Smelly, slimy and slithering

We have a wildlife-friendly garden, and part of that is a pesticide-free compost heap. When it needs to be turned over, the usual creatures are plentiful - earthworms, slugs, woodlice and so on - but sometimes something less familiar appears, brought to me by my loving wife...

A mass of about 20 worm-like creatures attached to a decaying slug.

I usually deal with invertebrates with legs, but I like a challenge so, holding my nose (the slug-remains were highly fragrant!) looked more closely.

One of the 'worms' off exploring.

It was clear very quickly that these weren't leeches. Although they moved like them, they didn't have the segmentation or mouthparts - instead they were a type of flatworm. A quick look at Jones (2005) told me they were Kontikia ventrolineata, an Australian species introduced through the ornamental plant trade.

Kontikia ventrolineata - the pair of grey lines on the dorsal surface is a key identification feature.

Kontikia ventrolineata - the series of light and dark bands on the ventral surface is another key identification feature, and give it its specific name.

The garden plant trade has introduced several Australasian flatworms to Britain,and some such as the Australian flatworm Australoplana sanguinea and the New Zealand flatworm Arthurdendyus triangulatus can be problematic as they are predatory and hunt earthworms, and may impact on populations of our native species which are so important for soil quality. Fortunately K. ventrolineata is probably less troublesome as it feeds on small snails and possibly slugs, as well as (in this case) scavenging. As yet, I am unaware if it has an impact on our native molluscs, though it is widespread in southern and southwestern England (and as I understand it has been found as far north as Scotland, although the NBN currently holds no records). So, observations and data are always welcome, and if you see this species in England or Wales, let Hugh Jones know via the Natural History Museum in London (scroll down, he's a Scientific Associate), or in Scotland, you'll want Brian Boag who works on introduced and invasive species.

Reference

Jones, H.D. (2005). Identification: British land flatworms. British Wildlife 16(3): 189-194.

In spring the pond goes sproing

After a long, cold early spring, the last couple of weeks have been warm, dry and sunny - and the season has leapt at the opportunity. Butterflies are flying, birds are nest-building, bees are pollinating. All good things. As a microcosm of this rapid seasonal burst of activity the denizens of our garden pond have been distracting me from computer-based tapping and drawing me out to sit by and watch, camera in hand. Our pond's not huge, but a lot of care was taken to ensure variety of structure (and thus habitat) - here are a few of the occupants taking advantage of it.

The fly Elgiva cucularia. It is a member of the family Sciomyzidae that specialise in hunting snails - of which we have plenty. This species is associated with boggy conditions and I have recorded it several times on our pond vegetation, so the shallow, algae filled section is clearly doing its job.

A Large Red Damselfly Pyrrhosoma nymphula posing obligingly on my finger.

One of at least six smooth newts living in our pond. This shot was taken with a waterproof endoscope which I'm still new at using.

Some of this year's batch of frogspawn.

Two pond snails busily engaged in making more snails. The whitish structure top-right is indeed a snail penis AKA 'love-dart'.

Ghosts in the shell

You probably don't know, but I have a pet giant African land snail (Achatina fulica) called 'Chickpea' because that's what s/he (the species is hermaphrodite) looked like when young. S/he's now a bit larger and 'Brazilnut' might be more accurate.

Chickpea the not-yet-giant African land snail.

However, the keeping of exotic invertebrates is not today's topic. While cleaning Chickpea's tank, I noticed some tiny white dots moving rapidly around the snail's foot and disappearing behind it. Of course, I grabbed a camera and then went online. It didn't take long to find out that this was an infestation of Riccardoella mites, probably R. limacum.

Riccardoella mites.

More Riccardoella mites.

