Depending on the Liver

Stained specimen of Asian liver fluke Clonorchis sinensis, from here.

Every year, tens of millions of people worldwide (particularly in tropical Asia) suffer the effects of clonorchiasis and opisthorchiasis, conditions caused by infections with liver flukes of the family Opisthorchiidae. Exactly which condition the victim is suffering from depends on just which species of flukes they find themselves infected with, but there is little immediate difference between the clinical symptoms of either. Issues arising from clonorchiasis include fever, jaundice, diarrhoea and malnutrition. Long-term or heavy infections may result in cirrhosis, pancreatitis or even cancer (King & Scholz 2001). But just what is responsible for these debilitating illnesses?

Flukes are a diverse group of endoparasitic flatworms that reach maturity in association with vertebrates. As with other parasite lineages, different fluke species prefer different hosts and infect different parts of the host's system. Many have complex life cycles involving multiple larval stages and the successive infection of up to three distinct hosts on the way to maturity. Opisthorchiidae have such a three-host life cycle; their adult (or 'definitive') hosts span the gamut of vertebrates from fish to birds to mammals. Opisthorchiids in the strict sense are invariably associated with the liver of these hosts, taking up residence in the bile duct and gall bladder (however, phylogenetic studies have indicated that the closely related Heterophyidae, which infect the intestine, are probably paraphyletic with regard to opisthorchiids and the two families may be merged into an expanded Opisthorchiidae—Thaenkham et al. 2012). When mature they are elongate and flattened with the mouth near the front of the body surrounded by a sucker for attachment to host tissue. A second sucker is present on the underside of the body not too far behind the first (Dawes 1956).

Like other internal parasites, liver flukes are incredibly fecund. A female of Clonorchis sinensis, one of the main opisthorchiid species of concern to humans (yes, flukes reproduce sexually; I'll allow a moment for the disgusting implications to fully sink in), may produce up to 4000 eggs in a single day. These eggs are released into the host's digestive system, passing out in the faeces. They do not hatch until after they are ingested by the first larval host, an aquatic snail (many sources will say a freshwater snail but at least one opisthorchiid genus, Delphinicola, paratises marine dolphins so presumably has a correspondingly marine gastropod host). The egg hatches into a ciliated larva called a miracidium that over the course of the next few hours will find a likely spot in the snail's gut to develop into the next larval stage, the sporocyst. The sporocyst is immobile and mouthless, and feeds by absorbing nutrients directly from the host tissue. It also contains a mass of germ tissue that develops into multiple individuals of the next larval stage, the redia, that are released from the parent sporocyst after a couple of weeks or so. The rediae are worm-like and mobile, chomping their way through suitable sections of host tissue. They also develop multiple individuals of the next stage within them just as the sporocysts did. In this way, a single egg may eventually result in an exponentially increased number of larvae.

Life cycle of Clonorchis sinensis, from here.

The next larval stage is called the cercaria. In opisthorchiids, the cercariae look a bit like tadpoles with a dorsoventrally finned tail. I haven't found exactly how opisthorchiid cercariae are released into the water column but in other flukes they may be released with the discharge from the abcess or cyst that forms as the rediae feed on their host, or escape from the host tissue after the snail dies as a result of its infection. The cercaria is a dispersive stage that seeks out the next host in the life cycle. This they do by hanging head-down in the water column and allowing themselves to slowly sink until disturbed by contact with a potential host or water-currents created by its movement. At this point the cercaria rapidly swims upwards before allowing itself to sink again, hopefully onto the new hosts skin. The cercaria will then dig its way into the host's muscle tissue and transform into the last larval stage, a cyst called a metacercaria. Opisthorchiid cercariae most commonly attach themselves to some kind of fish but they are a bit less picky about their host than the other stages in their life cycle; opisthorchiid metacercariae have also been found in crustaceans and have been shown in the laboratory to even be capable of infecting mammals (specifically guinea pigs).

The developing liver fluke reaches its definitive host when the second larval host is eaten. A young fluke hatches from the metacercaria inside the definitive host's gut and make their way to the liver which they find by detecting the traces of its chemical products and/or by detecting the physical track of the bile duct. There they will mature into fully adult flukes, all ready to begin the cycle again (by doing the nasty in some poor sod's gall bladder).

