The morphlings versus the axolotls (how frogs have warped tadpoles into new shapes and sizes), part 3
...continued from https://www.inaturalist.org/journal/milewski/87002-the-morphlings-versus-the-axolotls-how-frogs-have-warped-tadpoles-into-new-shapes-and-sizes-part-2#
Large toads have tadpoles no longer than about 3 cm, which metamorphose into small ‘adults’ with snout-vent length not much more than 1.5 cm (and in some cases, I gather, as little as a third of this).
By contrast, the American bullfrog (Lithobates catesbeianus, https://www.inaturalist.org/taxa/65979-Lithobates-catesbeianus), which is about the size of a large toad in maturity and is probably the largest non-toad frog in North America, has tadpoles up to 17.7 cm long. I have not found data on snout-vent length at metamorphosis but if the tadpole is >15 cm then presumably the frog is initially at least 5 cm long, severalfold the corresponding size in toads.
The metamorphs of L. catesbeianus can weigh 5 g, compared to <0.05 g in the case of bufonids with similar mature sizes. That’s a difference of two orders of magnitude in body mass at the stage of metamorphosis.
I think it’s safe to say that in most comparisons of bufonids with ranids of similar mature body mass, we can expect a difference of an order of magnitude in body mass at metamorphosis.
Furthermore, ranids such as Lithobates seem to be a bit like centrolenids, in retaining more of the tail at metamorphosis (when the animal leaves the water) than is true of most families of frogs. Bufonids don’t seem to go in for the retention of a residual tail at all.
So typical toads and typical frogs (ranids, so familiar in Europe and North America, although absent from South Africa where their place is taken by pyxicephalids) are similar in their commonness and fecundity, but differ in their development: bufonids have morphlings whereas ranids do not.
The following illustrations show the body sizes relative to human figures for scale.
The species illustrated is the natterjack toad (Bufonidae: Epidalea calamita, https://www.inaturalist.org/taxa/65450-Epidalea-calamita) of Europe, but the body sizes and shapes are typical of many bufonids including Sclerophrys pantherina.
The natterjack toad is about average size for a toad, usually about 7 cm snout-vent length in maturity. The tadpoles, which complete their growth within two months, are small and this toad metamorphoses at about 0.7 cm snout-vent length.
Initially the morphlings (which are diurnal, presumably to avoid being cannibalised) are easily mistaken for invertebrates in the poolside herbage. The morphlings are so small relative to the fully mature animal that they take 3-4 years to reach sexual maturity.
The point of all of this is that toads typically metamorphose at remarkably small body size, which means that their life history can best be understood by dividing it into not just the three stages normally described for amphibians, viz eggs, larvae, and adults, but rather into four stages: eggs, tadpoles, morphlings, and adults.
The morphling can be thought of as an adult at larval size, and the development of toads can be interpreted, in a sense, as peramorphic – although I’ve never seen this suggested in any literature or anywhere on the internet.
Bufonidae: Epidalea calamita: mature individual:
http://i1.rgstatic.net/i/profile/60b7d54391f1a0ec3a_l_c69d9.jpg
Epidalea calamita morphling:
http://www.denbighshirecountryside.org.uk/files/Natterjack%20toadlet%202012%201.jpg
Epidalea calamita morphling:
http://farm7.staticflickr.com/6094/6349420673_f129805899_z.jpg
The following reference invokes peramorphosis in frogs of the family Ceratophryidae (https://www.inaturalist.org/observations?taxon_id=64727). From the abstract it’s not clear what the basis of peramorphosis is.
The ‘early onset of metamorphic transformations’ mentioned in the abstract is indeed what peramorphosis is all about, but the features concerned in this case seem too subtle or obscure to be specified in the abstract. So I’ll have to delve into the body of the paper itself.
I assume that the lungs of toads only develop at or after metamorphosis (?known as long ago as 1931 when Noble wrote his book about amphibians).
However, it’s worth noting that in many other families of frogs the lungs start to develop well before metamorphosis, along with the developing legs.
This means that the later stages of the tadpole already possess, and use, lungs in many frogs as well as in the salamanders.
