Ocean Sunfish

Image: Dive Concepts Bali via Flickr

It's the third biggest fish in the world! It's... wait a minute... where's the rest of it?

It's the biggest fish head in the world!

The Ocean Sunfish, Mola mola, reigns as the biggest of all bony fish. They are defeated only by the Whale Shark and Basking Shark, who, being sharks, are cartilaginous fish.

But nature is weird, and the Ocean Sunfish is weirder than most, so while being a bony fish, it has a skeleton made largely of cartilage. And it has no swim bladder. Or ribs. And it's most closely related to pufferfish.

Oh yeh, and it's basically a head. When I say it's the biggest bony fish, I certainly don't mean it's the longest, but it is the heaviest. The Ocean Sunfish simply isn't outstandingly long. Somewhere around here I probably have a piece string longer than the third biggest fish in the world!

So just how big is it?

The average is around:
1.8 m (5.9 ft) long,
2.5 m (8.2 ft) in height from fin to fin,
1,000 kg (2,200 lb) weight. A ton!

Particularly large individuals may be:
3.3 m (10.8 ft) long,
4.2 m (14 ft) in height from fin to fin,
2,300 kg (5,100 lb) weight. 2 and a half tons!

Those are some strange dimensions! Whoever heard of a fish that was taller than it was long? (you did, just now) Don't even bother with width, it's almost 2 dimensional.

And they look all soft and squishy because they don't have scales. Instead they have skin up to 7.5 cm (3 in) thick that is rough as sandpaper and covered in mucus. That's two reasons not to hug an Ocean Sunfish!

There is one part that's nice and smooth though. It's that weird thing on the end that replaces the tail fin. And tail. It's known as the clavus, which is the other name for the hard and horrible corns people can get on their feet. I sincerely doubt the Ocean Sunfish got like that because of "intermittent pressure and frictional forces."

Image: Dive Concepts Bali via Flickr

Inside the clavus are 12 fin rays and at the end are bumpy lumps of bony ossicles. It can be used as a rudder, but it's far too small to provide thrust. So too are the tiny pectoral fins that look like Charlie Brown's arms.

And so Ocean Sunfish is left with their dorsal and anal fins for propulsion. And they're huge! Each one is almost as long as the body is tall and they're both about the same size and shape. It gives the Ocean Sunfish a unique symmetry as it slowly swims, those mighty fins swaying to and fro.

What ARE you?

Though the pace be languid, they get there in the end. And when I say there I mean pretty much everywhere. Ocean Sunfish are found in all tropical and temperate ocean waters across the world. They can tolerate a bit of chilliness, but they certainly don't want icebergs floating above them. Quite fitting for something called a Sunfish.

And unlike the Basking Shark, the Ocean Sunfish really does bask in the sun. They turn onto their side on the ocean's surface and just drift along with the current. Ahhhh... lovely!

Aside from all the itching and scratching that is. Ocean Sunfish are infamous for the ridiculous number of parasites they harbour. They're like a walking oak tree, providing a habitat for an abundance of wildlife.

One reason for lying on their side is so sea birds land on this living island and start plucking out tasty beasties. Not a bad result from just floating around taking a rest.

On the opposite side of the energy scale, Ocean Sunfish can perform a rather shocking burst of speed to fling themselves out of the sea and 10 metres (33 feet) into the air! All those tons crashing back onto the water must be like an earthquake to all those parasites.

And toward the middle of the energy scale there is hanging around kelp forests to turn their blight into a bonanza for other, little fish. This is when they can end up getting quite close to shore. It seems that parasites are a constant consideration for poor old Ocean Sunfish.

There could, however, be some other reasons for basking. It's do to with food.

Image: fs999 via Flickr

In common with whales and the aforementioned giant sharks, the Ocean Sunfish eats lots and lots of not very much. The difference is that they don't have a huge mouth to catch huge amounts of tiny food. Instead, they have a tiny mouth for eating lots of nutritionally poor food.

Just like their pufferfish relatives, Ocean Sunfish have their teeth fused together to form a kind of beak. They mainly use it to crush and crunch their way through... jellyfish, which are some 95% water. It's like the Cucumber Diet or something... *Googled* Oh. That actually exists.

Since the Ocean Sunfish wants to be healthy and alive, rather than periodically lose half their weight on the X Diet and then put it all back on again on their normal diet, they must eat lots and lots of jellyfish. If they've been good, they can even treat themselves to small fish, crustaceans and squid. Mmmmmm, proper food! But then it's back to the jellyfish.

Image: Dive Concepts Bali via Flickr

Strangely enough, the Ocean Sunfish isn't actually averse to a bit of darkness. They regularly dive to depths of 300 m (984 ft) and they've occasionally been seen even deeper. One possibility is that basking in the sun allows them to warm up after entering the cold, deep waters.

Just as in the Whale Shark and Basking Shark, their giant size makes it all the more noticeable how little we know about their lives.

Here's something we do know though.

Many cartilaginous fish are a bit like mammals in that they devote a lot of energy to their offspring. Some sharks can even feed their unborn young unfertilized eggs so that they're big and strong on birth. For some, it goes to the extent where upon birth, some sharks may have already eaten their own siblings while they were still developing in mother's body.

Basically, big shark = big shark pups.

Image: G. David Johnson

Bony fish are different. They beat the odds through sheer weight of numbers. Instead of devoting energy into a few, well developed youngsters, they spawn millions of eggs.

Just as you would hope, the heaviest bony fish of all produces more eggs than any other vertebrate. Up to 300 million at a time.

I don't overly wish to know how much sperm the male produces, but 300 million cells is small change for that stuff and no-one seems to be particularly impressed with his output.

Still, did you know that "sperm are wonderful little cells and they can be loads of fun"? I always enjoy reading around my subject.

The eggs soon develop into fry some 2.5 mm (0.098 in) long. They don't look anything like their parents. Can you guess what they look like? Tiny pufferfish, of course! They have large pectoral fins, a tail fin and spikes.

They lose all that as they grow. This is particularly good news considering the spines. Can you imagine? They'd be a mile long! Always be grateful for gigantic mercies.