I use the word 'infestation' because these are parasitic. They were previously though to be commensal, feeding harmlessly on mucus, but are now known to be blood-feeders. This may not be too much of a problem unless they are numerous, but their feeding can lead to secondary infection via the tiny wounds. They are also difficult to remove as they are very swift and hide in the pneumostome (breathing pore). Rinsing the snail with water is likely to help limit them, and if lucky may remove them all eventually, but the only other known option is to buy the predatory mite species Hypoaspis miles which will target them, although these mites are only available in large quantities for use across whole gardens, allotments and so on up to larger scales. It is likely that the mites came in with the soil used in Chickpea's tank and they are common in the wild, being found on various native slugs and snails. So, I will see if washing works, and if not, maybe a 'mite-share' scheme will do the job. Until then, I don't have access to electron microscopy, but if you'd like more info and some excellent images, I recommend this page.

Snails - reaching the crunch-point

I like snails; I may garden, but I like them anyway, and I really hate standing on them as they roam across footpaths at night. However, there are many dangers awaiting snails - hungry corvids for example - and damage can occur that doesn't leave them squashed. However, apart from minor chips and so on, it often feels like they must die if their shell is broken, after all dehydration seems likely aside from any internal damage. This specimen of the common garden snail (Cornu aspersum or Helix aspersa depending where you look - I won't go into the taxonomic disagreements here) found in our garden yesterday shows that this is not necessarily the case...

A garden snail showing a large shell-injury

The shell injury showing where membranes have rejoined to it from the body. There are also cracks on the spire, indicating the considerable extent of the damage it survived.

Although it may not be obvious from the photos, this is not a new injury and the snail was highly active. The exposed surface is dry and has re-attached to the broken edges of the shell; although snails can, and do, mend minor shell-damage as they grow (by secreting new calcareous material), that is unlikely to happen with such a large hole. So, how did the snail survive?

Well, assuming no majo damage to the body itself, the main issue is water loss. The shell retains moisture but of course water can be lost through the main opening (aperture). Therefore, during dry and/or cold conditions, the snail withdraws and seals the aperture using a thin 'skin' of dried mucus, called an 'epiphragm'. Less obviously, it can also avoid damage from freezing, not only by seeking sheletered locations, but also through an antifreeze mechanism in its haemolymph (equivalent of blood). In warmer dry conditions, the edge of the internal 'skin' or mantle (i.e. where it meets the edge of the aperture) can change its permeability to water and thus further help prevent drying-out (Machin, 1966).

A broken shell however presents a whole new aperture through which water can be lost. A search for publications on snail-shell mending, show that Andrews (1935) looked at snails of the genus Neritina and noted a variety of mechanisms and examples, but Durning (1957) described the process from a medical perspective. He noted that the snail secretes a glycoprotein matrix in order to provide a substrate onto/into which calcium carbonate can be secreted in turn i.e. shell material can't be produced directly onto the edge of breaks, and there are few cells there to secrete directly. So, a matrix is needed. This is something that will be happening here to some extent, but the membrane around the mantle also seems to have toughened and, unless the damage is much more recent than it appears to be, this snail survived the recent long winter with this hole - a common species that I'm now looking at from a totally new perspective.

References

Andrews, E.A. (1935). Shell repair by the snail, Neritina. Journal of Experimental Zoology 70(1): 75–107.
Durning, W.C. (1957). Repair of a defect in the shell of the snail Helix aspersa. J Bone Joint Surg Am 39-A(2): 377-93.

Machin, J. (1966). The evaporation of water from Helix aspersa IV. Loss from the mantle of the inactive snail. Journal of Experimental Biology 45: 269-278.

2012 on the Spot

It's that time of year - the dribbly brained devotees of Mayan stonework-paranoia were wrong and we're still here on New Year's Eve, which means it's time for a run-down of some of my favourite Ecology Spot posts of the last twelve months.

January included a series of four posts about my pet Macleays's Spectre stick insects - I'm not sure I have a favourite as such, though part 2 (the males) does include a shot of one of the boys taking off which was seriously tricky to capture, plus one of my wife's head while she was being used as a launchpad for their aeronautical adventures.

February shifted from tropical-terrestrial to polar-marine with a look at gigantism in Antarctic sea-spiders, inspired by a visit to the Oxford Uni Museum of Natural History.

March was unusually warm and this got me out in the field (well, a churchyard) looking at the bees of a grassy bank, and very pleasant it was too.

