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Fishes
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Orange Spotted Stingray
(Potamotrygon
motoro)
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Although I always have a
difficult time answering the question, "which is your favorite pet?" my
stingray, Hippolyte
(below),
always springs to mind as a contender. The main reason she is
a
sentimental
favorite is that I’ve had her longer than any other of my
animals; I
purchased
her in the spring of 1995, soon after I moved to Chicago, and she has
been
doing great ever since.
Stingrays are chondrichthyans
– members of the same group of fishes that includes sharks
– and as the
name suggests, their skeleton is made of cartilage,
not bone. Like sharks, stingrays are easy to sex;
males have
long, modified anal fins known as claspers
and these are absent in females. Hippolyte belongs to a group of about
20 species
of
stingrays that lives in South American rivers. Most
people
are
surprised to learn that there are freshwater stingrays, especially
since they are related to sharks. In fact, stingrays have
independently
evolved to live in freshwater many times, and true freshwater stingrays
can
be found in Asia and Africa in addition to South
America. I
never
considered getting a saltwater stingray, mainly due to the greater cost
and time commitment associated with maintaining a saltwater
aquarium. In my experience, stingrays are one of the best pet fishes you can have. They are interesting, personable, and relaxing to watch as they glide through the water. They also seem to have an amazing immune system and ability to heal themselves; Hippolyte has never shown any signs of health problems and the few times that she has gotten injured (see below), she healed extremely quickly. They are also relatively long-lived, which also makes them a pretty good investment. I have no idea how old Hippolyte was when I purchased her; she is probably at least ten years old now. One of the most important factors in keeping a stingray is having the proper enclosure. Since they are large, round, flat fishes, they need a pretty big, horizontally-oriented tank. I keep mine in a 90-gallon acrylic aquarium that is four feet wide, two feet deep, and two feet high. The bottom is covered with four to five inches of pea-sized aquarium gravel in which are “planted” a variety of artificial plants. For live plants, I have only Java moss (Vesicularia dubyana, a plant that looks like algae but that is actually a type of moss); it grows very quickly and adds a more naturalistic look to the aquarium. I use two filters, a Magnum 220 canister filter (for mechanical filtration) and an Eheim professional canister filter (for biological filtration). I aerate the tank by adjusting the outflow of the Magnum filter such that it agitates the surface of the water, allowing for plenty of gas exchange. Light is provided by a four-foot fluorescent strip light. I don’t employ any sort of heater since my apartment is always reasonably warm. Maintenance of the aquarium is minimal. Every few weeks or so I do a partial water change using a gravel vacuum siphon, and every couple months I clean out the filters. Other than scrubbing algae off the sides of the tank (and pruning the Java moss), that’s about it. I don’t do a whole lot with water chemistry when I add water other than dechlorinating it and lowering the pH (relative to tap water). |
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In the wild, freshwater
stingrays eat snails, crustaceans, other invertebrates, and small
fish.
At my place, Hippolyte eats thawed frozen shrimp and
nitecrawlers.
For shrimp, I try to use ones that are small to medium size, peeled,
and
uncooked, but sometimes only precooked shrimp are available.
I
get
the cheapest I can find, usually about $5-6 per pound, and thaw out a
few
of these to give her every night. I generally hand feed these
to
her,
but when I’m in a hurry I just toss them into the tank and
she finds
them.
For nitecrawlers, I use regular, garden-variety ones that I catch after
rains or that I buy at the bait shop. A few times a week I
give
her a nitecrawler in the morning, in addition to the shrimp she gets at
night. Some folks recommend squeezing the dirt out of
nitecrawlers before feeding them to fish, but this is usually a messy
proposition and the dirt doesn’t seem to bother Hippolyte at
all.
I have tried fish with Hippolyte, but she mostly ignores
it or chews on it a long time and then spits it out. Stingrays
don’t
have
much in the way of teeth. Rather, they have a series of hard plates
designed
for crushing (rather than puncturing or slicing) relatively durable
foods.
