The Molids, part 2: Beyond Mola mola

In the previous post I argued that molids were among the strangest of fishes on the basis of peculiarities such as their loss of a caudal fin and the development of an analgous clavus from the dorsal and anal fins, non-bilaterally symmetrical flight from those dorsal and anal fins, half a trunk lateral line (related to clavus development?), a diet heavy in jellyfish despite body masses often in the hundreds of kilograms (and sometimes tonnes), and so forth. I forgot to work in the outrageous fecundity of females (300 million eggs in a 1.4 m individual) and the two larval stages which resemble pufferfish. Most of these figures pertain to Mola mola, but there are of course other species of extant molids.

It's always interesting how some clades have a "flagship" species of sorts with all the charisma and press while the majority of the diversity languishes in obscurity. Molid taxonomy has long been a mess with 54 proposed species and missing type specimens, but these days 3 "genera" and 3-5 species are typically listed (Bass et al. 2005, also citing Parente 2003, Fishbase). Bass et al. (2005) used data from the d-loop and cytochrome b of molids to establish a phylogeny of the group and were surprised that some Southern hemisphere Mola specimens were estimated to have diverged 2.8-7.5 mya from the other major clade (Bass et al. 2005). The authors resurrected the name Mola ramsayi for the divergent clade and noted that both species were recorded from South Africa (Bass et al. 2005). It appears that field workers are not able to distinguish the species (voucher specimens were lacking for Bass et al.'s study, however) which implies very similar morphology and prior workers suggested that M. ramsayi can be distinguished by more numerous fin rays and larger ossicles on the clavus (Bass et al. 2005, also citing Giglioli 1883, Fraser-Brunner 1951). Also unexpected was that the Atlantic and Indo-Pacific M. mola populations were estimated to have diverged very recently (0.05-0.32 mya) while the M. ramsayi populations in the southern portions of the same oceans apparently diverged much earlier (1.55-4.10 mya) (Bass et al. 2005). Further studies will be needed to clear up the biogeography and morphology of M. ramsayi, and things certainly haven't gotten any simpler...

Yoshita et al. (2009) examined mitochondrial D-loop data from 119 Mola specimens and found three distinct clades, two of which overlap off Japan. The group A clade (n = 20) mostly occurred off the Eastern coast of Japan (2 were from Australia) and was remarkable for being composed entirely of females (and unsexed fish) with an average total length of ~2.6 m (+/- 0.5 m) which increased with latitude - all 9 specimens from the Northeast of Japan were over 249 cm and one measured 332 cm* (Yoshita et al. 2009). The group A Mola clade also had distinctive morphological traits such as a well-developed head bump, a proportionally longer anterior portion of the body, a proportionally deeper body, 14-17 clavus fin rays, 8-15 clavus ossicles, and a clavus edge that was never wavy (Yoshita et al. 2009). It appears that the historical Fiona specimen (not the world record holder, see the footnote) displays this morphology - as opposed to this more typical aquarium specimen. The authors suggest that the infrequency, size, and possible sex bias of group A specimens caught off Japan indicates that the main population could occur near the Bonin Islands (a migration route could be present) and suggest that the clade may be primarily a Southern Hemisphere one (Yoshita et al. 2009). Alternately, it was suggested that group A could have a migration route from the Eastern or Northern parts of the Pacific to Japan since the northeastern Japanese and subtropical group A fish showed significant divergence (Yoshita et al. 2009). It is not clear if group A is synonymous with Mola ramsayi and if not, presumably either another name could be resurrected or one will be coined if none of the prior names provided an appropriate description.

* This is probably the largest reliably recorded bony fish to date. Fishbase mentions a 333 cm specimen (not the 10'2"/3.1 Fiona specimen) but I can't find the source to assess its reliability. The 332 cm female fish (caught in 2004) was no outlier as 323 cm (2004 - unsexed), 325 cm (1999 - unsexed), and 330 cm (2002 - unsexed) fish were all recently found in a small sample - all were from Japan and Group A (Yoshita et al. 2009). This fascinating article reveals that the Fiona specimen (3.1 m/2.2 tonnes) was probably never weighed since a 2.7 m Mola (from Japan, of course) weighed 2.3 tonnes - by my calculations the Fiona specimen was about 1.3 tonnes "short" and a 3.3 m Mola could weigh over, gulp, 4 tonnes.

Oh, and since Austria has a rather northerly latitude, it could be possible that the Austromola fossils represent a similar population consisting of very large animals (females?) and the actual median length could be the same as the Mola species.

Group B Mola were found to be widely distributed in the Kuroshio current and grouped with M. mola from outside Japan, including Atlantic specimens Yoshita et al. (2009). This clade was notable for having a much smaller mean size (1 m =/- 0.6 m, n = 86), a wavy clavus in larger individuals (1.9-2.7 m, n = 11), a smooth band at the base of the clavus (also present in group B), 12 rays and 8-9 ossicles on the clavus, recorded males, and no significant differences between the sexes (Yoshita et al. 2009). The morphology and genetics of this group match what was previously established for Mola mola (Yoshita et al. 2009)..

Group C Mola corresponded somewhat with the species Mola ramsayi established by Bass et al. (2005) - although one member of that clade from Australia was placed in group A by Yoshita et al. (2009) - oddly enough authors of the latter study did find a group C individual from New South Wales, Australia. The monophyly of group C was supported by a very high bootstrap by Yoshita et al. (2009) but it should be noted that the sample size was only 3 and it was found to be the sister clade to group A. While the evidence presented by the authors suggests of three Mola species, clearly group C needs many more samples in order to be convincingly demonstrated to be a distinct species. And then people need to argue over which clade gets to be M. ramsayi... there's a long way to go.

The re-recognition of multiple Mola species is going to necessitate a revision of basic biological information. Future studies will have to be careful to differentiate between populations/species as to not create chimerical data - hopefully then we'll get a clearer picture of just how these species are separated geographically and possibly behaviorally and ecologically. If large (entirely female?) group B individuals really are a minor presence in very heavily fished areas, perhaps this should be looked into as a concern for conservation.

I'm not done with molids yet - Mola is not the only one!

References:

Bass, Anna L., et al. 2005. Evolutionary divergence among lineages of the ocean sunfish family, Molidae (Tetraodontiformes). Marine Biology 148, 404-415

Wood, Gerald. 1982. Guinness Book of Animal Facts and Feats. Third Edition.

Yoshita, Yukiko, et al. 2009. Phylogenetic relationship of two Mola sunfishes (Tetraodontiformes: Molidae) occurring around the coast of Japan, with notes on their geographical distribution and morphological characteristics. Ichthyological Research 56, 232-244