Bark at the Moon - small invertebrates of timber (Part 6)

It's been a while since my last post about the small invertebrates that have made their home in our firewood store. These have generally been small beetles (Coleoptera) and barkflies (Psocoptera), but this one - found wandering along the coffee table just after a load of logs had been brought in belongs to a group I sometimes neglect a little (generally because of my tendency to favour the beetles); the true bugs or Hemiptera.This one was about 7mm long with striped legs and connexivium (the sides of the abdomen) and a hairy pronotum. These features, along with the overall appearance, indicate that this is the Nettle Groundbug Heterogaster urticae, one of the Lygaeidae (groundbugs).

Dorsal view of Heterogaster urticae

H. urticae is a common species in southern England - found on nettles in more open habitats during the summer, adults overwinter in various places, including in hollow stems and beneath bark which is likely source of this specimen which has now been placed back in the detritus in our woodstore. Adults emerge in the spring and then mate. The hairy pronotum and series of tibial stripes separate this species from the similar, though less widely distributed, H. artemisiae (Southwood & Leston, 2005). For more images, including nymphs, see here. As ever, it can be interesting to zoom in on some of the detail:

Head of H. urticae showing the bulbous compound eye, and to its left, the small, shiny red-pink simple eye or 'ocellus'.

The pale, erect hairs on the pronotum of H. urticae. The coarse punctures in the pronotal surface are also visible.

That's all for now - a fairly short post today - more soon...


Reference

Southwood, T.R.E. & Leston, D. (2005). Land & Water Bugs of the British Isles. Pisces, Newbury. Edited facsimile of the 1959 work.

Wheels of life

Straight in with a question today - have wheels evolved in nature? Now, I know it's been written about before, and there's no shortage of discussions on any number of online forums (or fora if you prefer), but it's something I've been musing on and coming up with some underlying questions - so, here goes with one my rare forays into the more speculative realms of biology and ecology...

Firstly, why might wheels be a useful adaptation? Well, they could provide an efficiency and simplicity of motion in some circumstances - I can certainly imagine animals using wide wheels to trundle across the soft sediments of the ocean floor for example (much like the wire-wheeled lunar rovers from Apollos 15-17). However, legs and fins generally work pretty well, with wheels really coming into their own on straight, smooth, hard surfaces. These are not common in nature, though humans produce plenty of them - and hence plenty of wheels. So, a lack of evolutionary advantage might be one reason why natural wheels are not widely seen.

Secondly, what do I even mean by a wheel? Here, I am only considering something that has an axle or bearing. There are plenty of organisms that roll - the South American pebble toad Oreophrynella nigra that tumbles down slopes to avoid predation, the wide variety of tumbleweed plants (and the rarer 'tumblefruits' such as Physaria) that disperse seeds as they roll with the wind, and the puffballs of the genus Bovista that are also blown around and so disperse their spores more widely. Ocean currents roll the coral Porites lutea across the sea floor and the small stomatopod mantis shrimp Nannosquilla decemspinosa can curl up and roll slowly like a wheel if stranded on a shallow damp sandy shore, thus returning to the sea. These are all interesting in their own right, and there are other examples, but none of them are wheels.

In fact, there don't appear to be any organisms that roll along on wheels in the way that humans' various vehicles do. As mentioned above, there may simply be no evolutionary pressure to produce a wheel, but there are also developmental constraints. For example, to have a wheel in a multicellular organism is tricky because, to be able to rotate freely, the wheel needs to be detached from the rest of the organism. If this is the case, how could it maintain a blood supply, neural connections and so on? Two options come to mind:

1. The wheel could be made of 'dead' material secreted by the organism, such as carapace material. This could grow as a toroid (doughnut-shaped) swelling on a limb/axle and gradually separate by thinning near the limb. This could produce a passive wheel on an axle much like a wood-turner produces a freely movable (but not removable) ring from a single piece of wood.
2. The wheel could be alive but self-contained. If a ring of cells developed as above and then detached, to be an effectively autonomous wheel, it would have to have its own energy supply (photosynthesis, chemosynthesis?) and so on.

