Liberian Tree-Hole Crab (Globonautes macropus)

A freshwater crab that lives in a tree!?

Liberian tree-hole crab (Globonautes macropus)

Liberian tree-hole crab (Globonautes macropus)

What is it?

When we think of freshwater biodiversity we naturally think of places such as rivers, streams, ponds and lakes. But not all freshwater animals live in these habitats. The Liberian tree-hole crab is an amazing species of freshwater crab that lives in the closed canopy rain forests of West Africa. Yep, it’s a freshwater crab that lives in a forest. Living far from permanent freshwater sources such as rivers or lakes, these crabs live in water-filled holes in trees to survive. At night they emerge from their homes in the trees and make their way down to the forest floor to forage for food, mostly small insects. Once they’re finished with their evening meal, they climb back up the tree where they spend their days.

Map of West Africa

Map of West Africa

Range and Habitat

This unique species was first documented in 1898 in Liberia (hence the name), but was not documented until 90 years later in 1988. It is one of only five species that belong to a rare group of freshwater crabs that are endemic to West Africa. Since 1988 sightings have increased but been restricted to small parts of the upper Guinea forest in western Liberia and Guinea and it is also thought their range extends to parts of Sierra Leone’s forest which lie between these two populations. The crab populations are solely restricted to rainforest in areas with rainwater filled natural holes found in suitably sized trees at a height of between 1 to 2 meters above the ground and they are known to exist in just five different locations across these regions. For an interactive map of the tree-hole crab’s range click here.

The Upper Guinea Forest extends from Guinea into Sierra Leone and eastward through Liberia, Côte d’Ivoire and Ghana into western Togo.

The Upper Guinea Forest extends from Guinea into Sierra Leone and eastward through Liberia, Côte d’Ivoire and Ghana into western Togo.


As such these tree-climbing crabs are extremely rare and can be hard to spot. The number of tree-hole crabs that still exist is uncertain due to a lack of information, however, it was estimated that before the civil war broke out in Liberia in 1989 there were between 5-10 per km² of closed-canopy rain forest. It is expected that this number has decreased since this time and the species is listed as endangered on the IUCN Red List. Current estimates put the number at fewer than 2500 mature individuals.

It is thought that the population numbers have been declining across its range due largely to habitat loss and deforestation related to years of civil war and political instability in the region. Due to the specific habitat of these cool crustaceans they are very vulnerable to habitat loss. Being a freshwater crab that relies on rainwater captured in tree holes, even the felling of a small number of appropriately hollowed trees in a particular area may threaten local populations. Other threats that these curious critters face include an increase in agriculture, mining and firewood collection, which again contribute to habitat loss. Given these threats, the tree-hole crab’s specialised habitat and small population size, it is of serious concern that this species is not found within a protected area, nor are there any conservation measures in place to protect this unique animal’s existence. Hopefully more research into this incredible creature will raise awareness of its plight and spur on effects to protect it.

Further information:

IUCN Red List of Threatened Species

Fantastic Fire Salamanders – Salamandra salamandra

Slimy and sublime, but do they really “live in fire”?

Fire salamander head shot with view of parotoid glands. Photo by Didier Descouens.

Salamandra salamandra

The name European (or fire) salamander covers a number of salamander subspecies generally found in the mountain forests of western, central and southern Europe, although some populations can be found in North Africa and the Middle East.  There are 13 subspecies; all varying in colour, behaviour and adaptations, but generally fire salamanders are sturdy looking amphibians with a variety of yellow to orange markings on their back. Adults can grow up to a foot long and have been known to live for as long as 50 years in captivity, but ages of 30+ have been recorded in the wild!

Most of these secretive salamanders inhabit moist woodlands where they like to hide under rocks and logs and dig into the leaf litter. Although the adults are terrible swimmers they are never far from the freshwater streams and small pools in which they begin their lives.  Fire salamanders are most active at night, but can be found out and about on damp, overcast days; any time that you are likely to find slugs, worms and other goodies creeping about for hungry amphibians to snack on!  Salamanders catch their prey by sneaking up on an unsuspecting bug and firing their super-sticky tongue out… Gulp! Look at the video below for some fire salamander hunting behaviour.

Water babies

Like all amphibians, fire salamanders spend part of their lives in water. As adults they are poor swimmers, but their larvae need to spend the first 3 months in water (breathing through gills) before metamorphosing into tiny brightly coloured adults and leaving their aquatic birthplace.

