Pondnet diary day 2

Continuing the Pondnet survey that began about a week ago, having done a preliminary survey of the main environmental features and whichever species could be spotted from the side, this time a net and white tray were required.

My wife/field-assistant doing some pond-netting.

the first thing we notcied was the great increase in frog tadpoles - not only in number (from a couple of hundred to at least a thousand as a rough estimate) but in size. A week previously the ones we saw were newly hatched - these had developed their typical fat-headed shape. They hadn't all hatched in the last week, so many must have been well hidden. The net also meant that we could confirm the species of the numerous small fish seen previously - they looked like sticklebacks but it's always worth checking, and all the ones we caught (and yes re-released) were indeed three-spined sticklebacks (Gasterosteus aculeatus).

A three-spined stickleback Gasterosteus aculeatus - the dorsal spines are just visible

Of course, it wouldn't be a proper sampling day (for me) if there weren't invertebrates involved. The netting meant we could see beyond the larger surface-dwelling species and maybe find some that, even if common, are less immediately familiar.

The water-slater or hog-louse Asellus aquaticus

The water-slater or hog-louse Asellus aquaticus is an isopod crustacean i.e. related to woodlice, and is common and widespread in Britain. It can be found in a very wide range of water bodies and qualities, but especially under aquatic foliage, stones and wood (Gregory, 2009), so is rarely seen by the casual observer without a net. It can be separated from the similar Proasellus meridianus by the head pattern although the photo doesn't show it as clearly as could be seen on the live specimen.

Another common-but-overlooked species is Plea minutissima, the 'least water-boatman'. It is broadly similar to other water-boatmen seen 'rowing' beneath the water's surface, but is tiny (around 1.8 - 2.8mm long) and its domed shape means that it is initially more likely to look like a small beetle than a water bug at a glance (Denton, 2007) - it certainly did to me when seen among the debris and other small species from the net. However, under the microscope (or even a squinted eye) it is quite different and the pointed mouthparts can be seen.

The least water-boatman Plea minutissima is the sole British species in the family Pleidae.

Lastly, I'd like to keep moving down the size scale to look at the water mites - arachnids of the suborder Hydracarina. These are mostly bulbous and the body in not separated into separate sections (cephalothorax and abdomen) as would be seen in the suborder Oribatei. Identification is tricky, but Hopkins (1961) can be downloaded from free here and is very useful for those beginning to study this tricky, and again often-overlooked, group. I won't go into detail about the identification here, but the water mite I investigated was Piona coccinea - a red species around 2-3mm long, red and globose in form.

That's where I'll leave pond-related matters for today - more soon!

The water mite Piona coccinea.

References

Denton, J. (2007). Water Bugs and Water Beetles of Surrey. SWT, Woking.
Gregory, S. (2009). Woodlice and Waterlice (Isopoda: Oniscidea & Asellota) in Britain and Ireland. NERC/BRC, Wallingford.
Hopkins, C.L. (1961). A key to the water mites (Hydracarina) of the Flatford area. Field Studies 1(3): 45-64.

The Lawn Shrimp cometh

I quite often receive invertebrates in the post, but they are usually leaf beetles (Chrysomelidae) sent to me for identification/verification in my capacity as organiser of the UK's Chrysomelid Recording Scheme - like here for example. However, a couple of days ago I came home to find something quite different awaiting me on the doormat - a crustacean looking like a small shrimp, or to be more precise an amphipod of the family Talitridae (a group usually associated with seashore habitats rather than inland terrestrial ones).

In this case, it had already been identified as Arcitalitrus dorrieni, the 'landhopper', 'woodhopper' or 'lawn shrimp' by the finder/sender, Dennis Trunecka of the Southampton Natural History Society. This is an interesting find as it is Australian in origin (New South Wales & Southern Queensland), with the first UK record being from the Scilly Isles in 1924. Since then, it has been found in a number of sites across southern England, and also in Ireland, the Channel Isles, west Wales and western Scotland (coastal when north of southern England). However, it is not entirely clear how widely it has established itself in the last couple of decades, although individuals can move tens of metres per day as well as being moved over longer distances by the plant trade etc. (Cowling et al. 2004).

