Biology of the Bigeye Ragged-Tooth

Shark (Odontaspis noronhai)

 

Field Marks Second dorsal and anal fins smaller than first dorsal fin; first dorsal fin origin closer to pectoral fin bases than pelvic bases; snout bulbously conical, relatively long (greater than mouth width); teeth slender and fang-like, with a lateral cusplet on each side of main blade; 2 large anterior teeth on each side of upper jaw, separated from lateral teeth by a small intermediate tooth; eyes relatively large; Color uniform dark chocolate brown, without spots; all fins ragged with dark margin on posterior edge; lining of upper jaw and palate dark.
Size Size of pups at birth unknown; most specimens about 11 ft (3.3 m) long; maximum length about 12 ft (3.6 m).
Range Known from only a few specimens, most from southern Brazil (= type locality); Indian Ocean record based on a set of jaws possibly collected at Seychelles.
Habitat Poorly known deep-water shark inhabiting continental and insular slopes; most records are coastal, but apparently also oceanic (based on a capture off Hawaii); known depth range 200-3 280 ft (60-1,000 m). Capture of several large specimens during spring from the same area of southern Brazil may indicate a local seasonal migration.
Feeding Virtually no data; prey apparently includes deep-water teleosts and squid (based on otoliths and a cephalopod beak in stomach contents of a specimen from the northern Gulf of Mexico).
Reproduction Almost certainly ovoviviparous; embryonic nutrition probably features oophagy and possibly embryophagy. No data on number of pups (2 - 4?), pupping season, or nursery grounds.
Age & Growth Males mature at a length of about 10.7-11.2 ft (3.3-3.4 m), females at about 11 ft (3.3 m); no data on age at maturity or longevity for either sex.
Danger to Humans Potentially dangerous; known to thrash and snap vigorously when caught.
Utilization Presumably taken occasionally by deepwater fisheries (holotype was taken on vertical longline set for Black Scabbardfish [Aphanopus carbo] at depth of 2,625-3,280 ft (800-1 000 m) off southern Brazil).
Remarks Due to rarity and poorly-known life history, declared a protected species in the western North Atlantic by US National Marine Fisheries Service (NMFS 50 CFR, Pt. 678), 

 

Known from only 15 or so specimens, the Bigeye Ragged-Tooth Shark is perhaps the most poorly understood living lamnoid. It is a veritable 'poster child' for the logistical difficulties that frustrate unraveling the mysteries of how a given shark goes about its day-to-day life. Piecing together the life history and basic biology of the Bigeye Ragged-Tooth is thus a challenge in inductive* logic worthy of Sherlock Holmes.

For quite a while, we were not even sure whether the Bigeye Ragged-Tooth Shark was a valid species. It was originally described in 1955 by ichthyologist G.A. Maul based on a 67-inch (170-cm) female specimen taken off Madeira on a vertical longline set by fishermen for Black Scabbardfish (Aphanopus carbo). Partly because specimens were so few and far between, as recently as 1981 shark systematist Leonard J.V. Compagno had speculated that the Bigeye Ragged-Tooth may simply be a morphological extreme of the (slightly) better-known Bumpytail Ragged-Tooth Shark (Odontaspis ferox). But by the mid 1980's it was clear to Compagno and others that noronhai was indeed a valid species, readily separable from ferox. Over a ten-year period, molecular systematists Gavin Naylor and his co-workers were able to obtain genetic material from all but one species of lamnoid, the Bigeye Ragged-Tooth. As a result, they could not evaluate the phylogenetic position of this species among the others in its order using mtDNA. In order to deduce the position of the Bigeye Ragged-Tooth for the above cladogram, I relied heavily on the work of Compagno, who bases his hypotheses about how various elasmobranchs are interrelated on strictly morphological characters.

As its name suggests, the Bigeye Ragged-Tooth has relatively large eyes. In sharks, large eyes are usually correlated with a deep-sea habitat and sometimes with nocturnal habits. The holotype (first specimen) was taken at a depth of between 2,625 and 3,280 feet (800 and 1,000 metres). This is certainly respectably deep, although specimens have been caught off Brazil at depths ranging from 400 feet (120 metres) to as shallow as 200 feet (60 metres), which is hardly 'deep-sea' by oceanic standards. Unfortunately, with so few specimens to go on, we cannot determine whether the Bigeye Ragged-Tooth is primarily an inhabitant of the mesopelagic zone (dimly-lit twilight layer of the ocean, occurring over an average depth range of 330-3 300 feet or 100-1 000 metres), the epipelagic zone (well-lit upper layer of the ocean, generally occurring from the surface down to a depth of about 330 feet or 100 metres), or whether it is a vertical migrator (spending the day in the mesopelagic zone, moving upward into the epipelagic by night, and returning to the mesopelagic before dawn). If only we could see through those large eyes, we could determine where the Bigeye Ragged-Tooth lives.

