Wasps with Fangs on their Feet?

Theronia septentrionalis, photographed by Stephen Cresswell.

The ichneumons are one of the more familiar groups of parasitoid wasps for the general public. The species in the photo above is a member of the Theronia group of ichneumons, which attain a reasonable size by wasp standards (a number of species seem to be in the range of 1.5 centimetres long) and are often brightly coloured in yellow or green. The Theronia group is primarily tropical in distribution, though some species are found in more temperate regions. Authors have differed on whether they treat this group as a single genus or divide it between about half a dozen genera; either option is complicated by the fact that both the group as a whole and some of its constituent restricted genera are doubtfully monophyletic (Gauld et al. 2002). Where their larval hosts are known, many members of the Theronia group are endoparasitic in moth cocoons (including some economically significant pests such as the gypsy moth), though at least some species are not parasites of the moth itself but are hyperparasites of other ichneumon larvae attacking the moth. One (sub)genus, Nomosphecia, includes parasites of vespid wasp larvae (Gauld 1984).

Male Theronia atalantae, photographed by Phil Huntley-Franck.

Bright colours are often a sign of danger in the animal kingdom, and the Theronia group seem to follow that trend. One of the group's distinctive features is larged, curved claws with an associated spatulate bristle. As noted by Gauld (1984), "When caught they sink their large claws into their captor." This sounds uncomfortable enough in itself, especially as said claws have a tendency to break and leave their tips embedded in the skin if the wasp is not allowed to remove them in her own time. But there's more: the inside of the claw bears a fluid-filled cavity, and the act of embedding the claws releases the contents of this cavity into the wound. In other words, the claws seem to function in much the same way as the fangs of a venomous snake.

Or do they? We know that the fluid injected by Theronia into would-be attackers can cause irritation to vertebrate epithelium (Gauld et al. 2002), but we don't seem to know just what it contains or how it acts. As such, we don't know how confident we can be that the fluid is indeed effective defensively. Theronia may have poison claws that act like fangs. Or it may just have big sharp claws, and that may be enough.

REFERENCES

Gauld, I. D. 1984. An Introduction to the Ichneumonidae of Australia. British Museum (Natural History).

Gauld, I. D., D. B. Wahl & G. R. Broad. 2002. The suprageneric groups of the Pimplinae (Hymenoptera: Ichneumonidae): a cladistic re-evaluation and evolutionary biological study. Zoological Journal of the Linnean Society 136: 421-485.

The Importance of Genitalia

Take a look at the figure above (taken from Mauriès 2003). What you're looking at is the intimate business of a male millipede, in this case a Bosnian millipede called Fagina silvatica. And if you ever had the pleasure of finding yourself working on millipede taxonomy, you'd be looking at a lot of these.

Fagina silvatica belongs to a superfamily of millipedes called the Neoatractosomatoidea, which is in turn part of the order Chordeumatida in the clade Helminthomorpha. Helminthomorph millipedes (as indicated by their name, which means 'worm-like') all cleave pretty closely to the classic image of their kind, with an elongate body bearing large numbers of relatively short legs. Chordeumatida are characterised by having silk-spinning glands on the telson, the very end segment of the body, and three pairs of strong bristles on the top of each body segment. Male chordeumatidans also have the eighth and ninth pairs of legs modified into the gonopods, the copulatory structures. Because millipedes are generally not extravagant animals in overall appearance, it is the gonopods that have become the primary structures for identifying them, and many millipede species cannot be reliably distinguished without examining them. In the Neoatractosomatoidea, the eighth pair of legs forms the gonopods proper that deliver the male's sperm to the female's vulvae, while the ninth pair form protective structures called paragonopods. The gonopods proper are divided into two branches that fold around each other, usually to guide a whip-like flagellum or other extended structure passing between them (one genus, Guizhousoma, lacks the flagellum—Mauriès 2005). One neoatractosomatoid genus, Osellasoma, also has the seventh pair of legs modified into protective structures (Mauriès 2003). Neoatractosomatoids have 28 or 30 body segments. Some neoatractosomatoids have the sides of the body extended into flattened processes called paraterga; others have the body more or less cylindrical. And no, I haven't been able to find a single photograph or illustration showing a neoatractosomatoid in its entirety. You'll have to content yourself with looking at their genitals (Wikipedia has photos of other Chordeumatida).