The economic impact of opisthorchiids around the world is estimated to amount to hundreds of millions of dollars each year. Unfortunately, as with many other illnesses more widespread in developing nations, there still remains a lot to be learned about their control. Cooking fish before consumption to kill metacercariae is one of the more obvious methods, though it should be noted that metacercariae can be devillishly difficult buggers to kill. Installation of sanitation and sewerage systems can also help by reducing the chance of egg-carrying faeces to make it into water bodies, though medically significant opisthorchiids may also infect animals other than humans such as cats, dogs or pigs. For now, it looks like liver flukes will be with us for some time.

REFERENCES

Dawes, B. 1956. The Trematoda, with special reference to British and other European forms. University Press: Cambridge.

King, S., & T. Scholz. 2001. Trematodes of the family Opisthorchiidae: a minireview. Korean Journal of Parasitology 39 (3): 209–221.

Thaenkham, U., D. Blair, Y. Nawa & J. Waikagul. 2012. Families Opisthorchiidae and Heterophyidae: are they distinct? Parasitology International 61: 90–93.

Trap-jaw Ants of Australia (and a couple from Africa)

Foraging worker of Epopostruma frosti, copyright Alex Wild.

Anyone who finds themselves travelling through regional Australia will soon find themselves convinced that this is a continent ruled by ants. During the course of the day, while the hot Australian sun drives other animals to seek shelter and seclusion, ants are often the only living things (other than plants) to be seen. To match this abundance, Australia's ants also come in a variety of distinctive forms, many of them unique to this country.

One distinctively Australian group of ants are the 'epopostrumiforms'. This is a small group of genera belonging to the tribe Dacetonini of the subfamily Myrmicinae (in the past the epopostrumiforms have been formally recognised as the subtribe Epopostrumiti, though Bolton eschewed the use of formal subtribes in his 1999 review of the Dacetonini). The Dacetonini are all predatory ants, with a distinctive large process inside the base of the mandibles that helps to lock them closed when holding struggling prey. The mandibles may be particularly long and slender, sometimes with only a few teeth present at the end. Where their habits are known, epopostrumiforms are predators of springtails; these are the most typical prey for the Dacetonini as a whole though some species of the tribe are more catholic in their tastes. Dacetonins live in small colonies, commonly in secluded habitats such as leaf litter; the epopostrumiforms include species that nest and forage either above or below ground (Brown & Wilson 1959). Dacetonins hunt their prey by stealthily sneaking up to it with the mandibles held open, followed by a quick lunge combined with snapping the mandibles shut. Once the prey has been successfully grabbed, those dacetonins with shorter mandibles rapidly bring the sting forward to quell it. Even after using the sting, however, hunters of springtails may find themselves flung into the air by flicks of the springtail's furca a couple of times before the venom takes full effect (hence the need for a firm mandibular lock). Dacetonins with longer mandibles may also deploy their sting or they may simply lift the prey above their heads until it gives up the ghost.

The African Microdaceton tanyspinosum, copyright April Nobile.

As already indicated, the majority of epopostrumiforms are endemic to Australia (one genus, Colobostruma, includes a few species found in New Guinea and the Solomon Islands). The only non-Australasian taxon to be assigned to the Epopostrumiti is an African long-mandibulate genus Microdaceton. Features uniting Microdaceton with the Australasian epopostrumiforms include the presence of lateral outgrowths on the petiole and postpetiole (the first two nodular segments of the metasoma) and the position of the petiolar spiracle (Bolton 1999) but some authors have suggested a closer relationship of Microdaceton to other dacetonin genera. Even if correctly positioned, Microdaceton is at most the sister taxon to the Australasian clade, members of which are united by features such as reduced antennae and an enlarged labrum.

Face of Colobostruma alinodis, copyright Estella Ortega.

Bolton (1999) divided the Australasian epopostrumiforms between three genera: Colobostruma, Mesostruma and Epopostruma. Less than fifty species of this clade have been described to date though others probably remain to be named. Even among the known species, many are rare and/or cryptic and some are known from only a very few specimens. Epopostruma resembles Microdaceton in having elongate mandibles with only a small number of interlocking teeth at the end (two in Epopostruma, three in Microdaceton). When hunting, Epopostruma may open their mandibles to a full 170°. Colobostruma has much shorter, triangular mandibles with numerous teeth; Mesostruma has triangular mandibles somewhat intermediate between the other two genera. The mandibles of both Colobostruma and Mesostruma cannot be opened to the same degree as those of Epopostruma; rather, species of these two genera will open their mandibles to a maximum angle of 90° when hunting. Whether the Dacetonini involved long mandibles on a single occasion, with a number of sub-lineages reverting to shorter mandibles afterwards, or whether the short-mandibled Dacetonini retain the ancestral morphology and long mandibles evolved on multiple occasions within the tribe, remains a question occasioning some debate.

