The Most-Traveled Cane Toad

Swinging in the tree o' life!

Questions answered (and bookmarked) on this page:

Classification overview. 

The details. 

Related species. 

How cane toads evolved.

Summary: phylogenetic tree.

 

The overview:  Scientists classify organisms in a nested manner, in which groups of organisms are successively broken down into smaller and more closely related groups.  Cane toads are domain eukarya, kingdom animalia, phylum chordata, class amphibia, order anura, family bufonidae.  The final categories of classication (genus then species) are variable for the cane toad, depending on the author and the age of the the source.  The species was named as Rana marina by Linnaeus in 1758.  Since then, the name has shifted to Bufo strumosus in 1858, Bufo agua in 1916, Bufo marinis [sic] also in 1916, Bufo m. marinus in 1966 (as potential subspecies became apparent), Bufo marinus marinus in 1972, Rhinella marina in 2006, and since then as Chaunus marinus.  Most literature refers to the species as Bufo marinus, but newer literature has switched over to Chaunus marinus.  These are all sub-classifications within the genus Bufo; Chaunus is a further subclassification of Rhinella.

 

The nitty gritty:

-Domain Eukarya (‘true nut, or nucleus’: Eukaryotes have cells containing nuclei and have functions confined to given locations within the cell.  Cane toads have 22 chromosomes in the nucleus of cells.

-Kingdom Animalia (‘living breaths’): This kingdom, the animals, is composed of multicellular organisms who do not make their own food but depend on either plants or other animals.  Animals tend to use sexual reproduction to create genetically unique offspring.    Cane toads eat both plant and animal matter, and love their sex lives. 

-Phylum chordata (‘cord’, as in nerve cord): Within kingdom animalia is phylum chordata.  Chordates have a backbone.  At some time in their lives, they have gills and a muscular tail.  Cane toads certainly have a spine, and as tadpoles they have both gills and a tail. 

-Class amphibian (‘dual lives’):Within phylum chordata is the class amphibia.  Amphibia has over 4500 known living species.  Amphibians do not intentionally produce body heat (heat may be generated by movement or metabolism, but body temperature fluctuates with the environment rather than being maintained at a set temperature).  They have 2 pairs of legs and a bony jaw and glandular skin lacks hair, feathers, and scales.  They either have lungs or they had lungs at some point in evolution, and all have moist skin for cutaneous respiration. 

Amphibians produce vast numbers of offspring.  They tend to have a lot of "junk"  DNA, but a limited number of chromosomes.  Juveniles develop into the distinct adult body by means of a process called metamorphosis.  Cane toads bask or hide in shade and seasonally migrate in order to exert some control over their body temperatures.  The adult body has 2 pairs of legs.  The skin contains huge parotid glands.  The toad depends on its lungs both for breathing and for regulating its ability to sink or float in water. 

-Order anura (‘without tail’): Within class amphibia is the order Anura.  These are known as frogs and toads, although all toads are frogs, and the term “toad” has no phylogenetic (evolutionary relational) meaning.  Anurans demonstrate a squatting position that results from fused spinal vertebrate in the postsacral region.  Their skulls are fused to their first vertebrae, which results in a rigid head position.  They have long hindlimbs but rather short forelimbs, and because of their pelvic anatomy, can walk, crawl, leap, and swim.   no tail (recall that anura means “no tail”).  [One anuran does have a “tail”, but it isn’t a muscular tail but an external sexual organ.]  

Males and females are visibly distinguishable from each other.  Anurans were the first to evolve the ability to vocalize; both sexes have a larynx, which enables sound production, but the female lacks vocal sacs which would amplify the sound.  They have large eardrums.  They use amplexus positions for reproduction and have free-standing zygotes that are not dependent upon the mother via a placenta.  They have may have horny tips at the ends of their digits but lack claws. 

The cane toad clearly exhibits both the sitting posture and variety of gaits in movement.  Cane toads have tails as tadpoles but lack tails in the adult form.  [When it is stated that anurans have no tail, we must remember that they are amphibians, and thus exhibit different bodies as they attain sexual maturity.]  Males have fewer warts and lack a cream stripe down the back, and all Bufo seasonally exhibit changes in throat coloration between males and females. 

-Family Bufonidae (‘tuneful’ or melodious, true toads):  Bufonidae are most characteristically distinguished by the presence of the Bidder’s organ in males and the lack of teeth in both sexes.  This organ, which is a rudimentary ovary, permits males to transition to sexually viable females.  There are 33 genera within family bufonidae and 450 species.  Not all have rough skin, and some are aquatic.  All have poison glands, although the size, location, toxicity of secretions, and number of glands varies.  Families are determined by anatomical features and breeding peculiarities.  It takes just one glance at the poison gland of a cane toad to know that it belongs to this family.

-Genus Bufo/Rhinella/Chaunus:  Confused over 3 genera?  The cane toad is known as Bufo marinus, Rhinella marinus, Rhinella marina, and Rana marina, Bufo marinus, Rhinella marina, and Chaunus marinus.  These are in order from oldest to newest and most specific classification.  All names refer to the same species but depend upon the manner in which relationships are determined.  Rhinella is a sub-genus of Bufo.  Bufo marinus is the most commonly accepted name for the cane toad.  Members of the genus Bufo tend to experience seasonal breeding, lack parental care for the young, and have horizontal pupils.  Genera (plural of genus) Rhinella and subgenera Chaunus are confined to South American species.