As defined by Mauriès (2003, 2005), the Neoatractosomatoidea only includes about 25 known species, mostly found in southern Europe. A single species, the aforementioned Guizhousoma latellai, is known from caves in China. Mauriès (2003) separated three families previously placed in the Neoatractosomatoidea into a separate superfamily Mastigophorophylloidea; if the mastigophorophylloids are included with the neoatractosomatoids, then the group includes further species found in northern Asia. Mauriès separated the two superfamilies on the basis that mastigophorophylloids possessed a flagellum on both the gonopods and the paragonopods, instead of only on the gonopods. The subsequent discovery of the entirely flagellum-less Guizhousoma could raise questions about the significance of this character, and the flagellum appears much reduced if not entirely absent on the paragonopods of at least one putative mastigophorophylloid, Kirkayakus pallidus, as illustrated by Mikhaljova (2004)*. However, I have to admit to having absolutely zero experience with interpreting millipede gonopods, so I am hardly one to be voicing an opinion.

*Mikhaljova (2004) illustrates this species under the name of Altajella pallida, but it has since been renamed by Özdikmen (2008) (yes, that Özdikmen) due to the original genus being preoccupied).

REFERENCES

Mauriès, J.-P. 2003. Schizmohetera olympica sp.n. from Greece, with a reclassification of the superfamily Neoatractosomatoidea (Diplopoda: Chordeumatida). Arthropoda Selecta 12 (1): 9-16.

Mauriès, J.-P. 2005. Guizhousoma latellai gen.n., sp.n., de Chine continentale, type d'une nouvelle famille de la superfamille des Neoatractosomatoidea (Diplopoda: Chordeumatida). Arthropoda Selecta 14 (1): 11-17.

Mikhaljova, E. V. 2004. The Millipedes (Diplopoda) of the Asian Part of Russia. Pensoft: Sofia.

Özdikmen, H. 2008. New family and genus names, Kirkayakidae nom. nov. and Kirkayakus nom. nov., for the millipedes (Diplopoda: Chordeumatida). Munis Entomology & Zoology 3 (1): 342-344.

Arthropods in the Precambrian?

The Ediacaran animal Spriggina floundersi, from here.

The Ediacaran biota has been touted as one of the great mysteries of palaeontology. Comprising the latest part of the Precambrian era, the Ediacaran is generally believed to have given us the earliest known animal fossils. However, palaeontologists have disagreed on just how the Ediacaran fossils relate to modern animals (see McCall 2006 for an exhaustively detailed review). Some see the Ediacarans as including the ancestors of groups that remain with us today: jellyfish, corals, comb jellies, sponges. Others see Ediacarans as outside the modern lineages: ancient animal groups that were swept aside by more modern animals at the beginning of the Cambrian. And some have even questioned whether the Ediacarans were even animals at all, suggesting links instead to fungi or Foraminifera, or even that they were an entirely independent lineage unrelated to any modern multicellular organisms.

In 1996, Benjamin Waggoner proposed the name 'Cephalata' for a clade uniting the arthropods with two groups of Ediacaran organisms: the Sprigginidae and the Vendiamorpha. These are among the most undeniably animal-like of the Ediacarans. The sprigginids (including Spriggina shown at the top of the post) have an undivided 'head' followed by a long segmented body. The vendiamorphs are shield-like organisms that also show evidence for segment-like divisions behind the 'head', such as branching internal structures that may represent side-branches of an internal gut.

The vendiamorph Vendia sokolovi, from Ivantsov (2004).

It is difficult to see these taxa as anything other than mobile animals. One supporter of non-animalian affinities for the Ediacarans, Adolf Seilacher, did suggest that Spriggina was a sessile organism, maintaining that the 'head' was in fact a holdfast while the 'body' extended upwards like the frond of a sea pen (I have seen a memorable reconstruction, though unfortunately I can't recall where, showing an individual of mobile Spriggina crawling past a cluster of sessile Spriggina). However, the numerous Spriggina specimens that have been found in Australia and Russia are invariably preserved lying flat, while sessile organisms from the same locations are preserved with the holdfast below the level of the body. Vendiamorphs, on the other hand, are simply not shaped in a way that allows them to be seen as anything other than lying flat. An immobile sprigginid or vendiamorph lying flat below the water would have been vulnerable to being buried by sediment, without any way of digging itself back out.

