It's as big as a rolling pin and can drain a tablespoon of human blood in a single meal, thanks to its razor-sharp hypodermic needle-like mouth

Around 680 species make up the subclass Hirudinea, segmented worms that are either parasitic or predatory and are more commonly known as leeches.

Of the 500 species that are found in freshwater, most range from the size of a fingernail to the length of a finger itself, but deep in the waterways of the upper Amazon system, lurks a leech that can grow considerably larger.

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The Amazon giant leech (Haementeria ghilianii) is appropriately named, regularly reaching 30cm in length and with specimens measuring 45cm recorded. They are parasitic, feeding on the blood of fish, reptiles and mammals that may enter the water, and will take a meal from a person if the opportunity presents itself.

Unlike many leech species, which attach to a host with a sucker-like mouth and then puncture the skin with small, bladed teeth, Haementeria ghilianii is equipped with a proboscis.

This measures up to 10cm long and acts similarly to a hypodermic needle, penetrating quite deeply into the skin. The saliva of the leech contains the enzyme hementin which acts as an anticoagulant, easing the flow of blood from the host and also preventing clotting within the proboscis.

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Haementeria ghilianii may go several months without feeding but can extract up to 15ml of blood in a single meal, enough to fill a tablespoon. Such quantity has led to speculation about the impact on host animals should multiple leeches attach simultaneously. However, historical claims that giant leeches could bleed a cow to the point of death have little substance.

As with many Amazonian species, Haementeria ghilianii faces an uncertain future as deforestation, pollution and habitat fragmentation continue to impact the world’s richest biodiversity, but the image of a 45cm long bloodsucker will always find a place in your dreams…

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Top image: Anonyme973, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons