Sandtiger — the Current State of
In late June, there were a series of list server postings regarding the Carcharias taurus. The specific topic of the discussion bounced around, touching upon aggression, feeding habits and regional dimorphism of the teeth. I eventually joined the conversation with an overview of the research and current state of knowledge of the Sandtiger.
One of the more common large, shallow-water sharks — and one of the few that adapt fairly readily to captivity — the Sandtiger (Carcharias taurus) has received scant attention from the scientific community and thus its basic life history, ecology, and behavior are relatively poorly known. For example, although the Sandtiger was originally described in early in this century (in 1910, by the great Turkish naturalist Constantine Samuel Rafinesque, based on a specimen caught off the coast of Sicily), the first really good study on age and growth in this species — tackling such fundamental life history parameters as age at maturity and breeding frequency — was published only a few years ago (Branstetter and Musick 1994).
But the litany of our scientific ignorance about this common shark does not stop there. Although the Sandtiger's embryophagous habits were known to science since 1948 (Springer), the he first comprehensive study of embryonic development and intrauterine nourishment in this species was not published until 1983 (Gilmore, Dodrill, and Linley). Despite relatively clear seasonal patterns of Sandtiger distribution being known to recreational anglers for nearly a hundred years, migration of this species has received little scientific study (an exception is off the eastern coast of southern Africa — see Bass, D'Aubrey, and Kistnassamy 1975 and Bass 1978). And although Sandtigers have been maintained in captivity since at least the 1930's and for periods of 16.6 years or more (Clark 1963; Govender, Kistnassamy, and van der Elst 1991), the first good studies on Sandtiger feeding habits and growth rates (Schmidt, Murru, and McDonald 1990; Govender, Kistnassamy, and van der Elst 1991) and tooth replacement rates (Overstrom 1991) in captivity did not appear in the literature until this decade. When it comes to the population biology and behavior of the Sandtiger, we are almost totally in the dark.
Feeding habits of large sharks are most readily and accurately studied in captivity, revealing patterns of food intake which may not correspond well to those occurring in the wild. A recent paper by Schmidt et al. (1990) suggests that, at Sea World of Florida (SWF), captive subadult Sandtigers (160 to 174 cm fork length [FL]) held at a relatively constant temperature of about 24 °C consume roughly 2% of their body weight in food per week (BW/wk) — fluctuating between about 2.1% BW/wk from June to November and 1.5% BW/wk from December to May — and grow at a rate of about 0.6 to 2.9 cm per month (or 7 to 35 cm per year). Two large (220 - 228 cm FL) female Sandtigers did not increase in length during the month-long trial. In March 1981, a 91-cm FL female Sandtiger pup was born at SWF which lived for 81 months and grew to total length (TL) of 210 cm, which suggests an average life-long growth rate of about 1.4 cm/month (or 17 cm/year). All 13 Sandtigers in this study gained weight during the first 5 month interval of the study (mean weight increase was 11±7.7 kg), however during the second interval, 10 sharks (77%) actually lost weight (average weight loss was 4±7.5 kg, ranging from 0.6 to 16.8 kg). Unfortunately, since the study was not maintained for more than one year, it cannot be determined whether weight loss in these captive Sandtigers reflects an endogenous seasonal pattern or an expression of some kind of captive stress (feeding inhibition due to increased size relative to the tank, disease, or changes in social dynamics among sharks cohabiting the tank, or some other factor[s]).
Recent work (Govender et al. 1991) on growth of captive Sandtigers at Seaworld, Durban, South Africa, permits comparison with wild Sandtigers from both the southwestern Indian Ocean and the western North Atlantic. Eight Sandtigers (known in South Africa as 'ragged tooth sharks' or 'raggies'), ranging from 95 to 248.4 cm TL and 0 to 16.6 years of age, were studied to investigate age and growth of this species in captivity. Age and length data for these captive sharks was entered into standard theoretical growth curves (general and special Von Bertalanffy), and various life history parameters estimated therefrom. The results predict a theoretical maximum length (Lmax) of about 250 cm TL, a growth coefficient (K) of 0.233, and a gestation period (t0) of about 2.24 years. Schwartz (1983) estimated the age of a 216.1 cm FL male Sandtiger from North Carolina to be about 8 years, based on the number and diameter relative to FL of growth rings on the vertebral centra. From a Von Beralanffy regression, derived from 63 data points (provided by Geremy Cliff of the Natal Sharks Board) from Sandtiger specimens ranging from 180 to 280 cm TL, a FL of 216.1 cm corresponds to a TL of about 248 cm. If ages of 8 and 9 years are substituted into the special Von Bertalanffy equation, estimated TL's of 227 to 232 cm result. Branstetter and Musick (1994) estimate Von Beralanffy parameters for northwestern Atlantic Sandtigers as follows: Lmax = 303 cm TL, K = 0.18, and t0 = 2.09 years.