Arcitalitrus dorrieni found in woodland leaf litter in Hampshire. The seven segments of the peraeon and the three segments of the pleon are indicated. Length (head to rear of body in this curved position) approx. 6.5mm.

It is most often found under stones and dead wood or among damp material (detritus, debris, leaf litter) in gardens, damp scrub and woodland. It most likely arrived (and to some extent spread) in the UK through the transport of plants/soil to and between plant nurseries and garden centres. It is uncertain whether this non-native species has a significant ecological impact in the UK, although it is possible that it competes with (and maybe replaces) native detritivores in woodlands. In some locations, it can be found in high densities - up to approximately 2,500 per square metre in Dicksonia antartica litter on the Scilly Isles (Richardson 1980). It certainly can be a significant detritivore, consuming 24.7% of annual litter fall in a coniferous woodland in Ireland (O'Hanlon & Bolger 1999) - more than any of the native macrofaunal species.

Identification is fairly straightforward, especially given the small number of possible confusion species. Orchestia cavimana  is an introduced semi-terrestrial Mediterranean amphipod (Konopacka et al. 2009) but much paler in colour - A. dorrieni is variably dark, and orange when dead as here, though pale if preserved). However, there is another introduced terrestrial amphipod, A. sylvaticus, although this is much rarer in the UK. Using the key in Peart & Lowry (2006), the two species can be separated by looking at the epimera (the three segments of the pleon, singular 'epimeron'). In A. dorrieni, the 2nd epimeron is longer than the 3rd while in A. sylvaticus they are more-or-less equal. In the top photo, this is unclear as the rear edge of eipermon 3 is obscured by one of the legs, but with some legs (re)moved, it is clear that this is A. dorrieni. There are other features which might be required to separate further species but these do not (yet) occur in the UK, although it is possible they could be imported with plants.

A. dorrieni - from the green lines, it is clear that epimeron 2 is longer than epimeron 3.

The head bears numerous appendages including two pairs of antennae (typical of crustaceans) and a complex array of mouthparts - I won't go into the details here but there are plenty of resources online and in print providing introductions to crustacean anatomy. The lateral compression is clear (flattened side-to-side) and is generally a good way of separating amphipods from isopods (e.g. woodlice which are flattened top-to-bottom i.e. dorso-ventrally). The antennae are inserted in front of the eye which is black and not especially well developed, being covered by a transparent plate. This is likely to be an adaptation to its life within/under leaf-litter and under damp material where vision is less likely to be useful than senses such as touch - note the long antennae and various bristles.

A. dorrieni showing its lateral compression.

Head of A. dorrieni (side view)

Mouth and mouthparts of A. dorrieni (ventral view)

So, an interesting find and thanks to Dennis for passing it on to me for closer scrutiny. As ever, finds such as this are useful in determining the distribution (and in this case, spread) of species, so it is worth keeping an eye out - especially in case a third Arcitalitrus finds its way here.

References

Cowling, J.E., Spicer, J.I., Weeks, J.M. & Gaston, K.J. (2004). Current status of an amphipod invader, Arcitalitrus dorrieni (Hunt, 1925) in Britain. Journal of Natural History 38: 1665-1675. 
Konopacka, A., Grabowski, M., Bącela-Spychalska, K. & Rewicz, T. (2009). Orchestia cavimana Heller, 1865 (Amphipoda: Talitridae) enters freshwater inland habitats in the Vistula River, Poland. Aquatic Invasions 4(4): 689-691.
O'Hanlon, R.P. & Bolger, T. (1999). The importance of Arcitalitrus dorrieni (Hunt) (Crustacea: Amphipoda: Talitridae) in coniferous litter breakdown. Applied Soil Ecology 11: 29-33.
Peart, R. & Lowry, J.K. (2006). The amphipod genus Arcitalitrus (Crustacea: Amphipoda: Talitridae) of New South Wales forests, with descriptions of six new species. Records of the Australian Museum 58: 97-118.
Richardson, A.M.M. (1980). Notes on the occurrence of Talitrus dorrieni Hunt (Crustacea: Amphipoda: Talitridae) in south-west England. Journal of Natural History 14: 751-757.