REFERENCES

Bolton, B. 1999. Ant genera of the tribe Dacetonini (Hymenoptera: Formicidae). Journal of Natural History 33: 1639–1689.

Brown, W. L., Jr & E. O. Wilson. 1959. The evolution of the dacetine ants. Quarterly Review of Biology 34 (4): 278–294.

The Monkey Orb of Asia

Just a quick entry for this week. And for the second week in a row, today's post will somehow involve monkeys.

Female monkey orb-weaving spider Neoscona punctigera, copyright Akio Tanikawa.

The orb-weavers of the family Araneidae are a highly diverse group of spiders, with well over 3000 known species. They are also one of the most familiar spider groups, often being relatively large as well as visible due to their construction of exposed and characteristic webs. The lady in the picture above represents one of the more moderately sized species, being about a centimetre in length (Tikader & Bal 1981). Neoscona punctigera is a widespread species in Asia, with a range extending from Madagascar and surrounding islands to Japan, as well as south into New Guinea and northernmost Australia. Vernacular names for the species include ghost spider or monkey orb-weaver. Like many other orb-weavers, N. punctigera only puts up its web at night; it sits in the web head downwards. When morning comes, the spider consumes the previous night's web and finds a concealed spot to hide until evening. On the underside of the body, N. punctigera has one or two pairs of bright white spots. When the spider is hunkered down for the day, these spots are concealed but when the spider is out on its web at night they are very visible; Chuang et al. (2008) found that these bright spots appear to attract prey, as spiders who had had their spots painted over caught less moths than usual.

Male Neoscona punctigera, copyright Suresh Kumar.

The name 'monkey orb-weaver' refers to the appearance of the male, which like the males of other orb-weavers is quite a bit smaller than the female (I have no idea where the name 'ghost spider' comes from; perhaps something to do with the spider's appearance on a web?) Resting males tend to adopt a pose with the front legs bent close together and the rear legs crossed behind the abdomen (as in the photo just above). Combined with eye-like spots on the abdomen, the overall effect has been compared to a monkey lying back with its legs crossed and its hands behind its head.

Orb-weaver taxonomy can often be confusing. Early authors tended to dump a large number of orb-weavers in a broad genus Araneus; though this genus is now used in a much narrower sense, many orb-weaver genera are difficult to distinguish without examining the genitalia. Individual species can also be quite variable in superficial appearance with a lot of variation in colour pattern, so many species were initially described under a number of names. Female Neoscona differ from Araneus in the presence of a longitudinal groove on the cephalothorax, as well as the presence of one or two lateral lobes at the base of the scape (a projecting process over the epigyne, the sclerotised structure around the female genital openings). Distingushing N. punctigera from other species of Neoscona requires even closer inspection of the genitalia. In a number of older sources the species now generally referred to as Neoscona punctigera (including in the World Spider Catalog) is commonly referred to as 'Araneus lugubris'. Confusingly enough, the latter name actually has priority (it dates to 1841 whereas the name pectinigera was only published in 1857) but has fallen out of disuse since Grasshoff (1986) stated that it was preoccupied in a review of African Neoscona. I'm not sure if he was correct—I suspect that he thought it was antedated by Aranea lugubris, published in 1802 for what is now a species of wolf spider, but as the 1841 species was originally placed in the now-obsolete genus Epeira I don't think they actually conflict. Nevertheless, the rules governing how preoccupation affects the use of older names can be complicated and if N. pectinigera has been settled as standard then it may be best to let it be.

REFERENCES

Grasshoff, M. 1986. Die Radnetzspinnen-Gattung Neoscona in Afrika (Arachnida: Araneae). Annalen Zoologische Wetenschappen 250: 1–123.

Chuang, C-Y., E.-C. Yang & I.-M. Tso. 2008. Deceptive color signaling in the night: a nocturnal predator attracts prey with visual lures. Behavioral Ecology 19 (2): 237–244.

Tikader, B. K., & A. Bal. 1981. Studies on some orb-weaving spiders of the genera Neoscona Simon and Araneus Clerck of the family Araneidae (=Argiopidae) from India. Records of the Zoological Survey of India, Occasional Paper 24: 1–60.