Field of Science

Crabs in Rivers, Crabs in Trees

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Freshwater crab Potamon ibericum, copyright Philipp Weigell.

Crabs are among the most recognisable animals one can find at the sea shore; any child who spends time at the beach will soon come to recognise their brandished pincers and sideways walk. But, as has been discussed by this site before, crabs are not only a coastal phenomenon. In warmer parts of the world, it may be possible to find crabs some distance inland.

Interestingly enough, there is at least circumstantial evidence that crabs made their way into fresh water relatively recently. The Old and New Worlds are each inhabited by a completely independent lineage of freshwater crabs that presumably originated after these continents went their separate ways. In the tropical Americas, rivers and streams are home to the Trichodactylidae, close relatives of the marine swimming crabs of the Portunidae. In the Old World, comparable habitats shelter a distinctly freshwater lineage comprising the superfamilies Gecarcinucoidea and Potamoidea.

Freshwater purple crab Insulamon palawanense, copyright Jolly Ibanez.

The classification of Old World freshwater crabs has (as with almost every other taxonomic group on this planet) shifted around a bit over the years. Many older references will combine all the Old World freshwater crab families into the Potamoidea but some more recent authors have tended to restrict this latter group to a single family, the Potamidae. There are other families, such as the African Potamonautidae and Deckeniidae, whose position superfamily-wise appears to be debated. The differences between the superfamilies Potamoidea and Gecarcinucoidea are primarily expressed in the structure of the males' second gonopods, the modified legs that the crabs use in transferring sperm during mating (Brandis & Sharma 2005). In the Gecarcinucoidea, a basal projection of the second gonopod surrounds the main body like a funnel for much of its length, while the tendril-like distal projection past this funnel is grooved and open on one side. In the Potamidae, the covering projection is restricted to the dorsal side only, and the distal part of the gonopod forms a closed tube.

Socotra limestone crab Socotra pseudocardisoma, copyright Gaëtan Rochez.

The Potamidae are most diverse in the Oriental bioregion with over seventy of the nearly eighty recognised genera being found there (Yeo et al. 2008). A couple of genera are found in the Afrotropical region. Only one genus, Potamon, makes it to Europe with modern species found in Italy and the Baltic peninsula, though the fossil record indicates a broader distribution on this continent in the past (Klaus & Gross 2009). Potamids are found in all types of water bodies, from fast-flowing streams and rivers to calm lakes and ponds, though they are inhabitants of the littoral zone rather than deep waters. The distinctive species Socotra pseudocardisoma is found on semi-arid limestone uplands of (surprisingly enough) the island of Socotra. Crabs of this species spend most of their time sheltered within cracks and crevices in the rocks that remain reasonably cool and damp year-round; they only emerge to the surface to forage during the rainy season while the surface briefly holds pools of standing water (Cumberlidge & Wranik 2002).

Another unusual lifestyle is found in a recently discovered species of the family Potamonautidae. This species from the Usambara Mountains of Tanzania specialises in living in phytotelmata, pools of water that accumulate in hollows in trees (Bayliss 2002). Though phytotelmata allow the crabs to inhabit regions of the rainforest that might otherwise be off limits, they are not the most forgiving of habitats. The combination of their small size together with an accumulation of organic matter means that the water in them tends to be quite acidic, a definite problem for a crab that relies on its calcitic exoskeleton for protection. The crabs feed on snails found in litter of the rainforest floor, and emerge from their home hollows to hunt at night or on cloudy, wet days. After eating a snail, they carry its shell back with them to their phytotelma and drop it in. The lime from the snail shell helps to neutralise the acidity of the water in the phytotelma, as well as supplying much-needed calcium that the crab will itself absorb when the time comes for it to moult to a new exoskeleton.


Bayliss, J. 2002. The East Usambara tree-hole crab (Brachyura: Potamoidea: Potamonautidae)—a striking example of crustacean adaptation in closed canopy forest, Tanzania. African Journal of Ecology 40: 26–34.

Brandis, D., & S. Sharma. 2005. Taxonomic revision of the freshwater crab fauna of Nepal with description of a new species (Crustacea, Decapoda, Brachyura, Potamoidea and Gecarcinucoidea. Senckenbergiana Biologica 85 (1): 1–30.

Cumberlidge, N., & W. Wranik. 2002. A new genus and new species of freshwater crab (Potamoidea, Potamidae) from Socotra Island, Yemen. Journal of Natural History 36: 51–64.

Klaus, S., & M. Gross. 2009. Synopsis of the fossil freshwater crabs of Europe (Brachyura: Potamoidea: Potamidae). N. Jb. Geol. Paläont. Abh.

