the evolution story.  Usually we would not expect to find any fossils of jellyfish or similar organisms, because they have no solid body structures to fossilize and so would be expected to be completely broken down by bacteria long before any sediment is able to build up on top of them, or to be crushed within the sediment before fossilization can occur.  However, the fossil record shows jellyfish-like creatures from around 600 million years ago, and the image below is of a fossil which is ca. 525 million years old. It is likely that this jellyfish was stranded on a tidal flat, and so the imprint of its body was fossilized, much like there are fossilized wave patterns from shallow tidal flats.

Fossilized jellyfish from Winsconsin. Image from UCL Berkeley 

Anyway, I said at the beginning that I consider Portuguese Man O’War to have more in common with corals than with jellyfish, and I had probably better explain myself.

As I wrote in the posts about corals and sea pens, they are colonial organisms, made up of a lot of tiny polyps which perform different functions within the colony, and together, act as one unit.  Portuguese Man O’War are also colonial, although it is not the just the polyp which is involved in the colony building.

Portuguese Man O’War zooids (the term for a single organism within a colony) are integrated within the colony to the degree that they are unable to survive individually, so if one part is damaged too much, the entire colony dies.  This is in contrast to corals, where individual polyps may die, but the reef as a whole will continue.

It has been suggested that colonies such as Portuguese Man O’War may provide an evolutionary link between colonial organisms where each individual can survive separately, and complex multi-cellular organisms such as fish, which are comprised of many different cells, all performing a function, but which are definitely one organism. Each zooid within the colony is so specialized that it can only perform one function.  The ones responsible for feeding are unable to swim, and the ones responsible for swimming are unable to feed.  In addition, each member of the colony is genetically identical.  This is in contrast to the sea pen, which is an accumulation of individual organisms.

The polyps and medusae which make up the Portuguese Man O’War (and other Siphonophores) are found as free living individuals in other Cnidarians.

Whilst researching this post, I got to thinking “If it is a genetically identical colony, how does it reproduce?”, so off to the combined knowledge of the internet I went….  I found out that they reproduce sexually, through zooids which are known as gonozoids.  These are either male or female, and so you can have a male, or female Portuguese Man O’War (this is known as dioecy, and they are therefore dioecious organisms).

So, once I found this out, I then wondered, “if it is a colony, and they are all genetically identical seperate organisms, how does this work with sexual reproduction?” Well, the answer is that once the egg has been fertilised, and larva has developed, asexual reproduction occurs.  In complex organisms, this would be similar to the way cell division occurs after fertilization to produce the different cells which have different functions.  This process is called mitosis

The balloon structure on top of the tentacles is one polyp, known as a pneumatophore, and is filled with gas.  In other Siphonophores, this gas is similar to atmospheric gases, but in the case of the Portuguese Man O’War, the gas has a higher concentration of carbon monoxide.

I apologise for the text heavy post today, I hope it was not too long or heavy to get through, I just wanted to get across that there is more to this amazing organism than being a danger on beaches in certain parts of the world.

On Monday, I will move on from Cnidaria to the next stages of the evolutionary journey.  As I said way back at the beginning of this “Life on Earth” section, I am working my way through David Attenboroughs Life on Earth series, and we are just about at the end of the first episode now.   If you get a chance, I highly recommend you watch the series, even if you know most of this stuff already, it is a really interesting series, just as good as Carl Sagans Cosmos series.

 

Further Reading:

These links are to sites I have used whilst writing this, so if you want to read in more depth, these are interesting starting points.

http://www.thecephalopodpage.org/MarineInvertebrateZoology/Physaliaphysalis.html

http://siphonophores.org/SiphOrganization.php

http://www.brown.edu/Faculty/Dunn_Lab/

http://www.ville-ge.ch/mhng/hydrozoa/sipho/siphonophora.htm

http://www.aloha.com/~lifeguards/portugue.html

 

Sea pens

Today I thought I would move on from corals, still staying within Cnidaria, as there are a few more organisms I want to cover before moving on to the next set of cool animals!

Today is about Sea Pens, which are Anthozoa, so still “plant animals” and in the same class as corals and sea anemones.  This means they are also stationary (sessile) in their adult form.

Sea pen. Image from Aquaviews

Sea pens belong in a subclass known as Octocorallian corals. These are soft corals, which means they do not have the stony skeleton of the corals we previously covered, and therefore are not involved in reef building.  They are known as Octocorallions because each adult (the polyp stage of life) has 8 tentacles, or octometrous symmetry

Close up of polyps from a sea pen.  Image from Bellenews.com

As with some of the other Cnidaria we have covered, sea pens are colonial, meaning that the first image shows many individual animals, which make up the fringes, and these are shown in the image above. As you can see, they have 8 tentacles each, and this is why they are octosymmetrical.