A load of old Balanus

We're all familiar with barnacles, at least in broad terms - we see them on rocks, ships, whales etc, they turn up in A-level college biology projects, and if we slip on them, they are sharp and they hurt. However, how often do we really look at them?

A typical view of a barnacle-encrusted rocky shore (Start Point, south Devon, England)

They are of course crustaceans (of the class Cirripedia, order Thoracica) and come in three main forms, the suborders Lepadomorpha (the stalked 'goose' barnacles), Verruomorpha (like 'typical' barnacles but with only two opercular plates - I'll get on to that later), and Balanomorpha (the 'typical' barnacles'). I'm only going to focus on the latter, though I can't let the opportunity pass by without mentioning the existence of the order Rhizocephala. These are evolutionarily derived relatives of the barnacles and parasitise decapod crustaceans (such as crabs), their free-swimming juveniles settling on their hosts and developing into an 'interna' of root-like growths and an 'externa' which is a sac of reproductive parts. They lack as body as such, and their name means 'root head', and as with many internal parasites, they redirect their hosts' physiology and behaviour to their own ends. If I find any, I'll definitely write about them, but until then, here's a fine little 4-minute video introducing their life cycle through the medium of hand-drawn cut-out animation.

Anyhow, back to barnacles as we know them... The rocky shore above was well encrusted with barnacles of several species. There were many of the small limpet-shaped ones (such as Chthamalus stellatus in the family Chthamalidae), but what grabbed my attention were some larger, less flattened specimens typical of the the family Balanidae.

A barnacle of the family Balanidae.

As you can see, this barnacle is fairly tall (rather than being a low flat cone). Although the joins are rather obscured in this mature specimen, there are six plates forming the outer wall. It isn't clear from the photo, but the base forms a calcareous layer on the underlying rock (rather than a membranous layer) and hence this is in the genus Balanus. Identification past this point is a little tricky because the colours are so variable and corrode to a greyish mishmash in old specimens, but the obscure sutures, sharp 'beak' (tergum) and ribbed outer surface suggest either B. perforatus or B. balanus.

The scutum (seen here as a 'keel' beneath the sharply beaked tergum) is slightly saw-edged, and the smaller barnacles (which I assume are the same species) below the large one have yellowish rims with brown banding below/around them. Also, the opening in the large specimen isn't especially small. These features combine to suggest that this is B. balanus - a common species although south Devon is at the very SW extent of its range as it is absent from Cornwall, but is a place where this species has been regularly recorded. A closer look should also help clarify a couple of the features mentioned here.

The operculum, tergum and scutum of Balanus balanus

Aside from the smaller barnacles that are using it as a substrate, the beaked tergum is clearly visible as is the rough, serrated edge of the scutum. Both these structures are paired (you can see the join at the top of the tergum), forming four internal plates (collectively the 'operculum') that can move and open to allow the feeding structures (feathery 'cirri') to waft in the water and catch food particles.


So, a closer-than-usual look at a familiar organism - and a relatively rare (for me) foray into marine and littoral/intertidal habitats. Maybe I'll have to do more on this as it was the habitat that first grabbed my attention in terms of the invertebrate fauna...

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.

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.

Some are squat and some are squatters: more of the intertidal

About a year ago when my blog was just getting going, I posted photos of some specimens from a Southampton Natural History Society (SNHS) visit to Calshot Beach in southern England, a site with areas of stones, gravels, sands and shingle as well as nearby saltmarsh (the latter not covered here). That visit produced some excellent views of spectacular species such as the Dahlia Anemone Urticina felina, and we were keen to see what could be found this year. This is an annual SNHS event and the findings go towards building up a late summer/early autumn species list for the site - this year, although we picked low tide, it wasn't as low as last year's so we expected a somewhat different range of species.

Some species, or their signs, were familar from last year, such as the shells of Haminoea sea slugs, the Snakelocks Anemone Anemonia viridis, and the introduced North American bivalve, the Quahog Mercenaria mercenaria. Many others were different however - some are presented here and I hope you enjoy them. The first group I want to cover are the molluscs, starting with some primitive armoured species (the chitons) and then a larger (and edible) introduction.

