Fish of the Month: Spotted Eagle Ray Aetobatus narinari

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FISH OF THE MONTH - JANUARY 2004
[Note: the scientific name of the Indo-Pacific species is now Aetobatus ocellatus.
The Atlantic species remains A. narinari. 11-10-2010]

SPOTTED EAGLE RAY
Aetobatus narinari (Euphrasen, 1790)

Kahe Point. O`ahu. 20 ft.

SPOTTED EAGLE RAY · hailepo; hihimanu
Aetobatus narinari (Euphrasen, 1790)
Family Myliobatidae
     These magnificent rays can attain 6 ft. or more from wingtip to wingtip. Their back, light brown, gray or black, is beautifully spotted with white. The underside is mostly white often with a faint mazelike pattern under the wings. The long slender tail can equal three times the width of the body if not broken or bitten off, and bears 2-5 venomous spines at the base. Under the large protruding head is a wide fleshy lobe somewhat resembling a "duck bill" which is used to dig for molluscs and other organisms. When not foraging, Spotted Eagle Rays swim well off the bottom, sometimes in small groups. One of the most beautiful of all underwater sights is a formation of Spotted Eagle Rays "flying" together in synchrony. In some parts of the world schools of 50 or more have been reported, but such behavior is certainly not common in Hawaiian waters. Occasionally Spotted Eagle Rays will leap from the water, either dolphin style or by cartwheeling with wings outspread. Leaping by pregnant females is said to facilitate the birth of young.
     Spotted Eagle Rays occur in warm seas around the world and have long been considered a single species. Recent research, however, reveals that the eastern Pacific and Indian ocean populations host different species of tape worms in their gut. Spot patterns and body proportions also differ between various geographic populations. All this suggests that what we now call Aetobatus narinari is actually a complex of several similar species, but the details have yet to be sorted out.
      The Hawaiian word hihimanu means "lavish," "magnificent," "elegant." In ancient times these powerful animals, which weigh up to 500 lbs., were forbidden to women as food. The species name, narinari, is a Brazilian Indian word meaning "stingray." The family Myliobatidae, to which this ray belongs, includes 22 species of eagle rays, bat rays, duckbill rays, cownose rays, manta rays and mobula rays.

Hanauma Bay, O`ahu, 20 ft.

Eagle Ray reproduction
      Spotted Eagle Rays pursuing one another may be about to mate.
       Writer/photographer Rod Canham describes "a large eagle ray hotly pursued by two smaller ones" at Molokini Islet, Maui. "The male latched onto the larger female and coupled 10 feet in front of us, as the pair rapidly glided out of sight."
      Dr. Tim Tricas of the University of Hawai`i observed two episodes of unsuccessful courtship at Enewetak Atoll, Marshall Islands. In the first instance two males swam behind a female on either side, nipping at her posterior margin. The female rose to the surface and vigorously slapped her wings on the water causing the males to back off. When she submerged the males continued to follow and nip. Occasionally one would swim wide circles around her.
      In the second episode, Tricas watched as a single male followed a female, periodically gouging her back with his dental plates and attempting to mount her. At each attempt she either shied to one side or surfaced. When she swam at the surface, the male followed either swimming conspicuously from side to side or up and down. Eventually the female left and the frustrated male swam in a wide circle alternately surfacing and diving at a 45 degree angle, his white underside producing bright "flashes" visible even after the outline of the ray disappeared in the distance.
     In their book Sharks and Rays of Hawai`i (Mutual Publishing, 2002), Gerald L. Crow and Jennifer Crites write: "In Hawai`i, spotted eagle rays are born in October, November and December in Pearl Harbor and Kane`ohe Bay, O`ahu, and on the leeward side of Moloka`i. During the birthing process, these rays have been seen leaping from the water and dropping their young in midair." Up to four pups (disc width 10-20 inches) are born per litter.

Eagle Ray feeding
     Spotted Eagle Rays feed mostly on molluscs. They will also take worms, crustaceans, echinoderms, and small fish. The mouth of an Eagle Ray contains platelike teeth used for crushing shelled prey. Special papillae on the floor and roof of the mouth separate the broken shell fragments from the edible flesh before it is swallowed. Foraging rays are thought use electroreceptors to "scan" for buried prey, which they dig out using their shovel-like nose (the "duck bill"). As they dig, sand may be stirred up in clouds or ingested and expelled out the gill slits, enveloping the entire head. Fishes will sometimes follow feeding Eagle Rays, presumably to snatch scraps.
     Spotted Eagle Rays are not limited to foraging in sand. Sometimes they will overturn large chunks of coral or rock in search of prey, or rip prey animals directly from rocks. Pauline Fiene, of Mike Severns Diving, reports: "I was diving alongshore Maui one morning and watched as an eagle ray worked at tearing something off a large lava rock. With its head down and tail up in the 'air' it worked away until it tore something off the rock with its lower jaw and then swam off, chewing. When I went over to see what it had gotten, I could see the byssal threads of the ark clam, Arca ventricosa, still attached to the rock."
     Nor are they afraid to tackle large molluscs. Kendra Choquette-D`Avella, of Dive Makai, reports: "Twice I have seen eagle rays eating Triton's trumpet shells! I actually have an old (crappy) photo of a ray with the pointy end of the shell sticking out of its mouth! Both times the ray picked up the shell (and these are BIG triton shells--not babies!) by the bulbous end and proceeded to smash it against rocks to try to break it open. Both times I was not able to see the end result but it is safe to assume the Tritons got the worse end of the deal!"

