Candiru (Vandellia cirrhosa)

There are some nasty things that lurk in bodies of water. Whether you’re in a lake, river, or ocean, there’s always something you should look out for, always something swimming quietly in the depths to make you just that little bit nervous while you’re paddling around. Today’s animal, the candiru, is a particularly nasty critter.

Luckily, candirus are only found in the Amazon River Basin, so you don’t have to worry about them unless you’re swimming in the Amazon. Candirus have been found in Bolivia, Brazil, Colombia, Ecuador and Peru. They like slow moving, shallow areas, particularly those with muddy bottoms.

Candirus are quite small, only growing to maximum lengths of 17cm. They are not very noticeable fish, as they are translucent except after feeding. They are long and thin, with large eyes on top of their heads. The most interesting part about candirus is the backwards-facing spines that project from their gill covers. These have quite a sinister purpose, I assure you.


It used to be thought that candirus were attracted to urea released from their hosts’ gills, but this is untrue. They hunt using visual cues, hence the large eyes. Image source

You see, candirus have a taste for blood, and the way they get it is definitely creepy. They lodge themselves into a fish’s gills, where their spines pierce the thin gill membranes, letting blood flow right into the candirus’ mouths. It used to be thought that Candirus were attracted to urea released from their hosts’ gills, but this is untrue; they hunt using visual cues, The spines also keep the candirus stuck under their hosts’ gills; the backwards-facing nature of the spines means these parasites are incredibly difficult to dislodge.

Sounds pretty nasty, right? Well that’s not the worst thing about candirus, according to prevalent rumours. You see, it’s said that sometimes when people are urinating in the river, a candiru gets a little confused and swims right into a person’s urethra. And because of those spines, they are extremely hard to remove. Not pleasant experience I’m sure.


Some recently fed candirus. Image source

So yes, candirus are horrible fish. But the accounts of candirus swimming into peoples’ urethras are rare, and are not terribly reliable. The only modern case occurred in 1997, and has a few inconsistent details that makes it unlikely that a candiru did actually lodge itself in a man’s urethra. Still, I’m not going to be the one to go in the Amazon to test whether or not the tales are true.

Common Hatchetfish (Gasteropelecus sternicla)

Fish come in all shapes and sizes, from pretty standard-looking minnows to ocean sunfish, or electric eels. Still, every so often I stumble across a fish that is so strange looking I have to do a double-take. Today’s animal, the common hatchetfish, is one of those fish.

Common hatchetfish are freshwater fish found in the Amazon River and its tributaries. They prefer to stay near the surface of the river, and generally hide in vegetation by the rivers’ banks. Hatchetfish will school, and only when they have the protection of their friends will they venture into open river waters.


Look how strange he looks! Image source:

Hatchetfish don’t get to be very big, reaching maximum lengths of 6.5 cm. They have extremely strangely shaped bodies, that are laterally compressed with a huge abdomen. This area houses the hatchetfish’s pectoral muscles, which can make up 25% of the fish’s body weight.


Hatchetfish are quite social, and prefer to be with other hatchetfish. Image source:

These muscles are used to power the hatchetfish’s wing-like pectoral fins. When hatchetfish are in danger, they will use these fins to jump out of the water, and ‘fly’ for a few meters before dropping back into the water. Hatchetfish will also spring from the water to catch flying insects, though they will eat insects that fall into the river as well. Hatchetfish are unique in that they actually use their fins to aid in flight, flapping them like birds’ wings.

Likely due to their funny shape, hatchetfish are popular aquarium fish. They do fairly well in captivity, but care must be taken to have a well-secured lid on the tank, as the fish will jump out of the water when startled. I know fish leaping out of a tank would scare the hell out of me, so I definitely wouldn’t want to keep any hatchetfish around.

Giant Ameiva (Ameiva ameiva)

Lizards are fun animals. They’re like snakes, but better, because they have legs. I imagine when a snake looks at a lizard the snake gets insanely jealous. And I can’t help but think every time a lizard glances at a snake they smirk at the poor legless creature.

Today’s animal is a lizard that belongs to the whiptail family, Teiidae. Giant ameivas live in Central and South America, and have been introduced to Florida. They are found in a range of habitats, but prefer rain forests that have recently been disturbed.

A male ameiva. He's got some pretty nice camouflage going on there.  Image credit: D. Gordon E. Robertson via Wikipedia

A male ameiva. He’s got some pretty nice camouflage going on there.
Image credit: D. Gordon E. Robertson via Wikipedia

Despite what their name might make you think, giant ameivas don’t get super big. Though I guess giant is a relative term, so maybe an ant named them. Giant ameivas grow to be a maximum length of 16 cm in females and 18 cm in males. Males are mostly bright green, with a brown head and light underbelly. Females are less vibrantly coloured than males.