The horizontal range of the Bigeye Ragged-Tooth Shark is not much better known than is its vertical distribution. Only one additional specimen has been collected from off Madeira, a specimen even smaller than the holotype. Most Bigeye Ragged-Tooth records to date (10 individuals, or 67% of known specimens) have come from southern Brazil. There is a frustratingly vague record, based on a set of jaws with dentition nearly identical to the holotype, collected either from the Indian Ocean (possibly from the Seychelles) or perhaps the South China Sea. Several Odontaspis teeth have been collected from bottom deposits in the central North Pacific, but they are not clearly identifiable as being from O. noronhai. The only definite record of the Bigeye Ragged-Tooth occurring outside the Atlantic is based on a 7-foot (2.2-metre) male captured on tuna longline gear in July 1985, set at a depth of about 1,480 feet (450 metres) some 340 miles (550 kilometres) southwest of the Big Island of Hawaii. So, the Bigeye Ragged-Tooth's known range includes the eastern North Atlantic, western South Atlantic, the central Pacific, and possibly the Indian Ocean or the western Pacific.

The first (and thus far only) record of a Bigeye Ragged-Tooth Shark from the western North Atlantic is based on 10.5-foot (3.2-metre) female caught on a commercial Swordfish (Xiphias gladius) longline some 70 miles (113 kilometres) east of Port Isabel, Texas. (Port Isabel, coincidentally, is also from whence came the infamous Whale Shark [Rhincodon typus] eggcase that misled shark biologists into thinking this species was oviparous [egg-laying].) This specimen was identified by fisheries biologist Steven Branstetter and elasmobranch systematist John McEachran as O. noronhai based on its tooth formula and the posterior position of its pelvic fins. But making the identification was far from straight-forward: perplexingly, it differed from the holotype in several aspects.

The Port Isabel specimen lacks upper symphyseal teeth (small teeth at the center of the jaw), has the second dorsal fin originating over the posterior third of the pelvic fin base, and the anal fin originating under the mid-point of the free rear tip of the second dorsal fin. In the holotype, however, upper symphyseals are present, the second dorsal origin is over the middle of the pelvic fin base, and the anal fin is posterior to the free rear tip of the second dorsal. Such subtle differences in proportional measurements are often useful in determining a species' identity. In doing so, the holotype serves as the ultimate standard for comparison. But in this case, the holotype is much smaller than the Port Isabel specimen (only about half as long) and had been stuffed.

In many sharks (as in many other creatures) bodily proportions and other features can change -sometimes dramatically with growth, a phenomenon known as allometry. Stuffing a specimen often results in its distortion (not only due to the process itself, but also over time as moisture content fluctuates, causing it to stretch, shrink, or warp to varying degrees), and thus illustrations and measurements based upon it may be somewhat misleading. Therefore, the proportional differences noted between the Port Isabel specimen and the holotype may have been due to a combination of allometry and a warping due to the latter's having been stuffed. The absence of upper symphyseal teeth was somewhat harder for Branstetter and McEachran to explain. Lack of symphyseals had been noted in the larger but not the smaller of the south Brazilian specimens reported in the scientific literature. Branstetter and McEachran thus tentatively concluded that loss of symphyseal teeth may be a feature of allometry in the Bigeye Ragged-Tooth Shark. But with so few specimens representing 'snapshots' of a lifetime spectrum of growth, differentiation, and changes in proportion, it is impossible to be certain.

The known vertical distribution of the Bigeye Ragged-Tooth is extensive, ranging from 200-400 feet (60-120 metres) off Brazil to 2,600-3,300 feet (800-1,000 metres) off Madeira. Atlantic captures of the Bigeye Ragged-Tooth suggest that the species is primarily a coastal inhabitant, but the only known Pacific specimen (from off Hawaii) was caught over deep oceanic waters far from shore. Of the 6,000 or so sharks caught during the extensive longline fishing program conducted throughout the central Pacific during 1952 through 1955, not one Bigeye Ragged-Tooth was reported. Fishing depths surveyed during that program ranged from the surface to about 500 feet (150 metres), suggesting that this species typically inhabits deeper waters in the central Pacific.

When it comes to daily or seasonal movement patterns in the Bigeye Ragged-Tooth Shark, we are almost totally in the dark. Capture of large specimens, all from the same area of southern Brazil and only during the spring season, led ichthyologist Victor Sadowsky and his co-workers to suggest that this pattern may reflect some manner of migrational movement, though they could not suggest in what direction or for what purpose. If the sharks themselves know, so far, they ain't telling.

All known specimens of Bigeye Ragged-Tooth Shark were collected by commercial fishing gear. This fact underscores the important contribution that fishermen can make in our knowledge of new and rare shark species. Fortunately for shark scientists, many commercial fishermen are fine and observant marine naturalists, possess an intimate knowledge of the seas where they fish, a keen eye for subtle differences in their catches, and a willingness to bring unusual specimens back to port and report them. Without the knowledge and good-natured curiosity of commercial fishermen, we would certainly know a lot less about the Bigeye Ragged-Tooth than we do.


* Induction and deduction are basically opposite strategies of logical inference. For all his pontification about the virtues of deduction (reasoning from accepted general principles to a specific conclusion), Sir Arthur Conan-Doyle's fictional supersleuth generally relied on induction (reasoning from specific facts or observations to a purported general principle). Scientists generally prefer deduction over induction, but one has to work with what data one has. [Return to text]

 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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