Mating between male and female fire salamanders usually takes place in the cooler months, before the winter hibernation.  After blocking the female’s path and rubbing her with his chin the male plants a sticky spermatophore onto the ground and then sala-manhandles (sorry) the female until her cloaca is positioned over it. After fertilization the eggs develop internally.  When the eggs are ready to hatch into larvae, often in the following spring, the female will lay them directly into water where they will immediately hatch.  This process, called ovoviviparity, is common in many aquatic vertebrates, but a few sub-species of Fire salamander also give birth to live young – viviparity!

Creatures of habit, most fire salamanders return to the same cave, crevice or log every day and generally stick to foraging from this location. Some individuals have even been recorded using the same hibernation place for over 20 years! The actual distances that an individual salamander can range for food is quite large, on average around 500m².

Toxic to the touch

An animal which is fairly sluggish and confined to the ground, the fire salamander is vulnerable to predation from other vertebrates, but has a cunning adaptation to discourage being eaten. Upon closer inspection, the upper dermis of all subspecies of fire salamander is covered in small glands which secrete both protective mucus and a powerful neurotoxin.

The compound samadarine isolated from fire salamander skin secretion. Image from Wikipedia.

Two major alkaloids, samandarine and samandarone have been isolated from these skin secretions. These compounds are skin irritants and also disrupt the vertebrate nervous system causing hyperventilation and convulsions. Some species are capable of actively squirting this poisonous cocktail from the parotoid glands just behind the head.  The yellow and black warning colouration helps predators to identify the potential prey as toxic.

Fire salamanders and people

Despite Salamandra coming from the name for a mythical fire lizard, and their “fiery” colouring, the fire salamander would not survive any type of extreme heat.  It’s thought that the common name for these amphibians originates due to their sudden appearance from logs that have been collected for firewood, which is probably the only time that people normally encountered them.

With 13 subspecies distributed over such a large area, people and salamanders are bound to come in to contact with each other. Although currently designated as a species of “least concern” by the IUCN, fire salamanders are at threat from habitat loss, pollution and climate change.  For a species so dependent on cool, wet conditions long-term changes in weather patterns could influence the range of current salamander populations.  Perhaps their major disadvantage lies in the fact that comparatively little is known about many of the subspecies.  Some, such as Salamandra salamandra terrestris, are now commonly kept as pets so much of what we know about their behaviour has been recorded from captive populations.  But those subspecies found in more remote areas are poorly understood and therefore difficult to protect. There is also a tendency to focus only on the aquatic stage in conservation research, but the adult population is just as vulnerable to habitat change.

Fancy seeing a fire salamander in the flesh?  Head to your nearest zoo or wildlife park, they make great exhibits so most places will have them! Check out the video below for some more fire salamander action in their natural habitat.

Arapaima – Freshwater Giants of South America

Ancient armored freshwater fish crushes prey with a toothed, bony tongue

Guest curator: Daniel Gurdak (SUNY-ESF)

Arapaima sp. from Guyana. Image: D.J. Stewart

Arapaima are the largest scaled freshwater fishes in the world. Known by several names, including pirarucu (Portuguese) and paiche (Spanish), they can grow to an amazing 3 m in length and weigh up to 200 kg! These tropical giants are naturally found in the rivers and floodplain lakes of Brazil, Columbia, Ecuador, Guyana, and Peru, but have been introduced to other parts of South America and around the world.

A little bit about a big fish

The groups of species found in the genus Arapaima are part of the family Osteoglossidae, an ancient group of fishes known as the “bony-tongues”.  These freshwater monsters have not changed much in the last 13 million years!  Large, powerful and covered with an armor of hard, overlapping scales, arapaima are well equipped to survive attacks from piranhas, crocodilians and even people. Arapaima are fearsome predators -prey are sucked in and crushed between their bony, toothed tongue and a bony plate on the roof of their mouth! (See the feeding video below).

Unlike most fish, arapaima need to come to the surface every 15-20 minutes to breath air. They have a weird swim bladder lined with blood vessels which works as a primitive lung. Indeed, if an adult arapaima can’t surface to breathe it will drown! Baby arapaima hatch with working gills but can only breathe under water for just over a week. In the tropics the ability to breathe air is an advantage. This is because the combination of slow moving water, high temperatures and decomposing plant material often deprive the water of dissolved oxygen.

