Shells? Who needs shells?

Exams all done now, so back to the fun of blogging about awesome animals!

So far, the molluscs we have looked at have had some sort of shell, but not all molluscs have taken the shelled path.

Today, and in the next post, we are going to meet some which prefer to be without shells, and are, in my opinion, some of the most beautiful molluscs around: So, without further ado, let me introduce you to todays topic: the nudibranch (from the Latin nudus meaning naked, and the Greek brankhia meaning gills, so literally, “naked gills”)

Berghia coerulescens, a species of nudibranch. Image from wikipedia

This group of animals has a whole lot of awesomeness going on, some of which we will cover today, and then it will spill over into the next post too!

For today, lets take a look at the one in the video, it is called Glaucus atlanticusm and the reason it is awesome for me, is that, despite being a squishy little sea-slug like thing, around 3cm long, it eats these:

Physalia physalis (Better known as the Portuguese Man O’ War) Image from wikipedia

Now, if there is one thing we know about Portuguese Man O’ War, it is that it has a very nasty sting, due to the trailing tentacles being full of cells which can fire toxic darts into its prey (or into an unsuspecting persons leg) (See link HERE for post about Man O’ War)

So how does Glaucus cope with these stings? There is a hypothesis that it releases mucus while eating the tentacles (Yes, because it is small, and the PMOW is much larger, it nibbles its way up the tentacles), which protects its insides from the stings, which can remain active for a period of time after the Man O’ War is dead.

To make it even cooler, it takes some of these stings, and uses them in its own defense. It goes a little something like this:

Glaucus eats the tentacle with the stinging nematocysts within it.  Some of these pass through digestion, and end up in growths on the outside of the animal, called cerata. These then become part of the animals defense systems, meaning it can fire them at any attackers.

It has primitive teeth (denticles) which it uses to chomp through its prey, but also to hang onto them

So, this is a fairly cool animal so far, but, it gets even more amazing when you find out some other details about it.

This lovely little animal floats upside down, so its top side is in the water, and its underside is at the surface.  It floats because it has a bubble of air in its stomach, but this means it has no way of steering, so it floats around as the winds or currents take it

In common with many fish which are found near the surface (Sharks for example), it uses a form of camouflage which is known as countershading

Its “top” (Which is in the water) is light silvery grey, so it is difficult to spot from below, whilst its “bottom” is a deeper blue, or blue and white, which helps it blend in with the water when seen from above.

I will leave this last image of Glaucus atlanticus for you, and next post we will look at some of the other animals in this family.

Glaucus Atlanticus. Image from EOL (Encylopedia of Life)

Back with bivalves…

So, its the new year now, I hope everyone had a nice new year, with not too many hangovers!

Last time, way back in October, I wrote about bivalves (HERE), and wanted to pick up where we left off, so today is about their vision and movement, as promised.

I am going to focus on scallops (Pectinidae) as they have both interesting eyes, and strange movement.

Let’s start out with the eyes.  You and me have two eyes, and this is a fairly common number of eyes to have, Scallops however, seem to be quite fond of eyes, usually having between 40 and 70, but they can have up to 100!

Eyes on a giant scallop, the dark blue dots along the rim of the shell. (Image from Wikipedia)

Now, not all eyes function the same way as ours do, we have already met organisms with eyespots, which are just a bunch of light sensitive cells able to distinguish between light and shade so the organism can move away from predators, or towards the light for photosynthesis. (HERE)

The eyes of scallops are what is known as concave mirror eyes.  These are “simple” eyes, which have a reflective layer at the back of the eye, which bounces light back onto the cells which are able to process the light. As they have so many eyes, positioned along the edge of their shell, they are able to follow an object as it moves past them, rather than having to move their eye to keep it in focus.  They also have two retina in each eye, one which responds to light, and one which responds to darkness.

