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Caecilians

2012; Elsevier BV; Volume: 22; Issue: 17 Linguagem: Inglês

10.1016/j.cub.2012.06.019

1879-0445

Mark Wilkinson,

Primate Behavior and Ecology

What is a caecilian? Think of an amphibian and you will most probably picture a frog or perhaps a salamander. Even if you had ever heard of them, much less likely would be that your first thought was of a caecilian. Caecilians are elongate, legless, snake- or worm-like amphibians of the old and new world tropics (Figure 1). Adults are mostly slimy-skinned burrowers in soils that feed upon soil invertebrates. Caecilians probably separated from the lineage comprising the frogs and salamanders (Batrachia) about 300 million years ago. Until some taxa dispersed out of India and into South East Asia, probably during the Oligocene, they were entirely a Gondwanan group. Why haven't I heard much about caecilians? Among vertebrate biologists caecilians are probably best known for being poorly known. Apart from a few aquatic species sometimes called 'rubber eels', caecilians have cryptic lifestyles, mostly hidden from view in muddy burrows and rarely crawling across the surface. They are consequently much less conspicuous than many other components of tropical rainforest ecosystems. Add to that their relatively inaccessible distribution in the wet tropics and complete absence of representatives in North America or Europe and you have a recipe for a kind of animal that is very rarely encountered and understudied. With no external trace of their presence in the soil, finding caecilians usually requires a lot of exploratory digging. How are caecilians adapted to life underground? Caecilians employ headfirst burrowing. Contraction of a specialised system of superficial body muscles within a double helix of connective tissue running along the body just below the skin squeezes the coelom and generates impressive hydrostatic force that lengthens the body and helps the head penetrate the soil. Their skulls are heavily ossified to withstand the pressure of burrowing, but at a cost of reduced size of ancestral jaw-closing muscles. To maintain a powerful bite caecilians also employ muscles that extend down the neck as the novel part of a dual-jaw closing mechanism. Once they have hold of their prey they can, like crocodiles, rotate along their long axes so as to tear their prey apart should it be too large to be swallowed whole. Another consequence of underground life is that their visual systems are rudimentary, both morphologically and physiologically. Small eyes may be completely hidden under bone and there is only a single light-sensitive photopigment and no colour vision. Reduced vision may be compensated for by unique, protrusible, sensory tentacles that seem to be analogous to snake tongues. Gymnophiona, the scientific name of the group, means 'naked snakes', a reference to their having been thought to have affinities with snakes while conspicuously differing from them in lacking scales. In fact, unlike other amphibians many caecilians have scales but these are embedded in pockets or folds in the skin and are invisible externally. This weird arrangement may also be associated with borrowing. Covering originally external scales with smooth skin may have reduced their resistance to especially backwards movement in underground burrows. How do caecilians reproduce? The class Amphibia is named for the biphasic lifestyle combining aquatic (eggs and larvae) and terrestrial (adult) stages that is typical of most temperate species of frogs and salamanders. Caecilians differ from other amphibians in that males have a copulatory organ formed from the eversible posteriormost part of the gut. Fertilisation is always internal and viviparity (live young bearing), which has evolved independently at least four times in caecilians, is much more common than in other amphibians. Caecilians that do lay eggs do so in terrestrial nests rather than in water, and many of these species have direct development, which bypasses the aquatic larval stage and metamorphosis completely. The foetuses of at least some viviparous caecilians are known to obtain nutrition from their mothers by feeding upon the lipid-rich epithelium of maternal oviducts using specialised spoon-shaped teeth with tiny cusps. Several direct developing, oviparous caecilians do something very similar which may have served as a precursor to viviparity: in these species, hatchlings also have specialised teeth that, over a period of three months or more, they use periodically to peel and eat the entire outer layer of skin of their mothers. While this might sound unpleasant for the mothers, their milky skin is specially modified for its role in rearing, and mothers are totally unhurt by their rapacious offspring. Although this maternal dermatophagy and extended parental care was only discovered very recently, it may be quite widespread in caecilians and appears to have been around for more than 100 million years. What is interesting about these amphibians? Caecilians may be most interesting by virtue of their phylogenetic relationships. As their sister group, they are equally as important as Batrachia (all the frogs and salamanders put together) to any attempt to infer features of their common amphibian ancestors and the history of early terrestrial vertebrate life. For example, that all oviparous caecilians lay their eggs on land rather than in water makes it plausible that the last common ancestor of the living amphibians and that of all living tetrapods also practiced terrestrial oviposition. If so, the origin of the amniotic egg would have been preceded by a long history of terrestrial amphibian eggs. Similarly, recent studies have revealed that frog and salamander skin secretions are rich in bioactive peptides with potential biomedical applications. Given their phylogenetic position, we might expect study of the slimy skin of any caecilian to enhance the known diversity of such compounds more than would study of any additional batrachian. As an independent lineage, caecilians provide many opportunities for comparative biologists to test theories on the evolution of diverse traits that were developed from studies of better-known taxa. For example, caecilian skin feeding and viviparity may provide useful analogues in the study of the evolutionary origins of lactation. Why are there so few species of caecilians? Good question. Currently there are only about 190 species of caecilian that have been described, compared to more than 600 species of salamander and over 6000 species of frog. Given recent discoveries, the actual numbers of species in each of these groups are far from certain, although the apparent differences in the orders of magnitude in the species diversity of frogs compared to caecilians and to salamanders is very unlikely to change. Perhaps frog speciation rates have been higher because of their use of song in mate recognition and courtship, or due to occupancy of relatively diverse habitats. In contrast, caecilians have no vocal communication and mostly occupy a more homogenous environment. However, caecilians remain poorly studied taxonomically and many areas in which they occur have been very incompletely surveyed. An entirely new family and radiation of caecilians was found recently in northeast India and it seems entirely plausible that there could be at least twice as many species worldwide as are currently recognised. I've heard there are global declines in amphibian populations, is that true for caecilians? There have been severe declines, and even extinctions, of some wild populations of frogs and salamanders in recent years. However, despite some anecdotes, there are no good data for caecilians. We do not know if caecilians are troubled by pathogenic chytrid fungus and the IUCN Red List compendium of conservation assessments lists just six caecilian species as threatened while the majority (66%) are 'data deficient'. This ignorance is no basis for being sanguine. Some caecilians seem well-suited to traditional agriculture in the tropics, maintain healthy populations in cultivated areas, and do not seem threatened. In contrast, less adaptable species may have already gone extinct due to changes in land use and large-scale habitat change must be considered a threat to caecilians as it is to many kinds of animal and plant.