Neither of these options have been discovered in nature, though this does not mean they never will - my feeling is that the lack of need is more likely to prevent wheels evolving than developmental problems. So far, I have not differentiated between passive and active wheels i.e. whether they simply roll like a cart (reducing the friction that would be caused by dragging) or are actively rotated by an energy source. Active wheels are developmentally even more problematic as a torque needs to be applied - in animals, motive force is produced by muscles, but this would not work on wheels as they need to be freely rotating. However, in bacteria, the problems of producing motive force, overcoming inertia and so on have been solved. In fact, the only example discovered so far of a true biological wheel (an active one that produces continuous propulsive torque around a fixed structure), is the bacterial flagellum, the  a propeller-like thread used for locomotion. Where the flagellum enters the cell membrane, there is a motor protein that works like a rotary engine, powered the flow of hydrogen ions (i.e. protons) across the bacterial cell membrane down a concentration gradient created by a proton pump. A similar system using a sodium ion pump exists in the genus Vibrio.

The structure of the flagellar base showing cutaway details of the 'motor'. Thanks go to Mariana Ruiz Villarreal for putting this and other diagrams in the public domain.

At an even smaller scale, the enzyme ATP synthase (which is involved in energy storage and transfer within cells) is somewhat similar to bacterial flagellar motors and is likely to be an example of modular evolution i.e. where two separate structures or sub-units (which evolved and previously functioned separately) become joined or associated, and in doing so gain a new function.

So, although true wheels have not been discovered in multicellular organisms, and both developmental and utility constraints make their evolution highly unlikely, maybe impossible, there are ways that wheels might be used in nature:

1. Through symbiosis, joining two otherwise unrelated structures/organisms in order to get round the developmental problems preventing direct evolution of wheels. This could be instinctive (imagine an extension of dung-ball rolling by dung-beetles) and is an idea which has been explored in fiction, e.g. in the Amber Spyglass (Philip Pullman, 2000). In this book, an alien race known as the Mulefa use large, round seed pods as wheels. They put these on sideways-oriented claws (which act as axles) on two of their legs, using the other two legs to push themselves along. The symbiotic aspect occurs because the trees that produce the seed pods depend on the rolling action under the weight of the Mulefa to break open the pods and allow the seeds to disperse and germinate. A number of other science fiction novels consider biological wheel use in a variety of ways, but Pullman's is probably my favourite so far, though other examples include David Brin's Brightness Reef (1995) and Infinity's Shore (1996), and Wheelers (2000), co-authored by Ian Stewart and Jack Cohen (who happen to be a couple of Terry Pratchett's collaborators if you like a bit of nerd-trivia).
2. Through tool use. Humans do this, using wheels widely - could other species do the same, even if with less technological sophistication? I'm just waiting to see corvids start rolling past...

OK, I think that's enough speculation for one day - if anyone out there does know of other examples of 'bio-wheels', I love to hear about them, so feel free to add a comment.

OMG in the OMZ: massive marine microbes

Today, I'm drawing inspiration from the Census of Marine Life, a decade-long project which has produced a huge inventory of marine life - a baseline catalogue to be used for further research and to inform the management and conservation of marine life. The Census looked at all scales from microbes to whales, at all latitudes and at all depths. The Census has produced a range of books, both popular and technical - one of the most straightforward and non-specialist, 'Citizens of the Sea' (Knowlton 2010) provided a couple of snippets that induced me to delve into the detail rather more...

First up, megabacteria - not the disease of budgies (which is actually a yeast), but very large true bacteria discovered off the coast of Chile and Peru in the 1960s. Placed in the genus Thioploca, the bacteria are filamentous and 2 to 7 cm (yes, cm) long. Secreting mucus, they form vast mats (the largest covering 130,000 km-sq) in/under the 'oxygen minimum zone' (OMZ), an area at 40-280 m with very little dissolved oxygen; instead they have to rely on hydrogen sulphide in the sediments. They oxidise this using nitrates (from sea water) which they can concentrate up to 500 mM in the liquid vacuole that occupies over 80% of their cell volume, even though the concentration of nitrates in sediment is only around 25 μM. Mucus-sheathed transport filaments send this nitrate 5–10 cm down into the sediment and reduce it, thus oxidising the hydrogen sulphide and creating a coupling of the nitrogen and sulphur cycles in the sediment (Fossing et al. 1994), producing pyrite and elemental sulphur as a result (Ferdelman et al. 1997). Thus, organic matter (in the form of anaerobic dissolved organic carbon) can be oxidised at low oxygen concentrations. The mats also provide food and shelter for a range of animals including squat lobsters (Pleuroncodes monodon), amphipods, and ophiuroids (Grupe 2011). As the OMZ shares features with conditions during the Proterozoic period (2.5 bya to 650 mya), and similar microfossils have been found, such bacteria may provide an insight into ancient life forms and ecology as well as performing a still little-known but key function in nutrient cycling. Research is ongoing with one recent example investigating Thioploca found in Danish waters where (in the species T. ingrica) nitrate accumulation was lower at around 3 mM, with bicarbonates and acetates used as carbon sources, and no mat being formed (Høgslund et al. 2010). 