Scallop Eye, showing the reflecting surface at the back which bounces light to the rods. (Image from rewiring-neuroscience.com)

I have been trying to find other diagrams showing this, and have come up with a couple of sites, depending on how techy of an explanation you want:

Opthobook.com has a very simple diagram (HERE), and a little video explaining the basics of eye variation (HERE).  For the more technical diagrams and explanations, this paper by M.F.Land (1965) is interesting (HERE), as well as this illustrative paper by Colicchia et al (2009)   (HERE). One final paper on comparative morphology (similarities and differences between the parts in different organisms) from Speiser & Johnson (2008) (HERE)

So, apart from having more eyes than that teacher at school who always spotted you chatting (Not that I was ever talking during class, of course!), what else is cool about these bivalves?

Well..I think sometimes a video says way more than I can by rambling, so lets take a look at how these scallops move:

This movement is done by using muscles at the back of the shell to open and close it, this pushes water out of the shell, and the scallop shoots off like one of those bottle rockets you make as a kid with fizzy drinks!  They can control the direction of the jet of water, so it is not a completely random motion.

 

 

Stalks, eyes and feet

Last time I wrote about life on Earth, we covered Cowries, and how they were used as currency.  We are staying within the molluscs still, as there are a couple more organisms I want to show you, to illustrate the different solutions that have been found to the same problems, namely feeding, and getting around.

Today, we are going to meet the cutely named Lambis lambis, or one of the Spider Conch molluscs, we may also meet some of its relatives.

Everyone knows what a conch shell looks like, it is the famous “Listen to the ocean in this” shell, and the ones you may have seen probably look like this:

A shell of Lobatas gigas, the Queen Conch. Image from wikipedia

This is the shell of Lambis lambis:

Lambis lambis shell. Image from gastropods.com

I have already covered how shells are formed, by secretions from the mantle, the fleshy part of the mollusc, so that is not what I am writing about today, although I am sure I will write a post about how the different shell shapes arise at some point!

The shell above has spikes all around it, and one long spike at the bottom.  The long spike at the bottom is where we will start today, because it is actually functional, it is called the Anterior siphon canal, and it provides support for an appendage called a siphon, which is an extension of the mantle.  This structure draws water into the cavity within the animal, and passes water over the gills, assisting in getting oxygen from the water, but also acting as a “nose” (Otherwise known as a chemoreceptor) to help the animal find food.

Lambis lambis is a herbivore, but this does not mean it is not guided towards food by what we would know as smells.

Here is what the external (outside the mantle cavity) body parts of Lambis lambis looks like:

Morphology of Lambis lambis. Image from University of Queensland

There are several features of this mollusc (and all conchs), which I want to elaborate a bit on, as I think they are fairly cool.

Firstly, the radula (the toothy tongue thing we met HERE), which is usually contained within the cavity, is actually on the end of the part labelled “proboscis”, so it has a stalk with its mouth on the end, and the radula too.

Here is a video of one chomping away happily on some algae, note the eyes which are also out on stalks, these are checking out the area for predators while it is eating, and if one is spotted, it pulls itself back inside its shell.  I personally like the eye stalks, as they look a little bit like cartoon alien eyes.

All true conchs are herbivores, (that is, ones which are members of the Strombidae family).  Other animals are known as conchs too, and not all these are herbivores, for example the crown conch (Melongena corona) feeds on oysters and clams by using its proboscis to pry open a bit of their shell and eat them from inside their shell.

The other very obvious structure in the morphology picture is the foot, and the part labelled “operculum”.  This is used to dig into the sediment to..well..propel the animal, or turn it over if it gets upside down.  I think this is easier to see than to explain, so here are two videos showing this in action:

Finally, here is a video showing this funky creature moving along a flat surface:

It is not just us humans who like these shells…hermit crabs also use them as mobile homes, as these two videos show:

I hope I have shown you some things so next time you see one of these pretty shells, you also think about the very cool animal which lives inside it.

Shiny, curly shells

We are staying in the mollusc family for a while, but moving on from the limpets we met in the last two posts.  Today and Wednesday are about a particularly interesting mollusc, which has been valued by humans for a very long time!

I would like to introduce you to the Cowry, or Cypraeidae.  When I first started writing this post, I was not sure how much I would have to put in it, but, as I have been researching it, I have found more and more interesting stuff!

