Those Ridiculous Giant Ocean Sunfish Start as Tiny Larvae

ocean sunfish mola mola
Image: Getty Images

You may be familiar with the ocean sunfish after it gained notoriety when a Facebook rant decrying its very existence went viral in 2017.

In the infamous Facebook post, published by a Scout Burns, the world’s largest bony fish is described as the “biggest joke on Earth” and a “complete failure of evolution”. Pretty damning stuff, but it’s easy to see why.

The ocean sunfish, or Mola Mola, is a strange sight to behold and many of its features make people question how it’s survived the cruelty of natural selection. With some creatures weighing in at more than two tonnes and measuring more than three metres between fins, the species really is the Big Friendly Giant of the sea.

What makes it particularly intriguing is that it exists without many of the features many other fish need in order to survive. The ocean sunfish has no tail fin – instead, its body looks like it’s abruptly cut off, with a makeshift rudder it uses to direct itself through water.

It also lacks a swim bladder needed to control buoyancy, which is likely what’s behind its unconventional and ungraceful swimming habits. To be fair, however, sharks and tuna get on fine without one too, though the ocean sunfish would never exist in the same category as them.

It’s also seen floating sideways on the surface, leading many to think it’s dying — it isn’t — and has been implicated in causing accidental boat sinkings.

The giant ocean sunfish starts from humble beginnings

New research from the Australian Museum and Auckland War Memorial Museum has identified that these mammoths of the sea, or “giant floating dinner plates” as Burns cruelly described them in 2017, actually start out as tiny larvae.

That fact had never been confirmed before as the larvae share no features with their bigger, bonier elders. DNA was extracted from an extremely rare larval specimen, about five millimetres in length, and compared to sequences available of different adult ocean sunfish around the world. It matched with an adult Bump-head Sunfish found in Australian waters.

Despite being observed in waters all around the world, including in Australia, little was known about the sunfish or its life cycle before the creature achieved its ‘iconic monstrosity’ status, fueled by viral posts and videos.

“These beautiful giants of the sea are found worldwide in the open ocean of tropical and temperate seas,” Kerryn Parkinson, an Australian Museum fish expert that worked on the discovery, said in a media release.

“The classification of the species from the genus Mola has long been confused, despite the large amount of interest these fishes create. This is mainly due to their rare occurrence to scientists, and difficulties in preserving them for research.”

The findings give experts a greater insight into how the ocean sunfish reproduces, given it does an incredible job at that too. Females of these awkward marvels can release up to 300 million eggs, so they’re certainly busy keeping the family tree well and truly alive.

“Given sunfish are so incredibly fecund, it is an enigma why their eggs have never been found in the wild, and why sunfish larvae are so few and far between — where are they?” Dr Marianne Nyegaard, a sunfish expert from Auckland War Museum, said.

“A genetic ID of one of these larvae is incredibly important but only one step on the long journey towards describing the early ontogeny of all three Mola species — an endeavour which will require global collaboration.

“If we want to protect these marine giants we need to understand their whole life history and that includes knowing what the larvae look like and where they occur.”

Maybe then we’ll finally understand one of the Darwin Theory’s biggest anomalies.