A core from a Thioploca bacterial mat. The core is about 8 cm across and the mat about 1 cm thick. The mat is made up of many bacterial filaments with individual cells visible to the naked eye as white threads. Huge for bacteria! Photo courtesy of NOAA/Lisa Levin.

Now, ocean acidification due to carbon emission from fossil fuels may affect marine microbes - with microbial ecosystems responsible for between 50 and 90% of all marine biomass and over 95% of marine respiration, they maintain Earth's habitability though their influences on climate (they can sequester atmospheric carbon dioxide), nutrient cycling and the decomposition of pollutants (Leahy 2012). So, this could be very serious indeed and current research is looking at the  sensitivity of marine microbes to acidification. If I find links to results from this research, I'll post an update, plus I have some more bacterial and marine posts (among others) in the pipeline.

References

Ferdelman, T.G., Lee, C., Pantoja, S., Harder, J., Bebout, B.M. & Fossing, H. (1997). Sulfate reduction and methanogenesis in a Thioploca-dominated sediment off the coast of Chile. Geochimica et Cosmochimica Acta 61(5): 3065-3079. Fossing, H, Gallardo, V.A., Jørgensen, B.B., Hüttel, M., Nielsen, L.P., Schulz, H., Canfield, D.E., Forster, S., Glud, R.N., Gundersen, J.K., Küver, J., Ramsing, N.B., Teske, A., Thamdrup, B. & Ulloa, O. (1994). Concentration and transport of nitrate by the mat-forming sulphur bacterium Thioploca. Nature 374: 713-715.
Grupe, B. (2011). Sea Floor Habitats of the Chile Margin. NOAA Ocean Explorer [accessed 15/02/2012].
Høgslund, S., Nielsen, J.L. & Nielsen, L.P. (2010). Distribution, ecology and molecular identification of Thioploca from Danish brackish water sediments. FEMS Microbiology Ecology 73(1): 110-120.
Knowlton, N. (2010). Citizens of the Sea: Wondrous Creatures from the Census of Marine Life. National Geographic, Washington DC.
Leahy, S. (2012). Giant Bacteria Colonize the Oceans. Tierramérica. [accessed 15/02/2012].

Antarctic sea spiders: polar or abyssal gigantism?

It is well known that in certain situations, some species evolve to be unusually large members of their taxonomic groups - the phenomenon of gigantism. Two such situations are polar and abyssal (deep-sea) gigantism, but why do large species evolve in polar and/or deep sea waters? As we will see, the answers are not always straighforward and are not necessarily the same for both situations. To illustrate this, I want to look at sea-spiders - not actually spiders but marine arthropods in the class Pycnogonida. A brief but informative introduction to this group can be found here, but in summary they are mostly free-living and are found at all latitudes and ocean depths. Superficially resembling spiders (though their taxonomic link to other groups is unclear), they have a cephalon (head) and a 4- (sometimes 5- or 6-) segmented body, each segment with a pair of walking legs (the rear segment bears a small abdomen), while the cephalon has various feeding appendages and palps, plus in males a pair of ovigers (leg-like appendages primarily used for carrying eggs & caring for young, but also for cleaning and courtship) which are found only in the Pycnogonida. Males also have 'cement glands' which they use to form eggs into round masses that are carried on the ovigers (Barnes 1980). Their biology is poorly studied - see Arnaud & Bamber (1987) for a useful review - but they reproduce by hatching as larvae or post-larvae with some being dispersed by medusae (jellyfish) and appear to feed on sessile animal prey or algae. Having such small bodies, their guts extend into their legs and in females, eggs are carried inside the femora. Around Britain, one common sea-spider is Pycnogonum littorale, a temperate shallow-water species (distribution given here) with a body around 5mm long and hence a leg-span of around 20mm.