But if sprigginids and vendiamorphs were definitely animals, what kind of animals were they? It is at this point that things get a bit more vague. Their segmented appearance immediately suggests arthropods (and onychophorans) or annelids, but there is not a great deal to suggest one or the other. The differentiated head of sprigginids suggests the head of an arthropod, while vendiamorphs have been compared to the larvae of arthropods such as trilobites. However, it is unclear whether the Ediacaran taxa possessed anything like the limbs of arthropods and related taxa. The segments of sprigginids may be separated at the edges, and some have argued that folds in vendiamorph fossils are suggestive of limbs underneath a dorsal shield, but there is nothing that one would call unequivocal. Lateral outgrowths of sprigginids may correlate to annelid parapodia instead of arthropod limbs, and folds in the bodies of vendiamorphs may be nothing more than that. We recognise relationships between fossil and extant animals on the basis of whether they have features in common, but our assessment of what features they have may be coloured by what features we expect to see.

Another possible vendiamorph, Parvancorina minchami, from here. Note the fine parallel lines on the body, which some have interpreted as the outlines of limbs.

Some authors have drawn attention to a feature of both vendiamorphs and sprigginids that is visible in the image of Vendia above: their so-called 'glide reflectional symmetry'. Though their bodies appear segmented, the segments do not go straight across the body as one might expect. Instead, the left and right sides of the body are slightly offset from each other. For this reason, some authors have claimed that these animals do not show true bilateral symmetry and hence argued for placing them outside the Bilateria crown group, along its stem. However, others have suggested that the offset between sides may be an artefact of preservation. Even if it was indeed a feature of the living animal, glide reflectional symmetry may not necessarily force the sprigginids outside the Bilateria: a number of living bilaterians also show a certain degree of symmetry offset either as adults or during development, including basal chordates (Waggoner 1996).

During the period of the Cambrian, directly after the Ediacaran, we have access to beautifully preserved fossil deposits that have allowed us to characterise many animals from that period in exquisite detail. No such fossils exist for the Ediacaran; instead, Ediacaran animals are mostly preserved in coarse sediments that preserve only relatively broad features of the fauna. This can turn the Ediacarans into tantalising shadows, and what we see in them can say more about our assumptions than the animals themselves.

REFERENCES

Ivantsov, A. Yu. 2004. New Proarticulata from the Vendian of the Arkhangel’sk region. Paleontologicheskii Zhurnal 2004 (3): 21–26 (transl. Paleontological Journal 38 (3): 247–253.

McCall, G. J. H. 2006. The Vendian (Ediacaran) in the geological record: enigmas in geology's prelude to the Cambrian explosion. Earth-Science Reviews 77: 1-229.

Waggoner, B. M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45 (2): 190-222.

Star-Grass

Common star-grass Hypoxis hirsuta, photographed by Merel R. Black.

It does not require a great deal of insight to understand why the plant pictured above has acquired the vernacular names of 'star-grass' or 'gold-star'. This small native of North America is one of the few representatives in that region of the family Hypoxidaceae, a group of about 150 species of thin-leaved monocots that is most diverse in the Southern Hemisphere, particularly in Africa. Most Hypoxidaceae are small like the North American gold-star, though the Asian hill coconut Curculigo latifolia may be over a metre in height. All grow from underground corms or rhizomes. Their flowers have the typical monocot arrangement of three sepals and three petals, and are most commonly yellow to pink in colouration. They mostly produce little scent (some have a faint sweet scent), and usually attract pollinators by offering pollen as a reward. Some species are grown as ornamentals, but for the most part the Hypoxidaceae are not that significant economically. The tubers of the African potato Hypoxis hemerocallidea (which, despite its vernacular name, does not seem to be eaten as a vegetable per se) have been used to make a medicinal tea; it has become particularly widely used in recent years to supposedly alleviate the symptoms of HIV, but tests of its actual efficacy remain in progress.

Lemba or hill coconut Curculigo latifolia (previously Molineria latifola), photographed by Ahmad Fuad Morad.

