The hunt for the elusive Giant Sunfish larva

Imagine the scene, some 250 years ago in St. Petersburg, Russia, when a German naturalist held a small, spiky ball-shaped specimen in his hand, wondering if he had discovered a new and rather unusual species of fish. In fact, what he held was a sunfish larva - it was equally unimaginable to naturalists back then as it is to us today that these rotund Pokémon look-alikes could be at all related to the giant sunfish, which would require them to increase their weight by a factor of not hundreds, not thousands, but millions, to reach the size of a car. 

_{Image: An adult sunfish with a car as comparison; Steve O'Shea}

After all, what makes sunfish recognisable is precisely their enormous size and unusual body shape, resembling huge pancakes with wings, and a peculiar “rudder” where you would expect a tail. They meet the public more often than you would think when they wash up on beaches in New Zealand, confounding onlookers who aren’t quite sure what they’re looking at.

_{Image: Mola larvae; Amy Rose Coghlan}

Unlike their somewhat grotesque-looking parents, the larvae are downright adorable, with a perpetual wide-eyed look of innocence and surprise. These early life stages have not been studied in much detail, making identifying Mola larvae to species-level a real challenge. They are seldom encountered and when they are, they are most often fixed in formalin in preparation for museum collections, which makes examining the DNA to identify the species difficult.

_{Background image: Obtaining a sample for genetics from a sunfish; Jonathan Anderson}

To date, two of the world’s five sunfish species have been successfully ‘married up’ with their larvae (namely the sharptail sunfish, Masturus lanceolatus, and the slender sunfish, Ranzania laevis), leaving the three remaining Mola species still a mystery. Even the adults have been tricky to get a handle on, with a brand new species described only three years ago - found in New Zealand waters. 

Genetics is essential to get the species identification right for these mysterious sunfish larvae. We had been trying for years to acquire a specimen fixed in ethanol rather than formalin so we could examine the DNA. Fantastic news arrived via email late one night from Andrew Stewart from Te Papa - he had found a Mola larva during a research cruise off the New South Wales coast of Australia and knew to keep it well away from the formalin bottle. I cannot describe how exciting this was!

What happened next is a perfect example of how the research community can share knowledge to solve some of these scientific mysteries. A single eyeball was removed from the specimen by technical officer Kerryn Parkinson from the Australian Museum in Sydney. DNA was then extracted from the eyeball and analysed by Mark King at the genomics lab, who successfully matched it to known genetic sequences in the database. Et voilà: the world had its first identification of a Mola alexandrini larva!

_{Image: Sunfish larva; Kerryn Parkinson}

Astoundingly, this species hatches at <2 mm but can reach a body weight of 2.3 metric tonnes, which means a potential increase in weight from larva to full-grown by a factor of over 600 million. A similar increase in weight by a human baby would result in the adult reaching far into space. 

Our team here at Auckland Museum is now working on CT scanning an exceptional larvae collection from Australia, currently on loan here, to allow us to study their size, shape, and inner workings without the need for a microscope.

While larvae sampling is a real challenge, getting a genetic sample from an adult sunfish is no mean feat either. One approach is to wait for a sunfish to strand on a beach near you, or near someone who is willing to help you out (and by "help out" I mean asking strangers to store bits of sunfish in their freezer next to their frozen peas). Another technique is to sneak up on the sunfish from behind while it is busy having skin parasites removed by cleaner-fish and covertly take a swab.  

One of the largest sunfish ever found anywhere in the world originated from Whangarei Heads in New Zealand, and was dissected – fittingly! – in the car park at the University of Auckland by marine scientist Steve O’Shea and a small army of helpers. This Mola alexandrini measured 3.3m in length and weighed more than 2.2 metric tonnes. It was likely a female, and just how many eggs she would have spawned in her lifetime is anyone’s guess as her age was unknown, but the number was likely astronomical: sunfish are the most fecund vertebrate on the planet, with 300 million eggs found in the ovary of a Mola mola that measured a mere 1.5 m long. 

This scale of fecundity makes it all the more mysterious why Mola larvae are so few and far between in museum collections and plankton trawls worldwide – where are they hiding? 

_{Image: A sunfish being cleaned by "cleaner fish"; Marianne Nyegaard}

_{Read more about Dr Nyegaard's sunfish research at The Spinoff and New Zealand Geographic}