The Molids, part 1: Intro and Mola mola

Of all the variations on the vertebrate "fish" body plan, molids are among the most bizarre. Yes, I'm well aware of how strange some other "fish" are. Members of the already oddball clade Tetraodontiformes, molids have the general appearance of swimming heads (German: "Schwimmender kopf"), have the fewest vertebrae of any "fish" (16-18) and lack the following structures*: caudal bones, ribs, pelvic fins, [fin] spines, girdles, and swim bladders (Fishbase, Bass et al. 2005 - citing Tyler 1980). Their skeletons have to be seen in order to be believed and it's hard to imagine how reconstructions would look if these animals were only known from fossils. While other large aquatic vertebrates have generally similar tapering/streamlined body shapes (sea turtles excepted), molids have taken a more... attenuated approach:

Taken from here.

* Although Fishbase reports the lateral line to be absent, this was recently discovered not to be the case. Nakae and Sasaki (2006) noted that there are six cephalic and one trunk lateral lines; the latter structure has only 27 superficial neuromasts and is limited to the anterior portion of the fish. Interestingly, the same study discovered that what appears to be the caudal fin of molids (the clavus) is in fact derived from the dorsal and anal fins.

Mola mola is the largest extant actinopterygian ("bony fish") with a reported average length of 1.8 m (counting the clavus) and weight of one tonne (Wood 1987); they can reach 3.32 m and probably exceed four tonnes (see the next post for an explanation). Molids have a cartilaginous, weakly ossified, spongy skeleton and as a result were only known from fossil jaws and dermal plates until recently when three extraordinary upper Miocene (~22 mya) specimens were described from Austria (Gregorova et al. 2009). The Austromola angerhoferi individuals were estimated from Mola mola proportions to have total lengths of 1.5-1.7 m, 2.4 m, and 3.2 m - this would seem to suggest that Austromola had a much larger average size than the extant species (However - a Mola subpopulation off Japan has an even larger average size) (Gregorova et al. 2009). Potentially phenomenal size aside, Austromola bridges the gap between the Eocene (~42 mya) Eomola and Miocene to Pliocene fossils of extinct Mola and Ranzania species; Austromola is the sister taxa to Mola + Masturus which is in turn the sister clade to Ranzania (Gregorova et al. 2009). More on those non-Mola mola molids in the next post.

Much like the basking shark, recent tracking data demonstrates that molas are much more mobile than was previously anticipated. Despite the common name "oceanic sunfish", three small (~14-18 kg) specimens showed a general diel pattern (deeper at day and vice versa) and had a maximum recorded depth of 472 m - excluding one incident where the tracker went below 2000 m and presumably indicated the death of the fish (Sims et al. 2009). The diel pattern wasn't strictly adhered to as some fish stayed below 200 m for most of the day and at other times fish moved from their maximum depth to the surface in 4 hours (Sims et al. 2009). Vertical movements in Mola may be related to finding maximum prey concentrations (when the prey is on the move); other hypotheses for surfacing (not necessarily mutually exclusive) include warming, recovering from time spent in sub-oxic waters, or even to have parasites removed by sea birds (Sims et al. 2009). Prey densities also appear to be the driving force of Mola movements to high latitudes during summer and movements back are possibly due to thermal tolerance issues (Sims et al. 2009). Global warming could thus expand the seasonal thermal window at high latitudes for Mola and overfishing and eutrophication could increase jellyfish blooms - molids are some of the few large predators which have top-down control over the cnidarians (Sims et al. 2009). Exactly what sort of impact bycatch* has on molid populations is also an interesting question - this certainly seem like a species worth looking into.

* So exactly how numerous are molids? Aerial observations in the Irish and Celtic seas observed small individuals (0.5 to 0.7 m) at a density of about 1 individual per 100 square kilometers working under the assumption that they are not at the surface 3/4 of the time (Houghton et al. 2006). The authors note that the largest Mola specimen from British waters only weighed 363 kg (slightly above average?) which would either suggest that adult individuals don't bask often or are rarely present - or both. It's worth noting that molids are very commonly caught as bycatch (29-93% of all catch, in some instances), and while fish are released alive, this activity must have substantial (but as yet unknowable) impacts on the fish (Houghton et al. 2006). It takes an estimated 20 years to grow a large 3 m Mola mola in the wild, but one aquarium fish went from 26 kg to 400 kg in only 14 months.

Another Mola tracking study took a novel approach by including the leatherback turtle Dermochelys coriacea. Unexpectedly, the giant species have converged to feed on low-nutrient jellyfish prey - however Mola can feed extensively at depths around 500 m while the turtles spent time almost exclusively in the upper 200 m (Hays et al. 2009). Unlike Sims et al. (2009) the Mola specimens were closer to the reported average (1.08-1.60 m, 67-213 kg) - the turtle were fair-sized as well (~1.7 m and ~450 kg for 2 specimens) (Hays et al. 2009). It is interesting that in South Africa at least, molas stay broadly in the same location and leatherback turtles migrate extensively despite having a similar diet (Hays et al. 2009). The reason why there are multiple large jellyfish feeders (there are other molids after all...) wasn't explored by Hays et al. (2009) but this is of course an area of ecology that we are just beginning to understand. I'm going to assume that the turtles have some sort of advantage in shallower waters (capable of handling larger prey?) and that an equilibrium between the species probably varies between geographical locations (if oxygen levels at depth play a role, for instance).

But on a different note, as tracking studies imply, molids are not the planktonic weak swimmers they were long assumed to be. Watanabe and Sato (2008) used larger Mola specimens for their study (48, 59, 153 kg) and found that the tracked fish swam actively by stroking their fins and had cruising speeds roughly comparable to sturgeons, salmon, marlin, and blue sharks; larger fish swam slower to maintain the same Reynold's number (Watanabe and Sato 2008). Mola acceleration data indicated a one-stroke cycle with lift-based thrust similar to that previously reported in penguins - this is the only known instance of "wings" which are not bilaterally symmetrical (Watanabe and Sato 2008). The dorsal and anal fins are symmetrical in shape with identical muscle mass although the muscle morphology differs markedly; the aspect ratio of the fin/wings decreases possibly due to reasons of mechanical strength (Watanabe and Sato 2008). Watanabe and Sato (2008) also determined that Mola is neutrally buoyant due to a layer of gelatinous tissue (similar to deep-sea fishes); the incompressible tissue gives stable buoyancy at every depth unlike fish which rely on swim bladders for neutral buoyancy (Watanabe and Sato 2008).

More molids soon!

References:

Hays, Graeme C., et al. 2009. Vertical niche overlap by two ocean giants with similar diets: Ocean sunfish and leatherback turtles. Journal of Experimental Marine Biology and Ecology 370, 134–143

Houghton, Jonathon D. R., et al. 2006. The ocean sunfish Mola mola: insights into distribution, abundance and behaviour in the Irish and Celtic Seas. J. Mar. Biol. Ass. U.K. 86, 1237-1243

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.

Nakae, Masanori and Sasaki, Kunio. 2006. Peripheral nervous system of the ocean sunfish Mola mola (Tetraodontiformes: Molidae). Ichthyological Research 53, 233-246. Available

Sims, David W., et al. 2009. Satellite tracking of the World's largest bony fish, the ocean sunfish (Mola mola L.) in the North East Atlantic. Journal of Experimental Marine Biology and Ecology370, 127–133

Watanabe, Yuuki and Sato, Katsufumi. 2008. Functional Dorsoventral Symmetry in Relation to Lift-Based Swimming in the Ocean Sunfish Mola mola. PLoS ONE. Available

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