About their ecology and behavior

Arapaima live mostly in lakes, quiet backwaters of large rivers and adjacent floodplains. The tropical floodplain is a unique ecosystem with high and low water seasons, it is neither “terrestrial” nor “aquatic”, but both and somewhere in between. Floodplain plants and animals in the Amazon are highly adapted to annual changes in water height.  For example, when water is low, fish can become concentrated in river channels and lakes. However, as waters rise (by more than 10 m in some areas), fish move into the floodplain and feast on newly available plants, fruits, and insects.

Many fish, including the arapaima, reproduce during the beginning of the high water season.  Arapaima breed along the edges of lakes and channels in flooded forests. These are no ordinary fish – they build nests by digging a hole using their mouths, sometimes brushing away nearby leaves and branches! What’s more, arapaima parents work together to protect their eggs and young throughout the flood season.

The perils of being a large, tasty fish

For people of the Amazon Arapaima are great eating. The meat has few bones, firm texture, large fillets and tastes delicious. Sometimes called the “cod-fish” of the Amazon, it can be cooked fresh or used later by freezing or salting and drying.

Traditionally, arapaima were captured by fishermen with a harpoon or a bow and arrow.  Skilled fishermen wait patiently, and strike quickly when the fish rises up to breathe. Commercial fishing for arapaima began in the early 1800’s, and since then, over-fishing, in combination with increasing habitat degradation, has caused sharp declines in arapaima populations across much of their range. The video below shows fishermen catching arapaima.

Fishermen catching arapaima in Brazil. Images: Rafael Sá Leitão Barboza.

Arapaima today

Today, arapaima are faced with continuing habitat loss and insufficient legislation for their protection. In the depths of the rainforest any regulations are tricky to enforce. As a result, arapaima are recognized on two international endangered species lists as Arapaima gigas: IUCN Red List as “data deficient” and CITES “Appendix II”. To ensure the diversity and uniqueness of this genus is preserved, much about arapaima biology and its ecological relations in the wild still needs to be discovered.

Arapaima research in action. Image: D J Stewart

Where to see arapaima

Aside from tropical lakes and rivers (and some restaurants or fish markets), arapaima can be found in public aquaria and even in some pet shops around the world.  Keep in mind they will outgrow the average aquarium and probably the average aquarium keeper within a couple of years. With enough space and food, they can grow to 1 m in just a year!

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Diatoms: a freshwater curiosity

Imagine an organism that takes on a bewildering array of beautiful and complex forms and is common to almost all aquatic ecosystems, yet is invisible to the naked eye.  Organisms that have existed since the Jurassic period yet are being intensively researched by scientists looking for new breakthroughs in computer chip technology.   Look beneath the surface and discover the incredible diatom…

Asterionella formosa (Image: wikipedia)

Diatoms are a widespread group of algae that can be found almost anywhere there is water across the world.  With known records extending back over 180 million years ago, there are now thought to be over 100,000 species of diatoms globally, exhibiting an incredible variety of unusual and beautiful forms.

Diatom image from "Water Lives..." showing Gomphonema acuminatum var. coronatum animated by Adam Proctor (

For such a tiny organism, diatoms are incredibly important .  In freshwaters, diatoms are at the bottom of the food chain, and they are exceptionally sensitive indicators of water quality.  Tiny diatoms also provide a window into the past: as millions of individuals die and fall to the bottom of the lake they leave their fossil remains in the mud which carry clues to environmental history.  Living diatoms have very particular preferences for environmental conditions such as water pH, nutrient levels and water salinity.  This means that by examining the diatoms preserved in the layers of mud, scientists can ‘reconstruct’ past environmental conditions, information that is very useful for modern day environmental managers.  You can read more about this historical diatom detective work here.

Fragiliaria crotonensis (image:

New research in the semi-conductor industry is seeking to learn from the dense, complex shapes formed in the diatom silica shell as a means of designing faster computer chips.  Similarly, research is also being carried out on the lightweight but incredibly strong diatom cell structures by the aerospace and car industries, hoping to gain insights for designing new products.  You can read more about both these exciting research projects here.

