The leopard shark (Triakis semifasciata) is a species of houndshark, in the family Triakidae. It is found along the Pacific coast of North America, from the U.S. state of Oregon to Mazatlán in Mexico. Typically measuring long, this slender-bodied shark is immediately identifiable by the striking pattern of black saddle-like markings and large spots over its back, from which it derives its common name. Large schools of leopard sharks are a common sight in bays and estuaries, swimming over sandy or muddy flats or rock-strewn areas near kelp beds and reefs. They are most common near the coast, in water less than deep.
Active-swimming predators, groups of leopard sharks often follow the tide onto intertidal mudflats to forage for food, mainly clams, spoon worms, crabs, shrimp, bony fish, and fish eggs. Most leopard sharks tend to remain within a particular area rather than undertaking long movements elsewhere, which has led to genetic divergence between populations of sharks living in different regions. This species is aplacental viviparous, meaning that the young hatch inside the uterus and are nourished by yolk. From March to June, the female gives birth to as many as 37 young after a gestation period of 10–12 months. It is relatively slow-growing and takes many years to mature.
Harmless to humans, the leopard shark is caught by commercial and recreational fisheries for food and the aquarium trade. This species is mostly fished in the waters off California where, after a period of population decline in the 1980s, new fishing regulations in the early 1990s reduced harvesting to sustainable levels. The International Union for Conservation of Nature has assessed this species as of Least Concern, while noting that local stocks may easily become overfished because of the shark's slow growth and limited migratory habits.
Taxonomy and phylogeny The first scientific name applied to the leopard shark was Triakis californica, coined by British zoologist John Edward Gray in the 1851 List of the specimens of fish in the collection of the British Museum. However, Gray did not furnish the name with a proper description, rendering it a nomen nudum. In December 1854, American ichthyologist William Orville Ayres gave a lecture describing the species as Mustelus felis, which included the first scientific description of the species. His lecture was reprinted first in The Pacific, a San Francisco newspaper, and then in the journal Proceedings of the California Academy of Natural Sciences. In April 1855, French biologist Charles Frédéric Girard published another description of this species, naming it Triakis semifasciata.
Despite M. felis being the senior synonym, an error in recording the dates of publication resulted in the widespread use of T. semifasciata as the leopard shark's scientific name. As a result of this long-standing error, Triakis semifasciata came to be recognized as the valid name (as a nomen protectum) and Mustelis felis was invalidated (as a nomen oblitum). The specific epithet semifasciata comes from the Latin words ("half") and ("banded"), describing this shark's dorsal pattern of saddle-like markings. In older literature, this species may be referred to as "tiger shark" or "catshark".
The genus Triakis contains two subgenera, Triakis and Cazon. The leopard shark is placed within the subgenus Triakis along with the banded houndshark (T. (Triakis) scyllium). A 2006 phylogenetic analysis by J. Andrés López and colleagues, based on protein-coding genes, revealed that Triakis and Cazon are in fact not closely related, and additionally that the leopard shark may be the most basal member of its family.
Distribution and habitat
The leopard shark occurs in the Northeastern Pacific Ocean, from the temperate continental waters of Coos Bay, Oregon to the tropical waters of Mazatlán, Mexico, including the Gulf of California. It favors muddy or sandy flats within enclosed bays and estuaries, and may also be encountered near kelp beds and rocky reefs, or along the open coast. Numbers have been known to gather near discharges of warm effluent from power plants. Leopard sharks generally swim close to the bottom and are most abundant from the intertidal zone to a depth of , though they may be found as deep as . Many leopard sharks, particularly in the north, leave their coastal habitats in winter and return in early spring. A study in Tomales Bay in northern California determined that they depart when the water temperature drops below ; one tagged shark was found to have swum some south.
While a few leopard sharks have been documented traveling hundreds of kilometers, most individuals tend to remain in a localized area for much of their lives. This low level of dispersal has led to genetic divergence across its range. Seven discrete gene pools have been identified along the Californian coast between Humboldt Bay and San Diego. Of these, the Humboldt Bay subpopulation is perhaps the most isolated, with the sharks there maturing at a larger size and producing fewer offspring than those from other areas. By contrast, the area around Los Angeles represents a genetic transitional zone between subpopulations whose boundaries are more diffuse. Off Baja California, the leopard sharks on the Pacific side are probably distinct from those in the northern Gulf of California. Although there is equivocal evidence for natal philopatry (returning to one's birthplace to breed) in this species, proximity to established breeding grounds likely contributes to the structuring of these different subpopulations.
