28 April 2008

What's in a name?

One of the many ways in which Marmorkrebs is an oddity is that it's extremely unusual for an organism to be the subject of multiple scholarly research papers... but have no name. It's even more unusual that nobody, as far as I know, is in any sort of hurry to give Marmorkrebs a proper scientific name.

Scientific names for organisms are also known as Latin names. Carl Linnaeus (pictured) started the naming scheme used in biology to today, with a few modifications. At the time, Latin was still a fairly dominant language of scholarship. So Linnaeus gave plants, and later animals, names in Latin. I think names based on Greek are also acceptable.

Today, for animals, the rules for names are handled by the International Commission on Zoological Nomenclature (ICZN). There are similar but separate organizations for plants and microbes and, I believe, fossils.

I'm going to do my best to summarize other people's arguments about giving Marmorkrebs a scientific name. (Some of these are based on half-remembered conversational snippets from conferences, so forgive me if I make errors.)

Marmorkrebs seems to belong in the genus Procambarus. The major crayfish taxonomist who described many of the members of this genus was a man with the wonderfully alliterative name of Horton H. Hobbs, Jr.. He based many of his species descriptions on the basis of the male sex organs... which Marmorkrebs, being all females, do not have.

The next plan of attack might be to use genetics. But there really isn't a lot of precedent for describing species on the basis of genes alone. I am not sure whether the ICZN code allows it. Regardless of whether it is permissible, there are strongly established traditions in taxonomy, and formal species descriptions are supposed to have detailed morphological descriptions, designate a type specimen for safekeeping in a museum, and so on.

And as you consider the problem of whether to give Marmorkrebs a scientific name, it leads to a very core question: What is a species? This is a question that has resulted in a lot of ink being spilled in scientific papers, and simply put, there is no consensus on how to define a species.

Based on some comments Keith Crandall has made, Marmorkrebs are genetically extremely similar Procambarus alleni... so following some species concepts that revolve around morphological separation, this might qualify as putting Marmorkrebs in with P. alleni.

But because Marmorkrebs reproduce asexually, they do not interbreed with sexual species... so following species concepts that revolve around interbreeding, this would suggest that Marmorkrebs should get its own species name.

As the preface to the latest edition of the code of zoological nomenclature notes:

One should always keep in mind that an important function of classifications is information retrieval.

So maybe it's not really critical to have a scientific Latin name immediately in these days of search engines and databases... but what is definitely important is to have a consistent name, one that can be found in databases.

For obvious reasons, I suggest "Marmorkrebs" be that name for use in titles and keywords.

23 April 2008

Great moments in crayfish research: Electrical synapses

If neurons are separate, how do they communicate?

The first early decades of the 20th century saw two major ideas emerging, which the great comparative physiologist Ted Bullock characterized as "soups versus sparks."

Bullock's "soups" were chemicals released by neurons, which are now called neurotransmitters. Bullock's "sparks" were electrical signals, which were thought to transmit directly from cell to cell.

Otto Loewi provided strong evidence for chemical neurotransmission between nerves and the heart. Loewi considered the matter conclusively proved. After he won the Nobel prize for his experiments, one might have reasonably considered the matter settled in favour of the "soups" camp.

So it was a bit of a surprise when two neurophysiologists, Ed Furshpan and David Potter, working on giant crayfish neurons found evidence for neurons communicating through sparks -- electrical synapses.

Furshpan and Potter were studying the giant neurons involved in escape responses. In particular, they were examining the connections between the lateral giant neurons and the motor neurons that go on to connect to muscle. Work on chemical synpases had shown short delays whenever there was a synapse between two neurons: the time needed for the chemicals to diffuse between the small gap between the cells. Furshpan and Potter found the delay between the crayfish neurons to be much, much shorter -- a small fraction of a millisecond (picture). But it was easy to see how such connections could be advantageous when the neurons were responsible for a rapid escape response.

Although some may have thought this would be peculiar to invertebrates, they were soon shown to be in vertebrates, too.

Thus, crayfish provided the first conclusive evidence for the existence of a different way that neurons could communicate.

References

Furshpan EE, Potter DD. 1957. Mechanism of nerve-impulse transmission at a crayfish synapse. Nature 180(4581): 342-343. http://dx.doi.org/10.1038/180342a0

Furshpan EE, Potter DD. 1959. Transmission at the giant motor synapses of the crayfish. Journal of Physiology 145(2): 289-325.

