Bivalves…Sucking and Sieving

Today we pick back up with our journey through evolution and natural history.

Last time we met the Spider Conch and today, we meet some of its relatives, the bivalves.   As the name suggests, these animals have two shells, and the ones you probably know best are oysters and clams.  Today I will write about the various feeding methods of these animals, and then the next post will be on movement and vision.

There are bivalves which resemble a 2-shelled animal we met earlier, the brachiopod (HERE), and so can be easily confused.

The first bivalve I would like you to meet today is Pedum spondyloideum, or the blue-lipped coral oyster. I am mostly showing you this one because I think it is spectacularly beautiful

Blue lipped coral oyster. Image from wikipedia

This is a teeny tiny scallop (or Pectinidae to give it the proper family name), which lives between corals.  These are in the same order as oysters (Ostreoida), so are related to them, but are in a different family to what me and you know as oysters.

There is an important differences between these, and the other molluscs we have met so far;

General anatomy of a bivalve. Image from Merriam-Webster

I mentioned before (HERE) that molluscs have a rather cool tongue called a radula, which is essentially lots of rows of tiny teeth that they use for scraping food off of surfaces.

If you look at the diagram above, there is no label saying “radula”.  This is because bivalves do not have one!  (They also do not have a head!) The image below shows the internal structure of a clam, and will help me explain what they do instead of scraping food:

Internal anatomy of a clam, image from Encyclopedia Britannica

In the image above, you can see something labelled the “incurrent siphon” and the “excurrent siphon”. As these animals breathe (by extracting oxygen from the water), they cause small currents around their gills.  These currents contain not just water, but yummy particles of food, which get moved towards the gills.  There are cilia (those small hair-like wavey things we have bumped into a lot) on the gills, which move these currents towards tiny pores.

If you take a peek at the top diagram, there is something labelled as the “labial palps”.  These, and the gills produce mucus (like you do when you have a cold), and this covers the food particles and they fall down towards the mouth where they are eaten.  So yes, they do eat food covered in snot!  Large particles like sand fall down into the mantle, and are carried out by cilia again (those little hairs just get everywhere don’t they?). Sometimes these particles get stuck in the mantle, and become irritating, at which point they become pearls (although not the sort we use for decoration, they are formed differently).

This method of feeding is known as filter feeding, and is how most bivalves eat. There are some species however, who obtain their food using a method known as deposit feeding.

This is thought to be the original form of feeding for bivalves.  Instead of the gills assisting in filtering food, they are used purely for breathing, whilst the labial palp has tubes attached to it which stick out to grab food from the sand or mud.  Food which is caught in currents moving towards the gills is also grabbed and eaten.

Still other species use symbiosis with small organisms (a lot like the corals do) whereby these organisms carry out photosynthesis and the bivalve gets most of its nutrition that way, while doing a small amount of filter feeding.  The most well known example of this is the giant clam, which is a huge animal, up to 1.2m or so long.

Giant clam, image from wikipedia

These animals are so huge that they are not able to move, so they sit on the sea floor, often in places like the Great Barrier Reef:

Giant clam on the Great Barrier Reef. Image from National Geographic

The bacteria, and dinoflagellates which I wrote about HERE obtain food by photosynthesis, like plants do, and then the Giant Clam feeds on the by-products produced, as shown in this video:

One final point about bivalve feeding.  Because they filter feed, they also perform a role in cleaning water, which benefits other organisms in their ecosystem, and mussels can be used as an indicator of how polluted a body of water is.  This is because as they feed, heavy metals and other pollutants are filtered, and build up within their bodies as they are unable to process them (like us with mercury etc).  So, if you measure the levels of these pollutants in mussels and other bivalves, it gives you an idea of how polluted the area is.

This video shows oysters and how they can function as filterers of water:

As mentioned in the video, populations of bivalves are decreasing in some areas, and this means they are less able to filter the water, which in turn has an impact on the other animals and plants in the ecosystem.


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

More flourescent corals! Image also from

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:

“Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica” Rumpho et al:

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

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

Coral larva. Image from

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!

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

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

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

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

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!