Ocean Sunfish, Mola mola (Linnaeus, 1758)

The Ocean Sunfish, Mola mola, has been known from waters worldwide for hundreds of years and are a huge round-bodied fish that is sometimes seen 'basking' on the water surface. These beautiful creatures can grow to over 3m in diameter and over 2500kg. In fact the name Mola comes from the Latin word for millstone and refers to the rounded shape of the fish.

Identification

Sunfish are from the family Molidae and are the largest bony fish in the world. They have attracted interest for centuries because of their unique shape and large size. The teeth in each jaw are fused to form a plate, and the mouth is small in comparison to the body size. Sunfishes are found worldwide in the open ocean of tropical and temperate seas. These interesting fish are completely without a caudal fin, instead replaced by a ‘clavus’, which in latin means ‘rudder’.

A number of different characteristics are used to separate the Mola species and one of them is the clavus. Mola mola can be identified from its wavy (frilly) clavus edge, 11-14 clavus fin rays and its body scales are conical shaped (the clavus is round without an indent in Mola alexandrini and rounded with an indent in Mola tecta).

Five species of sunfish are found in Australian waters, the Hoodwinker Sunfish - Mola tecta, the Giant Sunfish - Mola alexandrini, the Ocean Sunfish - Mola mola, Slender Sunfish - Ranzania laevis, and the Point-tailed Sunfish, Masturus lanceolatus.

Habitat

Ocean Sunfish are usually found in oceanic waters, but occasionally come inshore. Sunfishes in general are often seen at the surface where they may be mistaken for sharks, because of the large dorsal fin.

In their 2010 paper (see References, below) on Mola mola in the north-west Atlantic, Potter and Howell stated that "There was no observed relationship between the amount of time per day that fish spent in cold water (<10 °C) and the amount of time fish spent near the surface (0–6 m), indicating a lack of evidence for M. mola basking at the surface as a mechanism for behavioral thermoregulation." They also stated that "Fish spent greater than 30% of their time in the top 10 m of the water column, and over 80% of time in the top 200 m. The maximum depth recorded by any fish was 844 m."

Feeding and diet

Ocean Sunfish feed on jellyfish, salps, ctenophores and occassionally small crustaceans and fishes. Recent research states that they feed on hydrozonans ("which are small groups of predatory animals related to jellyfish"). The research sheds light on sunfish feeding behaviour, which may also explain why they bask in the sun on the surface. See also Nakamura, Goto and Sato (2015) in references, below.

Other behaviours and adaptations

The Ocean Sunfish swims by moving its dorsal and anal fins back and forth. Both fins are moved in the same direction at the same time. View an Ocean Sunfish swimming in the video below.

Larval specimens:

Sunfishes are amazingly fecund fishes. A single adult female can produce up to 300 million tiny buoyant eggs. Fertilization occurs when eggs and sperm are shed into the water.

While the adult sunfish species have been described and their taxonomy determined, very little is known about the early life history of these amazing fishes. The features that are used to determine which species an adult sunfish just aren’t visible in larval specimens. Research is currently underway to determine which species is which, using not only traditional morphological features but super cool DNA analysis. Until then we just have to be content with looking at these super cute images of larval Mola species

Sunfishes are harmless to people.

These Sunfish often meet their end by being struck by a ship. On 18 September 1908, the Steamer Fiona, was 65 km from Sydney when it suffered a ‘violent concussion’. A boat was lowered over the side and the men onboard saw a Sunfish jammed in the framework of the port propeller. The fish was the largest known at the time, measuring ’10 feet, 2 inches’ (3.1 m) in length and ’13 feet, 4 inches’ (4.1 m) in height.

Distribution

The Ocean Sunfish occurs in temperate marine waters worldwide.

In Australia, it has been recorded from the central coast of New South Wales to Tasmania and west to Mandurah, Western Australia.

The map below shows the Australian distribution of the species based on public sightings and specimens in Australian Museums. Click on the map for detailed information.