In addition to this, the first polyp to settle becomes the main “stem” of the sea pen (known as a rachis). This polyp loses its tentacles, and instead supports the rest of the colony. This is known as polyp dimorphism, where one polyp takes a different form to the rest.  Aside from the first polyp, other polyps retain their tentacles, but perform different functions. Some become feeding polyps, known as autozooids, whilst others take water in, and circulate it within the central polyp and the rest of the colony to keep it upright. These are known as syponozooids (zooid is the term for any individual which is part of a colony).

Unlike most coral types, sea pens settle into soft sediments, often sand instead of on rocks like other corals do.  This means the sea pen needs a more stable environment in relation to currents and tides, and unlike reef building corals, is found in waters over 10 metres deep, and often up to 2000 metres depth.

Although we usually associate these creatures with “exotic” waters, they are found in British waters too!  There are 3 species commonly found, and another found in deeper water.  The ones found in British waters are: (Details from UK Marine Areas of Special Conservation

Virgularia mirabilis, (slender sea pean) which is found between 10 and 400m depths around the coasts of the Northern UK primarily, but also in some harbours, such as Holyhead in North Wales, and in a number of Scottish lochs. It is also found around Western Europe, including in Kattegat in Denmark.  You can see if you find them off your coast with this link to WoRMS (World Register of Marine Species)

Slender Sea Pen. Image from MarLIN

Pennatula phosphorea (Common sea pen) (Link to Marine Life Information Network, MarLIN).  This is found in slightly shallower waters to 100m depth, in the UK it is found predominantly to the north of the coast and in lochs, but is also found in the Mediterranean as well as other areas of the North East Atlantic.

Common Sea Pen. Image from MarLIN

Funiculina quadrangularis (tall sea pen)occurs at a deeper range than the previous two, usually between 20 and 2000m. This is reported in both the Northern and Southern Hemisphere, from New Zealand, via Madagascar, to the North Atlantic regions.

Tall Sea Pen. Image from MarLIN

 

Video from the BBC Oceans.

Finally, when sea pens are touched, they emit a green light, caused by the combining of two chemicals known as Luciferase, and Green Flourescent Protein (GFP). Some species also squirt water as a defence.

The two pictures below show a sea pen under normal light, and in the dark.  Both pictures are from NOAAs Ocean Explorer site.

Sea Pen under normal light. Image from NOAAs Ocean Explorer

Sea pen in dark conditions. Image from NOAAs Ocean Explorer

When corals attack!

Alright, so now I have your attention with the headline, I promise there will be corals attacking in this post!  There will also be less aggressive aspects of life as a coral.

The polyps, once attached to a surface, grow by depositing calcium carbonate (CaCO3) at the base and sides, whilst the living part of the coral is at the top of the reef.

Coral Polyps. The tube-like structure they live inside can be seen just behind the heads of the polyps (Like a flower stalk). Image from Ocean World

So, how do corals feed, why are they these awesome colours, and what do they do apart from sit there looking very pretty!

The majority of the energy used by a polyp is produced via photosynthesis.  This is something that is most associated with trees, flowers and other green plants.  There are some animals which use photosynthesis, but they cheat a bit, and use chloroplasts (The cells which are responsible for photosynthesis) from algae which they eat. The ones we know about which do this are in a group known as Sacoglossa, or endearingly, sap sucking sea slugs. There is a brief New Scientist article on them here and a link to the paper referenced in the New Scientist article is HERE in case anyone wants to read further on it.

So, since coral polyps are not sap sucking sea slugs, or plants, how do they perform photosynthesis?  In a previous post I linked some pictures showing a coral egg with something called Zooxanthellae in it.  These are tiny micro-organisms which live in symbiosis with the polyp.  It is not certain how mutual the benefit is, but, from the perspective of the coral, there is a great benefit to these organisms living within it, and the Zooxanthellae gain a benefit from being protected within the coral.

Why do corals get a big benefit? Because the regions in which we find coral reefs are waters which are low in nutrients, and the photosynthesis provided by the Zooxanthellae enable the corals to survive in this environment, and this in turn leads to the coral reef areas being among the most productive regions of the ocean, despite having very low nutrient availability.

This symbiosis between the coral and the Zooxanthellae is also the reason why we find coral reefs in very shallow, clear waters, as the organisms need sufficient light to perform photosynthesis.  This however, causes its own problems: Exposure to high levels of sunlight also means exposure to UV which is harmful, and could damage both the coral, and the Zooxanthellae.   It appears that some corals have a defence against this, they are able to absorb UV at certain wavelengths. This means, that despite having a bright white skeleton, which would usually reflect back the light, some corals absorb UV light in their skeletons, whilst reflecting back the light in the wavelength used for photosynthesis (known as PAR, or photo-synthetically active radiation). So, instead of getting a double dose of incoming UV radiation, the coral skeleton protects the tissues by absorbing these wavelengths.  The result is, that under UV light, the corals fluoresce, as the image below shows.

There is a short article on this in Science Mag (LINK), or for the full journal article, the link to PLoS One is here

Corals fluorescing under UV light. Image from Science Mag

This flourescence is usually yellow, but there are some striking examples of other colours, as shown below.