Onyx Cowry (Erronea onyx), found in the warm waters of the Pacific and Indian Ocean, from Australia to Madagascar. Image from Wild Singapore

This amazing looking animal is Talparia talpaand the shiny glowy thing you can see is its shell!  The black bit is its mantle (The fleshy bit from which the shell is secreted)

Talparia talpa, with fully extended mantle. Image from EOL (Encyclopedia of Life)

This animal has a very glossy shell in comparison to other molluscs because, instead of secreting from the top of the mantle (The mantle is the fleshy part protecting the internal organs), this shell is secreted downwards, other molluscs secrete their shells from the bottom up, so the inside of their shells is nice and glossy and shiny (the inside of a Conch shell for example), Cowrys start secreting from the top down, meaning the top layer is the newest, and so nice and shiny and glossy.

Most mollusca (the group which the Cowry belongs to) have a mantle as a single layer of flesh under their shell, as in this diagram:

General Mollusc Anatomy: Image from Marine Education Society of Australia

In this group of molluscs however, the mantle forms two halves, which can be clearly seen in the picture with the black mantle.  This wraps around the shell from each side.  The reason it is not fleshy coloured, as might be expected, but can come in a wide range of textures and colours is for camouflage.  Many of these animals will have mantles exactly matching the sponges and corals on which they live!

I have been digging around to find out why these animals may have their shell on the inside, rather than the outside, and have not been able to find a definitive answer, although, if I was asked to suggest a reason, I would say that the camouflage possibilities of a bit of skin are more diverse than those of a shell, which once secreted, is fixed, and that it does not really matter if the shell is exterior or interior if its function is to protect the internal organs.  Also, a shell is not really very good for sensing surroundings, and the mantle could maybe be sensitive to changes in the surroundings indicating predators, other dangers or food approaching.

Next time I will write about how these animals have played a role in human society for a very long time, and their current global status.

Limpets….Going where squirrels fear to tread!

 

Ok, so when we left off yesterday, I had introduced you to some Limpets, and we had looked a bit at how they are not as static as they seem.

Today we are going where squirrels fear to tread, to some truly bizarre habitats.  I hope I can show you some new places which you did not know about, and if you knew about them, then I hope I can show you something interesting  anyway.

Our Limpet friend for the day is Neolepedtopsidae (don’t worry, I can’t pronounce it either, so I will go with “neoleppytopsy” or Topsy for short).  This is a newishly described family of limpets, first described in 1990, and the name means “New lepedtopsidae”, which does not make it any clearer, but, the Lepedtopsidae are an extinct group of limpets, so this is a family which has evolved from the ancient group.

I said yesterday that limpets are found on beaches, which makes it a little strange that we discovered this family only in 1990, as, most beaches are fairly well explored.

This is an image of a species called Eulepetopsis vitrea, which are part of the family of Neolepedtopsidae.

Eulepetopsis vitrea, image from EOL (Encylopedia of Life)

So, where was this lovely little animal hiding?  Let me show you what these animals consider an upmarket address:

The East Pacific Rise, home to our lovely little limpet. Image from Wikipedia

As that is a little zoomed out (Sort of a neighbourhood shot), lets zoom in a little, and see an image from one of my favourite institutions (I used to really want to work there, but I have a thing for water after a bumpy ferry journey in the UK, so not so into that now), the Woods Hole Oceanographic Institute:

Black Smoker on the East Pacific Rise. Image from Woods Hole Institute

Yes, that is a robotic arm that you see in the photo, but it is not because Terminator has happened and the robots are now running the world, it is because these are at a depth of  anything up to 2-3km underwater.  Not quite as deep as the mid-ocean trenches (These reach 10km in the deepest parts!), but still a bit beyond dive range!

