Monday, 26 September 2011

Aquarium Tour: Freshwater stingrays

Opposite the piranha tank is another large tank containing a variety of Amazonian fish, of which the most spectacular are our group of freshwater stingrays, Potamotrygon motoro. Practically all rays, and their close cousins the sharks, are either confined to the sea or at best occasionally enter fresh or brackish water at estuaries, but the Amazon is home to several species of rays which are fully adapted to fresh water and can no longer enter the sea. It appears that this process began about 15 million years ago, when the rise of the Andes and other earth movements isolated a fairly standard coastal species. Today, around 23 species in three genera can be found across the Amazon basin. Most of these are in the same genus Potamotrygon as those at Bristol. Many are seldom seen in captivity, but P.motoro is perhaps the most widely kept.

All the rays have a fairly similar lifestyle. They spend much of the time, especially during the day, buried in the sediment with only their eyes and the spiracle through which they breathe showing. If disturbed or threatened, they can defend themselves by lashing out with their tails, which carry a barbed and venomous spine about half way along. The tail is very flexible, so the sting can be jabbed into whatever animal is threatening them, at least as long as it has a hide the barb can penetrate. As the barb is capable of penetrating bone, most animals will approach them cautiously, although large caiman are probably safe.

When active, rays will patrol the river bed on the search for their prey, which is basically any form of animal life that will fit in their capacious mouths. Invertebrates in the form of snails and freshwater shrimps are probably the main food items, but they are quite capable of eating small fish, especially if they catch them sleeping while on the prowl at night. Species which build a nest for their eggs and fry, such as most of the cichlids, need to have string defence behaviours against rays and other bottom feeders.

Rays are quite large, and the Motoro ray can grow to around 50cm across the disk. Some other species can grow much larger, to nearly 1 metre, with a nose to tail length of perhaps 1.5m or more.

Some marine rays are egg layers, but like many other sharks and rays the various Amazonian species are all live bearers. Adult males are easily distinguished by the claspers, modifications of the anal fin that are used in mating, and the female can produce up to 4 pups at a time. The new born rays, each about 15cm across, are independent from birth and immediately go about searching for food as soon as they have uncurled from the rolled up shape they are born in.

Bristol has bred Motoro rays in the past, and reports of captive breeding of this species are fairly common, but many others have not been bred at all and several others have only had already pregnant wild caught females give birth in captivity. As there is not a big demand for them in the aquarium trade, there is no motive to produce captive bred stock, and all rays seen for sale are wild caught. Mostly these are very young or new born rays, and there is a high mortality, especially if the water conditions they are kept in have even a slightly raised level of nitrate or nitrite.

For a good video of a stingray giving birth look here:

Rays are one of those species, like Pacu and other giant species, which most aquarists should not even try to keep. Even a very large home aquarium is inadequate to care for these species, and one with an unobstructed floor area of at least 4 square metres should be considered the absolute minimum. Basically, you are looking at a small heated swimming pool to keep them in. In a warm climate this is not too much of a problem, but the costs of heating and filtering a water body of this size in a temperate climate is simply too much for most hobbyists. There is also the issue of the venomous barb. Rays become quite tame and are not aggressive fish, but an unexpected startle response is always possible, so they should always be treated with respect.

Beside juvenile being caught for the aquarium trade, probably the main threat is over fishing. Rays take at least three years to reach maturity, and have a low birth rate, so they could easily be fished out, especially as they are considered a desirable food fish. As the range of many species is unclear, and there are probably several undescribed species as well, every effort should be made to study their requirements for breeding, in case replenishment of locally exterminatred stocks is required from farmed fish.

(Images from wikipedia)

1 comment:

  1. Hot news! Both our female rays have produced pups, which are being raised off show.