A display of Pycnogonum littorale at the OUMNH

In contrast, polar and/or deep-water species may have leg-spans up to 750mm, especially in the family Colossendeidae.

Colossendeis wilsoni, A large Antarctic sea-spider also in the OUMNH collection. For scale, the label font is the same size as in the photo of P. littorale above.

So, why this gigantism in polar and abyssal marine environments? As noted in the excellent Deep Sea News, it is unclear whether the cause is the same in both cases, and the two tend to be confused in scientific reporting by the media. Also, although giant sea-spiders are familiar examples of Antarctic gigantism, many are found in deep water and therefore a species may be subject to the processes of adaptation to both polar and abyssal conditions, making it difficult to separate the two effects. For example, specimens of C. wilsoni (photo above) in the Smithsonian Museum were found at depths between 36m and 801m, while the most common Antarctic species in this genus, C. megalonyx, has been found between 3m and 4,900m (Wu & Mastro 2004)! Hence, it is not clear whether gigantism in this species is polar or polar-and-abyssal.

A widely cited paper by Chapelle & Peck (1999), found that the maximum size of amphipods (shrimp-like crustaceans) was related to dissolved oxygen rather than temperature or salinity, with polar waters being high in dissolved oxygen, because water can hold more oxygen at low temperatures. Similar effects in bivalve molluscs have also been found (e.g. Pörtner et al. 2006). The reasoning behind this 'oxygen hypothesis' is that as the size of an organism increases, its surface area:volume ratio reduces. This means that larger animals have more tissue volume requiring oxygen, but relatively less surface area with which to sequester it from the surrounding water. In warmer waters, not only is their less dissolved oxygen, but the oxygen needs of animal tissues is higher. Thus polar gigantism occurs due to cold water temperatures and high levels of dissolved oxygen. However, more recent research involving the self-righting abilities of 12 different-sized species of sea-spider (Woods et al. 2008) did not fully support the oxygen hypothesis, although it did agree that oxygen availability was likely to be one important factor, just not the only one. A possible explanation is that, being an apparently early branch of sea-spider evolution (Arango & Wheeler 2007), Colossendeis species have been adapting to cold, well-oxygenated waters for a longer period that other genera and have oxygen delivery systems which are more finely tuned to such conditions. If this is the case, then climate change is a potentially serious threat to a specialist groups of species functioning with narrow oxygen safety margins i.e. warmer waters leading to higher oxygen demand and lower availability could push Colossendeis beyond these margins more quickly than it can adapt.

So, although it seems that polar gigantism is a result of oxygen availability plus other factors, abyssal gigantism is in some ways more mysterious. Firstly, as noted by Deep Sea News, much work has looked at deep sea dwarfism rather than gigantism because so many taxa show this reduction in size, suggesting that the deep ocean is primarily a small-organism habitat (McClain et al. 2005, Kaariainen & Bett 2010). Thus, the incidence of abyssal gigantism (seen particularly in crustaceans, but also a range of other taxa) contrasts strongly with what appears to be the 'normal' situation in the deep ocean.

Abyssal dwarfism has generally been attributed to low food availability, with most animal communities (away from seeps and vents) relying on the 'marine snow' of detritus sinking from the surface, with occasional larger localised inputs such as dead whales. Thus little food arrives, especially away from productive shallow coastal waters. However, several possible explanations for the rarer gigantism have been proposed e.g.:

• Higher oxygen availability (Chapelle & Peck 2001) as the amount of available oxygen determines the amount of sustainable tissue, with cell size and number both increasing with higher oxygen concentration in Drosophila fruit flies (Frazier et al. 2001) and freshwater amphipods (Peck & Chapelle 2003). In gastropods, a link between larger size and more oxygenated deep-sea sites has been noted (McClain & Rex 2001), but giant isopods Bathynomus sp. are known from low-oxygen regions in the Gulf of Mexico.
• Longer lifespans due to reduced predation (few predators) and slower growth rates in cold water with larger cell size in crustaceans (Timofeev 2001) with a similar process suggested for other taxa (e.g. Van Voorhies 1996).

