Biology and behaviour

There is still much information to be gathered about this species and many gaps in knowledge to be filled. However, below is a brief summary regarding the biology, ecology and behaviours of white spotted eagle rays. See useful links for more detailed species information.

CLASSIFICATION

KINGDOM: Animalia, PHYLUM: Chordata, CLASS: Chondricthyes, ORDER: Rajiformes, FAMILY: Myliobatidae

Scientific name: Aetobatus narinari

Some interesting common english names: 'Lady Rays', 'Bishop Rays', 'Leopard Rays', 'Speckled pancakes' (Bermuda) and here in TCI they are called 'Flatbars' by local fishermen.

NOTE: Although globally distributed across subtropical and tropical seas, recent genetic analysis has shown that there are in fact distinct populations between the West/\Central Pacific and the Eastern Pacific and Central Atlantic (Richards et al., 2009). It has been recommended that these be classified as sub-species. Different populations also exhibit variations in morphology and parasite communities.

QUICK STATISTICS

  • Maximum length: around 5m (10ft)
  • Maximum width (tip to tip): around 3m (10ft)
  • Maximum size: around 230kg (507lbs)
  • Average lifespan: unknown but estimated at possibly 20 years based on similar species and aquarium exhibits.

FEEDING BEHAVIOUR AND ECOLOGY

Spotted eagle rays feed on shelled invertebrates which in the Caribbean includes juvenile conch, clams, oysters, crabs, sea urchins and bivalves. They may also feed on octopus and small fish. They locate their prey via electro-sensory cells on their snouts which allow them to detect electrical signals given off by living organisms as well as using their sense of smell. They have perfectly adapted bony plates in their jaws for coping with these prey items and small protrusions on their flat teeth called papillae allow them to manipulate their food so that the shells are removed.  Analysis of stomach contents reveals only fleshy parts of food items remain as the shells are spat out and discarded. 

In Bimini, Bahamas, scientists observed tidal mediated behaviour. At high tides eagle rays moved into shallow lagoonal areas where they forage for food in sand flats, seagrass beds and coral reefs and at low tide refuge in deeper reef edge sites (Silliman & Gruber, 1999). This has also been observed of the populations here in South Caicos. 

PREDATORS

The main documented predators of spotted eagle rays are sharks such as the great hammerhead (Sphyrna mokarran) and, in the Indian Ocean, the silvertip (Carcharhinus albimarginatus)Slide 13. It is likely that other large sharks such as tigers, lemons and bulls also feed on these species. An observation of the behaviour of a great hammerhead feeding on a spotted eagle ray was documented in Bimini, Bahamas, where the shark pinned the ray to the ground so that it could pivot its mouth around the rays head (Chapman & Gruber, 2002). Spotted eagle rays can often be observed leaping high up into the air, often in rapid succession. This behaviour can be observed when they are being pursued by a predator but could also be a an action to rid themselves of parasites or another form of social behaviour.

 REPRODUCTION

Most information about courtship and mating behaviours comes from specimens in aquariums (Uchida, 1990) but there are also couple of documented scientific observations in the wild (e.g. Tricas, 1980).  The mating season is thought to be during the summer months based on personal observations from South Caicos, Bimini (Corcoran & Gruber, 1999) and aquarium studies. During this period the male, often more than one at a time, will chase the female and bite at her dorsal surface. If the female is willing, the male will then bite at her pectoral fins to turn her over and achieve an abdomen to abdomen position. The male then inserts either one of his claspers and they will remain in this position for up to 3 minutes. Claspers are the male reproductive organs found on all sharks and rays which are used to transfer sperm to the female.

The gestation period for a spotted eagle ray is thought to be around 12 months (males will try and mate with females as soon as they have given birth). This species is ovoviviparous which means that eggs hatch inside the mother but they feed on the yolk rather than via a placental connection.  Juveniles are born live at around 30cm in size and there are around 1-4pups per litter. Uchida (1990) gives an account of a birthing event in an aquarium in Japan in 1979. There are thought to have been around 10 successful births of spotted eagle rays in aquariums, the last two being born in Tenessee in Sept 2008.  Juveniles in the wild are rarely seen although there has been documentation of them occuring in nursery like estuarine habitats in Northeastern Brazil (Yokota & Lessa, 2006).

SOCIAL BEHAVIOURS

This year around late spring in South Caicos, up to 40 spotted eagle rays were seen schooling around the drop-offs. It is unknown as to why they school as mating or courtship behaviours are not observed (Corcoran & Gruber, 1999). The groupings are also independent of size and sex. Spotted eagle rays have the highest brain size to body ratio of any fish and so they are believed to be capable ot complex social interactions. They are often seen hanging out together in pairs and small groups. The understanding of associations between different individuals and the components of social behaviour make up some of the current research goals of studies in South Caicos.