Lepidochitona cinerea (family Ischnochitonidae). Probably the commonest North European chiton, this species is often found on the underside of stones, though this one was on a large bivalve shell. Though it looks smooth, the valves (sections) are rough to the touch, like fine sandpaper. This specimen is about 15mm long - the maximum is about 24mm.

The fairly common Acanthochitona crinitus (family Acanthochitonidae). The valves are less smoothly arranged than in L. cinerea and there are 18 tufts of coarse bristles. This specimen is about 20mm long - the maximum is about 34mm.

Chitons (class Polyplacophora) have a mantle skirt which forms a toughened 'girdle' around the whole edge of the animal and this is where the fringing spines etc are found. The head is small and covered by the girdle and the dorsal surface generally has armoured 'valve plates' as seen here. They graze plant and algal material from hard substrates and, like limpets, are able to withstand wave shocks without being dislodged.

The introduced Mediterranean/Biscay species Crassostrea gigas, the Portuguese Oyster. The shell has several large wrinkles and smaller concentric lines. It can grow to 180mm in length and is attached to the substrate at the hinge end - here it is attached to a dislodged stone.

Sticking with shelled species, but adding legs, a number of crustaceans were also found. As well as the common shore crab Carcinus maenas, some possibly less familiar species were worthy of a photo or two...

OK, this probably is quite familiar - it's the common hermit crab Pagurus bernhardus, using the shell of a Netted Dog-whelk Hinia reticulata. Gotta love hermit crabs! Note the larger right claw which is covered in small knobs or 'tubercles'.

One of my favourites, the Hairy Crab Pilumnus hirtellus. It is covered in hairs which are broader at the tip than the base and help camouflage it among the sediment and detritus of the intertidal zone. The shell is up to 20mm across, which is about the size of this specimen, doing its best to hide in a white tray with a few scraps of seaweed.

Another favourite, and less commonly seen, the squat lobster Galathea squamifera. The rostrum is triangular and pointed with 4 spines on each side, the rear ones also being the smallest. These spines are usually red-tipped (as here) as are those on the outer edge of the claws. The claws are also covered in flat scale-like tubercles. Despite the claws, it filter-feeds on suspended detritus.

The shrimp Palaemon elegans with a straight (rather than clearly up-curved) rostrum and dainty little claws. Note the telson (the flap at the end of the tail) doesn't have any lateral spines. These features separate it from similar species such as P. serratus.

Lastly, I'd like to introduce a couple of soft-bodied species - not the large showy ones, but a couple that are often likely to be overlooked.

The sea anemone Sagartia troglodytes. Similar to S. ornata (some specimens may be extremely difficult to separate to species), but this one clearly shows the pale brown column darkening at the top with speckles. Note the numerous tentacles; there may be up to 200 arranged in a roughly hexagonal pattern (here it is clear that the arrangement is not circular). The attachment to a buried stone is typical.

The Leathery Sea-squirt Styela clava. This is an introduced Pacific species and can be found attached to stones (as here) and pilings around the south-west coasts of Britain.

As with last years post, this is only a snapshot of what can be found on a diverse section of intertidal habitat, but hopefully an interesting one. However, it does illustrate how important volunteers are for recording wildlife, especially less 'popular' groups like many in marine and intertidal habitats - and with knowledge comes at least the potential for conservation.


References

Crothers, J. & Crothers, M. (1988). A key to the crabs and crab-like animals of British inshore waters. Field Studies 5(5): 753-802 (revised reprint).
Gibson, R., Hextall, B. & Rogers, A. (2001). Photographic Guide to the Sea & Shore Life of Britain & North-west Europe. OUP. Oxford.
Hayward, P.J. & Ryland, J.S. (eds) (2000). Handbook of the Marine Fauna of North-West Europe (2000 revision). OUP, Oxford.

