The Ocean Sunfish, members of the family Molidae, are related to pufferfish, and possess a number of unique anatomical features. The most distinctive feature is the lack of almost all the bony elements of caudal fin structures, which have been modified to form the clavus. Ocean Sunfish also lack ribs, pelvic fins, and spines, and pass through two distinct larval phases, in their transformation from a pufferfish-like early developmental stage, to the characteristic laterally compressed shape of the adult. Molids are thought to be the most fecund of living vertebrates, with each female capable of producing 300,000,000 eggs in a season. Globally distributed throughout temperate and tropical oceans, these pelagic fish feed on jellyfish and other zooplankton.
Although several researchers had proposed phylogenies for the molids, based on morphological characteristics, no molecular studies were published before 2004. Yamanoue and colleagues obtained complete nucleotide sequences for the mitochondrial DNA isolated from the three recognized species of Ocean Sunfish: Mola mola, Masturus lanceolatus, and Ranzania laevis. As for all other teleosts, the molid mitochondrial genome contained 37 genes, in a conserved order; there are 13 protein-coding genes, 22 transfer RNA genes, the 12S ribosomal RNA gene, and the 16S ribosomal RNA gene. Monophyly of the Molidae was confirmed in this study, and the figure below shows that Mola and Masturus are more closely related to each other than to Ranzania. The researchers cautioned that the identity of sister-group of the Molidae was ambiguous, given these mitochondrial DNA data, and the tetraodontids included in the study had a low probability of comprising a sister group (31% Bayesian posterior probabilities).
Molidae phylogeny (Yamanoue et al., 2004)
In 2005, Bass and colleagues reported additional molecular phylogenetic analyses of the Molidae, using the non-coding mitochondrial control region, or d-loop, as well as the cytochrome b mitochondrial gene. Their results indicated that Ranzania is the basal member of the monophyletic Molidae, and that Tetraodontidae (Takifugu) + Diodontidae (Diodon) comprise the sister-group to the molids. Moreover, the genetic divergence between sunfish collected in northern and southern hemispheres were consistent with the existence of a separate Mola species, M. ramsayi, off the coasts of Australia and South Africa.
Although Ocean Sunfish are large, abundant, and widespread marine animals, relatively little is known about their behavior and natural history. Unfortunately, molids comprise a large percentage of the bycatch in drift gillnet fisheries, particularly those targeting swordfish off the coast of California. To track the daily movement patterns of Mola mola near Santa Catalina Island, Cartamil and Lowe (2004) attached temperature and depth-sensing acoustic transmitters to eight Ocean Sunfish (total length range 73 to 151 cm). Each day, the fish traveled a mean horizontal distance of 27 kilometers, with a decrease in the rate of movement during the first six hours of night. The figure below shows that the sunfish displayed a diel pattern of vertical movement, limited to the warmer near-surface waters during the night, and ranging much deeper during daytime periods. In the daytime, the Mola dived repeatedly below the thermocline, for 2 to 20 minutes duration, to depths of 40 to 150 meters. The researchers concluded that Ocean Sunfish are active swimmers, and do not simply drift along with prevailing currents. They also speculated that the diel vertical movements of sunfish maximize foraging on migratory zooplankton, and are followed by return visits to the warm mixed layer to recover body temperature. Most importantly, these telemetry studies on Mola mola provide possible strategies for reducing bycatch, by lowering the minimum depth at which drift gillnets are set at night.
Vertical migration of Ocean Sunfish; time of day on x axis
From Cartamil and Lowe (2004)
Bass, A.L., Dewar, H., Thys, T., Streelman, J.T., and Karl, S.A. (2005) Evolutionary divergence among lineages of the ocean sunfish family, Molidae (Tetraodontiformes). Marine Biology 148, 405-414.
Cartamil, D.P., and Lowe, C.G. (2004) Diel movement patterns of ocean sunfish Mola mola off southern California. Marine Ecology Progress Series 266, 245-253.
Yamanoue, Y., Miya, M., Matsuura, K., Katoh, M., Sakai, H., Nishida, M. (2004). Mitochondrial genomes and phylogeny of the ocean sunfishes (Tetraodontiformes: Molidae). Ichthyological Research, 51(3) DOI: 10.1007/s10228-004-0218-6