However, although key effects such as the link between oxygen levels and cell size/number have been described, these are the result of work on unrelated taxa and it remains unclear precisely why Colossendeis sea-spiders (let alone giant isopods) should exhibit gigantism while others do not - and so it is tie for a little (hopefully not too idle) speculation:

• Through development of fat reserves, larger size may allow longer gaps in feeding when food is scarce (although sea-spiders do not appear to have much space for such storage) or larger foraging areas. 
• It may be that gigantism is linked to the species' evolutionary past as island biotas also show a mixture of dwarfism and gigantism related to the size of their mainland ancestors (e.g. Lomolino 2005). Could Colossendeis (or Bathynomus) be descendents of larger ancestors from warmer and/or shallower waters and thus display gigantism rather than dwarfism when adapted to polar/abyssal conditions? 
• Is their large size actually adaptive or is it simply a random evolutionary trait which happens to serve them as well as dwarfism might? 
• With many abyssal species tending towards dwarfism, might it provide a form of niche-separation and thus reduce competition?
• Does large size itself reduce predation?
• Might large size (through the ability to exploit a large food patch or larger food items) reduce the need to move and thus expend energy? Would this be a successful trade-off against the need for more energy/food to maintain a larger body size?
• With the smaller surface area: volume ratio, larger bodies can mean easier temperature regulation, but would this be sufficiently adaptive and if so, why in only a few species?
• With some hydrothermal vent and seep species such as vestimentiferan tubeworms showing great longevity (e.g. Fisher et al. 1997), and gigantism being at least partly associated with slow growth over a long period in a stable, if food-scarce environment, might gigantism be linked to an adaptive function of increased individual longevity in areas away from vents and seeps?

I suspect I could go on, but that is enough speculation for now. As always, comments and suggestions are most welcome -  this is an area of ongoing research where the processes involved are, in part, poorly understood, so this may require an update at some point in the not-too-distant future. And the answer to the original question - polar or abyssal gigantism? Well, it seems likely that both are involved and linked to some extent by the influence of oxygen availability, but the relative 'weight' of each type of gigantism can not currently be determined for certain. However, my feeling is that, for Colossendeis at least, the fact that a single species can be found anywhere from the sea surface to depths of thousands of metres suggests that it is the polar aspect that is constant and having a greater effect. Could be wrong though!


Further reading

For a key to coastal British species: King, P.E. (1986). Sea Spiders. A revised key to the adults of littoral Pycnogonida in the British Isles. Field Studies 6(3): 493-516.

References

Arango, C.P. & Wheeler, W.C. (2007). Phylogeny of the sea spiders (Arthropoda, Pycnogonida) based on direct optimization of six loci and morphology. Cladistics 23: 255–293. Arnaud, F. & Bamber, R.N. (1987). The Biology of Pycnogonida. Advances in Marine Biology 24: 1-96.
Bamber, R.N. & El Nagar, A. (eds.) (2012). Pycnobase: World Pycnogonida Database. [accessed 09/02/2012]
Barnes, R.D. (1980). Invertebrate Zoology (4th ed.). Holt-Saunders, Philadelphia.
Chapelle, G., & Peck L.S. (1999). Polar gigantism dictated by oxygen availability. Nature 399: 114-115.
Fisher, C.R., Urcuyo, I.A., Simpkins, M.A. & Nix, E. (1997). Life in the slow lane: growth and longevity of cold-seep vestimentiferans. Marine Ecology 18(1): 83-94.Frazier, M. R., Woods, H. A. & Harrison, J. F. (2001). Interactive effects of rearing temperature and oxygen on the development of Drosophila melanogaster. Physiological and Biochemical Zoology 74: 641-650.
Kaariainen, J. & Bett, B. (2010). Evidence for benthic body size miniaturization in the deep sea. Journal of the Marine Biological Association of the UK. 86(6): 1339-1345.
Lomolino, M.V. (2005). Body size evolution in insular vertebrates: generality of the island rule. Journal of Biogeography 32(10): 1683-1699.
McClain, C.R & Rex, M.A. (2001). The relationship between dissolved oxygen concentration and maximum size in deep-sea turrid gastropods: an application of quantile regression. Marine Biology 139: 681-685.
McClain, C.R., Rex, M.A. & Jabbour, R. (2005). Deconstructing bathymetric body size patterns in deep-sea gastropods. Marine Ecology Progress Series 297: 181-187.
Peck, L.S. & Chapelle, G. (2003). Reduced oxygen at high altitude limits maximum size. Proceedings of the Royal Society of London B (Suppl.) 270: S166-167.
Pörtner, H.O., Peck, L.S. & Hirse, T. (2006). Hyperoxia alleviates thermal stress in the Antarctic bivalve, Laternula elliptica: evidence for oxygen limited thermal tolerance. Polar Biology 29: 688-693.