In April, more fieldwork led to an unusual aphid/gall find and needed consultation with an aphid specialist to work out - quality collaboration!

May included a variety of invertebrates, including more collaboration, this time in order to identify a tricky pseudoscorpion specimen - a group I've never looked at before.

June saw a rare example of my use of video, looking at the leaf-mining fly larvae in Solomon's-seal.

Into July, and despite the horribly wet summer, I continued to record bee species in our garden (despite the weather, a range of bee-friendly features still attracted them), and reached the grand total of twenty species and counting.

As the summer finally dried out a bit in August, I shifted briefly away from British invertebrates and looked at a poison-arrow frog - you'll see why when you get to the end...

September saw me get a bit speculative (or odd, depending on your point of view) as I looked at why Smurfs are like slipper limpets. Yes, really...

In October, normal service was resumed and I wrote a series of three posts about tarantula anatomy, the third of which looked at their various appendanges (e.g. feet and fangs) - not one for arachnophobes!

November was a busy month - not only did I go on holiday and come back full of ideas realting to things I'd seen during an unseasonally warm week in Devon, but my key to leaf beetles of the British Isles was also published, oh yes :)

And finally onto December where a rather unusual Christmas-party gift led to some unexpected microscope work and a specimen from an unexpected kingdom...

That's all from me for 2012 - back after the New Year celebrations which should involve a ridiculous hat of some sort...

Yup, that's me in a splendid hat with a gun of rum...

Why Smurfs are like slipper limpets

Yes, I do mean Smurfs, those little blue Belgian cartoon characters... and slipper limpets are marine gastropods, Crepidula fornicata. So, why are they similar? Well, you probably know that, although there are lots of Smurfs (101 in fact), only one is female - Smurfette. Now, this could easily lead into pornographic territory (and undoubtedly has, somewhere on the Internet), but that's not what the Ecology Spot is about... instead I want to be a bit speculative and look at how this might affect Smurfs biologically if they were real...

One possibility would be that they became eusocial (like ants, bees and termites for example), with Smurfette as the only reproductive female (I assume Smurfs are viviparous, but maybe there are Smurf eggs - who knows?). However, Smurfette does not appear to be a large sedentary egg-layer (or large sedentary birther-of-live-young Smurflings), nor do there appear to be non-reproductive females rendered infertile by Smurfette pheromones. This is the case in, for example, the honey bee Apis mellifera, where the queen emits Queen Mandibular Pheromone (QMP), a pheromone set which, among other functions, inhibits ovary development in other females. So, the queen bee remains on the throne, and the princesses have to wait in line.With no other females present, and Smurfette running around actively, this seems unlikely. Instead, I think Smurfs might be an example of sequential hermaphroditism (SH).

One of the best-known examples of SH is C. fornicata. Though native to the eastern coast of North America, it has been widely introduced into the coastal waters of Europe, Japan and the NW Pacific, where it is invasive (having no predators away from its original range), competing with native filter-feeders for food. For more on its British history see here.

A stack of C. fornicata (plus a small chiton on the left) - photo by F. Lamiot, and used here under the Creative Commons Attribution-Share Alike 1.0 Generic license.

They can often be found in stacks and chains, their SH reproductive strategy meaning that the largest, oldest individuals, found at the base of  the stack are female, while the younger, smaller ones at the top are male, and some in between are 'transient'. If the female(s) die, the largest male becomes a new female.

Proestou (2005) showed that C. fornicata tended towards a 1:1 sex ratio, and that as a male's distance from a female increased, his reproductive success decreased i.e. that the males closest to the female have a competitive advantage. From this, it follows that if these males suffer a reduction in reproductive success (e.g. from competition with other males) that is greater than that due toswitching sex at a small size, then they should change. Only the lowest male in a stack can change sex, a process that takes around 60 days, during which the penis regresses and the pouches and glands of the female duct develop. If a juvenile settles on an existing stack, it develops as a male and may stay like this for up to 6 years due to pheromones released by females at the base of the stack (Fretter & Graham, 1981). Presumably the death of a female means this pheromone ceases to be produced and thus the male can change sex - another process must prevent others from changing, possibly pheromones from the new female-to-be? As there are 'transients' which complicate the picture, a pheromone gradient seems plausible.

