The Molids, part 3: Enough with Mola!

In the first two posts I've mostly discussed Mola, a clade of three genetically distinct species whose morphological and biogeographical differences have just begun to be investigated. While Mola mola was a "flagship" species which obfuscated [previously recognized] diversity, there are other molids which are effectively unknown to Western public consciousness.

The so-called sharp tail/fin mola Masturus, taken from here. The dorsal fin is of course heavily damaged and while the clavus also looks attenuated compared to those of other specimens, the rounded edge seems to suggest that it isn't damaged. Liu et al. (2009) recovered a linear relationship of total length and standard length (TL - SL = clavus length) and fairly similar proportions in males and females - the large sample of primarily immature fish may have masked differing clavus proportions in large individuals.

As the above photo illustrates, Masturus is very similar in appearance to the Mola species and can be easily differentiated by a projection of the clavus (hence the common name). Gregorova et al. (2009) found Masturus and Mola to be sister taxa with the only differences (4 characters of 57 total) relating to one less caudal vertebrae in Masturus and clavus rays terminating in bony plates exclusively in Mola. Bass et al. (2005) recovered the same topology with combined d-loop and cyt b data; d-loop data in the same study also revealed that the small sample (n = 5) of Masturus did not show clear differences across ocean basins. This seems curious since the same authors mention unpublished tracking data which showed no indication of ocean basin-scale movement or large-scale migrations in the species* (Bass et al. 2005). Also interesting was that the authors only discussed Ma. lanceolatus and didn't mention Ma. oxyuropterus; the small sample size leaves open the possibility of an additional species despite the wide geographic sampling (including the type locality for Ma. oxyuropterus). Who knows, there could be another Mola-like situation at hand here...

* Seitz et al. (2002) tagged a 1 m (TL) individual which travelled 9.7 km/day - although where it traveled was not mentioned. Contrary to prior suggestions, the fish was epi-pelagic (preferring waters <>2.7% of its time in the top 5 m of the water column and could apparently dive below 1000 m (Seitz et al. 2002). I'd assume that older individuals are more tolerant of of cold water, but due to the extreme rarity of surface behavior in Masturus it may be a while before this is confirmed.

Masturus has very recently become the target of a fishery off Eastern Taiwan and this gave Liu et al. (2009) an opportunity to examine the basic biology of the species. The population ranged in length from 42 to 192 cm (SL - the clavus was often damaged) with most individuals between 80 and 119 cm (Liu et al. 2009). All specimens over 158 cm SL were female although since only one gravid individual was recorded it appears likely that this locale is not a spawning ground for the species (Liu et al. 2009). The authors speculated that juveniles may frequent inshore nurseries (where the fisheries primarily operate) while the adults occur further offshore - they noted that specimens greatly exceeding the maxima in their study have been recorded (262.5 cm SL/313.8 cm TL - another 337 cm TL*) (Liu et al. 2009). Vertebra centra were examined for rings and working under the assumption that ring count correlated with age (it did correlate with size) this indicates that their largest female was 23 years old and longevities were estimated to be 105 and 82 years for females and males, respectively (Liu et al. 2009). The authors note that the lack of very young and old specimens can greatly affect growth estimates and of course it can't be emphasized enough that ring formation may be caused by events which do not occur annually. It seems unlikely that data pools as extensive as that of Liu et al. (2009) will be encountered elsewhere (fortunately, in a way) and it appears exceedingly unlikely that a mark-recapture study or study of captive specimens** can resolve the questions of growth and longevity in Masturus - presumably a model species can be used to answer the question that the authors brought up but couldn't answer: if the fishery will lead to a decline of stocks.

* Assuming this is a female, it would have a SL of 282 cm from the equation established in Liu et al. (2009) and 281 cm extrapolating from the largest fish with both TL/SL measurements - this seems to imply that the clavus length has the same proportion throughout life after all. A male, by the way, would have an estimated SL of 277 cm, implying that this is not a dimorphic trait.

What's especially confusing to me is that estimating the weight of the 282 cm SL individual from the equations set up by the authors yields weights of ~1000-1200 kg (the larger from a male) and scaling up from their largest individual (195 cm SL, 409) also yields ~1200 kg. This seems very strange since a 2.7 m Mola (TL?) weighs around 2.3 tonnes - how on earth can a Masturus of roughly the same linear dimensions weigh around half as much? More data on large Masturus individuals, with clearly differentiated total and standard lengths, is desirable to solve this puzzle!

** At least one Mola in an aquarium gained weight extremely quickly and possibly abnormally. If they can actually grow this quickly in the wild then perhaps the Masturus fishery won't totally wreck the population... in the near future.

It's amazing that a species for whom the discovery of a single specimen was notable several decades ago (e.g. Gudger 1935) is now being harvested by the ton - and hardly any more is known about it. While the cosmopolitan range of the species and lack of direct harvesting in most locales probably prevents it from being in any direct danger, exploiting an organism with little known about its basic biology is a dangerous game.

There's still one more molid left, the smallest, most basal, and arguably most neglected of them all!

References:

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

Gregorova, Ruzena, et al. 2009. A giant early Miocene sunfish from the North Apline Foreland basin (Austria) and its implications for molid phylogeny. Journal of Vertebrate Paleontology 29, 359–371.

Gudger, E. W. 1935. A photograph and description of Masturus lanceolatus taken at Tahiti, May, 1930. American Museum Novitates 778, 1-7. Available

Liu, Kwang-Ming, et al. 2009. Age and growth estimates of the sharptail mola, Masturus lanceolatus, in waters of eastern Taiwan. Fisheries Research 95, 154-160

Seitz, A. C., et al. 2002. Behaviour of a sharptail mola in the Gulf of Mexico. Journal of Fish Biology 60, 1597–1602.