Species Spotlight: Ocean Sunfish

If I asked you to picture a fish, chances are that most of you reading this would come up with a similar mental image with the same basic form and features. But, although a majority of fish can fit into that classic template of what makes a fish, fishes have diverse forms, body shapes, and characteristics, with perhaps one of the most divergent being the strange-looking, slow-moving ocean sunfish, or Mola mola.

Description

The ocean sunfish is one of the largest bony fishes in the world. Adult individuals usually weigh between 247 and 1,000 kg, although the maximum weight recorded so far is 2,300 kg. In addition, they have an average length of 1.8 m and an average height (or fin-to-fin length) of 2.5 m, although individuals of 3.3 m in height have been recorded.

The ocean sunfish (Mola mola).

The fish's body is flattened laterally and lacks the typical fish tail (or caudal fin) , which is instead replaced by a rounded, lumpy structure known as the clavus. It has elongated dorsal and anal fins, and the pectoral fins are relatively small and fan-shaped. The fish has a beak-like mouth formed of four fused teeth. It also has quite thick skin, which can be up to 7.3 cm thick in some places, and, like a shark, its skin is covered in tooth-like denticles rather than scales.

Habitat

The ocean sunfish can be found in tropical and temperate waters all over the world. They spend a large portion of their time in the deep sea, reaching depths of greater than 200 m. They tend to be found in waters warmer than 10 °C; however, spending a prolonged amount of time in water temperatures of 12 °C or below can harm the fish, causing disorientation and potentially leading to death.

Many of you who have encountered an ocean sunfish for yourself will have likely done so during its somewhat bizarre-looking surface basking behaviour, where the fish swims on its side at the ocean's surface. It is thought that one of the reasons for this behaviour may be as a way for the fish to warm itself after its excursions to deeper, colder water to feed.

Ocean sunfish basking at the surface (photo credit: Laurel Parshall).

Diet

Ocean sunfish are generalist predators with a broad diet. Their beak-like mouth is perfect for catching and gripping soft-bodied prey but they also have pharyngeal teeth in the throat that grind their food into smaller pieces. They mostly eat small fish, fish larvae, squid and other molluscs, and crustaceans but they are also able to eat jellyfish and other soft-bodied animals like salps. The sunfish pulls in and spits out water through its mouth in order to tear apart the jellyfish.

The diverse range of prey items that the ocean sunfish consumes demonstrates that the fish feeds within several different ocean layers, from the surface to the deep seas. There is even evidence that the ocean sunfish will occasionally hunt on the seafloor in some areas.

Life Cycle

A number of spawning areas have been suggested for the ocean sunfish in the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Oceans. The females can produce more eggs than any other vertebrate, up to 300 million at one time, which they release into the water to be fertilised externally by the males.

Once hatched, the larvae are only 2.5 mm long and weigh less than 1 g, so they have a lot of growing to do to before they reach their adult form. First, they develop into fry, which resemble mini pufferfish, complete with spines all over the body that they later lose as they grow.

An ocean sunfish larva (photo credit: G. David Johnson).

Young ocean sunfish tend to school together for protection. However, as they get older (and bigger), they go their separate ways, with the fully grown adults being mostly solitary. The fish's natural lifespan in the wild is not yet known.

How do ocean sunfish get around?

Contrary to popular perceptions, these fish are not passive drifters in the ocean. As we've already established, they spend a lot of time at depth, where they actively hunt for prey. Some ocean sunfish have been recorded swimming 26 km in a day, swimming at a speed of 3.2 km/h, not zippy by any stretch of the imagination but not sedentary either.

Due its body shape, the ocean sunfish has a different swimming style compared to most other fishes. The clavus is used much like a rudder, while the fish uses its dorsal and anal fins to propel itself. In terms of moving through the water column, the ocean sunfish has no swim bladder and so, to control buoyancy and move up or down, it instead has a stiff, gelatinous layer under its skin made up of water, collagen, and elastin.