So, although the sex ratio is different in Smurfs (100:1 rather than 1:1), an SH strategy fits well. If Smurfette dies, then as the oldest male, Papa Smurf should become Mama Smurf, with some of the others (who after all, could be 'transient' and we wouldn't know by looking at them) waiting in line.

Next post - normal service will resume!



References

Fretter, V. & Graham, A. (1981). The Prosobranch Molluscs of Britain and Denmark. Part 6. Journal of Molluscan Studies Supplement 9: 309-313.
Proestou, D.A. (2005). Sex change in Crepidula fornicata: Influence of environmental factors on reproductive success and the timing of sex change. Dissertation, University of Rhode Island.

After the rains and beyond the pale...

...or 'from floods and the 2012 wildlife apocalypse to meadow creation via the wordy worlds of genetics and biochemistry'.

If you've been in the UK during the middle of 2012, you will have noticed that it's been raining a bit. Well, I say 'a bit', I actually mean a lot. Really a lot - following an unusually hot spring, it was wet from April to mid-July, including the wettest April for a century, and the wettest June on record. It's well understood that this was because the jet stream looped south of the UK and stayed there, but what is less well known is why this happened - this is an area of active research (including links to climate change) and there is an excellent summary here.

It has also been widely reported in both national and local media (e.g. here and here) that this unusually lengthy and heavy rainfall has had a huge impact on British wildlife - some species such as ringlet (Aphantopus hyperantus) butterflies (which breed in damp grassy areas) can do well in wet conditions, but current predictions are that 2012 will be the worst year on record for British butterflies overall. Volunteer recording will be hugely important in determining the effects on this group and there is still time to join in with the Big Butterfly Count which runs until 5th August. Although slugs and snails have done well (much to the annoyance of gardeners and vegetable growers), winged insects (including those most eseential of pollinators, bees) have fared poorly, being unable to feed or find mates effectively in cold, wet conditions and therefore are also unable to reproduce successfully. Although some mollusc-feeders may have plenty of food, this may well not lead to a good year for amphibians as the hot spring dried up breeding ponds and the April rainwater was too cold for reproduction. Thus, despite a few winners, the effect has largely been an overwhelmingly negative one e.g. many birds have been unable to keep chicks warm and fed, while many species, such as puffins, have seen nests flooded.

A pair of common garden snails Cornu aspersum (often known as Helix aspersa)

With widespread breeding failures, local extinctions have been predicted and conservationists are rightly worried about how severe the impacts will be, especially as the summer of 2011 was also poor. However, with the weather changing to become hot and sunny about a week ago, hopes for a good late summer and autumn have been fuelled, and I have to wonder to what extent various species can make up for lost time while conditions are suitable. Certainly my own observations, and those of other naturalists I've been in contact with, suggest that there is currently a rapid burst of invertebrate adult emergence, including species usually encountered earlier in the summer. Some moths which have emerged late have been reported as unusually pale, although this is from a fairly small number of observations, so its importance remains unclear. Delayed moth emergence related to temperature is well documented (e.g. DuRant 1990) but there does not appear to be any mention in the literature of aberrant colouration due to such delays. Certainly, a pale colour does not suggest an adaptive response as darker colours are generally associated with cooler temperatures as they permit more rapid warming by absorption of solar radiation. So, my question is whether delayed emergence can lead to paler colouration through some effect on the mechanisms related to pigmentation. Although this idea is somewhat speculative, two possibilities (which are not mutually exclusive) spring readily to mind:

1. Delayed emergence affects the mechanism of pigment production.
2. A longer period as a pupa means energy reserves become depleted and less essential material (such as pigments) is metabolised to enable emergence to be postponed during unfavourable conditions.