I have cunningly managed to fit two of my favourite subjects, geology and ecology into this post, so here comes the sneaky geology part:

The East Pacific Rise runs from just north of Antarctica to California, and is the line running up the right hand side of the ocean floor on this image:

East Pacific Rise, image from Wikipedia

This is what the shape of the “land” looks like at the East Pacific Rise (This is called a bathymetric image, if it was above ground, it would be a topographical map, those circles you see drawn onto maps show height, and this does the same thing, only underwater…It is called bathymetry instead of topometry because it is underwater, and means “deep measures” in greek)

Bathymetric image of the East Pacific Rise, image from NOAA

The reds indicate the highest parts of the ridge, and as it goes through yellow, to green to blue, it gets deeper.  I am a bit fanatical about these undersea areas, because a) they are very very cool, and b) they have amazing features and creatures that we have only just begun to discover.

So, what on earth is this Pacific Rise, and what is it doing messing up the nice smooth ocean bottom?  Well….until the late 19th century, we had no idea what the ocean floor looked like, and many people thought it was flat and featureless, and probably not very deep.  Then an expedition was sent out to map the ocean floor by the Royal Navy, and they did this by chucking lengths of rope with distance markers over the side of the ship (this is known as sounding, which refers to any technique for measuring depth).  Luckily, they took a lot of rope with them, because, as they were going across the Atlantic, they hit a spot where the rope just kept going and going.  It finally hit the bottom at just over 8km deep, in an area known as the “Challenger Deep” (You might have heard of this from when James Cameron did his dive there earlier this year).

After the second world war, there was an increase in ocean mapping (Well, they had to put all those ships and sonar to good use), and as the data was compiled from these expeditions, it became clear that there were a series of lines running round the oceans, and that there were ridges as well as deep valleys.  This is one of the things which helped prove tectonic plate theory, and caused a lot of discussion at the time, about whether the results from sonar had been interpreted correctly, as obviously there was not a complete map of the ocean floor, so maybe they had filled bits in wrong.

The reason the discovery of these ridges helped prove plate tectonics is because, whilst it had been suggested that the continents had been closer together, and even joined into a super continent,and there was geological evidence for this, rocks from different continents matching together, but there did not seem to be a mechanism which could explain what the ocean was doing in the middle of the continents now.

The ridges are where new crust is formed, magma rises up from below the crust, and at the mid ocean ridges spills out, creating new ocean crust.  This is then carried across the oceans, until it reaches the edges of the continental plates, where subduction occurs, and it is pulled back down below the crust as the ocean plate goes under the continental plates.  This is also the reason why the ocean floor is so young compared to other rocks on the surface, with the oldest parts of the ocean floor (near the subduction zones) being a little under 200 million years old.  In contrast, some of the oldest continental rocks are 3.8 billion years old.  So, the big red ridge on the image is where new ocean floor is formed, and as the new material is continuously ejected from below the mantle, it pushes the other material away from the ridge, in the East Pacific Rise, this happens at a rate of 6-16cm per year.

The black smokers, which our limpets live on and around, are formed when sea water (Which is very cold at those depths), gets into the cracks in the ridge.  This means that very cold water under pressure  meets very hot molten rock.  The water becomes superheated, it cannot turn to gas because of the pressures at those depths, so it is liquid water at 400 degrees.  At this temperature, it is less dense than the surrounding water, and so shoots up in a column.  The reason that it is not clear water is because it has dissolved minerals in it from inside the crust, and is very rich in sulfur compounds.

This sounds like very hostile conditions for life….no sunlight, very hot water spewing out of cracks, lots of sulfur, and very high pressure (25 atmospheres at 2500m deep, at sea level, the pressure is 1 atmosphere), but, surprisingly, our very few expeditions to these locations has uncovered an amazing array of organisms thriving!

Instead of phototrophs like we have at the surface, the primary producers (equivalent of trees) are chemotrophs, which make their food from the sulfur and other chemicals emitted by the vents, and a whole food web, right up to fairly large predators, there has even been an octopus sighted near one of these vents!

These vents are of great interest to science, and a branch of biology called Astrobiology, which is concerned with the possibility of life on other planets, is interested in these vents as they illustrate how life can survive in an environment without oxygen and sunlight, and in acidic, sulfur rich conditions. (Astrobiology is also called Exobiology).