Ocean sunfish have also been observed breaching and can clear the ocean surface by roughly 3 m, possibly in an attempt to dislodge parasites from their skin. Parasite removal is another possible reason for the fish's basking behaviour too, as doing so gives seabirds access to the fish's skin so that they can remove any unwanted guests.

What gives ocean sunfish their distinct shape?

The ocean sunfish's spine is shorter in relation to the body, with fewer vertebrae, than that of any other fish, contributing to the fish's distinct proportions, where some individuals are as tall as they are long. Another interesting detail about their skeleton is that, despite evolving from bony ancestors, much of the ocean sunfish's skeleton contains cartilaginous tissues instead of bone, which is lighter and enables these fish to grow to the sizes that they do.

Skeleton of an ocean sunfish on display at the Museum of Natural History Vienna (photo credit: Sandstein).

As alluded to already, perhaps the most notable feature of the ocean sunfish is its lack of a caudal (or tail) fin. The clavus was formed during the course of the fish's evolution by a converging of its dorsal fin and anal fin.

Biggest Threats

The ocean sunfish is currently listed as Vulnerable on the IUCN Redlist of Threatened Species and its numbers are declining. Full-sized adults have few natural predators but they are sometimes eaten by sea lions, sharks, and killer whales. Meanwhile, the younger fish are vulnerable to large predatory fishes like tuna and mahi-mahi.

Ocean sunfish are considered a delicacy in some parts of the world, including Japan, Korea, and Taiwan, so they are also targeted by fisheries. However, the sale of sunfish-derived products is banned in most of Europe under EU regulations.

Life-sized model of an ocean sunfish on display at the National Museum of Scotland.

The main threat to ocean sunfish is the use of drift gillnets and, as a result, being caught unintentionally as bycatch. Ocean sunfish make up almost 30% of the total catch in swordfish drift gillnet fisheries in California, while, in the Mediterranean, a shocking 71 to 93% of the total catch in swordfish fisheries is made up of ocean sunfish. Unfortunately, fishery bycatch of ocean sunfish is currently unregulated worldwide and, unless that changes, their numbers are likely to continue to decline.

Why The Whale Came

When I was very young, I had a strong love for the film Free Willy. So, naturally, when my dad asked me on a grey evening in March 1997 if I wanted to go to Queensferry to "see the whale", I jumped at the idea. When we arrived at the viewpoint in South Queensferry, I expected an orca celebrating its freedom with joyful leaps. What I saw instead, off in the distance, was a sperm whale struggling in the shallow water on the north side of the Firth of Forth, with boats alongside him spraying him with hoses. It wasn’t majestic or inspiring. It was just sad.

Sperm whales are a large species, the largest of the toothed whales, and can be found in open oceans all over the world. They are deep-diving animals, reaching incredible depths of up to 3 km, where they hunt giant squid. They are certainly not suited for a shallow estuary like the Forth.

Sperm whale (Physeter macrocephalus) mother and calf (photo credit: Gabriel Barathieu).

After he was refloated near North Queensferry, the sperm whale, affectionately named Moby, headed further inland. A rescue team of boats, including BP tugs and the Maid of the Forth, formed a line across the estuary, revving their engines to deter Moby and send him back out to sea. After some to-ing and fro-ing, where it did appear that he had finally left the estuary, Moby eventually stranded on Airth foreshore, where he sadly died on 31 March 1997. He was the first sperm whale to be stranded in the Forth in over 200 years.

There are a number of factors that can cause a whale to strand. Illness, weather, chasing prey, being chased by predators, entanglement in fishing gear, and shipping noise and traffic are all things that can have an impact on whales. Moreover, looking deeper at human impacts, commercial whale watching, naval activities such as sonar and underwater explosions, oil and gas exploration, pollution, and the impacts of climate change can all contribute to strandings. In particular, whales are highly susceptible to noise pollution, with the noise from shipping and offshore activities driving animals away from their usual habitats and into less favourable ones, and the noise caused by explosions and sonar has the potential to temporarily deafen whales, impacting their ability to navigate properly.