She loves live ghost
shrimp, but these are too expensive to feed her on a regular
basis.
When I first got Hippolyte, she would only eat nightcrawlers; it wasn’t until later that she started eating shrimp. During the time she was on a nightcrawler-only diet, I noticed that her color seemed to be getting lighter and lighter. Conversely, when she started eating shrimp, her color became darker and darker; she is now much darker than she was when I obtained her. It seems, therefore, that her coloration partly depends on pigments (or other chemicals) obtained from the shrimp she eats; a similar phenomenon occurs in flamingos (which is why many zoo flamingos are dull in color), and it probably occurs in other animals, too. Whenever it gets close to feeding time, Hippolyte will hang out in the front corner of the tank and blow a jet of water into the gravel. (Water brought in through the spiracle is normally expelled via the gills, but can also be expelled through the mouth; see photo to the right.) This makes a pretty loud noise (audible in the next room) and I’m convinced it’s her way of saying that she’s ready to be fed; she does it more often when people are nearby and when it is getting close to feeding time. My guess is that this is a modification of a behavior used in the wild for foraging on river bottoms. I’ve never had any worries about getting stung by Hippolyte while feeding her or working in the tank. She has never shown any aggressive or defensive tendencies and from what I’ve heard, people generally only get stung by stingrays when they accidentally step on them. Getting stung is certainly something I can live without, especially given the wicked architecture of the stinger. Hippolyte’s stinger is a long (4 cm or so), thin, flat structure made of calcified cartilage that has a series of sharp, posteriorly-directed barbs along its entire length (see photos below). When it is used by a stingray, the barbs help ensure that the stinger is left in the victim (like a bee’s stinger). Unlike a bee, Hippolyte actually has two stingers; a longer, functional one on top and a smaller one growing underneath. When the top one is used (or shed), the one underneath takes its place and a new one starts growing under that one. This permits a stingray to replace a stinger that has been left in the ankle of some hapless swimmer. Because a new stinger is always growing underneath the functional one, “unused” stingers are periodically shed. I often find Hippolyte’s shed stingers on the bottom of the tank, but often not until long after the stinger was shed. Because there’s a lot of variation between when she sheds her stinger and when I actually find it, it’s difficult for me to estimate how often she sheds her stinger. Since I’ve had Hippolyte, I’ve collected about a dozen of them, an average of one every eight months or so. |
  A
typical stinger from Hippolyte is a
long, flat, thin spine about 4 cm long (left). It's edges are
lined with numerous
barbs (center) that make if very difficult to remove when lodged in
skin
or bone. When the organ that forms the stinger is damaged,
the
resulting
stingers can take on a variety of distorted shapes (right photo); the
leftmost
stinger (in side view) has an unnatural curve, the two in the center
are
small and twisted, and the rightmost stinger actually includes one
stinger
that has grown through the center of another. The injury that
resulted
in these distorted stingers was a bite to Hippolyte's tail by an
arrowana.
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Over the years I’ve tried
adding a few other fishes to the tank, but only one has worked out: my
armored catfish. I once had a plecostomus
in the tank to help keep algae down, but despite supplementing its diet
with
pieces of lettuce, it started rasping areas on Hippolyte’s
skin; I gave
that
fish away. I got an arawana,
thinking that an fish that swims in the upper part of the water column
would
be a nice addition to the tank, but it attacked Hippolyte’s
tail on a
couple
occasions; the second time it happened, it damaged her tail such that
she
couldn’t grow a normal stinger for a couple years (see
photo).
The
arrowana was gone soon after that incident. Most recently, I
added
a South
American lungfish to the tank. It wasn’t
the brightest of
animals,
and had a tough time distinguishing between the shrimp and Hippolyte;
after
it took a couple bites out of Hippolyte’s disk, I moved it
into its own
tank. For a nice site on freshwater stingrays with links to other sites, check out Bud's Freshwater Stingrays. Robert Fenner's page is also excellent. |
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This
page was last updated on December 12, 2006.
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