For me, these vents are one of the reasons I feel that we need to investigate our planet before venturing into space.  According to NOAA, we have covered less than 0.1% of the ocean ridges with our submersible expeditions (They tend to return to the same locations), and overall, we have explored less than 5% of the deep ocean, and the bits we have explored have revealed some amazing creatures, so what else is hiding down there, waiting for us to investigate?  The bottom trawling which occurs in much of the worlds oceans tends to flatten the bottom of the ocean where the trawlers fish, and damages seamounts, which are underwater mountains, home to many communities as yet undiscovered by humans, yet being damaged by deep sea trawling.

We only discovered these vents in the 1970s, and have barely begun investigating the deep ocean, so I look forward to many more exciting discoveries in the future.

Finally, here are some images of the communities which exist by these vents…the density of organisms is astounding!

Crabs and mussels on a deep sea vent, image from marinebio.net

An Octopus living near an Antarctic Hydrothermal vent. Image from Scientific American

Finally, here is a video showing a “Yeti crab”(Kiwa hirsuta) colony, again, near Antarctica.


I hope I have shown you some interesting things today, and that you did not fall asleep in my wall of text!

 

Limpets: Not just hanging around

Today, I introduce another group of animals which my fellow blogger Argylesock can add to the list of species to race with me when I visit the UK next!  By the time I get over there, hopefully we will have a list including some species which move a little faster, otherwise vast quantities of tea will be drunk whilst waiting for the race to finish!

Whilst a range of organisms are called Limpets, these next two posts will focus on one specific group of them, the “true” limpets, Patellogastropoda.

Today’s post is going to be about their shell, morphology (body features) and behaviour.  Tomorrow (extra post because I missed Monday) will be about their habitats.

Most people will be familiar with limpets, from the inter-tidal areas on beaches, where they are found on the rocks when the water recedes.  This lovely picture is of the one I am most familiar with from my time growing up by the sea, Patella vulgata or the common limpet:

Patella vulgata, hanging around on a rock. Image from Encyclopedia of Life

Many limpets have a slightly pointy shell, as you can see in the image above.  One of the very cool things about these creatures with shells is, that, unlike us, where you cannot point to a part and say “That is from when they were younger”, with molluscs, they secrete their shell starting from the centre, so the very centre of the shell is from when they first started secreting it.

As with all molluscs, limpets have a radula (the scratchy toothy tongue we covered before), which they use to scrape algae from the surface of rocks.

As anyone who has seen limpets will know, they are usually found sitting on rocks.  However, they do leave the rocks to go foraging for extra food between tides, and they follow their mucus trail (like a snail trail) to get back to their comfy spot on the rock.  You can see when limpets have been attached to rocks as they often leave a scar there from their shell edges.  This indentation from their shell edge means that when they settle down again, the shell digs into the top layer of the rock, which makes them more firmly attached than if the shell was just resting on the surface of the rock.

Although we do not usually see them moving around much, and so may think of them as fairly passive organisms, they are of course, living animals, and so have some defenses against predators (It is not a good evolutionary strategy to sit there and get eaten!).  The first video shows one of their defense responses, and illustrates the surprising flexibility of these animals, whilst the second shows feeding behaviour.

 

So, apart from homing behaviours and acrobatics, what else is cool about these animals?  Tomorrow I will cover the coolest thing about these animals (for me, at least), but there is one final thing I want to tell you today.

Patellogastropoda are hermaphrodite, that means they have both male, and female reproductive organs.  This is not unusual in invertebrates, or generally within nature, but what is interesting about these is that they are all male to begin with, after the larvae settle onto a rock and they start developing.  They mature as males after 9 months or so, and stay male for several years, before the majority of them change to female.

This means that the majority of sperm released is from younger animals, whilst the eggs are released by the older members of the group.

Ooops, I forgot!

It suddenly dawned on me when working out the next organism to cover, that I had completely forgotten to mention how we got to molluscs! (Panic not, I have doubled my daily allowance of tea and cheese and biscuits to make sure this never happens again!)

Today is about how we ended up at molluscs….so, hold onto your hats, there may be some taxonomy coming up!

