Zebrafish (Danio rerio)

Usually I pick animals for this blog because they look funny, or they have a silly name, or because there’s something really super special awesome about them. Today’s animal is a bit different, because although it is quite an incredible creature on its own, many of the most amazing things about it are the results of human experimentation.

Zebrafish are found in parts of India, Pakistan, Bangladesh, Myanmar and Nepal. They have also been introduced to some areas of the US and Colombia, inadvertently and on purpose. They are freshwater fish, and are generally found in slow-moving, shallow waters. They can live in rivers, streams, floodplains, and rice fields.


A wild-type zebrafish. Image source

Human disturbance is detrimental to many species of animals, but there are always a few that can cope or even benefit from human-changed habitats. Zebrafish are one of these lucky species, as they don’t seem to mind areas that have been altered because of rice cultivation. Growing rice often means waterways are dammed and irrigation systems are created, and zebrafish can be found in both of these altered ecosystems.

Zebrafish are quite small, innocuous fish. They can reach massive lengths of 6.4 cm, though sizes are more commonly a much more reasonable 2.5 cm. Zebrafish get their name from the stripes that run down their sides; each fish has five to seven blue stripes. Male zebrafish have gold colouring between their blue stripes, while females are silver-coloured.


I decided to try something a bit different for this post. Not sure how I feel about the fish, but I like the way the background turned out! 

Breeding in zebrafish occurs during the monsoon season, from April to August. The fish get pretty excited about the chance to mate, rising at dawn to start courting one another. Male zebrafish follow a female around, each of them trying to lead her to a spawning site. They do this by nudging her and swimming in circles around her, which must get extremely annoying. Once a pair has reached an appropriate site, they line up their genital pores and the female releases her eggs, and the male releases his sperm.

The fertilized eggs hatch after two to three days, with all hatchlings being female. Differentiation between the two genders starts to occur at five to seven weeks of age, though males need about three months for their testes to develop completely. What causes fish to become female or male is not yet known, though it is thought that food supply and growth rates influence gender. Slow-growing zebrafish grow up to be males, while faster growing ones become females.

Zebrafish, while seemingly simple and nondescript fish, have incredible regenerative powers. While they are still larvae, zebrafish can grow back their fins, heart, brains, and retinas. These abilities have been the subject of intense research, with possible applications in human medicine being explored.

Regeneration isn’t the only trait that makes zebrafish useful research subjects. They are hardy fish, with short lifespans and large clutch sizes, making them ideal for genetic studies. They were one of the first vertebrates to be cloned, and many mutated strains of zebrafish have been created. Among the more bizarre is a strain of transparent zebrafish that glows when the brain is undergoing strong activity, and a zebrafish that turns green in waters polluted by oestrogen.


Some GloFish… look how many colours they come in! Image source

Zebrafish aren’t just popular research animals — they are also extremely common in aquaria, especially since they come in many colours. They have even made florescent zebrafish, because why would you want a natural looking fish when you can have a GloFish®? And yes, they are actually called that.

Cover image source: Azul via Wikipedia

Saltwater Crocodile (Crocodylus porosus)

During the past two weeks I watched all four Jurassic Park movies, so I’m in a bit of a dinosaur mood. Unfortunately, Our Wild World is a blog about extant animals, so dinosaurs are right out. I’ve decided instead to write about a species of crocodile, because they’re basically the next best thing.

The species I’m going to focus on is the saltwater crocodile, because they are super cool. Saltwater crocodiles have a broad range, and are found from eastern India to Indonesia, Australia, and even around some Pacific islands, like Vanuatu and the Solomon Islands. As their name implies, saltwater crocs can tolerate high levels of salinity, and are often found in rivers, estuaries and coastal areas. More than any other species of crocodile, saltwater crocodiles are found in the ocean; they often travel long distances in the open ocean and can spend months at sea.