Diatom image showing Fragiliaria crotonensis and Asterionella formosa from "Water Lives..." animated by Adam Proctor (

The range of complex diatom forms has fascinated scientists and amateur naturalists since the invention of the microscope, and provided abundant inspiration for our “Water Lives…” animation.  It was popular in Victorian England to mount kaleidoscopic arrays of diatoms on slides, adding another layer of glass to the organism’s fascinating display. Diatoms have also inspired artworks such as Ernst Haeckel’s 1904 Artforms of Nature, Liz Douglas’ “Mire” series of paintings, Klaus Kemp’s slide mounts and David Mann’s series of etchings and mezzotints.  A recent art-science collaboration between Dr Paul Hargreaves and Fay Darling brought the diatoms to life in a dazzling array of shapes and colours through the “colourising” of a set of scanning electron micrographs of the organisms.

Example of the stunning art-science collaboration work on diatoms by Fay Douglas and Dr Paul Hargreaves. Image: Fay Douglas and Dr Paul Hargreaves (

Many diatoms are pelagic, which means they spend their lives suspended in open water, living a ‘boom and bust’ life-cycle where population numbers wildly fluctuate depending on the amount of light and nutrients available from season to season. All diatoms have the same basic form, a single cell (either “centric” with a radial symmetry or “pennate” diatoms with a long axis symmetry) within a silica shell, called a frustule, generally between 0.005 and 0.2 millimetres in size (but can be up to a “gigantic” 1mm).  Diatom frustules fit together in two slightly overlapping valves like a hat box or Camembert case.

Cocconeis molesta var. crucifera. (Image: UCL,

Silica is used (amongst many other things) to make glass.  It could be said that diatoms live their entire life in glass houses – natural display cabinets for their amazing forms.  Diatoms divide to create two smaller daughter cells, each taking one half of the parent cell.  This means that as diatom populations grow, the average size of each individual decreases (up to a point where sexual reproduction takes place to produce a new full size  cell before the whole process repeats itself– imagine if that were true in humans!

Many thanks to Dr Rick Battarbee (BioFresh, University College London) and Dr Alistair Seddon (Long-term Ecology Laboratory in Department of Zoology, Oxford) for their input, advice and corrections on this article.

More reading:

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Sea Lamprey – Petromyzon marinus

Sea lamprey. Petromyzon marinus. Photo from US EPA.

Parasitic, jawless ‘vampire’ fish with toothy suction mouth

The sea lamprey (Petromyzon marinus) is one of the most wonderfully weird freshwater creatures we’ve featured in the Cabinet in some time.  Translated from a mixture of Greek and Latin, lamprey means ‘stone licker‘, most likely due to the fish’s tendency to hug the bottom of the rivers of Atlantic Europe and America that it inhabits with its curious sucker-like mouth (see the amazing close up photo above).  Unlike many other lampreys, for the sea lamprey, this fearsomely toothy mouth serves a deadly purpose: to suck the blood of unwitting fish.

Sea lampreys attached to an American Lake Trout. Image: Wikipedia

Sea lampreys attach their wraith-like bodies to the skin of passing fish (such as the lake trout, above), gripping with rows of tiny sharp teeth embedded in their suction-cup mouth and probing with a keen tongue to drain blood from the host’s body.  The lamprey’s fantastically odd characteristics: its eel-like, jawless body, the suction-cup mouth and its anadromous behaviour (i.e. lives in both the sea and freshwater), have led some scientists to contest that it should even be described as a ‘fish’ at all!

Sea lampreys in an aquarium. Image: Wikipedia

Despite these fearsome, wonderfully odd adaptations, within a healthy ecosystem (for example in the River Usk in the U.K.), the sea lamprey is important to the complex web of life, both as a predator and as prey (e.g. to birds such as herons, and to people – indeed King Henry I of England (1068–1135) was said to be so fond of the taste of lamprey that he died after eating ‘a surfeit’ of them!).  However, accidental introductions of the sea lamprey to the Great Lakes of America through shipping canals and container ships in the 1800s have caused an ecosystem crisis as the lamprey feasts on native lake trout, as described in the video below.

Millions of dollars have been spent in trying to eliminate the sea lamprey from the Great Lakes.  In 2009, under the headline “Sex smell lures ‘vampire’ to doom” (!), the BBC reported on conservation managers in the Great Lakes trialling the use of a laboratory version of a male sea lamprey pheromone, released into waterways to trick ovulating females into swimming upstream into traps.  However, the picture is not so rosy for the sea lamprey on the other side of the Atlantic Ocean.  Over the last century, sea lamprey numbers have declined dramatically in northern Europe, with no current breeding sites known of in the Baltic region.   As a result, conservation efforts are aimed at increasing sea lamprey populations in the region.