Yeo, D. C. J., P. K. L. Ng, N. Cumberlidge, C. Magalhães, S. R. Daniels & M. R. Campos. 2008. Global diversity of crabs (Crustacea: Decapoda: Brachyura) in freshwater. Hydrobiologia 595: 275–286.

Delta Wasp

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Two views of the potter wasp Delta unguiculata, copyright Entomart.

Not so long ago, I found myself struggling with the challenge of identifying potter wasps. Potter wasps are close relatives of the social wasps, close enough that they are usually classified in the same family Vespidae, but they belong to a distinct lineage (the subfamily Eumeninae) of a more solitary bent, each female constructing its own individual nests in which to lay its eggs. The 'potter' part of their name refers to their preferred material for said nests which are sculpted from mud. Though they do not form the vexatious swarms that social wasps can, potter wasps still tend to be relatively large and impressive wasps, and like social wasps they are usually strikingly patterned in bold colours to give fair warning of their potentially painful stings.

Nevertheless, despite being the sort of thing that would be likely to attract interest, identifying potter wasps can be a definite challenge. For a large part of the twentieth century, eumenine genera were mostly divided very finely, with the features separating related genera often difficult to distinguish. Here in Australia, I found an approachable identification guide for most eumenines to be nigh on nonexistent. One potter wasp genus that I did successfully pull out, however, was Delta.

Female Delta campaniforme constructing a nest, from Brisbane Insects.

Delta is a genus of about fifty species of potter wasp found in warm regions of the Old World. At least one member of the genus, D. campaniforme rendalli, has become established in Florida after being introduced there from southern Africa (Menke & Stange 1986). Delta belongs to the Eumenes group of genera, in which the first segment of the metasoma (the petiole) is very long and slender. Distinctive features of Delta within this group include the second segment of the metasoma being relatively short with the associated tergum bell-shaped, and the males having the last segment of the antenna bent backwards to form a hook (Nguyen 2015). Females build their mud nests, which they stock with moth caterpillars, cemented to flattened surfaces such as the sides of buildings or along branches. The species introduced to North America possibly arrived in the form of a nest glued to some easily transportable substrate such as a shipment of lumber.

The names of Delta and many other Eumenes-group genera derive from the work of Henri de Saussure, who recognised a single genus Eumenes corresponding to this group but divided it into a number of sections that he labelled Alpha, Beta and so forth. Later authors raised these sections to the status of separate genera though some expressed the objection that Saussure may have never intended these alphabetical designations to be formal names at all. The validity of Saussure's 'genus-group names' was eventually settled by a decision of the International Commission on Zoological Nomenclature but authors such as Menke & Stange (1986) have continued to criticise the recognition of these difficult segregate genera, especially as, whereas the Eumenes group as a whole is probably monophyletic, many of its component genera may not be. Future classifications may yet see Eumenes gathering its prodigals back into the fold.


Menke, A. S., & L. A. Stange. 1986. Delta campaniforme rendalli (Bingham) and Zeta argillaceum (Linnaeus) established in southern Florida, and comments on generic discretion in Eumenes s. l. (Hymenoptera: Vespidae: Eumeninae). Florida Entomologist 69 (4): 697–702.

Nguyen, L. T. P. 2015. Taxonomic notes on the genus Delta de Saussure (Hymenoptera: Vespidae: Eumeninae) from Vietnam. Animal Systematics, Evolution and Diversity 31 (2): 95–100.

Metereca: Crossing the Divide

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Dorsal view and pedipalp of Metereca papillata, from Roewer (1935).

There can be little doubt that the continent with the least studied harvestmen fauna relative to its likely diversity is Africa. Africa is home to a wide range of harvestmen lineages, some of which are found nowhere else on earth, but many remain unrevised. Among these poorly known elements are numerous members of the family Assamiidae. Among these are the members of the genus Metereca, which I drew as the semi-random subject for this post.

The Assamiidae are a family of short-legged harvestmen found in tropical regions of the Old World: Africa, Asia and Australia. I've spoken enough in the past about the shadow of Carl-Friedrich Roewer that hangs heavy over harvestmen systematics. Recent years have seen a large amount of research being conducted on the harvestmen of the Neotropics, resulting in a vast improvement in our taxonomic understanding for that part of the world. The harvestmen of the Old World, unfortunately, are yet to attract the same attention. Assamiids were last extensively reviewed by Roewer in 1935. He divided them between 17 subfamilies but in the usual Roewerian way these were mostly based on fairly superficial features (numbers of subsegments in the leg tarsi, whether the palp femur has long spines or only short denticles, etc.) that may not be that significant. Staręga (1992) published a checklist of African harvestmen in which he synonymised assamiid 'genera' that Roewer had placed in separate subfamilies, thus implicitly synonymising the subfamilies they were tied to.