Further reading

Crothers, J.H. (1997). A key to the major groups of British marine invertebrates. Field Studies 9(1): 1-177. Very useful if you are new to marine invertebrates.
Hayward, P.J. (1988). Animals on Seaweed. Richmond, Slough. For those interested in generally small intertidal species found attached to seaweeds.
Hayward, P.J. (1994). Animals of Sandy Shores. Richmond, Slough. Not used here although Calshot Beach is sandy in places and supports some of the species in this book.
Hiscock, S. (1979). A field key to the British brown seaweeds (Phaeophyta). Field Studies 5(1): 1-44. Useful alongside Hayward (1988).

Spider crabs - spiny substrates on legs

We're all familiar to some extent with crabs and other crustaceans - the shell or carapace, the pointed walking legs, the first pair of legs with pincers and so on. Around the (mainly southern and western) coast of Britain, one large and distinctive species is the Spiny Spider Crab Maja squinado which can have a carapace up to 20cm long (not to mention the long legs), although the specimen below is about half this size. This is a fairly shallow-water species, found sublittorally to about 50m depth, but sometimes also in deep littoral pools low on the shore.

Dorsal view of the carapace showing bumps (tubercles) and spines, including the two larger frontal spines of the rostrum seen spreading apart at the top of the photo. These rostral spines are straight unlike those of M. verrucosa which are curved outwards. Note the growths of other organisms such as seaweeds, hydroids and sponges.

The spines have clear potential as protection against predators, and can also prove uncomfortable to the unwary paddler if a carapace is trodden on (fortunately they are not poisonous like some organisms such as weaver fish) and indeed may hide under loose substrates such as coarse sand. Their ability to hide is enhanced by the range of other organisms that use them as a substrate to grow on e.g. sponges, hydroids and seaweeds - this allows them to blend in with other substrates supporting similar organisms. For those of you who like fairy tales, think about Baba Yaga's walking hut from the traditional Russian story, or indeed the Studio Ghibli animation Howl's Moving Castle!

A close-up of the upper surface of the carapace.

Zooming in on the carapace, the tubercles can be seen to have tufts of bristles - a close look shows that where some have broken off, tiny holes remain in the surface of the carapace. Such bristles or hairs are also seen on the legs following a moult, though these are gradually rubbed off. More of these bristles later...

The inner surface of the carapace.

In contrast to the patchily pinkish, rough outer surface (could this texture be to aid the attachment of organisms?), the inner surface, with no need to be involved in camouflage, is smooth and white. The dimples show that the tubercles are hollow, reducing weight (even so, with their spindly limbs and large bodies, large spider crabs can't support their own weight out of water) while the wrinkled arc shows where muscles and other tissues were attached when the animal was alive.

Another view of the underside; this time some of the points of attachment for the mouthparts and related appendages. The basal segment of one such appendage remains - its pair shows an empty socket.

A close-up of one of the articulations on the underside of the carapace.

The first of these two photos shows the membrane forming part of the joint at the base of the appendage, but some hairs are also visible. The second photo shows such a brush of hairs in more detail. They are simple unjointed structures and are likely to be guard hairs with the function of keeping sand grains and other unwanted material out of delicate joints.

The V-shaped notch between the two rostral spines showing numerous bristles as well as the tiny holes where some are missing.

The spaces between spines around the edge of the carapace show similar bristles. These may again be there to trap unwanted material (and maybe they help with camouflage by breaking up the outline, especially if they collect seaweed fragments and similar), but the crab also has much tinier, finer sensory hairs (not visible here) which also need to be protected by guard hairs. Of course, crabs do not rely solely on sensory hairs - they also have compound eyes on mobile stalks.

One mottled compound eye on its mobile stalk. This mobility means the eye can be tucked away into a notch in the carapace to prevent damage.

A close-up of the eye showing individual lenses and the 'seal' around the edge. Red and green iridescence is also visible just below the centre of the photo.

The base of the eye-stalk again showing the guard hairs which help to protect joints from damage by unwanted materials.

This has been a fairly brief tour of spider crab morphology, even though it covers only the carapace - if I find some during a bout of beach-coming, I may write a follow-up covering legs and other appendages. Until then, enjoy the story so far and we'll see what the tide and strandline bring...