Timofeev, S.F. (2001). Bergmann's Principle and deep-water gigantism in marine crustaceans. Biology Bulletin 28(6): 646-650.

Van Voorhies, W.A. (1996). Bergmann size clines: a simple explanation for their occurrence in ectotherms. Evolution 50: 1259-1264.

Woods, H. A., Moran, A. L., Arango, C. P., Mullen, L. & Shields, C. (2008). Oxygen hypothesis of polar
gigantism not supported by performance of Antarctic pycnogonids in hypoxia. Proceedings of the Royal Society B 276: 1069-1075.
Wu, N. & Mastro, J. (2004). Under Antarctic Ice. University of California, Berkeley CA.

Birds in a box, beetles on pins

For those involved directly in taxonomy and species identification, the function of biological collections is well known e.g. to provide reference specimens, and more recently to create a potential source of genetic material for molecular research. For others, it may seem a somewhat outdated, even macabre, activity, but this is not the case as long as ethical guidelines are followed - such as a 'code of conduct for collecting' (there may also be legislation covering collecting that varies from country to country, so do beware and check for protected species and permit requirements). During my recent visit to the Oxford University Museum of Natural History (which I've heard referred to rather dismissively as 'the dead animal building'), I came across two excellent examples of why biological collections are of key importance in life science research. The first is an area you may have noticed me writing about quite a lot - small beetles; the second is quite different but more widely familiar, at least in broad terms.

A tray of scarabaeid beetles from the Hope Entomological Collections at the OUMNH

Trays of insects on cards and/or pins is one popular perception of a biological collection, along with stuffed animals, skeletons and 'pickled things in jars'. Although this is no some extent rue (visually at least), their purpose is not always well understood. For example, one reason for my recent visit to the OUMNH was to consult the Hope Entomological Collections. With over 5 million specimens this is the second most important such collection in the UK after that at the Natural History Museum in London. My reason for wanting to consult the collection was to help finish my key to identifying British Chrysomelidae, in particular the last few tricky species of Longitarsus flea beetles - L. curtus, L. fowleri and L. membranaceus. These are superficially very similar and I wanted to check some characteristics so that I could decide how to separate them in my book. They are also tricky because (a) I don't have my own beetle collection (I have nowhere to store one), (b) there is a very good collection maintained by the Hampshire Museums Service in Winchester; however although it is only 10km away, it tends to only be open when I am at work (an increasing problem in the UK as local government funding cuts reduce staffing and thus opening hours), and (c) these three species are not available as clear online photographs, even at the excellent European Chrysomelidae website. So, I went to Oxford to have a look at theirs - in particular fine details of the heads.

Longitarsus membranaceus

Longitarsus curtus

Longitarsus fowleri

I won't go into great morphological detail here, but the result is that I can now tell these three species apart from details of their heads - and so will anyone else be able to once my key is published - but in summary, L. membranaceus has a distinctive broad bar running down the front of its head, the sides of the bar being more or less parallel where it runs between the upper halves of the eyes, and it has a narrow process extending down between the antennal bases. In L. curtus, there is a broad wedge rather than a bar and this meets the upper edges of the eyes. In L. fowleri, the bar broadens towards the top of the head but is still separate from the eyes. So, with specimens and a microscope, a fairly straightforward way to separate some very similar species without needing to dissect them - and one that does not appear in existing keys, but could not have been determined without access to a collection. Plus, as I remembered to take my camera, there are now some useful photos that will appear when these species are Googled! Now, moving on to the second example, I enter the realm of an iconic vertebrate, the dodo (Raphus cucullatus).