Looking at pigment production, some relevant research has been undertaken by Sawada et al. (2002) who looked at the expression of an mRNA-encoding guanosine triphosphate-cyclohydrolase I (GTP-CH I), the first key enzyme in the synthesis of pteridines during pigment formation in the wings of the butterfly Precis coenia. The biochemical details are not important here, but the key result was that gene expression was strongest one day before butterfly emergence. So, if pigment production is timed to peak around emergence, could a delay lead directly to reduced pigmentation? From this example, I have to say that it doesn't appear so - the onset and duration of gene expression appears to be controlled by the decline in the ecdysteroid hormone 20-hydroxyecdysone (linked to moulting and metamorphosis and usually called 20E), and a short delay in emergence simply led to a later peak in expression such that pigmentation and emergence remained synchronised. It is still possible that low temperatures prevent pigment formation though this implies the retention of higher levels of 20E and/or one of more effects elsewhere in the chain of biochemical processes involved. Essentially it seems that no-one knows if such effects occur.

Moving onto pigment breakdown, I'll stick with pteridines as there are a number of other pigment groups and I want to keep this at least reasonably simple. However, as Watt (1967) shows, the pteridine pathway is anything but simple, which in turn means that there are many points at which it might be disrupted, and as above, there is no indication that anyone has looked at the effects of delayed emergence on pigmentation, including the breakdown of pigment compounds.

So, although this has been a somewhat limited look at possible pigmentation effects, it has at least shown that it is an area where future research is needed. However, this does not mean that there is no every-day or real-world relevance here. Coming back to the 'volunteer' aspect mentioned earlier, it is not just recording that is needed, but habitat creation - in particular, the small-scale improvements that anyone with a garden can make (or indeed councils who own open spaces) by cultivating more-or-less natural 'meadow' areas rather than ecologically sterile mown lawns. Our garden is not large but it does include a meadow patch with scabious, bird's-foot trefoil, clovers, meadow clary, cornflower, lady's bedstraw, knapweeds and others. Not only is it more interesting and attractive than a billard-table lawn, but it has been a haven for flying insects throughout the wet summer - I have still identified around 25 bee species alone this year, with flowers being used during even the briefest of lulls in rainfall, and very actively during genuinely warm conditions. Also, you don't have to be an expert/experienced entomologist, botanist or gardener to do this. Wild flowers are in fashion at the moment (let's hope they stay that way rather than gravel, paving-for-parking and swathes of decking) with thoughtful gardening writers and TV presenters such as Sarah Raven promoting this important subject, including easy how-to guides if you want a garden meadow - and this means that garden centres and plant nurseries are likely to be well-stocked with native insect-friendly species. If you do this, not only will invertebrates reap the benefits, but so will you.

The bee Andrena flavipes on white clover during the wettest summer on record.

References

DuRant, J.A. (1990). Influence of temperature on spring emergence of European corn borer moths (Lepidoptera: Pyralidae). Journal of Agricultural Entomology 7(3): 259-264.

Sawada, H., Nakagoshi, M., Reinhardt, R.K., Ziegler, I.& Koch, P.B. (2002). Hormonal control of GTP cyclohydrolase I gene expression and enzyme activity during color pattern development in wings of Precis coenia. Insect Biochemistry and Molecular Biology 32: 609–615.

Watt, W.B. (1967). Pteridine biosynthesis in the butterfly Colias eurytheme. Journal of Biological Chemistry 242(4): 565-562.

Hampshire's newest slug, a lover of logs

Yesterday at the annual HBIC Recorders' Forum, local members of the Conchological Society brought a display stand including a live specimen of Limacus maculatus. This was found in the Lyndhurst area of the New Forest and represents a species new to Hampshire as of 2011.

