The Coral Eating Starfish - Crown of Thorns

Although the Crown of Thorns Starfish (COTS), Acanthaster planci, is not something to be excited about in the slightest, it is worth blogging about, as they are quite interesting creatures. There are actually over 2000 different species of starfish or sea stars. Most have the remarkable ability to consume prey outside their bodies and they can also regenerate entire limbs! The general consensus about starfish though, are that they are nice, pretty, colourful  little animals, but the COTS is far from it. The second largest starfish in the world, it has a diameter of up to 3 feet and up to 21 arms! There bright colours make them quite beautiful and nice to look at, but beneath their fortress of protective poisonous spines is something quite deadly and destructive.

Crown-of-thorns starfish are one of the most damaging creatures of tropical coral reefs. They are corallivores, preying on living coral polyps! COTS especially like to eat the fast- growing Acropora species, particularly table corals. With hundreds of sucker-like tube feet they climb onto living corals and then throw out their stomach through their mouth to digest the coral tissue alive so it can be absorbed! Not so nice! This leaves a white scar of dead coral skeleton. One starfish can consume up to 6 square meters of living coral reef per year. Natural numbers of these starfish on the reef are not a huge problem, however, if there is a huge population boom, whereby their numbers are increased in their hundreds, well they can ultimately devastate entire reefs.

So it was not the best news when one of the dive instructors reported seeing many of them on the outside reef of the atoll! What to do? COTS do have natural predators, titan trigger fish, napoleon humphead wrasse and the huge triton snail, but not so many on the reef as to eat all the starfish. We decided the best plan of action was to collect and remove the starfish from the reef before they wipe it out for themselves. It was the right thing to do. During a 5 minute swim along the reef I counted 300 of them. I have never seen so many before, it was shocking. They had devoured part of the reef leaving just the white skeletons of dead corals in the wake of their feeing path. Our mission was on.... we did back to back dives and managed to collect most of the them so not bad for a days work!!

Though scientists do not know for certain what causes COTS outbreaks, there are a number of possibilities:

• the outbreaks are part of a natural cycle
• overfishing and the decline of the starfishes natural predators
• human disturbance such as agricultural  run-off of nutrients from land leading to phytoplankton blooms which provide more food for the starfish larvae.

Hopefully that will be the last we see of them for a long time. This is definitely one animal that i do not feel guilty about digging a hole and tossing them all in! 

 COTS devouring a large table coral. The white skeleton is the dead coral and the brown is still living, for the time being anyway!

Mission on - collecting as many as possible. We used fins or sticks to pick them up as they are covered in protective venomous spines which can give a nasty sting

So many on the reef consuming all the coral in their paths. It was heartbreaking :(

Greedy buggers! The coral has no chance...

Their sucker tube like feed to move around with a mouth in the middle

 A dead white coral skeleton, Acropora clathrata, which use to be beautiful and alive!

 All the dead white coral skeletons. It was like swimming over a grave yard!

 Bringing them up to the surface

  

Whoop whoop, mission accomplished. They dont smell too good on the surface though

We collected about 250 all together. Was a great team effort

My First Oceanic Manta

I’m very happy to report that I saw my first oceanic manta a few days ago!! It was nice to see a manta at all as manta season is usually from December to April but sometimes you get the odd one passing by.

Manta rays are cartilaginous fish which means they are closely related to all other rays as well as sharks. Until quite recently there was only thought to be one species of manta ray and in 2009 they were re-classified into two separate species; the resident reef manta (Manta alfredi), and the giant oceanic manta (Manta birostris). Oceanic mantas are the bigger of the two species reaching a wing span of up to 7m while resident mantas grow to about 4m.

We were diving at Bodu hithi Reef which is an important manta spot during the season. However, we were not there to look for mantas… a tiger shark had been spotted at the same reef a few days before so although it was unlikely to see it again it was worth going to check out as you just never know. I have always wanted to see a tiger shark or any big shark will do, you probably think I’m mad, but sharks are such fascinating and impressive creatures, and well I’m bored of seeing black tips and grey reefs now… I want to see something big!!!

Well I did see something big, not a tiger shark but a 4m giant oceanic manta ray will do. I was in the middle of a fish survey which unfortunately got binned when this huge manta swam over my head. It was pretty curious and circled around us all twice while we were hovering in the blue captivated by its pure magnificance and grace. At first I was like that’s a pretty big manta and then when I looked closely I realized it was an oceanic. Although I have never seen one before I knew how to recognize the different markings on the body. Just to make sure I sent it to my friend Guy Stevens who has been researching and identifying mantas in the Maldives for over 10 years now. Oceanic mantas are not spotted too often, they spend their entire lives cruising large distances across open oceans, so he would be happy to hear about this elusive giant. You can identify each individual manta by the spot pattern on their bellies so I was interested to know whether this one had been sighted before….. and she hadn’t, she was a first sighting which is great and it means I get to name her.

She was spotted just past the cleaning station so maybe she was on her way to be cleaned!! Cleaning stations are certain areas on a reef, usually a prominent reef outcrop or coral bommie, were small cleaner fish hang out. These cleaner fish are usually members of the wrasse family. They have a very important job as they clean all the fish that come specifically to visit these stations. Obviously fish don’t have hands so they cannot clean themselves… instead these cleaner wrasse do it for them. They eat all the dead skin cells, parasites and algae that cover their clients so essentially they get a “free” meal while their clients gets cleaned. It’s almost like going to the doctors or dentist or even your local spa! In the Maldives, there are hundreds of known manta ray cleaning stations were mantas will come to visit just to be cleaned but Bodu Hithi is definitely one of the best spots were you can have a reliable encounters with these magnificent animals, and it’s only a 25 minute boat ride away :)  

All of the above are photos of my first giant oceanic manta, luckily i had my camera!

A resident manta pictured above and below. You can see the different markings on the dorsal surface compared to the oceanic manta but they are quite subtle! 

    This one was at a cleaning station

Resident manta hovering at a cleaning station

 Manta rays are solitary animals but they come together at cleaning stations, so as well as being cleaned its also a good place to hang out!