Recent authors have recognised up to ten genera within the Hypoxidaceae, but a phylogenetic analysis of the family by Kocyan et al. (2011) lead them to suggest the reduction of that number to four or six, depending on how one might chose to deal with the position of Hypoxidia. This is a distinctive genus of two species found on the Seychelles. Flowers of Hypoxidia are a dark red-brown colour, and in contrast to the weak scent of other Hypoxidaceae they have a strong foetid odour that attracts flies as pollinators. Kocyan et al.'s phylogenetic analysis placed Hypoxidia as sister to the other species of Hypoxidaceae found on the Seychelles, Curculigo seychellensis, and the two together were placed as sister to a clade containing the remaining species of Curculigo (as well as species of Molineria, which Kocyan et al. suggested be synonymised with Curculigo). Curculigo species bear their flowers at the base of the plant; the ovary is actually found beneath the ground, with the corolla borne above the ground on an elongate tubular rostrum. This rostrum is particularly long in C. seychellensis, up to 12 cm. Curculigo seychellensis also has bifurcated leaves that make it look superficially like a palm seedling. It remains to be settled whether future authors will prefer to place C. seychellensis in its own new genus, or to sink Hypoxidia into Curculigo.

Pauridia capensis (previously Spiloxene capensis), photographed by Bob Rutemoeller.

The remaining Hypoxidaceae can be divided between the genera Hypoxis (containing Rhodohypoxis as a junior synonym), Empodium and Pauridia (containing Spiloxene and Saniella as synonyms, as well as some Australian species previously placed in Hypoxis). Members of the genera Empodium and Pauridia produce annual corms, while Hypoxis species have tuberous rhizomes. Empodium and Pauridia differ in features of the flowers and seeds (Kocyan et al. 2011). The two African Pauridia species previously classified as Saniella resemble Curculigo in having subterranean ovaries. It is perhaps unfortunate that the name Pauridia, previously restricted to two particularly small African species only a couple of centimetres in height, takes priority over Spiloxene, previously used for a larger group of about thirty species. But such are the vagaries of nomenclature, and that which we now call Pauridia capensis (Snijman & Kocyan 2013) will smell as... generally indifferent, actually, but it at least looks pretty specky.

REFERENCES

Kocyan, A., D. A. Snijman, F. Forest, D. S. Devey, J. V. Freudenstein, J. Wiland-Szymańska, M. W. Chase & P. J. Rudall. 2011. Molecular phylogenetics of Hypoxidaceae—evidence from plastid DNA data and inferences on morphology and biogeography. Molecular Phylogenetics and Evolution 60 (1): 122-136.

Snijman, D. A., & A. Kocyan. 2013. The genus Pauridia (Hypoxidaceae) amplified to include Hypoxis sect. Ianthe, Saniella and Spiloxene, with revised nomenclature and typification. Phytotaxa 116 (1): 19-33.

Caulerpa: Sea Grapes, Feather Algae and Other Variations on a Tube

Invasive Caulerpa taxifolia in the Mediterranean, from here.

There is an observation (it has been dubbed 'Gorton's Law') that any discussion of any marine organism will invariably lead to someone asking whether it can be served with chips. In most cases, the answer will be some variation on 'yes', 'no', 'eww' or 'that's just stupid'. Sometimes, however, the answer will be 'it depends'.

Caulerpa is a genus of marine green algae found in tropical and subtropical waters around the world. Over seventy species of Caulerpa are currently recognised, with many further divided into bewildering arrays of subspecies, varieties and formae. At least two of these species, C. taxifolia and C. racemosa, have become notorious as invasives in the Mediterranean and nearby parts of the Atlantic. Apart from the closely related genus Caulerpella, it cannot really be mistaken for anything else (at least if you have a microscope). The thallus of Caulerpa takes the form of a long branching, creeping tube (the stolon) from which arise numerous tubular, flattened or globular fronds. A Caulerpa thallus can grow reasonably large: the stolon may be over three metres in length, with fronds several centimetres high. Despite this size, the thallus is not divided into individual cells: the multinucleate cytoplasm is freely connected throughout. Instead of cell divisions, branching ingrowths of the cell wall called trabeculae provide strengthening for the thallus. Large Caulerpa individuals are therefore one of the leading contenders for the title of 'world's largest single-celled organism', though I've noted many times that the concept of 'largest' doesn't mean much when talking about multinucleate structures that are indeterminate in size.

Caulerpa racemosa, photographed by Guillermo Diaz-Pulido.

The two genera Caulerpa and Caulerpella that share this unique cell structure (there are other tubular, non-cellular algae, but they lack the trabeculae) are distinguished by their reproductive morphology. Caulerpa usually reproduces vegetatively: older pieces of a thallus die, separating the growing tips, or pieces of the thallus break off and settle separately. When sexual reproduction does occur, Caulerpa fronds do not produce dedicated reproductive structures. Instead, the cytoplasm within an entire frond becomes divided into gametes that are released through slender papillae that grow on the frond surface. In the single species of Caulerpella, C. ambigua, fronds may bear specialised reproductive structures formed from a compound whorl of tightly-branched lobes lacking trabeculae (Price 2011).

