The time spent in Ambergris Caye was truly incredible. For me, it was not just because of all the beautiful coral or colorful and intriguing fish, but the intensity and gentleness of the nurse shark. The nurse shark is a moderately sized shark, which makes it hard to believe that it uses suction feeding, rather than other feeding tactics typically thought of when the “shark” is in conversation. Their power led me to wonder how such a large animal could feed in this particular way, when their relatives fed so differently. From my curiosity, I hypothesized that nurse sharks evolved to be bottom feeders due to competition and therefore, their feeding mechanism evolved as well.
The anatomy of the shark is to be contributed to cause this suction feeding. Nurse sharks have a downward facing mouth that is positioned slightly lower on the face when compared to several other sharks. This allows them to be better equipped to feed along the ocean floor as bottom dwellers. The teeth of these sharks are broader and serrated, which especially allows them to tear, crush, and hold on to their prey, unlike great whites, who simply rip large chunks of flesh from the animal with incredibly large and sharp teeth. Nurse sharks have independent arrangement of teeth, as most sharks do, meaning that there is no overlap in the teeth and that shedding is not dependent on other teeth (FLMNH). Moss (1965) states that the sharks adaptations allows for a suction pump technique and the mouth possess thick lips that are removed from the jaw. The nurse shark possess all of the qualities primarily used for suction feeding. Each of these being a small, laterally enclosed gape, reduced tooth arrangement, rapid expansion of the cheeks/ sides of mouth, and top hyoid depression after peak gape. Motta et al. (2001) found that nurse sharks couple head elevation with depression of the jaw in order to open the mouth as much as possible as well as direct the prey toward the back of the opening, allowing the mouth to envelop the prey entirely. Although they do not exactly exhibit inertial suction feeding (the type of feeding used by bony fish), their tactics are extremely close to functionally replicating those found in bony fish. Motta also explains that when the nurse shark approaches a food source, the jawbone is rapidly depressed as the lips move back to laterally close the mouth. Often we believe that the larger the shark, the faster the attack may be. However, Robinson (1999) found that smaller nurse sharks tend to perform the same kinematic feeding more quickly than their larger counterparts. This is thought to be due to the physiological constraint on muscular contraction dynamics. It was observed to take approximately thirty-two milliseconds for the shark to open its mouth! One of the fastest recorded feedings of any shark.
Suction feeding of these sharks is characterized by an expansive, compressive, and recovery phase. Motta et al. (2008) found that during the expansive phase, depression of the lower jaw is effected by jaw muscles. There is also depression of the hyoid and expansion of the gill arches which are driven by contraction. The loose connection between a part of the hyoid arch (ceratohyal) and jaw allows for substantial cheek depression, unlike in most other sharks. Muscle activity by inner hyoid and inner jaw show that the hyoid and jaw are moved and the muscle from the mandibular arch move the jaws during the compressive phase. During what is thought to be the recovery phase, cartilage elements return to initial positions.
Two small barbels located near the mouth are used to search for prey by brushing them along the ocean floor. Some nurse sharks have even been found hovering over the ocean floor by resting on their pectoral fins. This creates a possible false cave in which to lure in crabs, lobsters, etc.. The power of their suction feeding is so intense that it has been known to dismember their prey. While nurse sharks would rather feast on the finer things such as fish, shrimp, and squid, their strong jaws are able to capture and crush shellfish and even coral. These sharks are typically nocturnal creatures and tend to “sleep” during the day under coral. However, their small mouths prevent them from consuming large fish. They hunt for their prey along the ocean floor and have even been seen grazing for algae and ground corals. Robinson and Motta (2001) also examined the effects of growth patterns on the scale of feeding kinematics. They found that a nurse shark’s feeding mechanism grows isometrically (same scale of growth).Their findings conclude that kinematic feeding will not differ with physiological growth and/or change of the shark.
It’s safe to say that nurse sharks are unique in many ways; from their suction technique primarily used for feeding, to the slurping/sucking sound this method produces. While nurse sharks are generally not hunted, a slight decline in population has still been found. Education programs such as TREC and even the worldwide phenomena Shark Week, all contribute to helping the world understand the beauty, rather than terror, of all sharks, not only the nurse. To me, they are such beautiful creatures and give a very different view of sharks than the one JAWS seemed to captivate. They are such a great contradiction by way of the incredible power of the jaws and feeding tactics, coupled with their docile nature to educated visitors.
Florida Museum of Natural History n.d. Ginglymostoma cirratum. Ichthyology Collection.
Motta, P.J., R.E. Hueter, T.C. Tricas, A.P. Summers 2002. Kinematic Analysis of Suction Feeding in the Nurse Shark, Ginglymostoma cirratum (Orectolobiformes, Ginglymostomatidae). Copeia 2002:24-38.
Motta, P.J., R.E. Hueter, T.C. Tricas, A.P. Summers, D.R. Huber, D. Lowry, K.R. Mara, M.P. Matott, L.B. Whitenack, A.P. Wintzer 2008. Functional Morphology of the Feeding Apparatus, Feeding Constraints, and Suction Performance in the Nurse Shark Ginglymostoma cirratum. Journal of Morphology 269:1041-1055.
Motta, P. J., C.D. Wilga. 2001. Advances in the study of feeding behaviors, mechanisms, and mechanics of sharks. Environmental Biology of Fishes 60:131-156.
Robinson, M.P., P.J. Motta 2001. Patterns of growth and the effects of scale on the feeding kinematics of the nurse shark (Ginglymostoma cirratum). The Zoological Society of London 256:449-462.
Snyderman, M. n.d. More Than a Mouthful: How Fishes Use Their Mouths. Dive Training Magazine.
Tanaka, S.K. 1973. Suction Feeding by the Nurse Shark. Copeia 60:606-608.