Flourescent corals. Image from sciencephoto.com

More flourescent corals! Image also from SciencePhoto.com

So, apart from all this photosynthetic goodness, what else do corals do to get nutrients?  Well, as they are Cnidaria, they have stinging cells which can fire, so they also hunt, well, hunt as much as a stationary organism can!  The video below shows corals preying on passing small animal at night, and also leads nicely onto the final part of todays post, and the promised coral attacks!

What the video does not mention, and I have tried to find a clip covering it, but have not been able to, is that as well as firing stinging cells at each other, the video (at around 1:51), shows corals extruding digestive filaments. Yes, this is as gross as it sounds, they are effectively turning parts of their stomach inside out to digest the other coral!   I have seen footage of this a number of times now, and it still strikes me as one of the most fascinating, yet  gross things I have seen.

If you want to see more about coral reefs, and the awesome creatures which live among them, and the amazing connections which these organisms form with each other, I highly recommend watching David Attenborough, Blue Planet, Episode 6, Coral Seas (BBC).

There are some fascinating creatures which find shelter within the reefs, and some of these provide direct benefits to the reef, such as the guard crab, a small (less than 5cm) crab which helps defend the reef from a large (over 50cm) starfish (the crown of thorns starfish) which feeds on the reef.

Then there are the fish which eat the coral, the bumpheaded parrot fish being one, as shown in this excerpt from Blue Planet

Even though everyone knows about the amazing colourful fish which live in reefs, for me the most interesting parts are the bits I have briefly covered above, the bits that not many people get to know about, which happen at night. I hope I have managed to show these in a clear way, and if you get the chance,do check out the Blue Planet episode I mentioned, it is really worth it.

References (direct links to the journal articles :

“Coral Skeletons Defend Against Ultra-Violet Radiation” Reef, Kaniewska, Ove Hoegh-Guldberg: http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0007995#pone-0007995-g002

“Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica” Rumpho et al: http://www.pnas.org/content/105/46/17867.full.pdf+html

Friday Documentary!

It is taking a bit longer than I thought to source material for my next post, so, here is a documentary I have been watching these past few days.  It goes some way to explain the reasons why I do the subject I do (Environmental Biology).  For me, it is all about finding the connections between organisms in an ecosystem, and the presenter of this documentary shares my enthusiasm for these links between animals and plants.

The documentary is Secrets of our living planet, from the BBC, presented by Chris Packham, and I have chosen Episode 3, on the seasonal northern forests.  This is the climate zone I am most familiar with, although this documentary focuses on North American forests, the forests are not so different from the ones in the region I live in (Northern Europe), although North America has far cooler wildlife, and WAY bigger forests!

If you have an hour to spare, grab a cup of tea/coffee, and sit down and watch this, if you like David Attenborough documentaries, you will like this, it is a bit more in depth than Attenborough, and is really fascinating, especially if you have never looked at these connections before.

I have also put a short second video up because, well, it is awesome, and even though it looks really insane, I would love to be able to do this! Having said that, I would need some serious breathing exercises before attempting it I think, even if I knew what I was doing!  It is Jeb Corliss base-jumping/gliding, and I want one of those squirrel suits!

Normal posting will resume on Monday!

Still corals!

Ok, a short post today, as my last one was a bit heavy on the text, and I am not sure that is such a good idea.  I got a bit carried away with it, so will try to be more graphical in this one.

Last post was about corals spawning, so this one picks up with what happens after the mass release of sperm and eggs into the ocean.

After the eggs become fertilized, they develop into a larva known as a planula, which, to me, look a bit like some bacteria that you see under a microscope

A coral planula, or larva. Image from sciencesummit

Here is another picture, showing a coral planula alongside a Brittle Star larva

Coral Planula (left) with a brittle star larva. Image from australianmuseum.net

The final two images of  planulae are probably my favourite, they show the zooxanthellae (micro-organisms which live in symbiosis with the coral, more about that later!) already within the larva.  These are present in the egg when released by the colony!  The first image shows the egg with zooxanthellae inside it, and the second shows the larva.

Coral egg, the zooxanthellae are in the lower half of the picture. Image from coralscience.org

Coral larva. Image from coralscience.org

These larva float in the water column, until they land in a suitable place, such as on a rock, or a wreck of a ship. They do have cilia (From the Latin for eyelash, these are small  hair like structures which beat rhythmically), which can propel them within the water column, but they are not free-swimming in the way that larger organisms are.

Once they find a suitable place to settle (assuming they are not eaten by predators, as many of them are), they develop into polyps. This is the same type of polyp as the non-moving (non-motile) stage that all Cnidaria have.

Diagram of a coral polyp, image from wikipedia

Orange Soft Coral polyp, image from world oceans 

Next post will cover the development of polyps within a reef, and how different types of corals co-exist, as well as some surprising habits of corals!