Toads seem to be an exception, which makes sense to me in view of the tiny size of the toad tadpoles.
So in toads the appearance of the lungs coincides with the loss of the external tail, whereas in salamander larvae there is no such coincidence because lungs are already present in the larva, and the tail is not lost; while in most tadpoles there is no such coincidence because the lungs develop before the tail is lost.
The paper below, Cohen & Alford (1993), gives data on the body sizes of morphlings for Rhinella marina.
I infer that a morphling can be defined in this species as having snout-vent length less than 3 cm.
The smallest morphlings seem to have snout-vent length of 9 mm, which is bigger than I thought, at least in the population these authors studied. I can’t understand how a morphling of 0.9 cm could possibly weigh as little as 0.025 g (which if memory serves is the minimum body mass given a paper by Shine). A morphling of length 0.9 cm is certainly at least blowfly size, not housefly size and certainly not fruitfly size. But I would still classify these as morphlings, because even at ca 1 cm they are still tiny relative to fully mature body sizes. So tiny that an additional, even tinier, larval stage seems ‘over the top’.
The significance of the morphling stage in toads is that:
this stage combines larval body size with adult form;
toads have essentially two consecutive ‘baby’ stages in their life history, viz tadpole and morphling;
morphlings are subject to cannibalism by juveniles regardless of whether they are also cannibalised by adults;
morphlings are part of a life history strategy of extreme fecundity (enormous clutches of eggs, up to 30,000 by a single mother), in which parental care is replaced by parental ‘hyperinvestment’.
http://www.publish.csiro.au/paper/WR9930001.htm
The morphlings of Anaxyrus terrestris (https://www.inaturalist.org/taxa/64988-Anaxyrus-terrestris), which I take to have snout-vent length of ca 8 mm, can be as small as 0.055g according to the figures in http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2435.2000.00386.x/pdf
For comparison, a house fly (length 6mm) has body mass of 0.012g. This means that even the smallest morphlings of Anaxyrus terrestris are about five-fold heavier than the average house fly.
Fully mature Anaxyrus terrestris reaches snout-vent 8 cm or up to 9.2 cm, which is large for a frog.
So the morphlings are small but certainly much larger than fruit fly size.
The hylid Litoria caerulea (https://www.inaturalist.org/taxa/517075-Ranoidea-caerulea) is a large frog. Its tadpole reaches about 5 cm long including the tail. The freshly metamorphosed (tailless) frog has a snout-vent length of about 1.6 cm, which means that it is initially about the same size as a man’s thumbnail. That means about the size of those big dung beetles one sees in elephant faeces. It can live >20 years, and over that time it grows to a snout-vent length of 10 cm.
Compare this with bufonids. Although the mature cane toad (Rhinella marina) is bigger than Litoria caerulea, its tadpole is smaller, reaching only 3.1 cm long including the tail. The freshly metamorphosed (tailless) stage is about 0.7 cm long snout-vent, about the size of a blowfly. This then goes on to grow even more than is the case in Litoria caerulea.
So there is a difference between this hylid and this bufonid, in relative size of tadpole and freshly metamorphosed frog. The difference in body mass is about an order of magnitude, in the case of both the fully-grown tadpole and the freshly metamorphosed ‘adult’ (tailless for the first time).
There is certainly a difference in life history strategy here. It remains debatable whether this difference justifies the introduction of a new term, viz ‘morphling’ (which needs objective criteria), for the extremely small freshly metamorphosed stage of the bufonid, which is more fecund than the hylid.
The value of making this distinction would be more apparent in a comparison with Pseudis (https://www.inaturalist.org/observations?taxon_id=24406), which has a much larger tadpole again and does not seem to grow much after metamorphosis.
Incidentally, Hyperolius (https://www.inaturalist.org/observations?taxon_id=23276) has a tadpole that is larger, relative to the mature size of the frog, than is the case for Litoria caerulea. This is partly because the fully mature Hyperolius is so small. No matter how we define ‘morphling’, Hyperolius would certainly not qualify as possessing such a stage in its life history. The whole concept of a ‘morphling’ probably only matters in large frogs, i.e. frog species in which the fully mature stage has a snout-vent length of say >5cm.
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