Mexican Tetra (Astyanax mexicanus)

The vast variety of environments on Earth gives rise to an incredibly diverse array of species, all which have adapted to live in specific ecosystems. Today’s animal is an excellent example of how environments influence species’ characteristics, as different forms of the Mexican tetra are radically different, depending on where they live.

Mexican tetras are found in Mexico (what a surprise!), but also occur in Texas. They live in the Rio Grande and the Neueces and Pecos Rivers, as well as in caves in northeastern Mexico. They are freshwater fish that like warm waters with temperatures of 20 to 25 degrees Celsius. In winter these fish migrate, moving to find warmer waters.

Mexican tetras can get as large as twelve centimeters in length (terrifying, I know). They are fairly normal-looking fish, and are somewhat compressed laterally. They don’t come in any particularly flashy colours, being silver with reddish fins.


Mexican tetras, looking pretty normal and boring. Image credit: haplochromis via Wikipedia

So what is so exciting about this little fish? Why am I blogging about it? So far the most exciting thing about it is that lives in Mexico, which is a pretty awesome place, or so I’ve heard. Well, Mexican tetras have two distinct forms, a normal form and a blind cave form. Both of them are members of the same species, but they have one very important difference.

You see, it’s not very helpful to be able to see in caves, because there isn’t much light underground. So Mexican tetras that live in caves have lost their sight. Some populations do retain some sight, while other cave tetras are completely blind, and have even lost their eyes.


The blind form of the Mexican tetra – doesn’t it look creepy? Image credit: JohnstonDJ via Wikipedia

There are other differences between cave tetras and normal tetras. Cave dwellers have taste buds on their heads, which lets them smell better, and they can store four times as much fat in their bodies. As food sources in caves aren’t particularly reliable, extra storage helps these fish survive long term. Cave tetras also are albino, having lost all the pigmentation in their skin.

The result is a two very different animals: one fish that is perfectly normal and perfectly bland, and one fish that looks like it is some kind of freak from a horror movie. Don’t worry though, both forms of the species are still the same species and so they can breed and produce fertile offspring.

Because of the weird differences between surface-dwelling and cave-dwelling tetras, scientists use these guys as a model to study different kinds of evolution. They are also popular aquarium fish, especially in their blind form. I don’t know if I’d want a blind cave tetra in my house, I think they look really creepy. But that’s just me!

Cover image source

Red-Bellied Piranha (Pygocentrus nattereri)

I’ve written about some pretty fearsome fish on this blog; barracudas, goliath tigerfish, and electric eels are some that come to mind. Yet it seems I’ve neglected a fish with a reputation to put all the rest to shame. I think it’s about time I wrote about piranhas.

There are between 30 and 60 species of piranhas, but I am going to focus on the red-bellied piranha, as they are the species that have the most fearsome reputation. Red-bellied piranhas are found in South America, in whitewater streams east of the Andes. They occur in every country in South America except for Suriname and Chile.

Red-bellied piranhas grow to be half a meter in length, and can weigh up to 4 kilograms. Adult piranhas have characteristic red colouring on their bellies, while young fish are simply silver with dark spots. They have strong jaws and nasty, sharp teeth.


The jawbone of a red-bellied piranha. Check out those teeth! Image credit: Sarefo via Wikipedia

While most people consider piranhas to be vicious flesh-tearing creatures, they are actually omnivorous scavengers. They mainly eat worms, insects, crustaceans and other fish. They will also consume debris, fish fins and scales, and plants. Young fish hunt during the day, while larger piranhas are active in the late afternoon and evening. Their primary method of feeding is ambush hunting, where they wait in vegetation and dark areas of the water to surprise a victim.

Though it is often thought that piranhas gather in large schools to attack big prey (a cow, for example), they actually do not hunt in groups. Red-bellied piranhas do form schools. But these are as a defence against predators, not for hunting. Stories of piranhas stripping a cow carcass in minutes are the exception — these types of events occur when the piranhas have been starved and food is very scarce. That being said, piranhas have attacked people, with some recorded fatalities. These occur when water levels are lower, and there is less food and piranha densities are higher.

Red-bellied piranhas use sounds to communicate with each other, using sonic muscles and their swim bladder to produce noises. They mainly use these sounds during aggressive interactions with one another. Low, drumming sounds are signals of mild or moderate attacks, while louder sounds are made during fierce attacks. So if you happen to hear a short, loud noise in a river in South America, get out of the water.


Look at the cute piranhas! Image source

Mating in piranhas coincides with rises in water levels and floods in the rivers they inhabit. Females lay thousands of eggs in a nest four to five centimetres in depth, near the bottom of water plants. Females and males will swim closely together in circles; this may be a courtship display but is more likely the two parents defending the nesting site. Fish eggs are pretty tasty meals, but I’m not sure I’d want to go after some guarded by piranhas. The eggs hatch after two to three days, and the young will remain hidden in the plants until they are large and fierce and able to fight off any hungry fish.