Metereca is a genus of about fifteen known species of assamiid found across Africa. Roewer (1935) placed it in his subfamily Erecinae, supposed features of which included simple claws and the absense of a pseudonychium (a 'false claw' between the two real claws) on the third and fourth tarsi, two subsegments in the first telotarsus, small denticles on the pedipalp femur, concealed spiracles, and no median spine on the front margin of the carapace. However, the Erecinae as defined in this way included genera from all three of the Old World continents. Considering that other harvestmen groups have turned out to have a strong correlation between geography and phylogeny, I'd be willing to put money on Roewer's Erecinae not being monophyletic.

That same doubt applies to Metereca (though I'm not sure I'd put money on it this time), which is one of the larger erecine genera currently recognised. Supposed features of Metereca include a lack of dorsal spines on the body, and a four-segmented first tarsus and two-segmented second telotarsus. Species have been assigned to this genus from widely separated parts of the continent: the Congo, Tanzania, Mozambique. But not only is this a genus defined primarily by the absence of features (always a bit suspect), but other groups of harvestmen have tended to show a division between western and eastern Africa. It would be worth someone's time in the future, I think, to confirm whether Metereca really does cross the divide that others don't.


Roewer, C. F. 1935. Alte und neue Assamiidae. Weitere Weberknechte VIII. (8. Ergänzung der "Weberknechte der Erde" 1923). Veröffentlichungen aus dem Deutschen Kolonial- und Uebersee-Museum in Bremen 1 (1): 1–168, pls 1–9.

Staręga, W. 1992. An annotated catalogue of Afrotropical harvestmen, excluding the Phalangiidae (Opiliones). Annals of the Natal Museum 33 (2): 271–336.

Checker Mallows

The crowdfunding campaign for my research on New Zealand harvestmen is still active. So far we're about 25% of the way towards the goal! Please click on the link above, and do your part to support your favourite arachnologist.

Flowering spike of Sidalcea nelsoniana, copyright Rhiannon Thomas.

Regular readers may have noticed that it's been a bit quiet around here lately. The last few weeks at chez Christopher have been... hectic. I have been writing posts but not had the time to publish them. So over the next few days, you'll be seeing a bit of a run of short posts in quick succession. Keep your eyes out.

The handsome plant you see above is a representative of Sidalcea, a genus of about thirty species found in the north of Mexico and the western United States. Members of this genus are commonly known as checker mallows (apparently because of the pattern of veins on the petals of some species); in the British gardening trade, they are also known as prairie mallows. As indicated by their vernacular names, Sidalcea species belong to the mallow family Malvaceae, and are hence related to other flowering plants such as cotton or hibiscus. These affinities are also reflected by their genus name, which is a portmanteau of the names of two other genera of Malvaceae, Sida and Althaea. Checker mallows differ from other members of the Malvaceae in having flowers with stamens that separate from the stamineal column in two tiers, an inner and an outer ring.

Most species of checker mallow are herbs; a few may develop into subshrubs. The genus includes both perennial and annual species. Stems of checker mallows are mostly more or less erect though they are often basally reclining or decumbent towards the base;it is not uncommon for decumbent stems to become secondarily rooted into the ground and develop into spreading stolons (or 'rhizomes'). Flowers of checker mallows are usually various shades of purple; a small number of species have white flowers (or white forms may occur in usually purple species). Many species of this genus are supposed to be difficult to identify: hybridisation is not uncommon, and some species are quite plastic in their own right. Young plants may also have a quite different appearance, including differently shaped leaves, from mature plants.

Sidalcea campestris, photographed by Amy Bartow.

The primary monograph of Sidalcea was published by E. M. F. Roush in 1931. She divided the genus between three subgenera, two of which contained only a single species each with all the remainder placed in her subgenus Eusidalcea (since the publication of Roush's monograph, a third non-Eusidalcea species has been recognised). These species are all perennials that, among other features, lack the variation in leaf shape with growth seen in Eusidalcea. More recent molecular analyses have supported Roush's arrangement arangement (Andreasen & Baldwin 2003). However, they have not supported Roush's division of Eusidalcea into separate sections for the annual and perennial species; instead, it appears that one or the other habit (it is unclear which) has arisen multiple times.

Like other diverse plant genera found in the California region, Sidalcea has attracted a certain degree of research into its evolutionary dynamics. Comparison of evolutionary rates between species has found that, as might be expected, annual lineages evolve faster than perennial ones (Andreasen & Baldwin 2001). Most species within each life-history class appeared to evolve at similar rates to each other, except for three perennial species: the three non-Eusidalcea species referred to above. One of these species, Sidalcea stipularis (the only one not known to Roush in 1931) showed evidence of an unusually high evolutionary rate for a perennial; this species is restricted to a very small population (only a few hundred plants may exist in the wild) and may have been subject to a higher rate of effective genetic drift. In contrast, the other two species have diverged more slowly than expected. One of these species, S. malachroides, is a presumably slow-lived subshrub; the other, S. hickmanii, commonly germinates after fires from seeds that may have remained in the ground for a number of years. In both cases, the overall result is that particular genotypes may persist in the population longer than in species with a more rapid turnover.

