BIG DEAL: the giant moray, Gymnothorax javanicus, is the largest of the moray eels, shown here amid glassy sweepers at Baa Atoll in the Maldives WATER COLUMN WATER BY DOUGLAS DAVID SEIFERT BY DOUGLAS DAVID EEL APPEAL Every diver relishes a decent moray eel sighting, but there’s more to these predators than meets the eye. Douglas Seifert investigates the secret world of the moray and the timeless appeal it holds for coral reef divers www.divemagazine.co.uk 25 WWaterater CColumn_DEC09.inddolumn_DEC09.indd 2525 22/11/09/11/09 118:21:358:21:35 FROM TIDAL pools formed at the lowest of low tides to fantastic and dangerous-looking teeth, has fascinated rocky habitats, wrecks, and reefs and down along coral people since the dawn of recorded history, with walls leading down to the abyss, wherever there is a narratives dating back to the time of the Roman Empire. suitable crevice, one is almost certain to find a moray eel Morays were said to be kept as pets or venerated as of one species or another. objects of curiosity, or, allegedly, as instruments of By day, they are shy and retiring, their bodies punishment or execution for slaves, criminals and concealed in labyrinthine lairs, with only their heads prisoners during the glory days of Roman civilization. peeking out from the rocky shadows towards open water. But the moray does not bare its open maw to display Their large, unblinking eyes stare fixedly and their jaws aggression – it is simply breathing. The eel’s gills are set open and close ceaselessly, rhythmically, displaying a well back from the mouth along its narrow flanks, hidden fierce array of multitudinous, backward-pointing, needle- beneath a narrow slit. In order for water to pass over sharp teeth set in long, tapered jaws. The sight of a those gills, the moray effectively gulps water by opening large aquatic creature peering out of a hole, with jaws its mouth wide and pumping the water through its gills. gaping wide and displaying a bear-trap mouth of Moray eels have a snake-like body but are bony fish, WWaterater CColumn_DEC09.inddolumn_DEC09.indd 2626 22/11/09/11/09 118:22:328:22:32 continuous band from just behind the head to and around the tip of the tail to terminate at the base of the anus. Their skeletal system is a bony vertebrate spinal column, similar to human or snake spinal chord but with flexible COLUMN WATER cartilage ribs instead of rigid bones. This anatomic flexibility enables the moray to bend and twist with fluid ease, with an ability to contort itself unmatched in the sea except only by its arch enemy, the octopus. Morays do not have scales, but instead a thick, smooth skin. This is covered in a mucus slime (toxic in some species) that forms a protective barrier to disease, infection and parasites. The mucus also serves to lubricate the moray’s body, facilitating passage over rough surfaces, through sand and rubble – and, in the case of sand-dwelling morays, to solidify sand grains into building material and form the walls of tunnels. The mucus even provides a kind of temporary exposure suit to keep the moray wet and lithe on the irregular occasions when it makes a crossing over land and terrestrial obstacles and briefly exits the ocean to move from one body of water to another. Additionally, the mucus covering is highly effective in making difficult a would-be predator’s purchase upon a writhing eel. The lubricating, protective mucus in conjunction with a highly flexible body make for an extremely efficient form of swimming called undulating. The eel moves in a rippling motion that can propel it backwards as well as forwards, a feat impossible for most other fish species. The moray’s tendency to hide during the day is largely the result of its sensory strengths and weaknesses. A moray’s eyesight is rather poor, suited to identifying light intensities but not colour. It sees more motion than detail and not very well at that. Its hearing is rudimentary. These diminished senses would give potential daytime predators such as sharks the advantage. Since the moray cannot see well, hiding is an obvious tactic. There are exceptions – such as the ribbon moray (Rhinomuraena quaesita) that is a hyperactive hunter by day, and relies on speed to ambush small fish prey – but most morays are daytime recluses. However, with the fall of night, the morays become active, prowling the reefs on a relentless hunt for prey. In the darkness, poor vision is of little consequence because, for the most part, fish and invertebrates see equally poorly at night. Morays vacate their lairs to openly members of the family anguilliformes, the true eels, HAPPY FAMILIES: probe the reef on a search-and-destroy mission, poking which contains more than 600 species in four suborders this