15 April 2008

Braband and colleagues, 2006

Braband A, Kawai T, Scholtz G. 2006. The phylogenetic position of the East Asian freshwater crayfish Cambaroides within the Northern Hemisphere Astacoidea (Crustacea, Decapoda, Astacida) based on molecular data. Journal of Zoological Systematics and Evolutionary Research 44(1): 17-24. http://dx.doi.org/10.1111/j.1439-0469.2005.00338.x

Abstract

The phylogenetic position of the four East Asian freshwater crayfish species of the genus Cambaroides is not settled. Morphological phylogenetic analyses suggest close affinities of Cambaroides with North American Cambaridae. This view is based mainly on characters of the male and female reproductive organs. In contrast, the only molecular phylogenetic analysis so far available leaves the phylogenetic position of Cambaroides unresolved. The question of whether Cambaroides is monophyletic or paraphyletic has so far been neither addressed morphologically nor with molecular data sets. Here we present the first comprehensive phylogenetic analysis of all four currently described species of Cambaroides in the framework of Northern Hemisphere freshwater crayfish (Astacoidea) relationships based on partial sequences of two mitochondrial genes (CO1 and 12S rRNA). Despite some evidence in favour of a monophyletic Cambaroides more data is needed to resolve this question. Our analyses
suggest a close relationship of the North American Cambaridae and the European Astacidae, leaving the Asian Cambaroides basal to them. If these results hold true the similar reproductive organs of Cambaroides and American Cambaridae must be either homoplastic or ancestral for Northern Hemisphere Astacoidea.

Keywords: mitochondrial genes • evolution • reproductive organs • Astacoidea • arthropods

08 April 2008

Keeping them alive

The biggest problem with rearing Marmorkrebs is that the high mortality in the early stages -- before individuals get to about a centimeter or so long. Getting embryos is not a problem. Getting hatchlings is not a problem. Nurturing them through to a moderate sized juvenile is the problem.

I don't know if there is anything that can be done about that, however. Most crustaceans do have a reproductive strategy emphasizing quantity of offspring rather than quality.

This suggests that at least initially, developmental research will probably proceed faster than other types of research that revolves around adult organisms.

02 April 2008

Marmorkrebs labs

Over on the left side of this blog, I've added a new list of links (you'll have to scroll down some distance) of labs that I know are working with Marmorkrebs. I apologize in advance that the list is incomplete. Please email me if you would like to have your lab listed.

01 April 2008

Great moments in crayfish research: The model organism

The great Victorian biologist, Thomas Henry Huxley, is famed for his defense of the concept of natural selection, for first proposing a relationship between birds and dinosaurs, and many other achievements. Less well known among general biologists was his book titled The Crayfish: An Introduction to the Study of Zoology.

In inimitable Victoria prose, he prefaced his book by saying:

I have desired, in fact, to show how the careful study of one of the commonest and most insignificant of animals, leads us, step by step, from every-day knowledge to the widest generalizations and the most difficult problems of zoology; and, indeed, of biological science in general.

It is for this reason that I have termed the book an “Introduction to Zoology.” For, whoever will follow its pages, crayfish in hand, and will try to verify for himself the statements which it contains, will find himself brought face to face with all the great zoological questions which excite so lively an interest at the present day; he will understand the method by which alone we can hope to attain to satisfactory answers of these questions; and, finally, he will appreciate the justice of Diderot's remark, “Il faut être profond dans l'art on dans la science pour en bien posséder les éléments.”

Ah, the old “throw in a quote in another language without a translation to show how smart you are” trick.

This may be one of the earliest explicit justifications for using a single organisms to study a host of biological problems. Those single organisms are known today as model organisms. The 20th century was when the concept of model organisms really rolled into force, perhaps starting with fruit flies (Drosophila melanogaster). Certainly, by the mid-century scientists had taken Huxley's lengthy justification and traded them for bon mots like this one from Jacques Monod, “What is true for E. coli is also true for the elephant.”

So, arguably, crayfish contributed to the model organism concept. From time to time, I'll be highlighting basic biological discoveries that were made by studying crayfish. Which, I hope, Marmorkrebs will be adding to as more labs begin to work with this organism.

Here's a link to an HTML version of Huxley's text, and here's the Google books version.

Additional (23 February 2009): Monod's quote is preceded by a similar one by Albert Kluyver: “From the elephant to butyric acid bacterium—it is all the same!”