Billfish Bills - What Are They Good For?

In the prior article, I discussed long-beaked "dolphins" (Eurhinodelphidae) and noted that I couldn't find hypotheses on the function of their uneven jaws in the literature... aside from a weird proposal involving Skimmers. The Theatrical Tanystropheus mentioned a couple ("digging for small, sand-dwelling organisms or as a bat with which to stun fish") which are plausible, but I don't know where they are from or what lines of reasoning are behind them. There are extant species with a superficially similar condition - billfishes - and it could be relevant to review what they do with their bills.

Atlantic White Marlin (Tetrapturus albidus) from Wikipedia Commons.

Swordfish (Xiphiidae) and Marlins/Sailfishes/Spearfishes (Istiophoridae) are living sister taxa¹ in the clade Xiphioidea; while traditionally included in Scombroidei, billfishes are presently regarded as phylogenetically distinct (Orrell et al. 2006) and possibly close relatives of jacks and... flatfishes (Little et al. 2010). Fish phylogenetics is scary business, and I suspect billfish relations will undergo further revisions as the monstrosity known as "Perciformes" is reasoned into pieces. Anyways, while xiphiids² and istiophorids look superficially similar, they actually have rather distinctive morphology. Swordfish have a bill which is flat in cross-section, toothless, blunt-tipped, and with central chambers (compared to rounded, denticulated, pointed, and chamber-less for istiophorids), a weak mandible much shorter than the rostrum, no scales, and no pelvic fins (Collette et al. 2006; Fierstine 2006; Fierstine and Voight 1996 citing Nakamura 1983). Strangely, most extinct billfishes have jaws of equal length, and if the proposed (Istiophoridae + Hemingwayidae) and (Xiphias + Xiphiorhynchus) clades (Fierstine 2006) are correct (see note 1), this would mean the unequal jaws of extant billfishes evolved twice. 

¹ A detailed cladistic analysis with the fossil members of the group has yet to be undertaken (Fierstine 2006).
² As for what the deal with them and ziphiids is, I have no idea.

Swordfish (Xiphias gladius) from Wikipedia Commons.

One infamous use of the billfish bill is impaling unexpected objects. One Blue Marlin was found with rostrum fragments from two other, different billfish species (Fierstine 1997). Other unfortunates include large fish, whales, bales of rubber, boats, ships, deep-diving vessels, people, and turtles (Frazier et al. 1994 - citing various). The billfish-on-billfish impaling has been interpreted as defense against predators (Fierstine 1997) and in the case of the turtles, it was hypothesized that the billfish accidentally impaled them when aiming for fish aggregated nearby (Frazier et al. 1994). Istiophorids can survive with a foreshortened rostrum (Fierstine 2006) so apparently these accidents are survivable. But this raises another question - do they need an elongated rostrum at all?

One study of 227 Blue Marlins (Makaira nigricans) stomach contents found that 38% of prey items showed evidence of damage from the bill, 11% of which were speared and 81% of which were slashed, and the rest of which were in multiple pieces (Shimose et al. 2007). Bizarrely, another study with 226 Blue Marlins found no evidence of prey being struck or speared (Vaske et al. 2011). Vaske et al. (2011) offered no explanation for this anomaly, and I can't see an obvious one either. Both populations (from Japan and Brazil, respectively) even primarily preyed on Skipjack Tuna (Katsuwonus pelamis), which were normally killed with the bill in the former population. I'm stumped.

Fierstine (2006) hypothesized that unequal jaw length in billfishes may have evolved to avoid suffocation when impaling large objects (predator or prey) and to avoid damage to the mandible. I don't buy the mandibular reasoning since extant billfishes get by just fine with them naturally foreshortened. The available evidence suggests impaling is a rather rare event and thus unlikely to be the main factor in the evolution of the characteristic billfish bill. An alternate hypothesis could be that the mandible was shortened so the rostrum could be "weaponized" (sword-like flattening in xiphiids and denticles in istiophorids³) to slash at prey. However, the population which apparently doesn't use bills to feed and healthy individuals with damaged rostra are problematic for both of these hypotheses. Perhaps future studies will show that the bill is generally important for feeding in the group and that the counterexamples are just freaks, but either way, it seems premature to make any conclusions about why billfish have their striking morphology.


³ The ichthyosaur Eurhinosaurus has teeth on the upper jaw which could be a similar instance of "weaponization". 

I really have no idea how eurhinodelphids fit into this framework since Fierstine's hypothetical suffocation would not be an issue (if they could impale at all) and the rostrum does not seem particularly dangerous (no teeth, denticles, or flattening). I wonder if this morphology evolved for different reasons, or if it evolved for reasons that have yet to be hypothesized.

References:

Collette, B. B., McDowell, J. R., and Graves, J. E. (2006). Phylogeny of Recent Billfishes. Bulletin of Marine Science 79(3), 455-468. Available.

Fierstine, H. L. (2006). Fossil history of Billfishes (Xiphioidea). Bulletin of Marine Science 79(3), 433-453. Available.

Fierstine, H. L. (1997). An Atlantic Blue Marlin (Makaira nigricans), impaled by two species of billfishes (Teleostei: Istiophoridae). Bulletin of Marine Science 61(2), 495-499. Available.

Fierstine, H. L., and Voight, N. L. (1996). Use of Rostral Characters for Identifying Adult Billfishes (Teleostei: Perciformes: Istiophoridae and Xiphiidae). Copeia 1996(1), 148-161. Available.

Frazier, J. G., Fierstine, H. L., Beavers, S. C., Achaval, F., Suganuma, H., Pitman, R. L., Yamaguchi, Y., and Prigioni, C. M. (1994). Impalement of marine turtles (Reptilia, Chelonia: Cheloniidae and Dermochelyidae) by billfishes (Osteichthyes, Perciformes: Istiophoridae and Xiphiidae). Fisheries Science 39(1), 85-96. Available.

Little, A. G., Lougheed, S. C., and Moyes, C. D. (2010). Evolutionary affinity of billfishes (Xiphiidae and Istiophoridae) and flatfishes (Plueronectiformes): Independent and trans-subordinal origins of endothermy in teleost fishes. Molecular Phylogenetics and Evolution 56(3), 897-904. doi:10.1016/j.ympev.2010.04.022

Nakamura, I. (1983). Systematics of billfishes (Xip­hiidae and Istiophoridae). Publications of the Seto Marine Biological Laboratory 28, 255-396.

Orrell, T. M., Collette B. B., and Johnson, G. J. (2006). Molecular data supports separate scombroid and xiphioid clades. Bulletin of Marine Science 79(3), 505-519. Available.

Shimose, T., Yokawa, K., Saito, H., and Tachihara, K. (2007). Evidence for use of the bill by blue marlin, Makaira nigricans, during feeding. Ichthyological Research 54(4), 420-422. DOI: 10.1007/s10228-007-0419-x

Vaske, T., Travassos, P. E., Pinheiro, P. B., Hazin, F. H. V., Tolotti, M. T., and Barbosa, T. M. (2011). Diet of the Blue Marlin (Makaira nigricans, Lacepède 1802) (Perciformes: Istiophoridae) of the southwestern equatorial Atlantic Ocean. Brazilian Journal of Aquatic Science and Technology 15(1), 65-70. Available.