Monday, December 5, 2011

Mola mola, the Ocean Sunfish

The Ocean Sunfish is a bony fish that has made it its life-long goal to set as many records as possible. It does this while having an appearance that is, in a word, odd. These fish have undergone so many specializations as to set them apart from nearly every other group of extant fish.

The largest species, the Ocean Sunfish Mola mola belongs to the genus Mola of which it shares with only one other living member, The Southern Sunfish Mola ramsayi. In turn, the genus Mola belongs to the family Molidae which contains three genera and five known species. Until more genera were uncovered, the genus Mola did not exist and was instead classified under the same genus that houses the pufferfish, Tetradon. However, with better study of these fish and improved classifications of the related genera and species, they were moved to their own family and genera. Though they are still classifed within the same order as the pufferfish, porcupinefish, boxfish and filefish, the Tetradontiforms.

The Sunfish's relation to the Tetradontiforms has led to an ongoing debate as to the toxicity of these fish. Some claim that the organs contain a low level tetrodotoxin, the same kind of toxin found within certain pufferfish (as well as the Blue-Ringed Octopus and certain newts) and the cause of Fugu poisoning. Despite the claims, there have been no confirmed cases of tetrodotoxin poisoning in any species of sunfish. Which is certainly reassuring for the multiple cultures (primarily Japanese and Thai) that find sunfish flesh to be a delicacy.

Seen next to a diver, the adult Ocean Sunfish's size can truly be appreciated.
These fish have an appearance that makes them seem like they are only half a fish. In a sense, they are as their cauldal fin (the tail fin) has degenerated to a clavus which barely has any use in locomotion, acting more as a stumpy rudder. The dorsal and anal fins have become the prime sources of locomotion, moving side to side to slowly propel the fish through the water with a speed of under 2 mph (3.2 hm/h) and a range of around 16 miles per day (26 km). In fact it is due to this slow speed and large size, combined with their habit to lay on the surface of the water on their side to sunbathe that they have become known as a boating hazard where they have damaged hulls and destroyed engines. But this is essentially the only threat these docile fish pose, with the lone example of direct injury occurring when of boy was injured when an Ocean Sunfish breached (jumped out of the water) and landed on his boat, knocking him into the water.

One of the first record breaking traits to be noted about the Ocean Sunfish is its size. These fish are both the heaviest bony fish as well as the largest ray-finned bony fish. With a weight of up to and sometimes exceeding 2,200 lbs (1,000 kg), these fish are massive. Also, due to their odd appearance, they often have a length equal to their height, often around 11 ft. (3.3 m).

Despite their large size, Ocean Sunfish feed primarily on small organisms.  Their primarily source of nutrition are sea jellies, but they have also been known to feed on small fish, crustaceans, squid and eel grass.  Their teeth have fused into a beak, similar to many other tetradontiforms, but the sunfish's has fused in a way to prevent it from closing its mouth fully.  So instead of biting at its food, it will suck it in where it will blow water back and forth rapidly to tear apart its soft bodied prey.  It will then partially swallow its food and allow its pharyngeal teeth to grind the food down to a more manageable size before swallowing it the rest of the way.  The pharyngeal jaw is a trait held in common by some 30,000 species of fish and is, as the name suggests, teeth or, in some rare cases like Moray Eels, fully mobile jaws used to pull food down the throat.  In the Ocean Sunfish, they are basic teeth in the pharynx that simply grinds bits of food and nothing more.  Due to their nutrient poor diet, the Ocean Sunfish must eat frequently.

Due to the primary prey item of the sunfish being sea jellies, these fish are often found dead after they have tried to eat a plastic bag floating in the water and are unable to swallow it further.  Making pollution, as well as ocean traffic and fishing the primary threats to these fish.  Due to their size, they have few natural predators.  The only ones of note being Sea Lions, Orcas and certain sharks.

One of the other record setting traits of these fish can be found in their rate of reproduction.  The Ocean Sunfish lays more eggs at a time then any other vertebrate.  At a time, a single female sunfish can lay as many as 300,000,000 eggs (yes, that is 300 million).  These eggs are then externally fertilized by a male and then left to drift with the rest of the plankton on the oceans currents.  These eggs are fed on by multiple organisms all along the food chain including fish such as tuna.  It is because of this rather wasteful method of reproduction that such a large number of eggs must be produced to maintain the species population.  Once the eggs have hatched, the tiny fry appear as 1 mm, spiny spheres, betraying their common ancestor with the pufferfish.

A newly hatched sunfish fry.
Another of the record setting traits of the Ocean Sunfish has to do with the size of its spinal cord.  Due to the shortening of the body that has occurred over time, the actual spinal cord only extends about 1mm from the base of the brain.  The rest of the body is connected through the peripheral nervous system.  Thus, the sunfish has the smallest spinal cord of any fish in relation to their body.  They have also lost many of their vertebrae leaving them with the fewest of any fish.  In fact, despite being a bony fish, the sunfish has lost nearly all of its bone, replacing it with cartilage.  This adaptation has allowed the sunfish to grow to such large sizes without causing damage to its skeleton.  Further showing convergent evolutionary traits with the cartilaginous fishes is the development of denticles, sharp, teeth like protrusions of the skin.  These give sharks and rays, as well as the Ocean Sunfish, their sandpaper like texture.  This acts as a kind of body armor as well as decreasing water friction allowing them to move more easily through the water.
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Unknown said...
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Unknown said...

I guys,

Check mola mola page at
a comprehensive catalogue of marine species to sea lovers.