Centipede segment shenanigans

The Guardian has an exciting-looking article, entitled “New to Nature”, which is about a centipede, Scolopendropsis duplicata, which has been discovered in Tocantins State, central Brazil. This discovery is described in this open access article from the journal Zootaxa by Chagas Jr, Edgecome and Minelli. (Incidentally, Chagas Jr’s first name is highly appropriate: “Amazonas”; his nickname, it appears, is “Amazing”.)

Scolopendropsis duplicata. An individual with 43 trunk segments. Photo by Amazonas Chagas, Jr. (The tail is on the left.)

S. duplicata is particularly interesting because it has “too many” segments. S. duplicata is part of the scolopendromorph order of centipedes, all of which have either 21 or 23 leg-bearing (= “trunk”) segments. As its name suggests, S. duplicata has done something rather odd – it has nearly twice as many segments as expected, either 39 or 43. None of the 700 other scolopendromorph species has more than 23. Furthermore, S. duplicata is unique in that the number of segments (39 or 43) varies within a population.

There have been previous reports of intra-specific variability in trunk segment number, (e.g. Scolopendropsis bahiensis, and species in the order Geophilomorpha) but this variability was always seen between populations, not within. So in the small world of centipede biologists, this is a cracking discovery!

Classification is a difficult business, and everything above the species (the “sapiens” in Homo sapiens) level is effectively a human construct, a way we use to classify organisms, and to describe the process of evolution, rather than something that has real biological meaning. Nevertheless, classification not only helps us make sense of the world, it also provides evolutionary hypotheses that can be tested by morphological and genetic studies

Centipedes as a whole are classified along with millipedes as part of the Myriapoda sub-phylum of arthropods. There are over 3000 species, grouped into five orders. The basal group is assumed to be the Geophilomorpha , which look most like millipedes. Species in this order can have up to 177 segments, but in all other orders 23 segments has hitherto been assumed to be the maximum. Unlike the millipedes, each segment has only one pair of legs.

So normally, if you found a species with such a radically different form – nearly twice as many segments, and unprecedented population-level variability – you’d tend to think they were in different genera (the “Homo” part of H. sapiens). However, everything else about S. duplicata clearly indicates its proximity to other scolopendromorphs, so the authors comment dryly:

“We note the paradox that variability in scolopendromorph segmentation is a remarkable discovery, and yet S. duplicata and S. bahiensis are so similar in other respects and their sister group relationship so highly corroborated that generic separation is unwarranted.”

It seems probable that the genetic basis of the segment variability seen within S. duplicata and between S. duplicata and closely related species is due to variability in homeobox (“hox”) genes that control the way that segmentation takes place. However, things are not quite so simple. In the Discussion, the authors note that all the 43-segmented individuals they dissected were females, while males were only found in the 39-segment group. This suggests that – like in some Geophilomorph species – this species may show sexual dimorphism in segment number.

So whatever is controlling the polymorphism, it would appear to be some interaction between the sex determination genes in this species (and I know nothing about sex determination in myriapods, but it would appear to be on the basis of XY chromosomes, as in most insects and most chelicerates) and the hox genes.

There has been a long argument – going back over 100 years – about how the various arthropod groups should be grouped together. The current wave of molecular data shows that insects and crustaceans are more closely related to each other than they are to the other arthropods (the “pan-crustacean” hypothesis), while insects + crustaceans group together with the myriapods to form the “mandibulata” because they have mandibles, rather than chelicera, which is the mouth appendage seen in the Chelicerata.

The saddest part of the description of S. duplicata by Chagas-Junior et al comes at the end, and suggests the centipede may no longer be extant:

“Most specimens of S. duplicata were found in pitfall traps for reptiles and amphibians in the dry, xeric “cerrado”, a vegetation typical of central Brazil. All specimens were collected before flooding of the Luis Eduardo Magalhães hydroelectric power plant, in the Tocantins River, and the type locality is now under water. Vegetation around the lake is the same as that at the now submerged type locality. An expedition organized by the first author in June 2007 failed to discover any specimens of S. duplicata, even though a forest patch 500 m away from the type locality was sampled. Thus, the original habitat of this species may have been impacted by the flooding of the hydroelectric power plant, and further expeditions are needed to seek additional individuals of this remarkable Brazilian species.”

Finally, why on earth are we talking about this now? The web is full of chatter about it – just try googling Scolopendropsis duplicata and you’ll see what I mean. Because, although The Guardian doesn’t mention it, S. duplicata is not “new to nature” – the Chagas, Jr article was published back in 2008…

The answer, it appears, is the Natural History Museum website, which had S. duplicata, as its “species of the day”, and has a great interactive page, based on the Zootaxa article, of which Gregory Edgecombe of the NHM was a co-author. The Guardian and other websites obviously picked up on this, as did I… The sudden interest must be a trifle perplexing (but pleasing) to “Amazing” Amazonas and his colleagues.

Chilopod facts:

• Most species are carnivorous (they can even eat bats!)

• Like insects, they have trachaea for respiration and mandibles for eating.

• Most species are oviparous (i.e. they lay eggs), but some are viviparous (i.e. they bear live young).

• Like most chelicerates (spiders etc; harvestmen – opiliones – are an exception) they do not have penetrative sex, but the male makes a spermatophore out of silk, which the female picks up and uses to fertilize her eggs.

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One response to “Centipede segment shenanigans

  1. Pingback: Centipedes segment | Islandinthesun

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