Bump-head Sunfish, Mola alexandrini (Ranzani 1839)

Introduction

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. These beautiful giants can grow to more than 3m and over 2000kg. 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’.

Five species of sunfish are found in Australian waters, the Bump head Sunfish - Mola alexandrine, Hoodwinker Sunfish - Mola tecta, the Oceanic Sunfish - Mola mola, Slender Sunfish - Ranzania laevis, and the Point-tailed Sunfish, Masturus lanceolatus. Mola alexandrini is found in oceanic waters worldwide, feeding on salps and siphonophores.

Identification

The Bump-head Sunfish, Mola alexandrini, was formally known as Mola ramsayi until recent research by Sawai et al (2018) redescribed the species, resolving the long held confusion between this species and the Oceanic sunfish Mola mola.

A number of different characteristics are used to separate the Mola species and one of them is the clavus. Mola alexandrini can be identified from its rounded clavus edge without an indent, 14-24 clavus fin rays and its body scales are rectangular shaped (the clavus is wavy or lobed in Mola mola and rounded with an indent in Mola tecta). It also has a large bump on the head and chin, hence the name Bump-head sunfish.

Sunfish are often mistaken for sharks due to their large dorsal fin which is often highly visible out of the water. This fish below was noticed off the south coast of New South Wales. The specimen was photographed as it cruised past with its companions known as Raft fish, Schedophilus sp.

A Bump-head Sunfish off Port Augusta, upper Spencer Gulf, South Australia, August 2003. The photographers captured this image from their kayaks. The fish was slapping the water with its dorsal fin and made no attempt to swim away. It may have been moribund, because a dead sunfish was found washed up on a nearby beach about two weeks after this photo was taken.

Sunfish are often found washed up on beaches such as this Bump-head sunfish specimen found near Sussex inlet, New South Wales. This Bump-head Sunfish was found on the rock platform near the southern entrance in August 2003. Note the ridge above and behind the eye and the fused beak-like teeth. (AMS I.42801)

Specimens in the collection:

Sunfishes are harmless to people, but sometimes they bump into ships with unfavourable results. On 13th October 1998, staff of the Australian Museum were called to examine an Bump-head Sunfish, Mola alexandrini, that was found stuck on the bulbous bow of the cement carrier, MV Goliath, as it tied up to the wharf in Sydney (left image). The huge fish, which weighed approximately 1400 kg was removed from the bow of the ship by the Sydney Waterways Authority. The fish became stuck on the bow off Jervis Bay, New South Wales. It caused the speed of the ship to slow from 14 to 11 knots. The skin of the Bump-head Sunfish was so rough it wore the ship's paint work back to the bare metal. The fish measured 3.1 m from the tip of the dorsal fin to the tip of the anal fin, and 2.5 m from the tip of the snout to the end of the clavus. Skin samples were taken from above the pectoral fin and near the tail. These are registered in the Australian Museum Ichthyology Collection as AMS I.38997-001 and AMS I.38997-002.

Larval specimens

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 specimens.

Distribution

Five species of sunfish are found in Australian waters, the Bump head Sunfish - Mola alexandrini, Hoodwinker Sunfish - Mola tecta, the Oceanic Sunfish - Mola mola, Slender Sunfish - Ranzania laevis, and the Point-tailed Sunfish, Masturus lanceolatus. Mola alexandrini is found in oceanic waters worldwide, feeding on salps and siphonophores.

Mola alexandrini was commonly known as the southern ocean sunfish and thought to be only found in the southern hemisphere. However due to the revision of the Mola species and clarification of their identifiable characteristics , it is now thought that specimens from the northern hemisphere may have been misidentified and are actually Mola alexandrini and therefore have a much wider distribution.

In Australia it is found in temperate marine waters from northern New South Wales, around the south of the country, including Tasmania, to south-western 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. Source: Atlas of Living Australia.

References

1. Glover, C.J.M. in Gomon, M.F., Glover, C.J.M. & R.H. Kuiter (Eds). 1994. The Fishes of Australia's South Coast. State Print, Adelaide. Pp. 992.
2. Hutchins, B. & R. Swainston. 1986. Sea Fishes of Southern Australia. Complete Field Guide for Anglers and Divers. Swainston Publishing. Pp. 180.
3. Hutchins, B. & M. Thompson. 1983. The Marine and Estuarine Fishes of South-western Australia. Western Australian Museum. Pp. 103.
4. Last, P.R., E.O.G. Scott & F.H. Talbot. 1983. Fishes of Tasmania. Tasmanian Fisheries Development Authority. Pp. 563.
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. 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
8. 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.
9. Thys, T.M. et al. 2013. First record of the southern ocean sunfish, Mola ramsayi, in the Galapagos Marine Reserve. Marine Biodiversity Records. DOI:10.1017/S175526723000377. Vol 6.