Limacus maculatus

L. maculatus (previously in the genus Limax as is the case in many key British texts) is also known as the Irish Yellow (sometimes Green) Slug; it is widespread in Ireland (see map) where it is found commonly in towns and gardens as well as being associated with rotting wood in natural habitats (NMNI 2010). emerging at night to feed and climb. It is medium to large (70-130 mm long), yellowish to greenish and blotchy, with grey to blue-grey tentacles and colourless to yellow or orange slime. A common pattern variation has the darker mottling more fragmented such that the animal has a spotty appearance, similar to that of Limacus flavus (the Yellow Slug) which is similarly often given as genus Limax. These two species were previously considered to be the same (Cameron et al. 1983), though L. flavus is usually a paler yellowish colour and spotty, with a pale zone extending above the fringe of the foot whereas in L. maculatus there is dark pigmentation to the fringe of the foot, and the animal usually has larger dark blotches (though note the spotty variant mentioned above). As a rule of thumb, though care is needed as they are variable, L. maculatus is usually a darker and blotchier green while L. flavus is usually a paler and spottier yellow. See here for photographs of extended specimens showing the tentacles.

More widely, as well as Britain, it is known from France, the Canary Isles, Romania, Bulgaria, Ukraine, Russia, the Black Sea coast and the mountains of Transcaucasia (Turkey to Azerbaijan and NW Iran). Its full distribution is not known with certainty (Kerney, 1999), though it is considered to have been introduced by humans into the British Isles (as well as around Moscow and St. Petersburg), with its native range being the deciduous forests of the Crimea and Caucasus (Wiktor & Norris 1982, Sysoev & Schileyko 2009, Schütt 2010).

In Britain, most records of L. maculatus (see map) are from northern England, with a scattering elsewhere. While L. flavus is largely associated with humans (garden rubbish, damp cellars and outbuildings etc) with occasional records in woodland, L. maculatus is more strongly associated with woodlands, particularly beneath large logs, bark or in tree-holes where moist conditions are maintained (although it can be found in situations similar to those of L. flavus, as well as under stones in fields). The Hampshire specimens seemed to be associated particularly with large logs. Though found sometimes on rubbish, food put out for other animals, dead plants, or on lichens on walls and stones (Cook & Radford 1988, Schütt 2010), its strong association with large logs means that L. maculatus probably feeds on wood-decay fungi, suggesting that large fallen timber may provide both food and shelter. This diet (fungi, algae, lichens, dead plant material) is common among slugs and although many gardeners and vegetable growers dislike slugs, only a few species such as the common Field Slug Deroceras reticulatum actually feed on living higher plants (Kerney & Stubbs, 1980).

As a final note, the 'new to Hampshire' tag is a close one as it was found in Christchurch in 1884 (Kerney 1986) - though now in the county of Dorset, back then the town was in Hampshire...

Found L. maculatus in Britain? Let your local Biological Records Centre know...

References

Cameron, R.A.D., Eversham, B. & Jackson, N. (1983). A field key to the slugs of the British Isles. Field Studies 5: 807-824.
Cook, A. & Radford, D. J. (1988). The comparative ecology of four sympatric limacid slug species in Northern Ireland. Malacologia 28: 131-146.
Kerney, M. (1986). A 19th-century record of Limax maculatus in the British Isles. Conchologists' Newsletter 97: 361.
Kerney, M. (1999). Atlas of the Land and Freshwater Molluscs of Britain and Ireland. Harley, Colchester.
Kerney, M. & Stubbs, A. (1980). The Conservation of Snails, Slugs and Freshwater Mussels. NCC, Shrewsbury.
National Museums Northern Ireland (2010). MolluscIreland: Limacus maculatus (Kaleniczenko 1851) Irish Yellow Slug. [accessed 04/03/2012]
Schütt, H. (2010). Turkish Land Snails. Verlag Natur & Wissenschaft, Solingen.
Sysoev, A. & Schileyko, A. (2009). Land Snails and Slugs of Russia and Adjacent Countries. Pensoft, Sofia.
Wiktor, A. & Norris, A. (1982). The synonymy of Limax maculatus (Kaleniczenko, 1851) with notes on its European distribution. Joural of Concholology 31: 75-77.

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.

Circus of the Spineless #68 - gifts galore!

It's December, so it's tempting to come up with a festive theme for this edition of Circus of the Spineless. However, I'm going for a 'birthday' theme instead because it's my blog's 'official' 1st birthday - 'official' (like the Queen's) because it's about a year since it really got going though I started it a bit before that. And, my 20,000th pageview just appeared, so thanks to whoever that was! Anyhow, I digress - please do click to take a giftbox, a slice of virtual cake and/or a glass of whatever suits you...