"scanning" for prey - Lana`i Lookout, O`ahu. 30 ft.

"scanning" for prey - Kahe Point, O`ahu. 30 ft.

Spotted Eagle Rays as food
     Sharks are the main predators of Spotted Eagle Rays. Species known to feed on them in other parts of the world include the Silvertip Shark(Carcharhinus albimarginatus) and Great Hammerhead (Sphyrna mokarran). Although neither of these is common in Hawaiian waters, the Tiger Shark (Galeocerdo cuvier) is. Consider the following story by Matthew D'Avella of Dive Makai Charters in Kailua-Kona, Hawai`i:
     "Kendra and I had heard word of a tiger shark that had taken a liking to Honokohau Small Boat Harbor. One day after multiple sightings of the shark at the mouth of the harbor, Kendra, our friend and co-worker Mark, and I, decided to do a beach dive at "Manta Ray Bay" just outside of the Harbor in hopes of catching a glimpse of this beautiful animal.
     We entered the water at about 4:00 in the afternoon. About 10 minutes into the dive we reached the pinnacle, about 45 feet deep, and came upon an eagle ray. I later named him Ray Ray. I got the idea that if I stayed close to the ray, and the shark came to visit, then the ray might be a more tempting meal for the shark than me. Besides, the ray was obliging my wishes and was staying close to me too.
     For the next 20 minutes I shot video of Ray Ray swimming and digging in the sand for food or facing me and looking at me. He was just staring at me, it was almost creepy to be looking at his face. At times he was swimming so close to me that I could not get his entire body in the view screen, or if I did get the shot, I had my fin tips at the bottom of the video because I was having to swim backwards to get away from him.
     Towards the end of my encounter with Ray Ray I met up with Mark and Kendra again. Kendra grabbed my hand and had me feel her heart beat. To call her heart beat "racing" would be an understatement. She is a very small girl and I could feel her heart beating quite powerfully through her double 7mil wet suit.
     We headed back into the beach and we surfaced in 5 feet of water and Kendra told me of her encounter with the tiger shark. Kendra said that the shark swam right over her head, close enough to touch. She got two outstanding photos of the tiger shark which later became known as Laverne. Kendra then proceeded to tell me of what she saw of my encounter with the eagle ray. It seems that while I was using the ray for cover from the shark, the ray was using me for the same reason. Laverne and Ray Ray were playing 'ring around the diver', and I didn't know it. All that I saw was the ray, and all that I thought was "this video is going to be GREAT!"
     Well the video of Ray Ray is outstanding. I have some good shots of him feeding and swimming around me. After watching the video a few times, I started to notice some jerking motions from Ray Ray, was this in an effort to hide from Laverne? I suspect yes. Should I ever get the chance to see an eagle ray up close and face to face again, I am sure that I will feel an urge to turn and look behind me. After all, I am a firm believer that my bravery in the ocean realm is directly related to the amount of tape and remaining battery power in my camera."
                   Matthew J D'Avella
                   Dive Makai Charters
                   Kona Hawaii

resting at the Mahi wreck, leeward O`ahu. 80 ft.

What does a Spotted Eagle Ray do all day?
Researchers in Bimini, Bahama Islands, tagged 17 individual rays with small sonic transmitters and tracked them for periods of up to 98 consecutive hours. They discovered two important facts: A) The rays' activity fell into four phases which were strongly correlated with the tidal cycle. B) The rays were strongly attached to three "core areas" around the island, where they rested at low tide.

a) Rest
The "resting phase" began at low tide and lasted 2-4 hours until the tide rose about half way to high. During this time the rays tended to remain in their core areas, typically circling slowly in small groups or swimming gently into the current .

b) Commute
As the tide continued to rise and approached its high point, the rays increased their swimming speed and began moving out of their core areas. Researchers called this the"commuting phase."

c) Feed and socialize
At high tide the rays entered the "foraging phase," moving in slow circles, feeding, and socializing. This lasted for several hours until the tide fell about half way to low.

d) Go home
As the tide continued to fall and approached low, the rays entered the "returning phase," and began swimming back to their core areas. Movements were slow and the rays were loosely grouped.

Interesting factoids
1) Spotted Eagle Rays have one of the largest brain to body weight ratios of any fish.
2) Spotted Eagle Rays are the only members of their family to have multiple spines on the tail.
2) The earliest reference to the native Brazilian name "narinari" from which the scientific species name was drawn appears in a book on the natural history of Brazil written by a French monk and published in 1613.