Caulerpa nummularia, photographed by D. S. Littler.

Species and varieties are distinguished by the morphology of the thallus, using features such as the shape of the fronds (which may be simple tubes, or branched and feathery, or disc-shaped, or broad and flat, or globular and looking like clusters of grapes, or any number of further variations) and their manner of branching. However, many species may vary considerably in habit, and in some species forms described as separate varieties may later be found growing as separate sections of a single thallus. In a detailed study of variation in four species of Caulerpa found in the Philippines, involving comparisons of specimens collected in the wild, thalli cultured in the laboratory, and molecular data, de Senerpont Domis et al. (2003) found that three of the four species showed morphological variation consistent with their previous classification, but the fourth species (C. racemosa) varied to the extent that previously recognised 'varieties' could not be reliably distinguished.

Caulerpa prolifera, photographed by Elizabeth Lacey.

But to get back to the important question: can it be served with chips? It depends. Many Caulerpa are widely eaten, particularly in eastern Asia, and particularly those with globular fronds (which are referred to as 'sea grapes'). Nevertheless, Caulerpa species have also been referred to as toxic, particularly in relation to their noxious weed status in the Mediterranean (where their invasiveness has been attributed to the absence of suitable grazers). Toxicity of Caulerpa, it appears, can vary between taxa and possibly between seasons (the taste of edible varieties in the Philippines becomes more peppery during the rainy season). The identity of the toxic compound(s) in Caulerpa has been subject to debate: a number of candidates have been identified, but studies have disagreed on their effects and severity, and it remains uncertain whether Caulerpa poisoning poses a serious risk to humans (Higa & Kuniyoshi 2000). One of these candidates, caulerpicin, was tested by Doty & Aguilar-Santos (1966) using the straightforward method of feeding it to 'volunteers'. They recorded the results as follows:

Different people respond differently to caulerpicin. Some merely obtain a mild anaesthetizing sensation which is not immediate but is delayed for a minute or two. Others also obtain a numbness of the tongue or lips. In one subject exposed to the substance at various times truly toxic symptoms have become stronger and stronger following each contact. Almost immediately on chewing the raw dried Caulerpa material, the subject felt a numbness at the tip of the tongue. This has developed to a point at which the reaction is one of numbness of the extremities coupled with a cold sensation in the feet and fingers, rapid and difficult breathing, slight depression and, finally, loss of balance requiring the subject to lie down. The symptoms wear off, depending on the dosage, in a few hours to a day. Coupled with these reactions to the impure and pure substance, the same subject has developed a sensitivity to oysters and crabs and eating them produces the same symptoms.

It's the 'following each contact' bit that worries me. "Feeling better? Good. Have some more!"

REFERENCES

Doty, M. S., & G. Aguilar-Santos. 1966. Caulerpicin, a toxic constituent of Caulerpa. Nature 211: 990.

Higa, H., & M. Kuniyoshi. 2000. Toxins associated with medicinal and edible seaweeds. Toxin Reviews 19 (2): 119-137.

Price, I. R. 2011. A taxonomic revision of the marine green algal genera Caulerpa and Caulerpella (Chlorophyta, Caulerpaceae) in northern (tropical and subtropical) Australia. Australian Systematic Botany 24: 137-213.

de Senerpont Domis, L. N., P. Famà, A. J. Bartlett, W. F. Prud’homme van Reine, C. A. Espinosa & G. C. Trono Jr. 2003. Defining taxon boundaries in members of the morphologically and genetically plastic genus Caulerpa (Caulerpales, Chlorophyta). Journal of Phycology 39: 1019-1037.

Dragons in a Desolate Land

Ring-tailed dragon Ctenophorus caudicinctus, from here.

The comb-bearing dragons of the genus Ctenophorus are an assemblage of 28 (and counting!) species of medium-sized lizards found around Australia. Darren Naish has recently been giving an overview of the Australian dragons; you can read what he's already said about Ctenophorus here. I'd suggest reading that first, then coming back here.