unique aggregation their noses in everything and everywhere. of fine-spotted moray and 19 families. One of these is the Muraenidae, the eels (Gymnothorax Morays are endowed with other senses enabling them family of the moray eels, comprising about 200 species dovii) is only found at to succeed as one of the reef’s top predators. They Malpelo Island in the of morays in 15 genera. Eastern Pacific possess a reduced lateral line system restricted to the Morays are found in all tropical and temperate seas head that is used to detect and analyse vibrations, pulses and come in a variety of colour patterns and sizes, from and movement. This effectively creates a three- the relatively small red-faced moray (Monopenchelys dimensional picture of the immediate surroundings – and acuta) at 20cm in length, to the giant moray all fishy and invertebrate activities taking place within it. (Gymnothorax javanicus), which can weigh in at 36kg. The moray eel’s dominant sensory apparatus is its There is also a freshwater moray eel that can be found in acute sense of smell: more than anything else, a moray southeast Asia and in the Great Lakes of the US. eel could be considered a swimming nose. Their sense of Lacking pectoral fins, morays have dorsal and ventral smell is in a state of constant overdrive: a moray eel fins fused with the caudal (tail) fin that run as a continuously monitors olfactory information by taking www.divemagazine.co.uk 27 WWaterater CColumn_DEC09.inddolumn_DEC09.indd 2727 22/11/09/11/09 118:23:008:23:00 WATER COLUMN WATER 26 www.divemagazine.co.uk WWaterater CColumn_DEC09.inddolumn_DEC09.indd 2828 22/11/09/11/09 118:23:288:23:28 in water from its surroundings, gathering waterborne GENDER BENDER: roof of their mouth to enhance gripping and crushing. scent molecules through its nostrils, via paired, tube-like a male (blue) ribbon One of the most extraordinary characteristics about eel (Rhinomuraena structures called nares. Water flows into anterior quaesita) transforming moray eels has only been recently discovered. The moray openings at the tip of their snout, scent molecules are into a female (yellow) eel possesses a raptorial second set of jaws deep within at Lembeh Strait, COLUMN WATER differentiated and analysed as they flow through the Sulawesi, Indonesia its throat. Why do eels have two sets of jaws? All the olfactory organ before being expelled through the [opposite page]; the better to eat you with, my pretty… posterior nostril tubes situated above the eyes. The subtle blackcheek has proved The moray’s heads and body is narrow and to be more aggressive differences in intensities of scent between the two sets of than most moray eels compressed, so it cannot achieve the suction technique nares are evaluated and used to pinpoint the source of [below] other fish use to swallow prey. At best, a moray can the odour with an astonishing accuracy. bite and grab, and if the prey item is too large to swallow, As a top predator, the moray has a cosmopolitan diet the moray is in danger of asphyxiation because their consisting of crabs, shrimp, lobsters, fish and their throat will be blocked. Other fish, with a larger oral irresistible prey: the octopus. The greater the scent a prey cavity, use a volume of seawater to help suck the prey animal emits, the greater the chance the eel has to find it down their throats and in the process also pass water and devour it. For instance, parrotfish are well known to over their gills. A moray eel cannot do that because the secrete a mucus cocoon enveloping their entire bodies prey is a tight fit in its mouth, leaving no space for water when they retire under coralline structures at night: the to be entrapped and pass along. mucus membrane is designed to keep the parrotfish Instead, morays rely on a second set of jaws, called scent in so an eel does not detect the slumbering fish. ‘pharyngeal jaws’, to transport prey from the mouth Moray eels and octopus are eternal enemies: they vie down their gullet. Other bony fish also have pharyngeal for the same habitat; they pursue the same hunting jaws that assist in swallowing, but they are usually strategy as nocturnal hunters, and they consume the grinding plates or small teeth. A moray eel’s pharyngeal same prey. It seems only natural that their jaws are muscular and have sharp recurved competition should turn towards each other. teeth, allowing the jaws to be projected far When an eel is a juvenile, it may fall prey to WHY DO EELS HAVE forward where they act like forceps, gripping an octopus, but once it reaches a good size, then dragging the prey down the the tables are turned and the moray TWO SETS OF JAWS? oesophagus, where it can be swallowed and becomes the invertebrate’s most deadly foe.