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.

Stunning Sea Slugs

Chromodoris willani, photographed by Samantha Craven.

The name 'sea slug' does not sound immediately enticing to those not in the know, but in fact they include some of the world's most fabulous animals. The Chromodorididae are a family of more than 300 species of ludicrously colourful sea slugs, with the greater number of species found in the Indo-West Pacific region. They are specialist feeders on sponges, preferring those sponges that do not contain mineralised spicules.

Cadlinella ornatissima, photographed by Barb Makohin.

Their bright colours, of course, are a signal to any would-be predators that they are highly toxic and distasteful, in the manner of monarch butterflies, poison-arrow frogs, or raspberry cordial. As noted by sea-slug researcher Bill Rudman in this forum post, toxic compounds are often concentrated in the mantle fringe that runs around the sides of the chromodorids. They will often flap this mantle, and Rudman speculates that this may make an attacker more likely to bite off a piece of the distasteful (and easily regrown) mantle rather than the main body of the animal.

Pair of Hypselodoris bennetti, photographed by Richard Ling.

Until the classification of the Indo-West Pacific species was reviewed by Rudman (1984), most chromodorid species were lumped into a single genus Chromodoris under the belief that there were few relaible characters available to distinguish higher taxa within the family. Rudman (1984) identified a number of features, including characters of the reproductive and digestive tracts, that could be used to separate genera. A molecular analysis of the family by Johnson & Gosliner (2012) led them to recognise 17 genera of chromodorids, with genera recognised by Rudman (1984) as pan-tropical proving to be polyphyletic.

Doriprismatica kulonba (previously Digidentis kulonba), photographed by Bill Rudman).

Though sea slugs are mostly thought of as tropical animals, there are some that make there homes in cooler waters. Doriprismatica kulonba, for instance, is found around south-east Australia.

Ceratosoma tenue, photographed by Steve Childs.

Long-term readers may have noticed that this post contains a somewhat lower text-to-imagery ratio than usual. For this, I make no apology.

Diversidoris flava (previously Noumea flava), photographed by Chad Ordelheide.

REFERENCES

Johnson, R. F., & T. M. Gosliner. 2012. Traditional taxonomic groupings mask evolutionary history: a molecular phylogeny and new classification of the chromodorid nudibranchs. PLoS ONE 7 (4): e33479. doi:10.1371/journal.pone.0033479.

Rudman, W. B. 1984. The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the genera. Zoological Journal of the Linnean Society 81 (2-3): 115-273.