-Species marinus/marina (‘marine’, or salty water):  The following information is an exact repeat of “Describe the appearance of a cane toad"– but you were warned, so read at your boredom or to refresh your memory.   OR, skip repeat.

Generally these toads sit rather erectly and are terrestrial (one pair has been observed 2.5 m up in a tree!).  First, consider the appearance of a cane toad.  Cane toad eggs are black.  Cane toad tadpoles are completely black and between 1 and 2.5 cm in length, with tail length equal to the body length.  New metamorphs are about 10 mm in size and lack large parotid glands.  They have relatively narrow bodies, broad heads with blunt-snouts and lateral nostrils, and *tend* to be slightly greenish. 

Both metamorphs and adults have unwebbed fingers on the front feet and moderately webbed toes and callouses on the back feet, light colored bellies, horizontal pupils, and have many warts on their backs.  Warts on the legs are smaller than warts on the back.  In males, each wart has at least one spike protruding from it.  There is a large ridge above the eye and often a less prominent, more incomplete ridge below the eye.  The ridge under the eye runs to the tip of the eye and then along the side of the flat portion of the skull towards the snout, where it merges with the ridge from the other eye; the ridge then runs down the center of the snout, between the nostrils.  Collectively this ridge design appears like an “M” shape. 

Behind and slightly below the eye is the tympanic membrane (external appearance of the internal ear), which presents as being slightly large than half the size of the eye and light in color.  Adults tend to lose the greenish tinge, instead being shades along the continuum of red, brown, and black.  After 5 cm, cane toad adults have a 4:3 length to width ratio.  For example, a toad might be roughly 6 inches long by 4.5 inches wide.  Their bodies are relatively flat.

 

Related species:  There are 40+ species in the South American genus Chaunus.  The resolution of evolutionary relationships is ongoing and in dispute.  What I *think* is the most recent follows.  From Hyloidea (a really old group) arose family bufonidae.  Within bufonidae arose genus Bufo, which was later divided into Rhinella, and later Rhamphorhyrne and Chaunus. Additionally, groups of B. marinus separated by the rise of the Andes mountains appear genetically distinct, likely due to genetic drift that would occur when these two groups fail to interbreed.  It is unclear whether these groups should be classified as separate species or subspecies.  Much resolution on the phylogenetic tree remains to be done.  Recent studies of the Chaunus marinus group (not species) has depended upon mitochondria DNA.

The evolutionary tale (to tail-less): 

About 360 million years ago, the first tetrapod (four-legged creature) arose from a fish with reinforced fins that could bear weight.  The fish propelled its body by means of these evolving legs, using its tail not for locomotion -as do most fish- but for balance.  This fish was likely an Ichthyostega (image below), whose fossils suggest that it may have occurred around Greenland.  It had primitive lungs, evolved a neck that enabled head rotation, and a more solid skeleton that happened to contain shoulder and pelvic girdles.  Life on land held the benefit of reduced competition, since these were the first land creatures.  It could escape any aquatic predators.  Additionally, this fish could then escape drying ponds that may have resulted from frequent droughts.

 

 

   Ichthyosteaga.     Pangaea. 

 

 

As vascular plants evolved to form forests, these primitive amphibians spread across the mega-continent of Pangaea, from which all current-day continents split.  This tetrapod spread took 135 million years.  Alternatively, because of the wide distribution of both Ichthyostega and other lobbed fish, amphibians may have evolved from different groups in different locations.  Regardless, as new habitats were encountered, tetrapods continued to diverge.  A group known as Labyrinthodonts arose.  These evolved the amphibian auditory ossicle, a broadened skull structure, and exhibited an ossified skeleton upon sexual maturation  -- which sounds sort of like metamorphosis.  250 million years ago, climatic change cooled the planet, and seasonal weather changes dictated not only hibernation, but narrowed the seasonal time frame for which adult reproduction result in successful offspring: offspring needed enough time to mature before the next seasonal hibernation. 

Skeletal changes occurred.  Over time some tetrapods evolved a more light weight skeleton, forming a theorized creature known as Lissamphibia.  Lissamphibia is hypothesized to have occurred during a 30 million year period from which no fossils have been discovered.  However, from this predicted ancestor arose Batrachia, which split to form Salienta . Fossils of this animal have been found in Madagascar which -230 million years ago- was near what would become South America and Africa.  Similar fossils in what became Poland suggest that early amphibians were very widespread.  Salientia resembled either a short-tailed salamander or an odd frog with a tail.  From Salientia arose the first amphibians without tails, the anurans, whose fossils date back 180 million years to Gold Spring Quarry, Arizona.  The bufonidae family itself has fossils dating back 57 million years, originating on the coast where South American continent once joined into the Africa continent.  They moved up South America, through Central America, into North America, through a land bridge connecting North America to Siberia, and into Eurasia and finally into Africa.  They are not present in Australia, New Guinea, or some nearby islands such as Madagascar.  They are the most widely distributed of anuran families.

The relational tree:

Note that evolution from Ichthyostega to Anura is based on fossil evidence.  Within Anura, various DNA analysis has been used to discern relationships.  Within Rhinella, relationships are derived from mitochondrial DNA.  

Below are some picture of other C. marinus group members.  Scroll over the photos to see the identity of each! 

  

 

 

 

    

 

 

 

 

 

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