Red-bellied piranhas are not currently endangered or threatened, though capture for aquaria may put some pressure on the species. Yes, that’s right, people like to keep piranhas as pets. Personally I wouldn’t want a piranha in my house, but I guess some people like them.

Cover image credit: Gregory Moine via Wikipedia

Giant Freshwater Stingray (Himantura polylepis)

I’ve always thought of stingrays as saltwater animals, though I don’t have any particular reason to think this. But there are freshwater rays, and today’s animal also has the distinction of being the largest of the freshwater stingrays. It is unimaginatively called the giant freshwater stingray.

These animals are found rivers in southeast Asia; in Thailand, Malaysia, and Borneo. Giant freshwater rays tend to hang out at the bottom of rivers, and especially like muddy and sandy areas. They are sometimes found in estuaries, but there have been no reports of these stingrays in fully marine habitats.


A map of the distribution of the giant freshwater stingray. Image source: Wikipedia

As I mentioned, these stingrays get pretty big. Did I say pretty big? I meant huge. Giant, even. They can reach widths of almost two meters and lengths of five meters (though much of this is thanks to the rays’ long tails), and weigh in at 500-600 kg. The stingrays’ tails are equipped with a spine near the base. This spine measures over 35 cm long, making it the largest spine found on any stingray. Giant freshwater rays are coloured perfectly for a life on muddy river bottoms, being brown on top and light on the bottom.

Giant freshwater stingrays mainly rely on electric signals to sense and catch their prey, which mainly consist of small, bottom-dwelling fish and invertebrates, such as crustaceans and molluscs. The stingrays themselves have very few predators, thanks to their massive size. Even small stingrays are hard to hunt, thanks to their colouring and that stinger on their tails, which is covered in a toxic mucus.


A giant freshwater stingray in captivity. They tend to do quite poorly in aquaria, because it is difficult to provide them with adequate food and space. Image source

Not a lot is known about reproduction in giant freshwater stingrays. It is believed that the rays find mates using the same electrical sensory organs that help them hunt. Once females have mated, they head into estuaries, which act as nursing grounds for the rays. They give birth to one to four live young, and care for them until they are ready to move into freshwater areas.

As is the case with most giant, interesting species, the freshwater stingray has suffered from population declines in recent years. They have been hunted for meat, aquaria and sport, and also are affected by habitat degradation. The giant freshwater ray is currently considered endangered, and thus far conservation programs have proved ineffective.

I have a mild fear of swimming in oceans, because I have an active imagination and can picture all the giant creatures swimming beneath me. I’ve always felt quite a bit safer in rivers and lakes, but after writing about some terrifyingly large freshwater critters, I’m not so sure. Maybe I should just avoid swimming altogether. Can you imagine one of these guys swimming up beside you as you take a leisurely dip in the Mekong? I think I would faint.

Nile Monitor (Varanus niloticus)

There are some pretty terrifying reptiles out there. I’m not really scared of snakes, though some of the more venomous ones would definitely be unpleasant to meet. Crocodiles and Komodo dragons are some I would avoid. And now I get to add another animal to the list: the Nile monitor.

Nile monitors are found along the Nile River, as well as in most areas in sub-Saharan Africa. They live in many different habitats, such as forests, swamps, savanna and scrubland. There are two important habitat features that Nile monitors need: proximity to water and an open area to bask in. Other than that, these guys are pretty adaptable, being excellent climbers and swimmers.


Nile monitors spend much of their time basking, which sounds like a nice way to make a living. Image source:

Nile monitors are big lizards, and are one of the biggest lizards in Africa. They can grow up to 2.4 m long, and weigh 20 kg, though most are smaller than that. They have well-muscled legs, strong jaws, giant sharp claws, and are just generally built to be badass. Nile monitors are usually dark brown or grey, with yellow and black patterns all over their bodies which can be quite varied.

They prey Nile monitors consume is varied and impressive. Their strong jaws can administer a powerful bite, and are capable of crushing mollusc shells and breaking the necks of small vertebrates. They will eat amphibians, birds, small mammals, lizards, turtles, crocodile eggs and young, and a wide variety of invertebrates. Nile monitors have been observed working together to raid crocodile nests, with one monitor distracting the mother crocodile and the other stealing the eggs.

The combination of a Nile monitor’s strength, size, and speed make them a formidable enemy for any animal. There are, however, some brave creatures that will take monitors on. Pythons are one of these, and it has been reported that a 4.5 m long python fought and ate a 1.4 m Nile monitor, which is pretty impressive. Crocodiles are another occasional predator of Nile monitors, probably because they are getting revenge for their eaten eggs.

