Perfect Little Killing Machines: The Jaws of Death

There’s a bit of a pardox of public opinion regarding the effectiveness of spiny dogfish as predators.  Depend on who you ask, they’re either forming a swimming wall of teeth annihilating everything in their path or they’re weak scavengers, poor excuses for sharks.  This much-maligned species gets the double-whammy of both being a pest and being unworthy of respect as a predator.  However, science (a lot of it coming out of the Wilga lab at my undergrad alma mater) seems to back up the “formidable predator” side of the argument, and these little sharks earn their place in the marine food web through a number of neat adaptations.  For this installment of Perfect Little Killing Machines, we’re going to look at the most obvious adaptation for a predatory lifestyle: the jaws.

First off, let’s get a “last thing a herring ever sees” view of the business end of a spiny dogfish.

The better to eat you with...The image above (from my Master’s degree field work aboard the NOAA R/V Henry B. Bigelow) shows a lot of the tools a dogfish uses to snag, chop up, and swallow its prey.  There are several nifty features of the spiny dogfish jaw that reflect the almost absurdly generalist feeding strategy of this species, and what it doesn’t accomplish through physical adaptations it can make up for through behavior.

In fishes it is typically thought that there are three main forms of feeding: ram feeding (in which prey is run down and swallowed whole), suction (in which prey is inhaled), and biting (in which larger prey is chopped up into tiny bits).  Most fish use a combination of these, and spiny dogfish may make use of all three in the capture and consumption of any given prey item.  Basically, a dogfish is physically capable of taking down and consume any available prey in its path.

First off, the jaw itself has evolved a combination of features found in other, more specialized sharks.  When comparing the skeletal and muscular structure of the jaw to other elasmobranchs, the spiny dogfish almost seems as if it was cobbled together from pieces of other species.  Work by Ramsay (2012) shows that the jaw of the spiny dogfish incorporates structures found in both the whitespotted bamboo shark (a primarily suction-feeding species) and the sandbar shark (which primarily relies on biting prey).  The combination of these structures allows spiny dogfish to open their jaws rapidly enough to generate low pressure within the mouth and suck prey in, while still giving their jaws enough stability to reliably hold onto prey as they bite into it.  In addition, the labial cartilages (I’ll let you go ahead an snicker at the name now), which can be seen on the sides of the dogfish mouth in the above image, snap forward as the dogfish opens its mouth, creating a tubular shape to the open mouth that isn’t seen in purely bite-oriented species (Wilga and Motta 1998).  This allows for a narrower opening to force water through when slurping up prey.  Not to be outdone in the ram-feeding department, in spiny dogfish the upper jaw is attached to the skull in a way that the front portion “hangs” from the skull, attached primarily by the muscle responsible for projecting the jaw forward.  This allows dogfish to project their jaws an impressive 30% of their head length as they bite, which is advantageous when running down elusive prey like schooling fishes and squid (Wilga and Motta 1998).

However, being generalists means that spiny dogfish aren’t necessarily optimized for either suction or biting, and there are some trade-offs.  Balaban (2013) found that the structures relating to gape size in spiny dogfish were smaller than in more ‘bitey” species like sandbar sharks and smooth dogfish, meaning dogfish can’t open their mouths quite wide enough to get their jaws around larger prey.  To compensate for this, the oropharyngeal cavity (basically the “throat” of the dogfish around where the gills are) expands more than in other suction feeding sharks to allow larger prey to be swallowed, but this comes at a cost of increasing the time needed to generate pressure (Wilga et al. 2012). However, Wilga et al. (2012) also noted that unlike other suction-feeding sharks, spiny dogfish attack on the run and usually don’t stop moving during the initial strike, which means ram-feeding behavior may compensate for their suction-feeding shortcomings.

Spiny dogfish incorporate their jack-of-all-trades feeding style by tailoring their attack and prey processing behavior to specific prey items.  As Wilga and Motta (1998) found, ram, suction, and bite-feeding all play a role in the process of a dogfish attack.