Isn’t it fascinating how perceptions and management of this much-maligned, incredible fish can vary so widely on different sides of the Atlantic? A true freshwater curiosity!

More information on the sea lamprey:

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The Iberian Peninsula: a European hotspot for freshwater diversity and curiosity

Achondrostoma: four tiny, beautiful fish species endemic to Iberia. Image: Ana Maria Geraldes

This month’s guest curator is Dr Ana Maria Geraldes from the Instituto Politécnico de Bragança in Portugal, who supplied the information about an incredible range of freshwater species found in the Iberian Peninsula (Spain, Portugal, Andorra and Gibraltar).

The Iberian Peninsula. Image: Wikipedia

The Iberian Peninsula is a hotspot for freshwater fish biodiversity and curiosity in Europe. The existence of geographical barriers such as the Atlantic Ocean, Mediterranean Sea and Pyrenees Mountains prevents fish from moving to other regions, creating a variety of different, isolated habitats, each subject to a wide range in climate.  This peculiar combination of factors has combined to produce a high number of endemic species through evolution.

What are ‘endemic species’?

Endemism is the word used to describe species that are only found in a very small habitat and nowhere else.  For example, the Galapagos tortoise is endemic to the Galapagos islands, because it is only found there (except in zoos, which don’t necessarily count as “native” habitats!).  Species become endemic to a small area because of geographical isolation, causing them to evolve in different ways to similar species found in other habitats (see for example, the differences between the Galapagos tortoise, and other tortoises found elsewhere).

Endemic fish species on the Iberian Peninsula: a hotbed of diversity and curiosity

The Iberian peninsula is home to 36 endemic species of cyprinids: tiny, shimmering and beautiful fish. Some are restricted to very small areas, such as the Southwestern arched-mouth nase (only named in 2005!), Portuguese arched-mouth nase, Western ruivaco, Squalius aradensis and Squalius torgalensis….not all of these fish are well-known enough to have been named yet!). More

Red bellied piranha – Pygocentrus nattereri

Red-bellied piranha. Image: Wikipedia

Infamous predators reveal more bark to their bite

What do we talk about when we talk about piranhas?  Huge teeth? Voracious appetites?  Frenzied feeding shoals?  All fascinating, and all true.  However, a new study by Belgian scientists has shown that one species of piranha has a similarly curious (and previous undocumented) characteristic: the ability to create an amazing, complex range of sounds.

Sandie Millot and colleagues at the University of Liege in Belgium used underwater microphones to record a shoal of piranhas during a range of different behaviours such as aggression, intimidation, food competition and chasing (all characteristics we’ve come to expect from Hollywood depictions of the fish…!).  Fascinatingly, the researchers found that the piranhas produced different, distinct sounds depending on their behaviour (play the audio below!).

The clip features three sounds.  The first is a “bark” produced in what the researchers called a “frontal display”, meaning where two fish swam quickly towards each other and stayed still, aggressively intimidating and staring at each other.  The second is a “drum beat” produced by the largest fish in the group when circling the shoal, mostly when there was competition for food.  The third “croak” was generally associated with a piranha chasing and biting another fish.

How the sounds are produced

The sounds are produced by the piranhas using their swimbladder – an organ which helps keep the fish buoyant and stable in the water.  The piranhas vary the sounds produced by quickly contracting muscles leading the swimbladder.  The rate at which the muscles contract varies the sound produced.

A fearsome reputation. A close up of the piranha's impressive teeth. Image: ARKive

Why is this important?

The piranhas in the study were silent for most of the time, only producing these weird and wonderful sounds during (very aggressive…!) social situations such as group feeding.  What this research shows, that hadn’t been seen (or heard) before, was that sound is a key part of how shoals of piranhas interact with each other.  The fact that all the sounds produced were associated with aggression only serves to reinforce the reputation of this fearsome little freshwater predator!  In fact, one of the researchers suffered a serious bite to their finger from the piranha when carrying out the recording…

Where does the red-bellied piranha live in the wild?

In the wild, the red-bellied piranha is native to the freshwater rivers of South America, congregating in large shoals.  Whilst the fish has a reputation for its aggressive feeding, it rarely feeds in groups, instead individually preying on fish and molluscs.  However, under conditions of extreme stress, the piranha shoals will exhibit a spectacular ‘feeding frenzy’ if presented with suitable food, potentially stripping a large item of prey to the bone in minutes.

More information:

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