The range of the saltwater crocodile. Image source: Wikipedia 

Saltwater crocodiles are big. You definitely don’t want to meet one of them in a murky river. In fact, saltwater crocodiles are the biggest of all crocodiles and are the biggest reptiles in the world. Males can reach lengths of six to seven meters, though females only grow to a paltry three meters. The largest male ever recorded weighed 1,075 kg. Adults are a dark greenish colour, with lighter bellies. Young crocodiles have more exciting colouring, being yellow with stripes and spots on them.

Crocodiles are big (as I mentioned), and they have large, pointy teeth. Saltwater crocodiles have between 64 and 68 teeth, the longest of which can measure up to nine centimetres in length. They use these massive teeth to hunt, feeding on a wide variety of animals such as fish, turtles, snakes, buffalo, birds, wild boars and monkeys. Crocodiles are ambush predators, hiding below the water’s surface with only their backs, nostrils and eyes visible, until an unlucky victim stumbles along.


A very fat looking saltwater crocodile. Image credit: fvanrenterghem via Wikipedia

When a prey animal does happen to wander too close to a saltwater crocodile, the crocodile strikes. They are surprisingly fast when striking from the water, using both feet and their tails to launch themselves at their prey. They can swim in bursts of 24 to 29 km/hr, so you really don’t want to be stuck in the water with a saltwater crocodile.

Once the crocodiles have an animal in their strong jaws, it is either swallowed whole, or, if it is too large, the crocs drag their prey underwater and drown it. Crocodile teeth are not made for shearing, so crocodiles rip chunks of meat off their prey by rolling in the water while gripping the prey to twist off pieces of flesh, or by jerking their heads to remove hunks of tasty meat.

If you do end up getting bitten by a saltwater crocodile, good luck getting away; saltwater crocodiles have the highest bite force of any animal, measuring a maximum of 16,414 N (which I’m guessing is a lot). Part of the bite strength of saltwater crocodiles comes from the design of their jaw muscles; they can clamp down extremely hard, but have weak muscles when it comes to opening their jaws. Apparently a few layers of duct tape is sufficient to hold a crocodile’s mouth shut. The tricky part, of course, is getting the duct tape on the crocodile in the first place.


Saltwater crocs can breach out of the water to try and catch food, so don’t think you’re safe just because you’re in a tree. Image credit: Matt via Wikipedia

Saltwater crocodiles breed during the wet season, from September to October. Though they often live in saltwater, saltwater crocodiles move to fresh water to breed. Males are very territorial in general, but are especially aggressive during the breeding season, chasing away any other males that encroach on their territory.

Female saltwater crocodiles lay between 40 and 90 eggs in mounds placed on river banks and shores. The eggs are laid raised from the ground, to prevent them from being washed away during floods. The eggs hatch after around three months, at which point calls from the young prompt the mother to help unearth the eggs. She then carries the hatchlings in her mouth to the water, and stays with her brood for a few months. Very few survive to adulthood, and those that do disperse at eight months of age. Sexual maturity is reached when crocodiles are 10 to 16 years old, and these remarkable reptiles can live to be over 70 years of age.

Because saltwater crocodiles are highly valued for their meat, eggs, and skin, this species was once hunted extensively. They have since come under protection in most of their range, and have made great recoveries. Thankfully they are not currently endangered or threatened, but habitat destruction is a concern for these magnificent beasts.


My drawing for the week (I apologize for the poor quality of the image – I’m away and do not have access to my scanner) – this is all you would see of a crocodile hiding in the water. 

I’d like to say a few last things about the saltwater crocodile. First, they are big, with large teeth, and are territorial and aggressive. So yes, they can and do eat people who come into their waters, so watch out. And secondly, even though these guys are giant man-eaters, you shouldn’t hate them, because not only are they very cool, they are also supposed to be very intelligent. They have extensive means of communication, can learn tasks quite quickly, and track the migratory patterns of their prey. So don’t hate saltwater crocodiles, and definitely don’t swim with them.

Cover image credit: Djambalawa via Wikipedia

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

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: http://saltytank.fuzzybox.co.uk/fishdb/index.php?ID=400

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: http://www.euplectes.com/Aquarium/

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.

