Goblin Sharks are Giants

Goblin Sharks (Mitsukurina owstoni) may be among the largest of cartilaginous fishes. While generally credited with a maximum total length of 3.8 meters (12’6″), one specimen appears to have exceeded that by far.

In July 2000, an enormous shark was accidentally captured in the northern Gulf of Mexico after being entangled in a line attached to a crab trap (Parsons et al. 2002). Only the jaws of the shark were kept by the fishers (Parsons et al. 2002) and it is unknown why they weren’t examined by the authors. Considering the fishers took the time to dissect the shark (Parsons et al. 2002) I’m puzzled that no measurements were provided. The above photo, while clearly demonstrating Goblin Shark morphology and giving the impression of great size, unfortunately lacks any landmarks which can establish scale. Fortunately, a second photograph focusing on the head was taken and it proved surprisingly informative about the Gulf shark’s size.

The rope proved to be the key. With a known diameter of 2.06 cm (0.8″) Parsons et al. were able to measure a snout to eye distance of 62.9 cm (2’1″). Scaling up from the previously largest known specimen yielded a total length of 5.4 m (17’9″) for the Gulf shark (Parsons et al. 2002). The authors suspected the figure may have been an underestimate as snouts become proportionally shorter with increased total length and so used an exponential regression to calculate a total length of 6.17 m (20’3″). I strongly suspect the latter figure is closest to reality. I calculated the above photograph shows about 3.5 meters (11’6″) of shark despite most of the tail being out of frame. The aforementioned 3.84 m specimen appears to have a proportionally much longer snout, lending credence to the notion that 5.4 m is an underestimate. Of course it would be nice if those jaws showed up – or better yet, a similarly-sized specimen – but the case for gargantuan Goblin Sharks seems compelling.

Is the Gulf shark some one-off freak? I suspect not. The Gulf shark was the first specimen ever recorded from the Atlantic coast of North America (Parsons et al. 2002) and is still apparently the only known (Castro 2011). Adult Goblin sharks have only been “occasionally reported” presumably due to their deep water habitat (Castro 2011) and this list suggests they are very occasional indeed. With such a small sample size, a lack of Gulf shark-sized individuals could just be a statistical quirk. Perhaps there is bias towards the capture of smaller individuals as a ~6 m individual (perhaps approaching a tonne in weight) could be prohibitively large for most vessels to catch, let alone haul on board and preserve. Here’s to hoping that a monstrous specimen scares the hell out of an ROV crew someday!

Goblin Sharks may be giants, but they are far from alone in the lightless depths and far from being the largest. More soon.

References:

Castro, J. (2011) The Sharks of North America. Oxford University Press.

Parsons, G. R., et al. (2002) First record of the goblin shark Mitsukurina owstoni, Jordan (Family Mitsukurinidae) in the Gulf of Mexico. Southeastern Naturalist 1(2), 189-192.

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Old Six Lobes

_Latimeria chalumnae_ from the Harvard Museum of Natural History. The famous _Kronosaurus_ lurks in the reflection.

What is going on with coelacanths? The first dorsal fin is a typically fishy rayed structure yet the second is fleshy and lobed. Considering that tetrapod limbs are derived lobed fins and the anal fin is also lobate this makes coelacanths hexapedes of a sort. It is baffling how the nickname “Old Fourlegs” got attached to a creature with such bizarrely arrayed and numerous appendages.

A phylogeny of lobed fins from Friedman et al. (2007).

Coelacanths can’t really be said to have “legs” or even proper limbs. Tetrapod limbs are defined as having mobile wrists, ankles and digits (Shubin et al. 2006) whereas coelacanth lobed fins are made out of a series of metapterygia with radials on the anterior and posterior margins (see above). The Devonian coelacanth Shoshonia had pectoral fins with some traits more typical of early tetrapodomorphs than extant coelacanths, most notably by having pronounced asymmetry (Friedman et al. 2007). This indicates that coelacanth lobe fins became less limb-like during their evolution and Friedman et al. (2007) argue that the extant fishes most likely to give insight into early tetrapod limbs are basal actinopterygians such as bichir and sturgeon. Thus extant coelacanths should not be viewed as evolutionarily stagnant and the nickname “Old Fourlegs” somehow manages to be even more wrong.

From Wikipedia Commons.

The medial second dorsal and anal fins of Latimeria are “practically identical” and mirror one another (Forey 1998). These lobed medial fins exhibit skeletal, muscular and innervational similarities with the paired lobed fins (Ahlberg 1992 – citing Millot and Anthony 1958) and all the lobed fins appear capable of considerable rotation (Forey 1998). Judging by videos it appears that the medial lobed fins beat in sync whereas the pectoral and pelvic pair does not. The medial lobed fins particularly resemble the pelvic fins and the basal plate supporting the former appears very similar to the pelvis of the latter rotated 90 degrees (Ahlberg 1992). Ahlberg (1992) suggested that the lobed median fins came into being through a switch in gene expression although noted more fossils and evidence from development and genetics would be needed. Unfortunately so far as I can tell this topic has yet to be explored further.

From Wikipedia Commons

The caudal fin of coelacanths is normally interpreted as a three-part structure (Forey 1998) however in the description of the Jurassic coelacanth Parnaibaia maranhaoensis Yabumoto (2008) interprets it as being composed of a third dorsal, second anal fin and a proper caudal. Third dorsals and second anals are not without precedent (e.g. cod) however the degree of disparity between fins that coelacanths display certainly is. Perhaps the medial lobed fins of coelacanths aren’t true dorsal or anal fins at all but totally novel replications of the pelvic fins. Alternately it could be possible that coelacanths had ancestors with a superfluous number of medial fins or that the caudal fin split into three parts for some reason; either way it seems likely that something very very strange is happening to coelacanths genetically and developmentally.

References:

Ahlberg, P. E. (1992). Coelacanth Fins and Evolution. Nature 358, 459. Available.

Forey, P. L. (1998). History of the Coelacanth Fishes. Natural History Museum: London.

Friedman, M. (2007). First discovery of a primitive coelacanth fin fills a major gap in the evolution of lobed fins and limbs. Evolution & Development 9(4), 329-337. Available.

Millot, J. & Anthony, J. (1958-1965) Anatomy de Latimeria chalumnae. Vol. I-II. Paris.

Shubin, N. H. et al. (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440 (6), 764-771. Available.

Yabumoto, Y. (2008). A new Mesozoic coelacanth from Brazil (Sarcopterygii, Actinistia). Paleontological Research 12(4), 329-343. Available.