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

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

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!

Cnidaria Part 4: More corals!

So, today we are back onto corals (Anthozoa), after a little detour yesterday, and starting out at the beginning, with the life-cycle.  Different types of corals within the reef structure will be covered later, this post is just going to be about how they spawn.

Most corals reproduce sexually, and the majority of them are hermaphrodite (meaning they have both male and female reproductive organs). Most corals reproduce once a year, and the results are very spectacular!

The corals which are the same species within a reef have synchronized spawning.  This means that all the corals of one species release their sperm and/or eggs at exactly the same time!  It is currently thought that environmental triggers such as temperature, day length, or the lunar cycle. An experiment carried out by NOAA (the National Oceanic and Atmospheric Administration) on the Flower Garden Reef in the Gulf of Mexico in 2008 showed that the although the day of spawning is determined by the lunar cycle, the exact time of spawning appears to be dependent on the time of sunset. (A link to the paper is here)  Other scientists have looked at how the corals are able to time the release of sperm and eggs, and have found that corals have cells which respond to light (photoreceptors).

They have found that some corals have light sensing cells (photoreceptors) of a variety known as cryptochromes, which respond to blue light.  These produce a protein in response to the amount of light available.  In the study (information in this link here) they found that the corals produce a lot of this protein when it is very light, and less when it is dark.  They do produce the protein at night, but a lot more of it when there is a full moon, as shown in the image below (the graph to the right is the amount of the protein produced during a new moon, or a full moon).

Corals response to light, the bars to the right show the level of gene expression, which is what controls the production of this protein. Image from

Why would corals respond to light, and how does this tie into their breeding cycle?

The cells which respond to light are found on the outer most layer of the coral (the ectoderm), and may be sensors for when there is too much UV light, allowing the corals to protect themselves against it, much the same way as we go brown in response to an increase in UV (Not red, that is when we burn, and the skin is damaged, but going brown is due to an increase in a chemical called melanin).

In relation to the spawn cycle of corals, most corals appear to spawn 6-10 days after a full moon, this is when the lowest tides occur (neap tides). Neap tides also occur in the same period after a new moon, but it appears that most corals time their spawn for the neap tides after a full moon.  The reason for the spawning events to occur during this tidal period is that the currents and waves are weakest during this period, and so the eggs and sperm are less likely to get spread across the ocean before they mix.

Ok, now onto the much cooler (and less technical) bit!

I mentioned at the beginning that many corals are hermaphrodites, having both male and female reproductive organs.  During spawning, these corals release a bundle of eggs, surrounded by a sperm package.  This does not fertilise the eggs, but keeps the bundle held together until it reaches the surface, where it breaks apart so that the sperm and eggs can mix with others from the same reef!

There is so much sperm and eggs released by the corals on a reef that it can form a sheen, like an oil slick on the surface, and can actually be smelled!

Coral spawn sheen on the surface of the sea at the Great Barrier Reef. Image from Perth now

Coral spawning, image from

Because it looks much cooler when you see it on video, I will leave you with a short clip from the same reef as the image above, and one slightly longer one with David Attenborough.

Cnidaria Part 3: Corals

Just a short introduction to corals today, I am heading off on a field trip this afternoon, so writing this just before I leave.  As it is a botany field trip, I may be blogging about plants as well as animals over the next few days.

Corals are in the same class as Sea Anemones, so Anthozoa (plant-animals), in the phylum Cnidaria.   We are all familiar with them from images such as the ones below

Tree Coral (Dendronephthya), from

Pillar coral (Dendrogyra cylindricus). Image from wikipedia

When I first saw corals, I thought they were rock formations on the sea floor, and indeed, some of them are very sharp, due to the Aragonite (calcium carbonate) they are made of (calcium carbonate is commonly found as chalk, or limestone, or limescale in the kettle, and can have different hardness depending on the way the crystals in the rock form).  Later, when I heard they were living, I assumed they must be some sort of underwater plant.  The reality is stranger than I possibly imagined, and I still have problems wrapping my head around it, even though I understand how they function, I still look at them and think “That cannot be how they work”

There are two types of coral, Hermatype or stony coral, and these build reefs, and Ahermatype corals which do not build reefs. The posts on corals will mostly be about the Hermatype coral, and Ahermatype corals will be covered under their other names (Sea Pen).

Corals have been around a VERY long time, with fossils recorded in the Cambrian period (this was approx 542 to 488 million years ago), although most existing fossils of coral are from later periods, the Ordovician (488-433 million years ago), and the Devonian (468-359 million years ago).  There is some evidence for soft corals in the Pre-Cambrian period, although whether these are corals, or polyps (one of the juvenile phases in Cnidaria) is debated, see Anthozoa Fossil Record for more details

Coral Fossil, possibly a tabulate coral (From UCMP Berkeley)

Rugosa (horn coral) fossil, from UCMP Berkeley

Next post will be about how corals are formed, and cover their life-cycle, and then later posts will be about feeding, and the different types of coral within each reef, the symbiosis they have with other organisms, and their place within the oceanic ecosystem (What they do in the ocean, and how they benefit the other organisms which live there), as well as the problems facing corals today in our oceans.  (See, I said there would be a lot of posts).