First up, Susannah of Wanderin' Weeta fame has provided some gift-wrapped goodies found tucked away in a vacant lot ('brownfield site' in UK-speak!) which goes to show it's always worth a look. To celebrate, why not start with a slice of tasty cake...

Next, a pair of splendid parcels arrived from John at 'Carp Without Cars' - the first arrived as 'snail mail' but not really (you'll see what I mean, just drink from the glass of finest red) and the second comes in a smaller package that rarely displays its contents quite as clearly as this...

My birthday cup already runneth over, but there's more to come... Here, Daniel at 'Notes from Dreamworlds' takes a close, close look at some freshwater critters courtesy of some high-quality optics (feel free to contrast this with the microscopy efforts on my blog!) and wraps the whole thing up as an 8-minute video - a veritable treasure-chest of precious things.

And lastly, I shall give myself a small gift of shameless self-promotion... Here, I look at the shiny jewelled contents of a box that really did turn up in the mail, so why not sit back with an ice-cold beer - mmm.... foamy...

So, that's all from my birthday-themed CoS #68 - thanks for coming along to the party; the more the merrier and there's no-one on the door to check for invitations. Next month, prepare yourself for some myrmecology as CoS scuttles off to Wild About Ants. Byeeee.

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).

Diversity: it's the talk of the chalk

The south-east of England is known for a number of things - too many rich bankers in over-priced houses, an excessive number of golf clubs (there's a correlation here I think!) and easy access to France by tunnel. However, from an ecological point of view, it is also well known for its calcareous grasslands growing on the cliffs and Downs pushed up when the ripples of the 'Alpine orogeny' (formation of the Alps) buckled what is now the south coast of England.

These grasslands are often exposed to the weather, hot when sunny, slightly alkaline (pH around 7.5-8), windswept, free-draining (so, often dry), low in nutrients due to leaching, and often closely grazed (by rabbits and sometimes sheep). Hence, though conditions are too challenging for more ruderal plant species, millennia of such conditions mean that there is a diverse array of plant and animal species adapted to grasslands of this type. Now, I don't intend to attempt a summary of the ecology, natural history and conservation management of southern England's chalk grasslands - these are covered elsewhere (e.g. Rodwell 1992, Crofts & Jefferson 1994). Instead I simply wish to provide a snapshot of what is visible during a brief visit - in this case during an Open University field trip in July 2011.

The site was the top of the cliffs just east of Birling Gap in East Sussex (near the more famous headland of Beachy Head). The cliffs are chalk with flint beds and have their own interesting flora and fauna such as the splendid lygaeid bug Henestaris laticeps - a specialist of cliff-faces where Buck's-horn Plantain (Plantago coronopus) grows, and easily identified by its stalked eyes.

H. laticeps (5-6mm long) showing stalked eyes. The similar H. halophilus is found in saltmarshes and has shorter eye-stalks. The function of the stalks remains unknown though noted by Southwood & Leston (1959).

However,cliff specialists might one day form a post (or series) of their own, so to keep the focus squarely on the cliff-top grasslands, let's take a walk uphill and see what can be found...

As this is a grassland, it seems sensible to start with the plants. Chalk grassland is noted for its high diversity - often a mosiac of mat-forming and rosette species with a scattering of upright species. These low-growing species are well placed to spread their leaves to maximise photosynthesis as well as spreading their roots to capture water as it swiftly drains through the soil. Those with rosettes are low-growing only until it is time to flower, at which point a spike grows quickly, flowers and sets seed, hopefully avoiding grazing or other potential hazards. Rosettes of some species such as P. coronopus may have downy hairs in the centre which trap moisture (including dew) and may also disrupt airflow sufficiently to reduce water loss through evapotranspiration.

Squinancywort (Asperula cynanchica), a prostrate mat-forming plant with small pinkish-white flowers and tiny linear leaves.