Oregon checkerbloom Sidalcea oregana ssp. spicata, copyright Dcrjsr.

Another feature of Sidalcea population dynamics to have attracted interest is the occurrence in several species of gynodioecy, a phenomenon where some individuals of a population have flowers with both male and female organs whereas other individuals have female organs only. The persistence of such an arrangement raises questions: because hermaphroditic individuals have the potential to contribute to more reproductive pairings than female-only individuals, shouldn't the former end up out-competing the latter and eliminating them from the population? This has lead to the inference that some factor(s) must give the female-only individuals an advantage that allows them to persist. Ashman (1992) found in germination tests of Sidalcea oregana spp. spicata that seeds that came from female-only plants tended to germinate into healthier, more vigorous offspring than those from hermaphrodites. It may be that plants that can only produce seed by outcrossing are less vulnerable to the effects of inbreeding, or perhaps not having to invest energy in making pollen means that the parent can put more energy into producing seeds.


Andreasen, K., & B. G. Baldwin. 2001. Unequal evolutionary rates between annual and perennial lineages of checker mallows (Sidalcea, Malvaceae): evidence from 18S–26S rDNA internal and external transcribed spacers. Mol. Biol. Evol. 936–944.

Andreasen, K., & B. G. Baldwin. 2003. Reexamination of relationships, habital evolution, and phylogeography of checker mallows (Sidalcea; Malvaceae) based on molecular phylogenetic data. American Journal of Botany 90 (3): 436–444.

Ashman, T.-L. 1992. The relative importance of inbreeding and maternal sex in determining progeny fitness in Sidalcea oregana ssp. spicata, a gynodioecious plant. Evolution 46 (6): 1862–1874.

Roush, E. M. F. 1931. A monograph of the genus Sidalcea. Annals of the Missouri Botanical Garden 18 (2): 117–244.

New Zealand Harvestmen: Please Help

The cave-dwelling Forsteropsalis photophaga, a remarkable harvestman species described in Taylor & Probert (2014).

As regular readers of this blog will be well aware, I've been working for several years now, off and on, on the taxonomy of long-legged harvestmen of the family Neopilionidae from Australia and New Zealand. In the past few years, this has been a bit more off than on: the necessities of earning a crust have meant that I haven't had the time to dedicate to full-time harvestman research. Nevertheless, I've been putting things together here and there where I can and an enormous amount of progress has been made. Back when I first decided to investigate this group of animals in 2000/2001, there were a handful of named species, often with descriptions amounting to nothing more than a couple of vague lines, all but unidentifiable in practice. Over time, I've redescribed each of these species in turn, as well as describing and naming a pile of new ones. We've learnt things about these animals we never knew before, such as the presence in many populations of a remarkable divergence within males to the extent that to the uninitiated they might be (and have been) mistaken for completely different species. We've seen the incredible range of forms in this group, from long-jawed monsters like to one at the top of this post, to heavily armoured cryptic soil-dwellers like in this photo by Stephen Thorpe.

After many years, I feel I'm finally approaching the point where I can put the finishing touches on my revision of the New Zealand neopilionids (for a given value of 'finish', of course, because there is no group of organisms for which the work is ever truly finished). Ideally, I would like to publish something incorporating a complete overview of this group of animals, a complete guide to all the known species offering a one-stop-shop to allow anyone, anywhere to confidently identify any specimen that might come to their hand. It's also important to me that I publish this guide in an open-access format so that it's also available at any time.

But to do that, I need your help. In order to be able to travel to the New Zealand museums that hold types and other crucial specimens that I need to examine, and to cover the publication fees of the resulting product, I've started a crowdfunding drive. Head over to and you'll be able to support my research, follow the results as they become available, and receive full acknowledgement in the resulting publication(s). Even if you can't support me directly myself, you would be helping immensely if you inform others of my campaign, whether through social media, in person, or any other medium that makes itself available. Together, we can bring this truly incredible group of animals the recognition they so richly deserve!

If you want to see some of my work on harvestmen that's already come out, check out the links below:

Remarkable things
Possibly the coolest thing I had published this year
Score one for biogeography
How to wipe out a family
The saga of Forsteropsalis fabulosa
More on the New Zealand Opiliones
Bye, bye, Spinicrus
The eater of light
New Zealand fills a biogeographical gap