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Author: (Gill, 1863)

Field Marks:
An unspotted, bronzy slender Mustelus with strongly cuspidate teeth, mostly tricuspidate denticles, long caudal peduncle, and broadly frayed posterior dorsal fin margins.

Diagnostic Features:
Body fairly slender. Head short, prepectoral length 19 to 22% of total length; snout moderately long and bluntly to sharply angular in lateral view, preoral snout 6.4 to 8% of total length, preorbital snout 6.6 to 8.4% of total length; internarial space broad, 2.6 to 3.4% of total length; eyes large, eye length 1.9 to 2.9 times in preorbital snout and 2.6 to 3.7% of total length; interorbital space fairly narrow, 3.9 to 5.3% of total length; mouth moderately long, subequal to eye length and 2.7 to 3.8% of total length; upper labial furrows longer than lowers, upper furrows 1.6 to 2.4% of total length; teeth cuspidate and asymmetric, with a rather strong primary cusp and usually cusplets present at all ages; buccopharyngeal denticles confined to anterior third of palate and surface of tongue. Interdorsal space 19 to 24% of total length; trailing edges of dorsal fins, and occasionally anal and caudal fins, naked with a broad, conspicuous dark band of bare ceratotrichia; first dorsal broadly triangular, with posteroventrally sloping posterior margin, its midbase closer to pelvic bases than to pectorals; pectoral fins moderately large, length of anterior margins 13 to 16% of total length, width of posterior margin 8.1 to 13% of total length; pelvic fins moderate-sized, anterior margin length 6.2 to 7.9% of total length; anal height 2.7 to 3.8% of total length; anal-caudal space greater than second dorsal height and 6.9 to 8.6% of total length; ventral caudal lobe more or less falcate in adults. Crowns of lateral trunk denticles more or less tricuspidate, with longitudinal ridges extending their entire length. Skeleton not hypercalcified in adults; palatoquadrates not subdivided; monospondylous precaudal centra 39 to 45, diplospondylous precaudal centra 54 to 65, precaudal centra 97 to 106. Colour usually iridescent bronzy-brown above, occasionally greyish, white below; no white spots or dark spots or dark bars. Development viviparous. Size moderate, adults 52 to 90 cm.

Geographical Distribution:
Eastern Pacific: Northern California to Gulf of California; Ecuador and Peru.

Habitat and Biology:
An abundant inshore to offshore, cold-temperate to warm-temperate or subtropical bottom-dwelling shark of the eastern Pacific continental shelves, found from the intertidal region to at least 200 m depth, and very common in enclosed, shallow, muddy bays. Of the three smooth-hounds found in Californian waters this is the most cold-tolerant, being apparently resident in cold-temperate northern California. In contrast, in normal years without an 'el Nino' or warm-water influx, M. californicus is a regular summer visitor to north-central California (Elkhorn Slough) and resident in warmtemperate southern California, although it was originally described from San Francisco Bay where it is normally absent (unlike M. henlei); while the more tropical M. lunulatus may not occur in Californian waters except in thesummers of warm-water years. However, all three species were collected by the writer in the Gulf of Carlifornia during the summer at Guaymas. Sonora. Mexico. In San Francisco Bay thisis apparently the most abundant local shark. One tagged individual migrated about 160 km in 3 months. The nature of the local movements of this shark is uncertain, but the writer suspects that in San Francisco Bay during wintertime, it may move out of the bay mouth into the ocean from its usual spring to autumn haunts in the shallows as the salinity drops with rainfall and increased influx of fresh water from the Sacramento-San Joaquin River system.

The microdistribution and population structure of this species may be very localized and spotty: centres of abundance in north-central California are apparently enclosed bays such as Humboldt, Tomales and San Francisco Bays, while it is uncommon in Monterey Bay and Elkhorn Slough.

Healthy individuals kept in captivity in a large circular tank at the US National Marine Fisheries Service Tiburon Laboratory proved to be aetive bottom-dwelling swimmers, resting on the substrate but often actively patrolling with their undersurfaces only a few millimetres above the substrate, although sometimes swimming in midwater or at the surface. The broad pectoral and pelvic fins and relatively flat undersurface of this and other smooth-hounds may allow them to ride on a ground effect close to the substrate, making for more efficient quartering of the bottom in search of prey. Brown smooth-hounds were found to be amazingly agile as they swam close to the bottom; they were observed to swim with considerable speed straight up to the vertical wall of the tank, but instead of hitting, they made high-speed 90 degree transitions and continued to swim vertically upward with their undersides close to the tank wall, and would often turn and swim horizontally with their body axis rotated 90 degrees and their ventral surfaces still riding just off the tank wall. This species is readily kept in large aquaria if not badly traumatized during capture.

Viviparous with a yolk-sac placenta; 3 to 5 young per litter.

Eats crabs and shrimp, ghost shrimps, manis shrimp, isopods, squid, polychaete worms, tunicates, and small bony fishes, including anchovies, surf perch, gobies and flatfish, as well as topsmelt eggs. Crustaceans, especially crabs, shrimp and isopods are the most important prey of these sharks, followed by polychaete worms and fish. Small grapsid crabs are more frequent prey items to smaller sharks below 60 cm while cancrid crabs, ghost shrimp, fish and squid are more readily taken by larger sharks 80 cm and above. The presence of shore (grapsid) crabs (Hemigrapsus oregonensis) in the diet suggests that this shark readily feeds in the intertidal, and is agile enough to readily capture these small, active crabs. When attacking large cancrid crabs in captivity the brown smooth-hound was observed to rush in and grab the defending crab by a cheliped, shaking it vigorously and usually causing its appendage to autotomize.

This shark is broadly sympatric in its northern range with another houndshark, the leopard shark, Triakis semifasciata, which occurs in similar microhabitats. The leopard shark differs noticeably in its diet, eating possibly less shrimp, more and larger fish, more fish eggs and considerable numbers of innkeeper worms (Urechis) and clam siphons.

Size:
Maximum about 95 cm, males maturing between 52 and 66 cm and reaching over 67 cm, females maturing between 51 and 63 cm and reaching over 89 cm; size at birth between 19 and 21 cm.

Interest to Fisheries:
An abundant inshore shark, fished commercially in the Gulf of California and utilized fresh or fresh-frozen for human consumption; commonly taken by sports anglers in California.

Remarks:
For details of the taxonomic history of the brown smooth-hound see Compagno (1970, 1979) and Heemstra (1975), who also recorded it for the first time from the Southern Hemisphere (Ecuador and Peru). Heemstra also noted that Bigelow and Schroeder (1948) described a specimen in the Harvard collection as M. schmitti, labelled as coming from Rio Grande do Sul, Brazil, which is actually M. henlei. With no other evidence on the occurrence of henlei in the Atlantic, Heemstra was inclined to regard the record as questionable (and possibly a result of mislabelling), which the writer concurs on.

Type material:
Holotype: U.S. National Museum of Natural History, USNM 4487, 230 mm. Type Locality: California.

Brown smooth-hound (Mustelus henlei)