In the face of limited water and nutrients, other species manage the energy balance a different way, growing a larger stem and becoming upright. Some of these, such as Viper's Bugloss (Echium vulgare) shown below protect their investment in biomass (which may be offset against less rapid reproduction and/or reduced energy reserves) by developing bristles or spines to deter herbivores.

A clump of Wild Mignonette (Reseda lutea)

The small yellowish flowers of R. lutea.

The blue and pinkish flowers of E. vulgare (bristly hairs can be seen).

In the absence of abundant nutrients, ruderals (which are stronger competitors than chalk grassland plants when conditions are favourable) are absent, or heavily disadvantaged, and so those species which compete poorly but can tolerate chalk grassland conditions are able to form the high-diversity mosaic mentioned, in some cases taking advantage of opportunities to colonise bare patches of soil (a thin rendzina) produced by erosion and other processes. In this case, for those interested in the NVC, or National Vegetation Classification (Rodwell 1992), although I have not formally surveyed/analysed the site, it appears to be close to a CG2 Festuca ovina-Avenula pratensis grassland; with abundant Carex flacca, Lotus corniculatus and Thymus polytrichus (among others) it may be CG2a, the Cirsium acaule-Asperula cynanchica sub-community. This is an informal assessment, so apologies to the site managers if it's wrong!

Leaving the NVC aside, such a high diversity of plant species naturally suggests a high diversity of invertebrates. I'm not aware of a species list for the site (I'd love to survey it!), but even a single visit on a moderately warm, fairly cloudy day with a little rain provided numerous interesting sightings.

Yes, I know, more beetles in copula... this time it's the Bloody-nosed Beetle Timarcha tenebricosa on the gorse fringing the grassland along its northern edge, parallel to the southern cliff edge. These are large (up to 18mm) leaf beetles which get their common name from their ability to ooze red liquid when disturbed. This pair was too busy to bother (as were several others).

A shell of the 'Garden Snail' Cornu aspersum (formerly Helix aspersa) which has been used as a nest by one of the Osmia bee species that make shell-nests.

These Osmia shell-nesters use the internal coils as they might any other handy crevice and there are three species relevant here; O. aurulenta, O. bicolor and O. spinulosa (also known as Hoplitis spinulosa). Although somewhat anecdotal, Andrewes (1969) notes that O. aurulenta closes its shell-nest with a plug of felt-like plant material as seen here (it is not dung, although it does appear so) while O. bicolor uses a fine 'rubble'  of shell and stone fragments and covers the shell with a mound of plant material (Edwards 1998) which was not seen in this case, though a shell hidden under the edge of gorse had been chosen, assuming no movement of it by humans. Edwards (1998) also lists H. aspersa as a shell used by O. aurulenta, but not O. bicolor; similarly, Edwards & Roy (2009) note that H. spinulosa uses smaller (i.e. medium-sized) shells such as Cepaea nemoralis and is not suited to the hard-grazed grassland seen at Birling Gap. So, my tentative suggestion is that this is a nest of O. aurulenta (a scarce/Notable species and probably the least common of the three mentioned here) - however, observations of adults would be needed to make a certain identification to species. Maybe on my next visit!

References

Andrewes, C. (1969). The Lives of Wasps and Bees. Chatto & Windus, London.
Baldock, D.W. (2008). Bees of Surrey. Surrey Wildlife Trust, Woking.
Crofts, A. & Jefferson, R.G. (eds.) (1994). The Lowland Grassland Management Handbook. English Nature, Peterborough/Wildlife Trusts, Lincoln.
Edwards, R. (ed.) (1998). Provisional Atlas of the Aculeate Hymenoptera of Britain and Ireland. Part 2. BWARS/BRC, Huntingdon.
Edwards, R. & Roy, H. (eds.) (2009). Provisional Atlas of the Aculeate Hymenoptera of Britain and Ireland. Part 2. BRC, Wallingford.
Rodwell, J.S. (ed.) (1992). British Plant Communities 3: Grasslands and Montane Communities. CUP, Cambridge.
Southwood, T.R.E. & Leston, D. (1959). Land and Water Bugs of the British Isles. Warne, London.