The monitor reproductive season begins in June and runs through to October. During this time, the testes of male monitors enlarge, which prepares them for mating. Male Nile monitors are also quite violent, engaging in vicious tussles to prove themselves worthy to mate. Females either lay their eggs in holes they dig or in termite mounds. The eggs can take up to a year to hatch, and often have to wait until it rains so their nest is soft enough for them to burrow out. Occasionally the hatchlings’ mother will come back and dig them out of the nest, but other than that she offers her young no assistance.

Despite their size and ferocity (or perhaps because of it), Nile monitors are reasonably popular as pets. I certainly wouldn’t want one of these beasts in my house, and neither should you.

Cover image source:

Hairy Frog (Trichobatrachus robustus)

When I stumbled across this frog, I knew right away that I had to blog about it. There’s just no way a frog this strange could be left out of this blog. So without further ado, let’s talk about the hairy frog, which is also known as the horror frog, or wolverine frog. Why this is will soon become clear.

Hairy frogs are found in southwest Africa, from Cameroon to Angola. Like most frogs, they love water, and are generally found near fast-flowing rivers. These can be located in habitats ranging from tropical rainforests to cultivated lands, such as tea plantations.

A preserved hairy frog specimen, showing the male's hair-like projections.  Image source: Wikipedia

A preserved hairy frog specimen, showing the male’s hair-like projections.
Image source: Wikipedia

Hairy frogs are reasonably large, reaching 11 cm in length, with males being much larger than females. They are brown in colour, with irregular black spots on their bodies. One of the strangest features of hairy frogs is their feet, which have ‘claws’.

These claws are where the ‘horror’ and ‘wolverine’ names come from. You see, the claws don’t always stick out of the frogs’ feet. For the most part, the claws are simply bumps at the tip of the frogs’ fingers. When they are threatened, hairy frogs will force these bones through their skin, by breaking their own toes. Once the threat has passed, the claws retract, and the tissues eventually heal. Just like Wolverine. But much less attractive.

The ‘hairy’ part of the frog’s name comes from strange, hair-like papillae that grow on the sides of breeding males. These are thought to increase oxygen absorption, something the males need while they guard their eggs underwater. These ‘hairs’ have the added function of making the frogs look very very odd, which always scores points on this blog.

Thankfully the hairy frog has not yet become endangered, though it is threatened by habitat loss. I’m glad something with its ridiculous appearance and badass way of defending itself is still around in abundance for us to admire.

Cover image source:

False Map Turtles (Graptemys pseudogeographica)

I like to think of false map turtles as animals that have maps on their shells, but wildly inaccurate ones that throw people off when they look at them. That would be the best kind of turtle ever.

Instead, false map turtles are actually part of the genus Graptemys, which is comprised of the map turtles. Map turtles are so called because of the lines on their shells, that connect to look like a road map. But why the ‘false’ is added before the one species’s name, I do not know. It’s even in the scientific name: pseudogeographica. My best guess is that when they were originally classified, false map turtles were thought to resemble map turtles but not actually be a part of the genus. If any of you know the reason, please feel free to enlighten me!

They are pretty interesting looking - just look at that skin colour!  Image source:

They are pretty interesting looking – just look at that skin colour!
Image source:

In any case, false map turtles can be found in the US, in the Mississippi and Missouri Rivers. They tend to be found in slow moving waters that have nice warm rocks or logs to bask on. They also like areas with lots of aquatic plant life.

False map turtles grow up to 27 cm in length, with females being quite a bit larger than males. Their shells are dark coloured, usually olive, black or brown, with yellow lines on them. It’s these lines that form the ‘map’ on the shell (which is almost certainly inaccurate). A notable feature of map turtles is the keel that runs along the length of the carapace.

Mating in false map turtles is kind of adorable — once the female sends out olfactory signals from her anal vent (okay that’s not the cute part), a male will come find her. To entice her to mate, he then strokes her head and neck in a loving manner. If she doesn’t move away, he proceeds to drum her over the eyes (presumably lightly). If she remains still, the pair will mate.

Females lay eggs on beaches, often waiting with a group of other ladies for conditions to be just right. They will digs holes 10-16cm deep and deposit 8-22 eggs into the nest. Map turtles mate two or three times a year, probably because they like that face stroking so much.

Basking is a favourite activity of false map turtles. They find a comfortable rock or stump to lie on and then stretch out their legs, spread their toes, and lit their heads up. This lets birds known as grackles remove leeches that have attached themselves to the turtles. While basking, map turtles feel particularly vulnerable, and so quickly leap in the water when approached. Any nearby turtles will also escape to the depths, after all, you wouldn’t want to be that one turtle that failed to get in the water and so got eaten or something.

Because they are fairly docile, false map turtles are quite popular in the pet trade. Despite pressures from this, habitat destruction and pollution, these guys seem to be doing quite well. It’s probably because people trying to find them get lost.

Cover image source: