Ocean sunfish: one of the strangest marine creatures visits Honduran waters

Among the creatures of the animal kingdom, there are some that make us question whether they are real. Among marine fishes, one particularly iconic example is the ocean sunfish ("moonfish" in Spanish), the name given to species of the family Molidae because of their strange shape, unique among fishes. Historical records of sightings date back as far as the Romans, when Pliny the Elder called it a “little pig,” due to a grunting sound that the author reported the fish made when removed from the water [1].

Although today the ocean sunfish has become a symbol of ocean protection, many mysteries remain regarding its biology. One aspect that is still not fully understood is its distribution across the oceans, and very recently its presence in Honduran waters was reported for the first time. In this Honduras Neotropical article we present some information about the biology of this curious animal and learn more about its reports in catracho waters.

What is an ocean sunfish and how many species are there?

Ocean sunfish are evolutionarily related to other families such as pufferfishes. Fishes in this group, known as Tetraodontiformes, usually have rigid bodies with distinctive shapes and rely more on the movement of their fins than their bodies to swim. But even within this group of unusual fishes, the sunfish family stands out for having a shape that resembles a head more than the full body of a fish.

Currently, five species are recognized across three different genera, varying in shape and size. Two of these genera are monotypic (they contain a single species: Masturus lanceolatus and the smallest member of the family, Ranzania laevis) [2].

Figure 1. The three genera in the family Molidae. A. Mola mola, ocean sunfish. B. Masturus lanceolatus, sharptail mola. C. Ranzania laevis, slender mola. A and C NOAA; B mounted specimen from the Natural History Museum of Lisbon. All photos taken from Wikimedia Commons.

Within the genus Mola, three species are recognized, but here we focus on Mola mola, the most common and best-studied ocean sunfish, and the species reported from the Honduran Caribbean.

What about that unusual shape?

There is no other fish like the ocean sunfish. Most fishes move using their pectoral fins and the movements of their tail and caudal fin. The ocean sunfish cannot do the latter because, during the evolutionary process that gave it its particular shape, it completely lost its caudal fin (the one found at the rear of most fishes).

The structure at the back of the sunfish is called the clavus, which means rudder in Latin and embryonically derives from the anal and dorsal fins. Because they lack a caudal fin, the swimming of sunfish is unique. The symmetrical position of their dorsal and anal fins and their use as wings or paddles is not found in any other group. For a long time it was believed that this made them clumsy swimmers dependent on ocean currents. Telemetry and video studies have shown that this idea is incorrect, and today they are considered active swimmers capable of vertical and horizontal movements in the open ocean [3].

In the earliest stages of life, the body shape of a sunfish is very different (Figure 2), and for many years it was believed to be a different species. A newly hatched sunfish is extremely small, weighing around 3.7 mg; the record for an adult Mola mola is 2,300 kg, equivalent to an increase of more than 600 million times [2]! Not surprisingly, they are considered the largest bony fish in the world (the whale shark, as well as all sharks and rays, belong to a different branch: cartilaginous fishes).

Figure 2. Larva of a species of Mola measuring 5–6 mm in length (Australian National Fish Collection, taken from [9]).

Another peculiarity is the absence of a swim bladder, an organ that facilitates buoyancy in most fishes. The absence of this organ is compensated by a layer of gelatinous skin with high water content, which in addition to aiding buoyancy also functions as a protective exoskeleton. This adaptation occupies a considerable percentage of their volume, but because it has little vascularization it does not generate a large energetic cost [4].

What is known about its ecology?

The ecology of the ocean sunfish is very similar to that of another vertebrate that swims in the Honduran Caribbean Sea, the leatherback sea turtle (Dermochelys coriacea), another large animal that can weigh more than 1,000 lbs. Like this large reptile, the sunfish forages from the ocean surface to depths greater than 800 m, following its main food source, gelatinous invertebrates such as jellyfish [5,6].

Although they do not need to breathe air at the surface, because they cannot regulate their body temperature and descend into the cold waters of the deep ocean, the sunfish must warm itself by basking near the surface [5]. In fact, in English its common name refers not to the moon but to the sun, and it is believed that the name originates from this curious behavior, which has also been associated with parasite removal by birds.

Due to their rapid growth, adults usually do not suffer predation from other organisms. However, their reproductive strategy relies on numbers, as they also hold the fertility record among vertebrates, with females historically and recently reported carrying 300 million or even more than 800 million eggs [7]!

The ocean sunfish in Honduras

Although the distribution of the ocean sunfish is thought to span the world’s oceans, including the Atlantic, they have very rarely been reported in the Caribbean Sea, and prior to the report discussed here there were no records in the scientific literature for Honduras.