Description The leopard shark has a moderately stout body, with a short, rounded snout. There are well-developed, triangular flaps of skin in front of the nares. The eyes are large and oval, with a nictitating membrane (a protective third eyelid). The line of the mouth is strongly curved. There are furrows at the corners of the mouth extending onto both jaws, with those on the lower jaw almost long enough to meet at the midline. The tooth rows number 41–55 in the upper jaw and 34–45 in the lower jaw; each tooth has a slightly oblique, smooth-edged cusp in the center and 1–2 small cusplets on either side. These teeth are arranged into a flat, "pavement"-like surface with overlapping ridges.Delius, B. Biological Profiles: Leopard Shark . Florida Museum of Natural History Ichthyology Department. Retrieved on August 9, 2009.
The large first dorsal fin is positioned about halfway between the pectoral and pelvic fins; the second is almost as large as the first and much larger than the anal fin. The pectoral fins are wide and triangular. The lower lobe of the caudal fin is well-developed in adults but less than half the length of the upper lobe, which has a strong ventral notch near the tip. The coloration is unique, consisting of prominent black "saddles" and large black spots running along the back, on a silvery to bronzy gray background. Adult sharks often have more spots and saddles with lighter centers compared to juveniles. The underside is whitish and plain. The average length of a leopard shark is . Rarely males may grow to and females , and there is a record of an exceptional female that measured long. The heaviest known leopard shark weighed .
Biology and ecology
An active species that swims with a strong undulating motion, the leopard shark is commonly spotted cruising in or just beyond the surf zone. It is more active at night than during the day, and sometimes lies still on the bottom. In Tomales Bay (among other similar regions), the leopard shark follows the tide onto mudflats to forage for food, retreating just fast enough to prevent being stranded or trapped as the water recedes. At Santa Catalina Island, resident sharks spend the day together in the shallows and at night, they disperse into deeper water, up to away.Manley, J.F. (1995). Diel movement patterns of leopard sharks, Triakis semifasciata, at Santa Catalina Island, California. MS Thesis. California State University, Long Beach, CA.
From birth, leopard sharks form large schools, generally segregated by age and sex; these groups may intermingle with grey or brown smoothhounds (Mustelus californicus and M. henlei) and spiny dogfish (Squalus acanthias). These schools are apparently nomadic, often appearing suddenly in an area for a few hours before vanishing just as quickly. In captivity, larger sharks have been observed establishing their dominance over smaller individuals via light nips to the pectoral fins. During summer days, large aggregations of mature females assemble in shallow bays and estuaries, dispersing at night. As these females follow the warmest patches of water (allowing them to raise their internal body temperatures by up to , they are speculated to be taking advantage of the heat to speed their own growth and that of their gestating young. The membership of these aggregations is inconstant, with individual females periodically switching between different sites scattered over several kilometers.
Compared to the related grey and brown smoothhounds that share its range, the leopard shark has smaller and more numerous red blood cells, allowing it to process oxygen more efficiently. This may be an adaptation for foraging in deoxygenated estuarine environments.Martin, R.A. Estuaries: Leopard Shark. ReefQuest Centre for Shark Research. Retrieved on August 9, 2009. Their eyes contain very few cone cells, likely due to the murky water they inhabit. Small leopard sharks fall prey to larger sharks such as the great white shark (Carcharodon carcharias) and the broadnose sevengill shark (Notorynchus cepedianus). On one occasion, a sevengill shark was seen ambushing a leopard shark on a tidal mudflat in Humboldt Bay, striking with such momentum that the larger predator momentarily beached itself.Martin, R.A. Sandy Plains: Broadnose Sevengill Shark . ReefQuest Centre for Shark Research. Retrieved on August 9, 2009. Known parasites of this species include the tapeworms Phyllobothrium riseri, Lacistorhynchus dollfusi and Paraorygmatobothrium barber, as well as the copepods Echthrogaleus eoleoptratus and Achtheinus oblongatus.
Feeding
The diet of the leopard shark consists of small benthic and littoral animals, most significantly crabs (Cancridae, Grapsidae, and Hippoidea), shrimp, bony fish (including anchovies, herring, topsmelt, croakers, surfperch, gobies, rockfish, sculpins, flatfish, and midshipmen), fish eggs, clams, and the echiurid fat innkeeper worm (Urechis caupo). This opportunistic hunter has also been known to eat ghost shrimp, polychaete worms, and the young of smoothhounds, shovelnose guitarfish (Rhinobatos productus), and bat rays (Myliobatis californicus). Eelgrass (Zostera) and algae may be swallowed incidentally.