The Oxford dodo display

This was not something I specifically went to visit, though it is an important exhibit, not just because the dodo is a popular metaphor for extinction, but because of the the information that can be gained by having the specimen in a biological collection. It is well known that the dodo was first discovered by Europeans on Mauritius in 1598 and that it was extinct by 1680 (though probably due more to pressure from other introduced animals rather than hunting by humans - apparently it wasn't very tasty!). However, despite being so iconic, little was known about its biology and ecology until recently. The best-known contemporary images are 17th and 18th century paintings but their portrayals of fat dodos are now known to be inaccurate, with research since the 1990s indicating a much slimmer bird even if precise estimates differ and work is ongoing (Kitchener, 1993a, b; Angst et al., 2011). The importance of the OUMNH's dodo specimen lies in the fact that it is the only specimen in the world with soft tissue preserved (skin on the head) from which DNA could be extracted. When this was analysed, the dodo, and its close relative the solitaire Pezophaps solitaria from the (relatively) nearby Rodrigues island, were found to be most closely related to the pigeons within the family Columbidae (Shapiro et al., 2002). This is a key result as the dodo had previously been taxonomically linked not just to pigeons, but also parrots, shorebirds and raptors - partly due to the lack of evidence/specimens and partly because of the considerable amount of adaptation and specialisation that occurred in its island location that rendered it superficially unlike any other bird, apart from the similarly poorly understood solitaire. The research indicates that the dodo and solitaire separated from south-east Asian relatives around 40 mya while able to fly, and dispersed to the Mascarene Islands. The dodo and solitaire then separated around 26 mya; Mauritous and Rodrigues are much younger (only around 8 and 1.5 my old respectively) which implies that the birds used the now-sunken Mascarene island chain as stepping stones, with the isolation of Rodrigues implying that the solitaire was able to fly as recently as 1.5 mya.

So, although genetics is only one area of research, and like any other needs to be applied and interpreted appropriately, this is an example where a modern technique and a traditional biological collection were both required for research purposes and combined to produce important results - the dodo is much more than a stuffed bird in a case, and genetics needs real-world applications beyond 'bar-coding' of species. It also highlights the point that when a specimen is collected, its use may be unknown as this specimens dates from long before the concept of the gene had been thought of. For an overview of some other applications of this technology, Nicholls (2005) covers some important points, and for much more detail about the 'Oxford dodo', have a look at this excellent OUMNH factsheet which I mercilessly plundered for background information.

References

Angst, D., Buffetaut, E. & Abourachid, A. (2011). The end of the fat dodo? A new mass estimate for Raphus cucullatus. Naturwissenschaften 98(3): 233-236.

Kitchener, A.C. (1993a). On the external appearance of the Dodo Raphus cucullatus (L.). Archives of Natural History 20(2): 279-301.

Kitchener, A.C. (1993b). Justice at last for the Dodo. New Scientist. (28.8.93)

Nicholls, H. (2005). Ancient DNA Comes of Age. Public Library of Science Biology 3(2): e56

Shapiro, B., Sibthorpe, D., Rambaut,A., Austin, J., Wragg, G.M., Bininda-Emonds, O.R.P., Lee, P.L.M. & Cooper, A. (2002). Flight of the Dodo. Science 295: 1683.

Four of the best: spiny trilobites

I visited the Oxford University Museum of Natural History a couple of days ago for the 9th Coleopterists' Day (more about that soon). While I was there, I had a wander round some of the exhibits and was distracted by a display of excellent trilobites. I have a couple of fossil trilobites at home, but nothing as spectacular as what was on show, so I felt compelled to take a few photos and find out a little more about them.

There are many good introductions to trilobites so I don't intend to spend long on this aspect, but to give alittle background, they are a group of marine arthropods that lived from the early Cambrian (about 526 mya) until the mass extinction at the end of the Permian (about 250 mya). They were incredibly successful and diverse with about 17,000 species currently known from the fossil record. Some scavenged, filter-fed or hunted on the sea bed while others swam and fed on plankton. Some, mainly in the family Olenidae, may have had a symbiotic relationship with sulphur-metabolising bacteria from which they derived nutrition (Fortey, 2000). Also, they grew as nymphs/instars through a series of moults, each becoming progressively larger, much like many modern-day invertebrates. Despite the rich fossil record, the taxonomy and phylogeny of trilobites remains somewhat uncertain - currently it seems plausible that they fit into the clade Mandibulata (i.e. Myriapoda, Crustacea & Hexapoda) (Scholtz & Edgecombe, 2005) although they have been popularly placed in the clade Arachnomorpha which includes all other arthropods. The jury remains out on which is correct (or at least the most appropriate).