Since, according to Gilmore et al. (1983), the gestation period of Sandtigers varies between about 9 and 12 months (less than half the t0 predicted from the Von Bertalanffy growth curves), it seems likely that embryonic growth in this species is considerably faster than post partum (after birth) — a conclusion which is unsurprising and fully supported by Gilmore et al.'s (1983) study. Further, since the TL between the ages of 8 and 9 years predicted from the Von Bertalanffy curve for wild Sandtigers from both southern Africa and eastern North America is greater than that observed in captive members of the same species, is would seem that captive Sandtigers grow more slowly than their wild conspecifics. It is not possible at this time to determine whether this apparent disparity is due to captive stress, some other factor(s), or — given the paucity of comparative data — represents a 'real' disparity at all.
Regional reports on the diet of wild Sandtigers are scattered and largely anecdotal, suggesting that this species preys on whatever bony and elasmobranch fishes are locally abundant (as well as the occasional crustacean or cephalopod). Hard numbers on the relative importance of prey types, feeding strategies, the amount of food consumed per year as a function of body weight, feeding periodicity, seasonal changes in diet and food consumption, and correlation of feeding habits to water temperature in wild Sandtigers are almost totally lacking.
We know from direct (Gilmore, pers comm.) and indirect evidence (hydroids actually grow on the teeth, suggesting prolonged lack of use) that pregnant Sandtigers cease to feed. Sandtigers from southeastern Australia — where they are known as 'Grey Nurse Sharks' — have noticeably stouter teeth than the (supposedly) same species from the western North Atlantic (pers. obs.), and there is a surprising degree of variation in the dentition of Sandtigers from the East China Sea (Taniuchi 1970), suggesting the possibility of dental adaptation to regionally different prey types or feeding strategies. There are numerous anecdotal reports of Sandtigers practicing co-operative feeding on schooling fishes, although — as much as I would like to believe it — I have expressed doubts whether two or more sharks that happen to be feeding on the same school are necessarily co-operating (schooling fishes seem to draw together instinctively in the presence of a predator; such a school would be further concentrated if two or more predators are independently stalking/feeding upon it at the same time without any need for co-operation, co-ordination, or communication on the part of the predators). We also do not know to what extent Sandtigers (or other sharks, for that matter) are inhibited from feeding by the proximity of human observers.
On the matter of aggressiveness toward humans, the Sandtiger is largely a victim of its own appearance. With its long, pointed snout, cold, staring eyes, and a mouthful of some of the wickedest-looking teeth in sharkdom, the Sandtiger looks downright ferocious. Because the Sandtiger looks so dangerous, people were (and are) more than willing to believe it so. In decades past, the Sandtiger has often been blamed — despite little or no supporting evidence — for numerous attacks on bathers, swimmers, and divers. But, in my experience — and that of many of my diving friends and colleagues — Sandtigers are generally placid and unaggressive toward divers. There are two situations which are possible exceptions to this: 1) individual Sandtigers may be stimulated to bite a person in the water in the presence of speared or hooked and struggling fish, and 2) male Sandtigers are reported to be more aggressive during their mating season. Based on my experience, Sandtigers sometimes seem to regard divers as a potential threat, displaying their uneasiness via repeated exaggerated yawn-like jaw gapes and sustained pectoral fin depression. If a diver does not heed these warning signs and increase distance from a displaying Sandtiger, he or she may very well be bitten — not out of hunger or aggression but in self-defense. (In captivity, this species has bitten other moderately large sharks, but these appear inhibited in nature and are probably delivered out of defensive threat rather than attempts to dominate or predate upon tank-mates.)
Human language has also played a role in fostering a nasty reputation for this shark. The name 'sand-tiger' has become commonplace largely due to the promotional efforts of North American public aquaria, which uniformly prefer this dangerous-sounding moniker to the comparatively bland name, 'sand shark'. Divers and sport anglers — even those who knew the truth about these ferocious-looking but mild-mannered sharks — have also done their share of damage, as those who could not resist the temptation to appear brave and heroic to landlubbers have extolled their ferocity and killed them in unprecedented numbers with spearguns and powerheads. This barbaric practice was largely responsible for sweeping the Grey Nurse from many parts of its range in southeastern Australia. The media is also somewhat to blame, especially in Australia — as Grey Nurse looks far more impressive in print than simply 'shark'. As a result of all this intentional and unwitting distortion, the popular mythos about Sandtigers is far better known than are the relatively few scientific facts.
In any case, I hope the above synopsis of Sandtiger behavior, feeding biology, and life history is of interest.
— R. Aidan Martin