The southern sunfish (Mola alexandrini), also known as the Ramsay's sunfish, southern ocean sunfish, short sunfish or bump-head sunfish in many parts of the world, is a fish belonging to the family Molidae. It is closely related to its congener, more widely known Mola mola, and is found in the Southern Hemisphere. It can be found basking on its side occasionally near the surface, which is thought to be used to re-heat themselves after diving in cold water for prey, recharge their oxygen stores, and attract gulls to free them of parasites.
In December 2017, it was demonstrated that Mola alexandrini may be a senior synonym of Mola ramsayi (Ranzani 1839) through both historically and newly published morphological data. In July 2020, building upon this scientific learning, the larval forms of these species were discovered for the first time and confirmed with DNA analyses by Australian and New Zealander scientific teams.
The ocean sunfish are in the genus Mola, currently composed of three species: Mola mola, Mola alexandrini, Mola tecta. Mola alexandrini was seen in the Oman Sea near Iran for the first time in August 2013 (Yasemi and Narari Bejgan, 2013). Also known as the southern ocean sunfish or southern sunfish, Mola alexandrini are commonly found in the epipelagic zone of the ocean which is the part of the ocean where enough light penetrates for photosynthesis to occur (Matsuura, 2002) although recent studies also suggest that the sunfish are more common in deep waters than previously thought (Phillips et al. 2015).
Mola alexandrini has a relatively small mouth and its teeth fused into a parrot-like beak. It can reach up to 3.3 m (11 ft) in length and 2,300 kg in mass, making it one of the two heaviest bony fish, only matched by its congener, the ocean sunfish. Their body is flat and round, with large fins that they swish back and forth to propel themselves with as they swim horizontally. Their skin has rough denticles, leathery texture, with brown and gray coloring with pale blotches until death when they turn white. Both mola species have no caudal bones, ribs, and pelvic fins and have fused vertebrae, leaving only their median fins to propel themselves. It can be recognized from the Mola mola by their lesser number of ossicles and lacking the vertical band of denticles at its base.
The fish of the family Molidae are characterized by their compressed shape, fused teeth, absence of spines in dorsal and anal fins, and a short caudal fin (Yasemi and Narari Bejgan, 2013) and can grow to great sizes.
M. alexandrini was found to be synonymous with M. ramsayi in July 2017 and can be distinguished by their unique characteristics of head bump, a chin bump, rectangular body scales, and rounded clavus. Although adult sunfish look generally similar, they are distinguishable using the seven characteristics: number of clavus fin rays (Yoshita et al 2009), number of clavus ossicles (Yoshita et al 2009), shape of clavus margin (Yoshita et al 2009), presence of head bump (Yoshita et al 2009), proportion of body length compared to body height (Yoshita et al 2009), shape of body scales (Fraser-Brunner 1951), and the presence of a chin bump (Sawai et al. 2017).
Mola alexandrini can be distinguished from Mola mola by a getaway bump on its chin and noticeable bump on its head and a differently looking tail, which is more rounded than being scalloped as that of Mola mola.
Mola alexandrini is found in the southwest Pacific, especially around Australia and New Zealand, and the southeast Pacific around Chile. Its range also extends to the southeast Atlantic near South Africa. This species is found in pelagic-oceanic temperate waters.
M. alexandrini have previously been found in the southern oceans, south-west Pacific near Australia and New Zealand, south-east Pacific near Chile, and south-east Atlantic near South Africa (Yasemi and Narari Bejgan, 2013). M alexandrini's distribution are not well studied but are predicted to be global (Phillips et al 2017). The sunfish are susceptible to by catch and are classified as vulnerable.
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