Researchers from Kanahau, Tom Brown and Alexis Manzanares, reported in 2024 in the scientific journal Caribbean Journal of Science [8] the discovery of a sunfish specimen in the “Lower Lagoon,” south of the island of Utila in the Bay Islands. The authors estimate that the specimen had been stranded for about a week, as it emitted a strong odor, showed a visible state of decomposition, and had signs of having been scavenged. Several of its bones were scattered around the site and others were not found.

Like assembling a puzzle, the researchers had to reconstruct the specimen and, through its bones, concluded that it was an adult individual of the species Mola mola. After conducting further inquiries within the Utila community, where nearly all respondents were unaware of the presence of sunfish around the island, they eventually found a diver from the Utila Dive Center who had a photograph of an encounter with a live specimen observed several years earlier in 2019 (Figure 3). The publication reports an additional sighting in Roatán in 2012 and mentions from diving forums from the mid-2000s, although none of these reports could be confirmed with photographs.

Figure 3. Encounter with an ocean sunfish by instructors from Utila Dive Center in April 2019. Photo taken by Maria Isabel Fernandes Chaves, from [8].

This report is undoubtedly fascinating, as everything indicates that observations of this fish in Honduran Caribbean waters are quite rare, but not impossible. Much remains to be learned about this impressive organism, including its migration routes. Researchers hypothesize that the sunfish may follow a route similar to that of the whale shark, which is observed more regularly in the Bay Islands.

The authors also point out the interesting coincidence between the appearance of the specimen found dead and Hurricane Beryl in 2024, a hurricane that reached Category 5 in July, an unusually early date in the calendar. This could indicate that the climate change currently affecting the planet may be influencing the biology of this fish in ways not yet understood.

You can observe this sunfish specimen found in Honduran waters the next time you visit Utila, where Kanahau has recently inaugurated the Utila Museum of Natural and Cultural History on Monkey Tail Road (Figure 4).

Figure 4. A. Skeleton of the sunfish found in Utila in 2024 and local school students learning about the sunfish at the Utila Museum of Natural and Cultural History. Photo by Kanahau.

By: Diego Ardón, Biólogo

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References

1. Thys TM, Nyegaard M, Kubicek L. 2020. Ocean Sunfishes and Society. En: Thys TM, Hays GC, Houghton DR (eds.) The Ocean Sunfishes: Evolution, Biology and Conservation. CRC Press

2. Sawai E, Nyegaard M, Yamanoue Y. 2020. Phylogeny, Taxonomy and Size Records of Ocean Sunfishes. En: Thys TM, Hays GC, Houghton DR (eds.) The Ocean Sunfishes: Evolution, Biology and Conservation. CRC Press

3. Watanabe YY, Davenport J. 2020. Locomotory Systems and Biomechanics of Ocean Sunfish. En: Thys TM, Hays GC, Houghton DR (eds.) The Ocean Sunfishes: Evolution, Biology and Conservation. CRC Press

4. Bemis KE, Tyler JC, Hilton EJ, Bemis WE. 2020. Overview of the Anatomy of Ocean Sunfishes (Molidae: Tetraodontiformes). En: Thys TM, Hays GC, Houghton DR (eds.) The Ocean Sunfishes: Evolution, Biology and Conservation. CRC Press

5. Sousa LL, Nakamura I, Sims DW. 2020. Movements and Foraging Behavior of Ocean Sunfish. En: Thys TM, Hays GC, Houghton DR (eds.) The Ocean Sunfishes: Evolution, Biology and Conservation. CRC Press

6. Phillips ND, Pope EC, Harrod C, Houghton JDR. The Diet and Trophic Role of Ocean Sunfishes. En: Thys TM, Hays GC, Houghton DR (eds.) The Ocean Sunfishes: Evolution, Biology and Conservation. CRC Press

7. Forsgren K, McBride RS, Nakatsubo T, Thys TM, Carson CD, Tholke EK, Kubicek L, Potter I. 2020. Reproductive Biology of the Ocean Sunfishes. En: Thys TM, Hays GC, Houghton DR (eds.) The Ocean Sunfishes: Evolution, Biology and Conservation. CRC Press

8. Brown TW, Manzanares AJ. 2024. Rare Occurrences of Ocean Sunfish (Mola mola) in the Bay Islands of Honduras; Notes on Distribution and Osteology. Caribbean Journal of Science 2024 (54): 410-420 p.

9. Nyegaard M. 2018. There be giants! The importance of taxonomic clarity of the large ocean sunfishes (genus Mola, Family Molidae) for assessing sunfish vulnerability to anthropogenic pressures. Master Thesis Murdoch University, Australia.