LATERAL LINE OF THE NURSE SHARK (Ginglymostoma cirratum): BRIEF REVISION

The nurse shark (Ginglymostoma cirratum), also known in Portuguese as “tubarão lixa” or “lambaru”, is found mostly in groups of variable size. Specimens inhabit the bottom of the sea, in warm, littoral waters. The nervous system of these sharks arises embryologically from the medullary plate, in which the cephalon develops. The cephalon is divided into three parts, prosencephalon, mesencephalon, and rhombencephalon. These sharks have very sensitive sensors, both mechanoreceptors and electroreceptors, that contribute to the exploration of their surrounding aquatic world and enhance their survival,

This animal may attain a length of 4 m and weight up to 500 kg. It is an oviparous species, in which the female may lay up to 30 eggs. Saville et al. (2002), using DNA methods, studied cases of multiple paternity in juveniles of this species, indicating that this reproductive tactic represents a way to guarantee genetic variability of the nurse shark, when one considers that their migratory movements are relatively restricted. Sexual dimorphism is observed, females ataining the largest sizes (1-3 m), relative to males (2.2-2,57 m) (FURTADO et al., 2018).
The mouth contains small and sharp teeth, similar on both dental arcades. Matott, Motta, and Hueter (2005) found that, although there is some specialization of the buccal apparatus, the nurse shark does not have a specialized diet. The snout is short and flat, with branchial folds on the posterior surface of the pectoral fins. Fins are rounded, the posterior dorsal fin being larger than the second fin. During the daytime the animals are less active and remain stationed on the sand bottom or within available caves. They become very active at night. They present the curious habit of piling themselves one over the other, forming clusters of up to 30 individuals.
Nurse sharks are found mostly reunited into groups. Although they are not eminently dangerous for humans, a safety distance should be observed when approaching them. They remain immobile for hours during the day (CARVALHO, 1992;FERREIRA, 2015). Their feeding items include bottom fishes, shrimp, squids, octopuses, crabs, and lobsters, among others. They have a structure popularly called a goatee, that helps locating prey mostly during the night (FURTADO et al., 2018).
Nurse sharks live in warm littoral waters, in depths of up to 60 m. They range from Brazil to North America and to Africa. They are also found along the West coast of the Americas, from Ecuador to the Gulf of California (CASTRO, 2000;COMPAGNO, 1984).
Nurse sharks have sense organs that make them well adapted to prey species sharing the habitats in which they live. This special character instigates research directed to understand their structure and function (FURTADO et al., 2018).

CENTRAL NERVOUS SYSTEM
The nervous system of sharks appears embryologically from the medullary plate, which is formed from the neural tube. Here is located the cephalon, that is divided into three parts, the prosencephalon, mesencephalon, and rhombencephalon. All three belong to and help to conserve the central nervous system (YOPAK et al., 2007).
The prosencephalon concentrates olfactive and visual functions. The mesencephalon coordinates the nervous system in general, and the rhombencephalon coordinates the muscles and the control centers for respiration, cardiac action and metabolism. Leaving the cephalon are two cranial nerves that connect to the sensory organs. Chondrichthyans (cartilaginous fishes, such as sharks) have well developed sensory organs (YOPAK et al., 2007).
The sensory organs provide sharks with a great ability to locate their prey. The nerves also connect to the muscles, especially those that attach to the head of the animal, the cephalon is connected to the spinal cord, from which several nerves ( Chondrichthyes also present a lateral line, which is a set of channels filled with water that have sensory cells, which function to detect variations in water pressure, water vibrations, and the movement of other fishes. These mechanoreceptors of the lateral line are able to perceive pressure and touch (TRICAS; CARLSON, 2012).
The system is formed by neuromasts (specialized receptor organs) (Fig. 1) The ampullae of Lorenzini are observed in the snout of the shark (Fig. 2). These are  Sharks have well developed sensory organs that are fundamental for an oceanic predator. They permit the finding of prey placed at considerable distance or in hiding. Along the millennia the shark has been able to perfect this sensory system of great biological significance, resulting in the efficient shark that has survived up to our days (TRICAS; CARLSON, 2012).

CONCLUSIONS
The nurse shark Ginglymostoma cirratum has sensory organs of great precision. Its mechanoreceptors and electroreceptors are formed by specialized ampullae located in lateral water channels located along the sides of the animal. They are filled by a gelatinous substance and end in surface pores on the epidermis that communicate with the surrounding water. The several functions of this system have been described in the literature (FIELDS; FIELDS; FIELDS, 2007. In this review we emphasize the pressure sensors (MURRAY, 1957) and those specialized in detecting electrical fields (KALMIJN, 1982). The shark uses its senses to record and to explore the surrounding subaquatic environment, resulting in a highly efficient predator, and contributing to its long-term survival.