Reproduction in giant ameivas depends on rainfall. In areas where there is a wet and dry season, the lizards will breed in the rainy season. When there is near constant rain, however, ameivas are able to breed year round. The female lays anywhere from three to eleven eggs,  which hatch in five months. Male ameivas tend to grow faster than females, and the young become mature around eight months of age. Neither parent bothers to look after the eggs once they’re laid.

Giant ameivas have pores on the underside of their hind legs. These pores are used primarily for communication. They secrete chemicals which are signals to other ameivas, and are used in territorial marking and defence, predation, and sexual behaviours. It’s a strange way to communicate, but I guess it works for the ameivas.

An immature male, skulking in some foliage.  Image credit: Dario Sanches via Wikipedia

An immature male, skulking in some foliage.
Image credit: Dario Sanches via Wikipedia

Giant ameivas mainly subsist on insects, though as they get larger their diet starts to include larger animals, such as other lizards. Ameivas are preyed upon by birds and snakes (so I guess the snakes have the last laugh after all). They don’t have any special defences against predation, except for their speed. Their bodies are designed to move quickly, and they rely on this agility to escape their numerous predators. This is why an alternative (and more accurate) name for the giant ameiva is the Amazon racerunner.

I don’t know if anyone has ever held an ameiva race, but people do keep them as pets. They aren’t a great species to have though: they tend to be aggressive and are wonderful carries of salmonella. Still, they are brightly coloured, so people want to have them. I for one would rather not get bitten and contract nasty diseases, but then I’ve never really understood the draw of exotic pets.

Electric Eel (Electrophorus electricus)

Sometimes when I find an animal to blog about I’m surprised I haven’t written about it before. This usually leads to me frantically searching through my former posts to double- and triple-check that I have not, in fact, blogged about said animal before. Today’s animal is one of the those, the electric eel. How can I not have written about such a strange and wonderful creature yet?

Electric eels aren’t actually eels — they are knifefish (I have written about one of these, the Black Ghost Knifefish (Apteronotus albifrons)). They are so weird that they are placed in their own genus, Electrophorus. Electric eels live in fresh water, and can be found in South America, particularly in the Amazon and Orinoco Rivers. They prefer to hang out at the bottoms of rivers and swamps, and the muddier the water, the better. They like areas with lots of shade for protection, and can survive in waters with low oxygen thanks to their air breathing capabilities.

An electric eel hanging out in some foliage. Image source:

An electric eel hanging out in some foliage.
Image source: Wikipedia

The reason electric eels are commonly thought to be eels is pretty obvious. They look almost exactly like an eel, being long and thin without the fins most fish wear so proudly. Electric eels can grow up to 2.5 metres in length, and the first twenty percent of the fish contains all of the eel’s vital organs. Though they have gills, electric eels cannot get all their oxygen from the water, and so must surface to breathe. They get about 80% of their oxygen from air breathing.

The remaining length of the eel’s body is dedicated to producing electricity. These guys are kings of electricity, with three different organs that create electricity. Using these, the eels can make both high and low voltage discharges. Electric eels can produce a shock of up to 600 volts and 1 ampere of current. Though the shocks are painful to people (and can be felt from a ways away, thanks to the conducting power of water), they are not enough to kill an adult human, thanks to the short duration of the shocks (2 milliseconds).

Electric eels are polarized - the front end is positively charges while the tail is negative. They're basically giant batteries. Image source:

Electric eels are polarized – the front end is positively charges while the tail is negative. They’re basically giant batteries.
Image source: 8559245ccc321c6cb77a2479a

Electric eels use their electric powers for a number of purposes. Of course, self-defence is a key purpose; I think it would take some guts to try and eat a fish that can shock you with 600 volts. The eels also use their electricity for hunting, with low voltage shocks from the Sach’s organ to electrolocate, and high voltage shocks to stun prey. Of course, the shocks could also hurt the eel producing them, so electric eels are covered in a slimy skin that protects them from being zapped.

Reproduction in electric eels occurs during the dry season. Electric signals are used by the fish to find mates, because why be called the electric eel if you aren’t going to use electricity for absolutely everything? The female lays her eggs in a nest of saliva produced by the male. Once the eggs are laid, the male guards them against any potential predators.

I’ve always thought that electricity is super cool — if I had a superpower it would definitely be control over electricity. The fact that a fish pretty much already has this superpower makes me more than a little jealous. Maybe in my next life I will be reincarnated as an electric eel.

Cover image source:

Black Ghost Knifefish (Apteronotus albifrons)

Is there another species of fish that has a more badass name than the black ghost knifefish? I doubt it. The combination of ‘black’, ‘ghost’, and ‘knifefish’ just make me think of some kind of underwater ninja. Which, in a way, the black ghost knifefish almost is. At least, they’re sneaky and hang around in the dark all the time. That’s what ninja do, right?