References

1. Glover, J.C.M. in Gomon, M.F, J.C.M. Glover & R.H. Kuiter (Eds). 1994. The Fishes of Australia's South Coast. State Print, Adelaide. Pp. 992.
2. Martin, F.D. & G.E. Drewry. 1978. Development of Fishes of the Mid-Atlantic Bight. An Atlas of Egg, Larval and Juvenile Stages. Vol. VI. Stromateidae through Ogcocephalidae. U.S. Fish and Wildlife Service. FWS/OBS-78/12. Pp. 1- 416.
3. Matsuura, K. & J.C. Tyler. Triggerfishes and their allies. in Paxton, J.R. & W.N. Eschmeyer (Eds). 1994. Encyclopedia of Fishes. Sydney: New South Wales University Press; San Diego: Academic Press. Pp. 240.
4. Nakamura, I., Goto, Y. & K. Sato. 2015. Ocean sunfish rewarm at the surface after deep excursions to forage for siphonophores. Journal of Animal Ecology. 84(3): 590-603.
5. Nyegaard M, Loneragan N, Hall S, Andrew J, Sawai E, Nyegaard M. 2018. Giant jelly eaters on the line: species distribution and bycatch of three dominant sunfishes in the Southwest Pacific. Estuarine, Coastal and Shelf Science 207:1– 15. Doi: 10.1016/j.ecss.2018.03.017
6. Nyegaard, M., Sawai, E., Gemmell, N., Gillum, J., Loneragan, N.R., Yamanoue, Y., Stewart, A.L. 2018. Hiding in broad daylight: molecular and morphological data reveal a new ocean sunfish species (Tetraodontiformes: Molidae) that has eluded recognition. Zoological Journal of the Linnean Society 182:631-658. Doi: 10.1093/zoolinnean/zlx040
7. Paulin, C., Stewart, A., Roberts, C. & P. McMillan. 1989. New Zealand Fishes. A Complete Guide. National Museum of New Zealand. Pp. 279.
8. Pope, E.C et al. 2010 The biology and ecology of the ocean sunfish Mola mola: a review of current knowledge and future research perspectives. Reviews in Fish Biology and Fisheries. 20(4): 471-487, DOI: 10.1007/s11160-009-9155-9.
9. Potter, I.F & W.H. Howell, 2010. Vertical movement and behavior of the ocean sunfish, Mola mola, in the northwest Atlantic. Journal of Experimental Marine Biology and Ecology. 396(2): 138-146.
10. Reuvens, C.L. 1894. Remarks on the genus Orthragoriscus. Notes Leyden Mus. 16:128-130, pl. 5.
11. Sanzo, L. 1939. Rarissimi stadi larvali di Teleostei. Arch. Zool. Ital. 26:121-151, 2 pls. [in Italian]
12. Sagara K, Yoshita Y, Nishibori M, Kuniyoshi H, Umino T, Sakai Y, Hashimoto H, Gushima K. 2005. Coexistence of two clades of the ocean sunfish Mola mola (Molidae) around the Japan cost. Japanese Journal of Ichthyology 52: 35–39. Doi: 10.11369/jji1950.52.35
13. Sawai, E. et al., 2017. Redescription of the bump-head sunfish Mola alexandrini (Ranzani 1839), senior synonym of Mola ramsayi (Giglioli 1883), with designation of a neotype for Mola mola (Linnaeus 1758) (Tetraodontiformes: Molidae). Ichthyol. Res. - Online DOI 10.1007/s10228-017-0603-6.
14. Stead, D.G. 1933. Giants and Pigmies of the Deep. A story of Australian Denizens. Shakespear Head Australian Nature Books. Pp. 108.
15. Stewart, A. 1997. Sunfishes - sculling superfish. Seafood New Zealand. June: 89-91.
16. Tortonese, E. 1956. Plectognathi in Fauna E Flora del Golfo di Napoli. Monograpfia 38: Uova, Larvae e Stadi Giovanili de Teleostei. "Monografi elaborata con l'uso del materiale raccolto e seriato da Salvatore Lo Bianco" Part 3B. pp.457-1064, pl. XXXVI-LI. [in Italian]
17. Watson W. 1996. Molidae. in The Early Stages of Fishes in the California Current Region, Pp. 1439-1441. Moser, H.G. (Ed.). CalCOFI Atlas No. 33.