Crayfish of the world united

by Sammy De Grave, head of research

How many species of crayfish can you name? Not many, or perhaps none? Well today, for the first time, a list of all the species of crayfish in the world has been published, thanks to a collaborative effort between Professor Keith Crandall at George Washington University and Dr Sammy De Grave, head of research here at the Museum.

The new list draws together much recent work and gives biologists access to a single, comprehensive summary of all the recognised species of crayfish for the first time. The new classifications group crayfish into 669 species, 38 genera, and five families, with two superfamilies corresponding to the Northern and Southern hemispheres.

Fallicambarus devastator. Image: Chris Lukhaup

On the occasion of this taxonomic triumph it seems like a good opportunity to take a look at some interesting crayfish from around the world.

Outside biological taxonomy, crayfish are much better known as a source of food. They are eaten worldwide, but especially in the southern US, Australia, and Europe, where the Red Swamp Crayfish (Procambarus clarkii) is most commonly on the menu. As a result, the Red Swamp Crayfish has been introduced into several countries and has out-competed the local species.

Several other species are also known as invaders. The Signal Crayfish (Pacifastacus leniusculus), native to North America, is now very abundant in Europe, and is out-competing the native Noble Crayfish (Astacus astacus).

The Noble Crayfish (Astacus astacus), above, is native to Europe, but is being out-competed by the introduced Signal Crayfish (Pacifastacus leniusculus). Image: Chris Lukhaup

Another remarkable crayfish is the Marmorkrebs, a species which still has no official taxonomic name. It was first noticed in the aquarium trade in Germany in the 1990s, but no natural populations are known. But the really interesting thing about this species is that all known individuals are female: it is parthenogenetic, which means the females reproduce from eggs without fertilisation – no males involved!

The Marmorkrebs crayfish has no official taxonomic name and is parthenogenetic – all individuals are female, genetically identical and reproduce without males. Image: Chris Lukhaup

Unfortunately, Marmorkrebs has escaped from aquaria in several countries, and is outcompeting local species due to its fast reproduction. Of most concern is its occurrence in Madagascar, where it competes for food and space with the endemic Astacoides crayfish, a much larger but slower-growing species.

Astacopsis madagascariensi, above, is being out-competed in Madagascar by the Marmorkrebs, which has escaped from several aquaria. Image: Chris Lukhaup

The Tasmanian Giant Crayfish (Astacopsis gouldi) is considered to be the largest freshwater invertebrate on the globe. Although its size has declined in recent years due to over fishing, historical specimens weighed up to 6kg and could reach 80-90 cm in length.

The completion of the new world crayfish list allows for further refinements to the conservation status of the animals too. Current Red List assessments show that 32 per cent of crayfish are already thought to be threatened with extinction, a similar number to freshwater shrimps and crabs.

It is really exciting to finally have a single source for the world’s freshwater crayfish taxonomy. Such a resource will impact a wide variety of fields that rely on crayfishes as study organisms. We hope it will also advance conservation efforts of these keystone species of highly endangered freshwater ecosystems.
– Professor Keith Crandall, George Washington University

The paper, An updated classification of the freshwater crayfishes (Decapoda: Astacidea) of the world, with a complete species list, is published today in the Journal of Crustacean Biology.

Out of order

Amphionides small

Our head of research, Sammy De Grave, is the lead author of a paper published in Scientific Reports last week. In this paper he and his co-authors propose to remove an entire ‘order’ of crustaceans. But just what is an order, and why would we want to get rid of one?

Biologists organise all life into ranked groups. The most familiar, and at the bottom level, are genus and species. These are recognisable in the format Homo sapiens, where Homo is the genus and sapiens is the species.

One of the higher groupings is called an order. For crustaceans, there are around 70,000 known species grouped into approximately 50 different orders. One of these orders is the subject of the paper mentioned above – it’s called Amphionidacea – and the odd thing about it is that it was created in 1973 for just one species, an enigmatic open-sea creature called Amphionides reynaudii.

Presumed adult female of Amphionides reynaudii (after Williamson, 1973)
Presumed adult female of Amphionides reynaudii (after Williamson, 1973)

Although the species has been known since 1833, relatively few specimens have been collected and almost none since 1973. Some larval stages have but recognised, but only three adult males have ever been found and no intact adult females have been collected. The reference illustration above is a composite of 43 damaged specimens.

Lacking good research specimens, the status of this creature has long been debated. Luckily, in 2011 Jose Landeira, a biologist on Gran Canaria, collected six specimens. As usual they were extensively damaged (you can see this in the photograph at the top), but a specialist genetic lab at National Taiwan Ocean University was able to extract some small fragments of DNA.

Building on earlier work by a US group, the sequences were analysed and the results show that Amphionides is not a separate order after all, but merely a shrimp. You can see where it fits in the taxonomic scheme of things by clicking the chart below.

Out of Order Fig3
Phylogram of the Decapoda order of crustaceans, which includes crayfish, crabs, lobsters, prawns and shrimp. The position of Amphionides indicated in red (click to enlarge)

In keeping with its oddball status, however, many questions remain unanswered. Amphionides larvae have been recorded across all oceans from the tropics to subtropics, but almost no known shrimp species have such a distribution as adults. And although the genetic analysis reveals a strong affinity to a single family of shrimp (Pandalidae), it remains unclear which genus or even species it could be the larvae for.

So Amphionides may be removed from its order, but the mystery of the little shrimp lingers yet…