Species of Ctenophorus are distinguished from other dragons by the presence of a row of tectiform (roof-shaped) scales running from behind the nostrils under the eyes, though in some species this row is only weakly pronounced (Melville et al. 2008). In most species, the tympanum (ear-drum) is exposed, though a few species have it covered over. I'm personally familiar with one species of Ctenophorus, the ring-tailed dragon C. caudicinctus. Where we've been doing fieldwork on Barrow Island, ring-tailed dragons are a common site perched on termite mounds or larger rocks, invariably just one dragon to a rock, monitoring the surrounding territory for food or mates. Not all Ctenophorus species engage in such behaviour: the species have been divided between three groups depending on whether they prefer rocky habitats, whether they prefer sandy habitats and use tufts of spinifex and other vegetation for cover, or whether they shelter in burrows. Phylogenetic analysis suggests that the burrowing habit was ancestral for the genus; rock-dwelling or scrub-dwelling habits may have each evolved more than once within Ctenophorus, though the possibility cannot be entirely ruled out that they may characterise monophyletic groups (Melville et al. 2001). These differences in ecology also correlate with morphological differences: rock-dwelling species have dorsoventrally flattened heads, while the scrub-dwelling species are long-legged and cursorial.

Military dragon Ctenophorus isolepis, a sand-dwelling species associated with spinifex, photographed by Stewart Macdonald.

While some species of Ctenophorus are widespread, others are far more restricted in range. Ctenophorus caudicinctus, for instance, is found across most of northern Western Australia and the Northern Territory, but Butler's dragon* C. butleri is restricted to coastal sand dunes between Shark Bay and Kalbarri in Western Australia (Cogger 2014). The most recently described species to date, the Barrier Range dragon C. mirrityana, is known from two locations about 100 km apart in western New South Wales (McLean et al. 2013). And it possibly does say something that new species continue to be described even in this not inconspicuous genus.

*Or should that be 'Butlers' dragon', as it was apparently named after both Harry and Margaret Butler?

Lake Eyre dragon Ctenophorus maculosus, photographed by Rune Midtgaard.

Perhaps the most hard-core of the comb-bearing dragons is the Lake Eyre dragon Ctenophorus maculosus, a specialised inhabitant of dry salt lakes in South Australia. This is a spectacularly harsh environment: searing hot sun, often at temperatures above 40°, beating down on a crust of crystallised salt. Few other animals can survive there without spontaneously combusting. The dragons protect themselves from the head by burrowing into the layer of unconsolidated sand beneath the salt-crust; Pedler & Neilly (2010) discovered one female with its head protruding from a burrow with an entrance too small for its body, and suggested that she must have gotten there by 'swimming' through the sand. The Lake Eyre dragons feed on ants such as Melophorus (themselves no slouch in the hard-core stakes) or other insects that have become stranded on the salt-pan. When the lake becomes filled with water (as it does about once a decade or so), the dragons are forced to flee into the habitats surrounding the lake shores and wait for the flood to clear. Two Western Australian species, the claypan dragon Ctenophorus salinarum and the Lake Disappointment dragon C. nguyarna, are also associated with salt-pans, but they do not have quite the level of specialisation of the Lake Eyre dragon.

REFERENCES

Cogger, H. G. 2014. Reptiles and Amphibians of Australia, 7th ed. CSIRO Publishing: Collingwood.

McLean, C. A., A. Moussalli, S. Sass & D. Stuart-Fox. 2013. Taxonomic assessment of the Ctenophorus decresii complex (Reptilia: Agamidae) reveals a new species of dragon lizard from western New South Wales. Records of the Australian Museum 65 (3): 51-63.

Melville, J., L. P. Shoo & P. Doughty. 2008. Phylogenetic relationships of the heath dragons (Rankinia adelaidensis and R. parviceps) from the south-western Australian biodiversity hotspot. Australian Journal of Zoology 56: 159–171.

Melville, J., J. A. Shulte II & A. Larson. 2001. A molecular phylogenetic study of ecological diversification in the Australian lizard genus Ctenophorus. Journal of Experimental Zoology 291: 339-353.

Pedler, R. & H. Neilly. 2010. A re-evaluation of the distribution and status of the Lake Eyre dragon (Ctenophorus maculosus): an endemic South Australian salt lake specialist. South Australian Naturalist 84 (1): 15-29.

Shock Me like an Electric Eel

Electric eel Electrophorus electricus, photographed by Stefan Köder.