References

Crothers, J. & Crothers, M. (1988). A key to the crabs and crab-like animals of British inshore waters. Field Studies 5: 753-806. Revised version available here.
Gibson, R., Hextall, B. & Rogers, A. (2001). Photographic Guide to the Sea & Shore Life of Britain & North-west Europe. OUP, Oxford.
Hayward, P.J. & Ryland, J.S. (eds) (2000). Handbook of the Marine Fauna of North-West Europe (reprint with corrections). OUP, Oxford.

Ants round my home, woodlice in theirs...

My entomological side is getting twitchy - after months of near-invertebrateless cold, I can see a variety of multi-appendaged beasts crawling, flying and generally being busy. So, during a trip to dig the allotment, I couldn't help noticing quite a bit of ant activity. They haven't found the house (i.e. the kitchen) yet but are busily working in the garden, around all kinds of cracks and various bits and pieces. As I left through the back gate, there were even some that had helpfully started work at head height (in and around two bolt-holes in a gatepost) allowing me to look at them closely without crouching down: most thoughtful...

Black ants busy around a hole in a gatepost.

Looking more closely, it seemed as if they were clearing out the remains of previous occupants - there looked to be bits of woodlouse and spider coming out. Closer still, building work was going on with 'cement' plates being formed inside the hole - luckily I managed to capture one ant placing a mouthful (mandible-ful?) of building material...

Note on the left - one ant placing building material

They look like one of the common dark garden species, but it's always worth checking so I brought a couple indoors to see if I could key them out (using Skinner & Allen 1996). The single-segment waist with well-developed petiole (a little plate sticking up from the waist), circular spiracle towards the rear of the propodeum (segment just in front of the waist) and a lack of rows of bristles on the underside of the hind tibia brought me to the genus Lasius (not unexpectedly...). From there, various features including the presence of standing hairs on the antennal scape and hind tibia (but not the double rows seen in other groups) took me to the common species L. niger.

Hind tibia with standing hairs

Head, including antennal scape with standing hairs (see below the eye)

OK, so I've been distracted - time to dig the allotment and off I go... digging (finally) takes place and I'm almost finished when I notice (a) some orange-yellow ants and with them (b) some tiny white woodlice. This looks straightforward enough - the ant is Lasius flavus nesting in the grassy path and the woodlouse is the Ant Woodlouse Platyarthrus hoffmannseggi.

L. flavus and P. hoffmannseggi - both around 3-4mm long

P. hoffmannseggi - the yellow colour is just due to lighting/image processing...

P. hoffmannseggi is an interesting beast - it is white (in the top photo you can just see dark gut contents through its dorsal surface), blind and lives closely associated with ants of almost any species (Hopkin 1991), often found in their nests where it may feed on pellets regurgitated by the hosts (Williams & Franks 1988) and/or may be a more general scavenger in the 'rubbish-dump' parts of nests (Pontin 2005). It has also been noted as following lines of ants when nests have moved (Donisthorpe 1927). This is unusual behaviour for a woodlouse (it is the only myrmecophilous - 'ant-associated' - woodlouse in Britain), but not unique - in Malaysia there is a species of Exalloniscus which 'hitchhikes' (maybe 'stows away' is more accurate) on the pupae of ponerine army ants. Although not often seen without digging, it is fairly common in southern England and Wales, becoming scarcer northwards (will climate change extend its range, and if so, how does it disperse?). Beyond this, little is known about P. hoffmannseggi - an area for a keen researcher to work in maybe?

References

Donisthorpe, H. St. J. K. (1927). The Guests of British Ants.Routledge, London. [interesting if you can find it, but can be expensive]
Hopkin, S. (1991). A key to the woodlice of Britain and Ireland. Field Studies 7(4): 599-650. [this has also be published in separate reprint form]
Pontin, J. (2005). Ants of Surrey. Surrey Wildlife Trust, Woking. [another volume in the excellent series of Surrey atlases]
Skinner, G. J. & Allen, G. W. (1996). Ants. Richmond, Slough. [excellent keys to British species]
Williams, T. & Franks, N. R. (1988). Population size and growth rate, sex ratio and behaviour in the ant isopod Platyarthrus hoffmannseggi. Journal of Zoology 215: 703-717.