The stages of a spiny dogfish attack. From Wilga and Motta (1998).

The above image shows the four main stages of feeding identified by Wilga and Motta (1998).  Column A is the initial prey capture, which is accomplished with a combination of ram and suction feeding.  Column B shows manipulation by biting as the dogfish essentially tries to orient the prey for easy swallowing.  In column C the dogfish has successfully captured and manipulated its prey and is now swallowing it via suction.  Finally column D shows the head shaking behavior typical of many sharks.  This behavior drags the prey back and forth across the shark’s teeth.  Spiny dogfish teeth, by the way, are arranged in interlocking rows that form a uniform cutting surface along the jaws.  This unique arrangement allows dogfish to literally saw apart prey that would otherwise be too large to swallow.  For really large prey (in some cases larger than the shark itself) a dogfish may be capable of latching on through a combination of biting and suction and then can use head-shaking to saw chunks off for easy consumption.  If that isn’t enough crazy feeding mechanics for you, spiny dogfish are also capable of moving the muscles on either side of their jaws independently, meaning they can choose which side of their jaws are biting harder and may actually adjust their bites to maximize the sawing action of their head-shaking behavior (Gerry et al. 2008).

Whenever the feeding behavior of sharks is discussed, sooner or later the issue of bite force comes up.  Pure bite force may be the only aspect of feeding behavior where spiny dogfish fall behind: Huber and Motta (2004) found that spiny dogfish bite with a fairly weak force of 20 Newtons (N).  This is greater than that of a bluehead wrasse (5 N), but below that of a lab rat (50 N), actual dog (550 N), human (680 N), silky shark (889 N), and American alligator (a bone-crushing 13300 N).  This is not, however, and indication that the spiny dogfish is a weak predator.  As Wilga and Motta (1998) showed, biting in the dogfish is more for the purpose of capture and manipulation while the actual cutting is accomplished by way of the head shaking behavior.  This makes the spiny dogfish well-adapted for a generalist fish-eating diet, and is apparently sufficient to allow these small sharks to attack and consume relatively large fish.

So to summarize, the dogfish makes use of all three of the main forms of feeding, and because of this is the ultimate generalist feeder in the shark world.  If they slack off in any department it’s in sheer bite force, but this is mitigated by both sharp teeth and behavior.  From a purely biomechanical perspective alone these are pretty impressive predators, so when handling dogfish you should probably keep your hands clear of the mouth.

This post is an updated version of this other post from way back in the first year of the blog’s existence.  It has been updated with new information and some additional editing has been done because, frankly, I thought it would read better.

References

Balaban J (2013). The morphology and biomechanics of jaw structures in chondrichthyes. Master’s Thesis, University of Rhode Island: Link

Gerry SP, Ramsay JB, Dean MN, & Wilga CD (2008). Evolution of asynchronous motor activity in paired muscles: effects of ecology, morphology, and phylogeny. Integrative and Comarative Biology, 48 (2), 272-282 DOI: 10.1093/icb/icn055

Huber DR, & Motta PJ (2004). Comparative analysis of methods for determining bite force in the spiny dogfish Squalus acanthias. Journal of Experimental Zoology. Part A, Comparative Experimental Biology, 301 (1), 26-37 PMID: 14695686

Ramsay JB (2012). A comparative investigation of cranial morphology, mechanics, and muscle function in suction and bite feeding sharks. PhD Dissertation, University of Rhode Island: 1081470345

Wilga C, & Motta P (1998). Conservation and variation in the feeding mechanism of the spiny dogfish Squalus acanthias. The Journal of Experimental Biology, 201 (Pt 9), 1345-58 PMID: 9547315

Wilga C, Stoehr AA, Duquette DC, & Allen RM (2012). Functional ecology of feeding in elasmobranchs. Environmental Biology of Fishes, 95, 155-167 DOI: 10.1007/s10641-011-9781-7

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