Black ghost knifefish are found in South America, from Venezuela to Paraguay. These are freshwater fish, and prefer waters that are fast moving, with sandy bottoms. Knifefish like areas with lots of vegetation and good places to hide.

Black ghosts adopt this slanted posture when searching for prey. It looks silly.  Image source:

Black ghosts adopt this slanted posture when searching for prey. It looks silly.
Image source:

Black ghost knifefish are black, with two white markings on their tails. Some fish have a white stripe that runs from the tip of the snout along the back. They are long and slender (hence the ‘knife’ part of their name), and can grow to a maximum of 50cm in length. Knifefish have one long fin on their undersides, and use this fin to move, waving it back and forth to propel themselves around.

To act more like ninja, black ghosts are only active at night. However, it’s pretty difficult to navigate streams that are thick with vegetation in the middle of the night, so black ghost knifefish have developed some senses to help them get around. Three particular systems are useful for this night-loving fish: active electrolocation, passive electrolocation, and lateral line sensing (found in most fish).

Active electrolocation involves the fish emitting an weak electrical signal, which allows the black ghost to locate objects from the disturbances in the signal. Passive electrolocation lets the fish sense electric signals from other sources, such as another knifefish. The fish have two different receptor organs for each of these senses. Finally, the fish’s lateral line detects changing movements and vibrations in the water.

The electric signals generated by knifefish aren’t just used for navigation; after all, why develop such a cool ability if you aren’t going to use it for more than one thing? Knifefish also use electricity to communicate — and these communications can be quite complex. Each species of knifefish (yes, there is more than one) uses a different frequency of signal, and within species signals vary with gender and age. Female black ghost fish have higher frequency signals, while subordinate fish gradually lose signal frequency when in the presence of dominant fish.

Black ghost knifefish are quite common as pets - probably because they look so cool.  Image credit: Derek Ramsey via Wikipedia

Black ghost knifefish are quite common as pets – probably because they look so cool.
Image credit: Derek Ramsey via Wikipedia

So though they may not be actual ninja, there’s no denying that black ghost knifefish are super cool. After all, they talk via electricity. Oh, and the ghost part of their name originates from the belief that the souls of the dead hang out in these fish. Maybe they try and communicate with us via their electrical signals. Spooky stuff.

Cover image credit: Erik Sviland

Mata Mata (Chelus fimbriata)

Most of the time I think turtles are pretty cute, but in the world of animals there are exceptions to every rule. The mata mata definitely falls into this category. I’ve never seen a funnier looking turtle.

The mata mata is so strange that it is placed in its own genus, Chelus. It is found in South America, in the Amazon and Orinoco river basins, as well as on the island of Trinidad. The mata mata lives entirely in water, usually in slow moving and brackish waters. Any disgusting looking pond will do, especially if the pond has a lovely muddy bottom.

As I’ve said, the mata mata is one odd looking turtle. Though it may look absolutely ridiculous, the mata mata is actually well designed to hide in its habitat. Its head is flat and triangular and covered with tubercles and barbels that make it look like a piece of debris. These projections, along with two flaps of skin on the turtle’s head, are thought to be involved in detecting water movements. The shell of the turtle can reach up to 45 cm, and has three ridges along it. Often algae will grow on the mata mata’s shell, further helping it camouflage in the water.

You can see how they might be good at hiding, especially in brackish waters with lots of debris.  Photo credit: Barbara L. Lundrigan via Animal Diversity Web

You can see how they might be good at hiding, especially in brackish waters with lots of debris.
Photo credit: Barbara L. Lundrigan via Animal Diversity Web

Mata matas are terrible swimmers, and prefer to walk on the bottoms of the waters they inhabit. Because of this, mata matas usually like to live in shallow waters, where they can reach the surface with their snout without needing to swim. Their laziness also extends to feeding – they sit camouflaged in the water until a fish comes close, and then open their mouths as wide as they can. This creates a vacuum, which sucks the fish into the mata mata’s mouth. All the turtle has to do then is snap its mouth closed and it has a tasty meal. Not too shabby if you ask me.

It definitely has a funny face.  Source: Wikipedia

It definitely has a funny face.
Source: Wikipedia

As with any funny looking reptile, the mata mata is highly desired in the pet trade. Fortunately this hasn’t had a huge impact on the species’ survival, as it is still common in its native range. Still, with the rate of habitat loss in the Amazon and Orinoco basins, we should probably start thinking about conserving these species before there’s a problem. But when have we ever been proactive when it comes to saving species? Anyway, hopefully these guys stay around for a while, because they are really great to look at.

Cover image credit: Joachim S. Müller