15. May, 2016

Nurse sharks: ‘lazy’ but efficient survivors

The nurse shark (Ginglymostoma cirratum) is not the most popular shark on the list of underwater photographers. They simply  miss the elegance of the requiem sharks, the Carcharhinus  and the hammerheads, the Spyrna. The shark has two small beady eyes, a broad snout and a small mouth, bracketed by two sensory barbels and deep oronasal grooves connecting the mouth and the nasal organs. Unlike most sharks its mouth is located in front of the eyes. During daytime nurse sharks often congegrate  on  shallow sandy floors at  submerged ledges or in crevices of the reef.  They have  a brownish spongy body with large rounded pectoral fins that they use to rest on the sandy sea floor, a long tail and two backward dorsal fins of almost the same size. Nurse sharks are ovoviparous, meaning that their eggs hatch in the mother until she is ready to give birth to a large  (20-30) number or pups. The size of the pups is around 30 cm. 

Nurse shark seem a bit dull and sluggish, a reason why they are sometimes considered as the "couch potato" of the sharks. The ones we met at Bimini were often grouping together on the sand, waiting for a moment to snatch  a small piece of fish from the baiting box (see insert). Sometimes pushed away with a pole to allow the  hammerheads a free access. Nevertheless, the shark is an efficient hunter, able to crush shells  of mollusks with its small  razor sharp teeth. With its small mouth  its capable to suck in its prey with a short, violent influx of water.  A nurse shark is seldom seen chasing its prey, probably because it takes advantage of dormant fish during the hunts at night, which would otherwise be too fast for the shark to catch. Similar to the white tip reef shark it is able to respire while stationary by pumping water through the mouth and gills.  Taken together these facts suggest that nurse sharks spend relative little energy.  Its metabolic rate, even while swimming is estimated to be only 18 percent of a similar measurement in the high-performance mako shark*

So, the  general  impression is that  nurse  sharks are well adapted to their environment and successful survivors due to their low  energy expenditure, efficient hunting strategy and large number of pups.  Which probably also  explains why they are still one of the most prevalent sharks in tropical and sub-tropical waters.