Nurse Shark, Common Nurse Shark.
Head broad. Snout short and bluntly rounded. Mouth small. Nasal flaps form long, thin barbells. First dorsal fin positioned far back on body, the origin slightly anterior of pelvic fin origins. Second dorsal large but smaller than first. Second dorsal fin origin midway between pelvic fin free rear tip and anal fin origin. Lower caudal lobe indistinct. Dorsal coloration tan to dark brown. Neonates and small juveniles (up to 45cm) may have subtle light and dark bands and small scattered dark spots that fade with age. Small juveniles are rarely seen. Ventral surface white, cream, or blotchy.
G. unami differs from G. cirratum (from the Caribbean) by having a shorter distance between the second dorsal fin and the the caudal lobe, and by the position of the first dorsal fin insertion. And in the form and number of keels on the dermal denticles
Maximum length 280cm. Size at birth unknown but probably similar to its Caribbean cousin i.e. around 27-30cm.
A tropical species preferring shallow sandy bays, coral reefs, and sea grass beds. Usually found on or near the bottom. Maximum depth unknown.
Confined to the eastern tropical Pacific Ocean between the Gulf of California and Peru.
The IUCN still lists the Pacific nurse shark as an isolated subpopulation of the Caribbean nurse shark but the two species have recently been split based on significant genetic and morphological differences.
The Atlantic nurse shark is considered NEAR THREATENED throughout much of its western range (except in the Bahamas) so the Pacific nurse shark will likely be given a similar status; or perhaps THREATENED or VULNERABLE based on how infrequently it is sighted.
Citations and References
Rosa, R.S., Castro, A.L.F., Furtado, M., Monzini, J. & Grubbs, R.D. 2006. Ginglymostoma cirratum. The IUCN Red List of Threatened Species 2006: e.T60223A12325895. https://dx.doi.org/10.2305/IUCN.UK.2006.RLTS.T60223A12325895.en. Downloaded on 22 December 2020.
Almost certainly an aplacental viviparous species (without a yolk-sac placenta). Litter size unknown.
Unknown but probably small fishes, crustaceans, and other invertebrates.
No migration data exists for this species. I repeatedly witnessed 5-6 Pacific nurse sharks resting in a shallow channel close to the intertidal line near Playa del Coco in Costa Rica. According to my dive master, the sharks rest in that particular spot during the cooler months, likely because it is warmer than other potential resting areas that sit below a severe thermocline.
Reaction to divers
I found this species to be extremely shy; bolting as soon as they noticed my presence. Likely this was because they were not used to being closely approached. However, I have heard of divers being able to make close approaches towards resting sharks.
Rarely encountered by divers. The Pacific nurse shark can be encountered by divers in Playa Del Coco at a dive site called the Washing Machine. This is a shallow site at Isla Catalina where up to a dozen or more animals congregate in a small hole in the reef. Likely, they remain in this surge swept area because it is noticeably warmer than deeper sites nearby. Unfortunately, the dive site is heavily affected by the omnipresent Pacific swells so it is difficult to reach unless conditions are very good. Even then, visibility can be poor, and the sharks tend to bolt almost immediately.
The nurse shark aggregation at the Washing Machine was only recently discovered so it is unclear if the sharks utilize the site all year long or if they remain there only when the surrounding water is too cold. Six sharks were present when I dove at this site in March 2019.
The Pacific nurse shark can also be encountered during the warmer months at one or two sites around Coiba Island in Panama.