Cladogram (relationship tree) for molluscs. Image from UCMP Berkeley

The diagram above shows the relationships between different groups of molluscs.  Now, these diagrams are constantly being redrawn, as new information comes to light.  This does not mean that scientists do not either a) know what they are doing, or b) know whether these organisms are all molluscs.  The groups within molluscs, and the animals in each group are largely agreed.  The differences come when trying to find out where the groups separated from each other, and once in a while merging or unmerging smaller groups as new molecular or genetic information comes to light.

What the diagram above shows is that the polyplacophora (the Chitons we covered in the last two posts) and another group, the Aplacophora, are related more closely to each other than they are to the other mollusc groups, and that squids are more closely related to snails than they are to mussels.

It is thought that the Polyplacophora (meaning carriers of many plates) and the Aplacophora (meaning without plates) are the oldest living forms of molluscs.  Neither of these have shells in the way many molluscs do (we will cover squids and octopus later on), the Chiton, as we saw in the last post, have a series of plates which are held in place by a girdle, and Aplacophora have no shell at all, but instead have spicules, which are tiny spikes.  As an example of how groups can be moved around, the Aplacophora are split into two groups by some biologists, and some data appears to show that one group may be related to the Cephalopods.  (The details can get a bit techy, but it is to do with molecular data, and embryonic development, links as always, are in the Further Reading section)

A lot of the lack of information on these, and many smaller organisms is due to the fact that not many people study them.  Whilst research can take you into some very interesting, and unusual species, if you are not aware of them in the first place, then you will not consider them as research topics.

Back to the Aplacophora: The following images show some close ups of one species, from that favourite resource of mine, Encyclopedia of Life.  The first image is a zoomed out view.  The spicules are just visible as slightly shiny bubbly looking bits.

Chaetoderma elegans. Image from EOL

Now for a zoomed in image, using a dissection microscope.  The spicules are more easily visible in this image.

Chaetoderma elegans. Image from EOL

Finally, a view of some spicules.  These are so many different colours because they are taken using a cross polarizer, which means that you are better able to see the structures.

Spicules from Chaetoderma elegans. Image from EOL

 

Ok, one more picture…I did promise no more worms for a bit, and technically speaking, this is not a worm…here is a picture of the same species as above, but zoomed right out so we can see its shape:

Not a worm…honest! Image from EOL

Now, eagle eyed readers will recognise this shape as one we have bumped into before, when we were with the Nemertea, and indeed, Nemertea are grouped into a Superphyla, known as Lophotrochozoa, due to molecular and developmental evidence.

So, we have a worm-like creature which can secrete spikes from its skin, very handy as a defence mechanism (Think about hedgehogs!).  As we saw in earlier posts, some organisms headed underground for safety, whilst others secreted a shell round one end of their body, which then stuck out of their burrows (HERE)

Now, both of these are effective strategies, but not without their drawbacks.  Living below the surface limits the food that is available to you, and is not really very safe once predators find out you are in the sediment.  Living vertically in a burrow with a shell around your protruding end allows for you to access more food, but limits you to food which is passing by.  As a defence mechanism it is fairly good, keeping your head attached is very important!

Secreting small spikes from your body offers less protection than a shell around your head, but allows you to move about much better, meaning you can increase your range when looking for food, or proximity to mates (many of these species use external fertilization, which means they release sperm and eggs, which are then fertilized by others nearby).

If these spikes widened, and joined together in bands, then you have something which may look like the plates we see on Chitons (This part is purely hypothetical on my part, so feel free to yell if I have got it completely wrong).

I think that is enough wall of texting for today (and rambling!).  Next time is moving on from Chitons to the next animal in our journey, and I will make sure to try to explain the links between them in future!

Further Reading:

UCMP Berkeley Paleo site, always a very handy site for me: http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/aplacophora.php

Paleo-biology article about very early molluscs: http://palaeo-electronica.org/2011_2/221/index.html

Blog post outlining the above article (less techy version!) http://www.palaeo-electronica.org/blog/?p=416

Phylogenic discussion of one of the Aplacophora species: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2828982/

Article discussing predation and shell evolution: http://www.biology.ualberta.ca/palmer.hp/pubs/79Evol/Palmer’79.Evolution.hi.pdf