Wednesday, September 9, 2015

An Inordinate Fondness for Fish Snouts. Part 2: More than just Sawfish — Paddlefish Proboscises, Marlin Muzzles & Swordfish Snoots

By Jason Seitz 

In Part 1 of this series we discussed the tooth-studded snouts of modern and Cretaceous sawfishes and saw sharks and how each group evolved separately and independently, a process called ‘convergent evolution’. In this final part of the series, we’ll discuss and compare beaked beasts that have unique adaptations and those that share similar anatomy due to common ancestry.

Fig. 7: The North American paddlefish (Polyodon spathula) uses the well-developed electrosensory system along its paddle to detect zooplankton which it strains from the water with its highly modified gill rakers.
One cannot talk about fish snouts without mentioning the unique paddlefishes (Polyodontidae), whose extremely long snout (longer than their head) is referred to as a paddle (Fig. 7). The paddle is covered with tens of thousands of ampullary organs (Fig. 8), an electrosensory system used to detect prey such as zooplankton or small schooling fishes. Although the genus of the North American paddlefish (Polyodon spathula) is Greek for ‘many teeth’, there are no teeth along the paddle and, indeed, the conical teeth in the mouth of juvenile paddlefish become smaller and less numerous as the fish grows, with large specimens appearing toothless. So why does the North American paddlefish genus refer to teeth if they have little or none? The answer is in the fish’s highly modified gill rakers, which are so specialized for food gathering, via the filtering of zooplankton from water, they effectively act as teeth. There are four extinct fossil species dating as far back as the Cretaceous.

Fig. 8: Close-up of the ventral surface of a paddle from a North American paddlefish (Polyodon spathula) showing the clusters of ampullary organs that it uses to detect zooplankton.
True to their name, the billfishes all have a well-developed snout. Three billfish families (Hemingwayidae, Blochiidae, Palaerhynchidae) are known only from the fossil record, with the remaining modern family (Istiophoridae) having existed since the middle Miocene. Modern billfish consist of about 11 species of familiar marlins, sailfish, and spearfishes. The bills of these great fish are round in cross-section, and most have small slender teeth set in sockets along the bill (Fig. 9). Marlins typically slash at their prey (typically pelagic fishes and squids) using a side-to-side motion and the stunned prey are then swallowed head-first. Fossilized vertebrae of tunas have been found that appear to have been punctured by the rostrum of a billfish. Also, sharks and other billfishes have been observed having billfish bills sticking out of their body, suggesting that the bill may be used not only for food gathering but perhaps also for defense. Alternatively, billfish may simply occasionally impale slow-moving objects by accident while feeding on smaller prey. 

Fig. 9: Close-up showing the small, slender teeth and some empty sockets (alveoli) along the bill of a marlin (Istiophoridae).
Swordfish are in a different family (Xiphiidae, 1 modern + about 10 fossil species) from the billfishes. As suggested by their superficial resemblance to the billfishes and their long, well-developed bill, swordfish are closely related to billfishes. The bill, or sword, is an extension of the upper jaw (technically called the pre-maxilla) that is flattened in cross-section and contains a central canal or series of central chambers. The long, well-developed sword of this fish surely made a strong impression on the naturalist Carl Linnaeus who described this species (in 1758)—the genus Xiphias is a Greek reference to the shape of a sword and the Latin specific epithet gladius also refers to a sword!  Swordfish do, however, have small, slender tooth-like spines along the lower surface of the sword (Fig. 10). These spines are rudimentary, rough projections of the rostral surface and are not set into sockets. They may represent the vestigial remains of past larger, better-developed teeth. The broken-off swords found in various living and non-living objects may be due to self-defense by the swordfish, or, alternatively, may be by accident while feeding under floating or slow-moving objects.

Fig. 10: Close-up of a portion of a swordfish (Xiphias gladius) sword, showing the rough projections along the surface. The spine-like projections may represent the remains of past larger, better-developed teeth.
Billfishes and swordfish are related groups that share a common ancestry, separate from the sawfishes that were discussed in Part 1 of this series. The similar form and function of the bills of billfishes and swordfish is an example of what scientists call ‘homologous structures’. The paddle of paddlefishes is different from the rest of the snouted fishes we discussed, and not closely related to the other groups.  The paddle represents a rather novel structure and not strictly a result of convergent evolution, nor is it a homologous structure. Researchers hypothesize how the structures came to be but there is still much to learn. Science will continue striving to learn more about these snouted oddities, fueled by unending fascination and a passion for discovery. Nature’s fascination with these snouted curiosities is perhaps matched only by our own enduring drive to learn more.

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