In Moby’s case, after a post-mortem, it was determined that there was nothing that any of the rescuers could have done to keep Moby from stranding; he was always going to die. He had been suffering from an infection of the blood from which he wouldn’t recover. It is possible that this illness had an impact on his natural behaviour and was perhaps even an influencing factor for him entering the estuary in the first place.

A part of Airth foreshore, Moby's final resting place.

Unfortunately, strandings of whales and other cetaceans are becoming more common. In the last few years, strandings in the UK have reached record levels, with a number of mass strandings and a greater variety of species being reported. The UK Cetacean Strandings Investigation Programme (CSIP) reported that the average number of strandings a year in the UK rose from 570 in the years 2010 to 2014 to around 890 a year in the years 2015 to 2019.

The Firth of Forth is no exception, with reported strandings becoming more frequent and larger species coming inland just like Moby did all those years ago. Northern bottlenose whales are not often seen on the Scottish coast, with only around 20 recorded sightings since 2000. However, in July 2023, two of these whales stranded on the shore at Culross, Fife, and passed away a day later, with no obvious cause for their stranding reported. Then, in October of the same year, a sei whale died after stranding at South Queensferry. This large species generally prefers deeper waters offshore but can occasionally be observed around Scotland coming closer to the shore to feed [8]. This particular individual was likely driven onto the beach by prevailing winds during Storm Babet.

A sei whale (Balaenoptera borealis) mother and calf.

So, why the increase in strandings? CSIP findings indicate that human impacts on the marine environment are having a significant effect on marine animals, including whales. They have stated that, out of around 4,050 post-mortems conducted on cetaceans stranded in the UK between 1990 and 2018, 782 were found to be as a result of fishing gear entanglement or being caught as bycatch. Moreover, there is evidence that pollution influences the frequency of strandings, with chemicals such as organochlorines and polychlorinated biphenyls (PCBs) being highly toxic to cetaceans. Even though they have been banned for decades, these chemicals persist in the environment and have entered the world’s oceans. As they move through the food chain, they become more concentrated and, as a result, more toxic, meaning they can have a much greater impact on large animals like whales.

On a more positive note, the number of strandings may be increasing simply because there are more whales. With concentrated conservation efforts and an improved awareness of our impact on the environment, whale populations that were previously threatened are starting to recover. For example, in 2008, the humpback whale was listed as “least concern” by the International Union for Conservation of Nature, replacing its earlier assessments of “endangered” and “vulnerable”, illustrating an improving outlook for the world’s whales.

I paid Moby another visit much more recently. His skull is a permanent fixture in the Grand Gallery of the National Museum of Scotland. Standing so close to it revealed the sheer scale of what looked so small and distant on the shore all those years ago; it was massive. It brought home the magnitude of what was lost that day. There’s something meaningful in the fact that, even now, people still feel such a connection to Moby and his story. It’s admirable that, in what turned out to be a hopeless situation, people tried their hardest anyway. And that that which we couldn’t save, we now honour in memory.

Moby's skull in situ at the National Museum of Scotland.

In the face of increasing environmental threats, that determination to try is as relevant as ever.

Update

This is just a quick post to say that the blog hasn't been abandoned, it was just on hiatus while I needed to prioritise other things.

For the past 2 years, I have been working towards a PgD in Science Communication and Public Engagement through the University of Edinburgh but, with my studies wrapping up soon, I'm looking forward to posting here more regularly again. In particular, I'm excited to put what I've learned to use and I already have so many ideas for the blog.

On top of that, having been inspired by my studies, I'm also starting a new project that I'm really excited to share, where I will combine science communication with one of my favourite hobbies, embroidery. If that's something that interests you, watch this space.

That's all for now but you'll be hearing more from me soon when the blog returns to its regular content.