Pacific Nurse Shark, Ginglymostoma unami
Pacific Nurse Shark, Ginglymostoma unami. Fish caught from coastal waters off Cabo San Lucas, Baja California Sur, August 2020. Length: 1.6 m (5 feet 3 inches). Catch, photograph and identification courtesy of Eli (obsessiveangling.wordpress.com).
Pacific Nurse Shark, Ginglymostoma unami. Underwater photograph taken in Zihuantanejo Bay, Guerrero, January 2020. Photograph and identification courtesy of Maude Jette, Dive Zihuantanejo, www.Divezihuatanejo.com.
The Pacific Nurse Shark, Ginglymostoma unami, is a member of the Ginglymostomatidae or Nurse Shark Family, and known in Mexico as tiburón gato. The origin of the common name Nurse Shark is unclear. Globally, there are two species in the genus Ginglymostoma, one from the Atlantic and one from the Pacific Ocean. The Pacific Nurse Shark has historically considered to be a Nurse Shark, Ginglymostoma cirratum, and in 2015 the Pacific population was separated into its own species. They are very closely related to the Rhincodontidae Family of Whale Sharks.
The Pacific Nurse Shark is a large shark with an elongated, slender body. The adults are a uniformly colored brown or grayish brown body that lightens ventrally. The juveniles are covered with small black spots with lighter pigmentation surrounding each spot. They become lighter in color in the dark. Their head is bulbous with very small eyes, minute spiracles, and a sub-terminal mouth that is found well in front of the eyes near the tip of the snout and has long nasal barbels on either side. Their teeth do not overlap and have the ability to replace lost teeth quickly. Their anal and spineless dorsal fins are broadly rounded and located close to the caudal fin, the caudal fin is more than 25% of the total length and is narrow with a large upper lobe and no distinct lower lobe. The first dorsal fin originates well behind the pectoral fins and over the pelvic fins. Their skin is smooth to the touch.
The Pacific Nurse Shark is a slow-moving bottom dwelling solitary shark that is found in warm shallow water around rocky reefs from the intertidal zone to depths up to 55 m (180 feet). They reach a maximum of 2.1 m (6 feet 10 inches) in length. They spend daylight hours resting on sandy bottoms or in shallow water cave and rock crevices and can be found in groups of up to 40-individuals, sometime piled upon on one another. They emerge at night for feeding but return to the same shelter at night. They prey predominately on small fish that are resting at night via a sucking action; they also prey upon crabs, octopus, sea urchins and spiny lobsters. Juvenile Nurse Sharks are preyed upon by Bull Sharks, Hammerhead Sharks, Lemon Sharks and Tiger Sharks. Reproduction in Nurse Sharks is ovoviviparous with a large number of males will mate with a single female. They have a high level of reproduction site fidelity. Embryonic development occurs in an egg case within the mother’s ovary without nourishment from the mother. Their reproduction cycles are biennial with 6-month gestation cycles. They hatch with the body of the mother with litter sizes ranging from 20 to 30 miniature pups that are 27 cm (10.6 inches) to 30 cm (11.8 inches) in length. The juveniles are found around shallow coral reefs, grass flats, and mangrove islands and as they mature they move to deeper reefs and rocky areas. The Pacific Nurse Shark is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, growth, longevity, movement patterns, diet, habitat use, and reproduction.
The Pacific Nurse Shark is a resident of all Mexican waters of the Pacific Ocean with the exception that they are absent from along the West Coast of the Baja.
The Pacific Nurse Shark is the only large brown shark found in the Eastern Pacific with barbels. They are very similar to the Atlantic Nurse Shark, Ginglymostoma cirratum (different first dorsal fin location versus pelvic fins).
From a conservation perspective the Pacific Nurse Shark has not been formally evaluated with limited information about their population levels. They are considered to be common rare with a widely distributed population noting that they are currently being strongly negatively impacted by human development and significant declines have occurred in the Eastern Pacific Ocean. In some area of Central America they are heavily fished to levels of extinction. They are considered to be an important species for shark research being robust and able to tolerate capture, handling, and tagging. Due to their docile nature they have become popular with the ecotourism trade in Costa Rica and Panama. They are considered to be non-aggressive and harmless to humans and try to avoid interactions by fleeing but will attack if provoked and can incur severe damages. These attacks have become more frequent that corresponds to the increase of feeding by the ecotourism trade. They are huge in size, have strong jaws filled with thousands of tiny serrated teeth and can be visually very intimidating to divers.