Goliath Tigerfish (Hydrocynus goliath)

There are some scary fish out there. Piranhas are pretty nasty, and I could do without running into electric eels. But I think after reading this post, you can probably agree that the goliath tigerfish is one of the scariest fish around. If I ever encountered this guy, I think I’d be scared away from water for life.

Luckily, goliath tigerfish only occur in a limited area of the world — the Congo River Basin and Lake Tanganyika in Africa. So as long as you don’t go swimming in waters in the Congo, you should be safe from these terrible creatures.


This one looks pretty tame, but wait until you see the inside of its mouth… Image credit: Loury Cedric, via Wikiepdia

Goliath tigerfish, as their name implies, are quite large creatures. They can grow to be five feet long, and weigh more than 50 kilograms. Specimens of over 60 kg have been rumoured to exist, but have not been authenticated. They are an inconspicuous silvery colour, and don’t look too scary at first glance.

But once these giants open their mouths, it’s easy to see why they have such a fearsome reputation. Goliath tigerfish have a mouthful of huge, sharp teeth that are no doubt very good at tearing flesh apart. They can have up to 32 teeth in their mouth, each up to an inch in length. Just one look at the tigerfish’s mouth is enough to give me nightmares.

Goliath tigerfish use these teeth to hunt pretty much anything that is smaller than them, including members of their own species. They prefer to hunt in fast-moving waters, where smaller fish have trouble swimming. The tigerfish then use their superior speed and strength to ambush their poor victim, their terrible teeth often slicing their prey in half. It’s pretty nasty stuff.

They have also been known to attack humans, probably because they thought the person was a tasty fish they could eat. Because it is large and terrifying, the goliath tigerfish is an extremely popular gamefish. It is notoriously difficult to catch, and has earned the titled of ‘greatest freshwater gamefish in the world’. I would probably give it the name ‘most nightmare-inducing horrible fish-but-mostly-just-teeth fish in the world’. And I would never, ever, try and catch one.

Four-eyed Fish (genus Anableps)

Yes, you read that right. Today’s animal is the family of fishes known as the four-eyed fish. I know this conjures images of fish wearing glasses (which is a hilarious thought), but these guys don’t wear glasses. Instead, they just have some very strange eye biology. Very strange.

There are only three species of four-eyed fish, the large-scale foureyes, the Pacific four-eyed fish, and the simply named foureyes. All species occur in a range spanning southern Mexico through Central America to the northern part of South America. The fish are found in brackish waters, usually in mangrove forests.

Four-eyed fish can grow to a maximum length of 32 cm, though they are usually smaller than that. They are pretty boring looking fish, being brown with a pale stripe running down each side of its body. But you didn’t click on this blog to read about what colour four-eyed fish are. You came here to find out about their eyes.

See how strange its eyes are? Image source: Wikipedia

See how strange its eyes are?
Image source: Wikipedia

So let’s get to it. Four-eyed fish don’t actually have four eyes, but what they do have is a pair of eyes that is split in two. Each eye has an above-water portion, and a below-water portion, with each having its own pupil. The top half of the eye is designed for seeing above water, while the lower half is best at seeing under water, with a thicker lens on the bottom to adjust for the different refractive index under water. Four-eyed fish sit just at the surface of the water, the top half of their eyes sticking into the air.

Another strange attribute of four-eyed fish is their reproductive behaviour. Both males and females have a ‘handedness’, a side on which they will mate. A right-handed male will only mate with a left-handed female, and vice versa. Four-eyed fish give birth to live young, keeping the eggs inside of them until they hatch.

An even weirder view of a four-eyed fish. Image source: http://www.practicalfishkeeping.co.uk/content.php?sid=4009

An even weirder view of a four-eyed fish.
Image source: http://www.practicalfishkeeping.co.uk/content.php?sid=4009

So although they don’t actually have four eyes, I think the four-eyed fishes definitely are right up there as some of the coolest fish around. After all, how many animals actually have split eyes?

Cover image source: https://quantumbiologist.wordpress.com/2011/04/26/four-eyes/