However, rather than diving into evolutionary biology/cladistics which isn't my area, I would like to look at some examples from the OUMNH collection and highlight some of the (to me) unexpected aspects of their morphology and the possible functions of such structures. I will however give some links to factsheets which provide details of the characteristics placing these species/genera in their currently accepted orders. Firstly, the frankly bizarre Walliserops trifurcatus with its unmistakeable trident...

Walliserops trifurcatus showing the long trident at the front end.

This species is in the family Acastidae within the Order Phacopida (factsheet). I have written about a member of this order before as I have a good-quality fossil of Phacops sp., including some fine details such as eye structure, but nothing quite as impressively odd-looking as this. The genus is unusual in showing a departure from strict bilateral symmetry, particularly in W. hammii with its sideways-curled occipital spine. However, the most eye-catching feature is of course the trident. Its function is uncertain due to a lack of data (Chatterton et al., 2006), although the presence of horns strongly suggests sexual dimorphism rather than having a primarily sensory, protective or hydrodynamic function (Knell & Fortey, 2005), though one or more of these could have occurred secondarily. Instead it appears most likely that it was used in competition between males (much like the horns and antlers of certain modern-day male beetles) and/or as a feature involved in mate selection by females ("ooo, what a big getting-in-the-way-when-feeding trident you have"). More data may eventually tell, but I can certainly imagine the trident being used to flip rival males. A close relative in the same family is Comura bultyncki which is well armed with spines, including some which curve off-centre, but no trident.

A specimen of Comura bultyncki

Moving on to the family Styginidae within the order Corynexochida (factsheet), Kolihapeltis chlupaci is another spectacular spined species, this one being from a relatively rare genus characterised by long backward-curving spines which grow from behind the eyes and the back of the head. K. chlupaci also has a ribbed 'tail' (more technically the 'pygidium' formed of rear body segments and articulating with the rest of the body). The role of the pygidum in the addition of new segments during growth and development is discussed in some detail here, with large pygidia such as this having a protective function as a shield when the trilobite rolled up (like a modern woodlouse). Some styginids have spiny pygidia (Holloway, 1996) which again suggests a defensive function, although these spines are not present in K. chlupaci.

Specimen of Kolihapeltis chlupaci

Lastly I want to briefly look at the family Lichidae within the order Lichida (factsheet), in particular, the genus Ceratarges (the species was given in the museum but was obscured in my photo, so I only have the genus for certain though I think it is C. armatus). This is another genus with prominent and impressive spines and horns. Some, such as C. spinosus have secondary spines - short 'thorns' pointing out at right-angles from the main spines. These again are likely to have an defensive function and I have to wonder whether they have a role in mate selection and competition between males as suggested for the tridents in the genus Walliserops.

Specimen of Ceratarges sp.

That's enough about these splendid beasts (for now). Given the cost of good-quality fossils of spiny trilobites, I don't think any will be added to my curio shelves soon, but if you are shopping for such things, beware fakes and learn how to spot them!

References

Chatterton, B., Fortey, R., Brett, K., Gibb, S. & McKellar, R. (2006). Trilobites from the Lower to Middle Devonian Timrhanrhart Formation, Jbel Gara el Zquilma, southern Morocco. Palaeontographica Canadiana 25: 1-179.
Fortey, R. (2000). Olenid trilobites: The oldest known chemoautotrophic symbionts? Proceedings of the National Academy of Sciences 97(12): 6574-6578.
Holloway, D.J. (1996). New early Devonian styginid trilobites from Victoria, Australia, with revision of some spinose styginids. Journal of Paleontology 70(3): 428-438.
Knell, R.J. & Fortey, R.A. (2005). Trilobite spines and beetle horns: sexual selection in the Palaeozoic? Biology Letters 1: 196–199.
Scholtz, G. & Edgecombe, G.D. (2005). Heads, Hox and the phylogenetic position of trilobites. In: Koenemann, S. & Jenner, R.A. Crustacea and Arthropod Relationships. Crustacean Issues 16: 139–165. CRC Press, Boca Raton.