The electric eel Electrophorus electricus is one of those animals that seem to border on the mythical. Most people will have come across some sort of reference to their existence, and may even have seen some sort of intended depiction of one in cartoon form. However, said depiction will probably bear little if any resemblance to a real-life electric eel. Most commonly, it will look more like a standard Anguilla eel, to which true electric eels are not close relatives. Instead, electric eels belong to a uniquely South and Central American group of fish, the Gymnotiformes.

The Gymnotiformes, commonly known as the Neotropical knife-fishes, are more closely related to catfish than they are to anguillid eels. They are characterised by an elongate body form, lacking the dorsal fin of other fish. The anus has been moved forward relative to other fish: in some gymnotiforms, the anus is actually in front of the pectoral fins, just behind the head. The anal fin that runs behind the anus has become greatly elongated, and instead of swimming by undulating the body from side to side like other fish, gymnotiforms swim by undulating the anal fin alone while the main body remains more or less rigid. This unusual swimming style is directly related to another distinctive feature of the gymnotiforms: their production of an electrical field. Many fish are able to passively sense electrical fields in the water: gymnotiforms take the next step and generate their own electrical field, which they use to sense their surrounding environment (Albert & Crampton 2005). As a result, they can live and hunt effectively at night and in turbid waters with poor visibility. They can also use their electrical fields for communication, signalling their moods and identities to other fish. The connection between electricity generation and swimming style is that, if gymnotiforms swam in the manner of other fish, their changes in body aspect would create changes in the shape of their electrical field. Holding the body more or less rigid means that the electrical field also remains constant, and any distortions must be caused by something external. Another group of fishes found in Africa and Asia that also navigates by electricity, the Notopteridae, has evolved a very similar appearance and swimming style to the gymnotiforms (and are also known as knife-fishes), but are entirely unrelated phylogenetically.

Tiger knife-fish Gymnotus tigre, from Trix.

The electric eel is something of an outlier among gymnotiforms. For a start, it's a monster: electric eels can be over two metres in length, while other gymnotiforms are all much smaller. The electric eel has also had a Susan Storm-style upgrade, and weaponised its electrosensory system. Electric eels can produce up to 600 volts of electricity, allowing them to stun reasonably large prey. The closest relatives of the electric eel are the banded knife-fishes of the genus Gymnotus; both are predators of fish and other aquatic animals. Males of at least some Gymnotus species and the electric eel build nests that the females lay their eggs into; males of Gymnotus carapo have been recorded to mouth-brood larvae.

The apteronotid Sternarchorhynchus mesensis, from here.

The remaining gymnotiforms were placed by Albert (2001) in a clade called the Sternopygoidei; these taxa have a smaller gape and feed on correspondingly smaller prey (some are planktivores). Two families, the Hypopomidae and Rhamphichthyidae, are united by the lack of teeth in the oral jaws; rhamphichthyids also have a very long and tubular snout. The other sternopygoids are placed in the families Sternopygidae and Apteronotidae; a distinctive feature uniting these two families is that they produce a wave- or tone-type electrical field instead of the pulse-type electrical field of other gymnotiforms. Pulse-type species produce discrete pulses of electricity at a lower frequency, while wave-type species produce a continuous series of electrical discharges at a much higher frequency (Albert 2001). While Albert (2001) regarded the pulse-type electrical field as ancestral for the gymnotiforms and the wave-type field as derived, other authors have preferred the opposite scenario. Sternopygids retain well developed eyes, in contrast to the reduced eyes of other gymnotiforms, while apteronotids are the only gymnotiforms to retain a caudal (tail) fin. If the wave-type families form a derived clade, then either these features were lost independently in the other families, or they represent reversals to an ancestral type.

One final thing to note is that the gymnotiforms have been going through something of a taxonomic boom, with many new species described in recent years. Albert & Crampton (2005) estimated that the total number of species out there could be nearly twice the 135 that had been named so far. In South America, it turns out, the streams are alive with the buzz of electricity.

REFERENCES

Albert, J. S. 2001. Species diversity and phylogenetic systematics of American knifefishes (Gymnotiformes, Teleostei). Miscellaneous Publications, Museum of Zoology, University of Michigan 190: 1-129.

Albert, J. S., & W. G. R. Crampton. 2005. Diversity and phylogeny of Neotropical electric fishes (Gymnotiformes). In: Bullock, T. H., C. D. Hopkins, A. N. Popper & R. R. Fay (eds) Electroreception, pp. 360-409. Springer: New York.