This is the 200th article at Tet Zoo ver 3 – thanks, pass the champagne, donation cheque etc. (hint hint). The plan is to produce a lengthy introspective-type article that includes links to all the content that’s appeared on Tet Zoo ver 3 so far. Alas, I just haven’t been able to find time for that and it’ll have to wait until conference season is over. Until then, here’s a short article about caecilians (to long-time Tet Zoo readers, it’s a section from one of the longer caecilian articles that appeared on ver 2 back in 2008). I so need to cover caecilians at length, once again. And in time I will.

All extant caecilians (pronounce it ‘say-see-lee-un’) are long-bodied, limbless, superficially worm-like amphibians with reduced eyes. [Image of Dermophis above from the CalPhotos database.] Caecilians are predominantly fossorial (adapted for burrowing), subterranean animals, although some are aquatic or semiaquatic and some terrestrial species have aquatic larvae. Limb girdles are entirely absent and skin folds (annulae) encircle the body. Despite their mostly subterranean habits, some caecilian species are visually striking, being blue, purple, green, bright orange, yellow, or with bright yellow lateral stripes. Others are decorated with blotches, or have a head that’s lighter in colour than the body. Poison glands are present (some species may even be quite toxic to humans), so it’s likely that the bright colours are aposematic.

While the smallest species (like the caeciliid Idiocranium russelli from Cameroon) are mature at just 70 mm, the largest – the very slender-bodied Caecilia thompsoni from Colombia – reaches an incredible 1.5 m [C. thompsoni shown in image below; photo by Taran Grant].

In the members of some caecilian clades (like the South American rhinatrematids), a short tail is present and the mouth is located at the front of the head; in others, a tail is absent and the mouth is situated further back, slung underneath the head. Among the weirder features of caecilians are their short, sensory tentacles. Unique to the group, these are paired structures that emerge from cavities on either side of the snout, between the eye and nostril. The tentacles are typically easy to see, even in the smallest species. Derived from the tear duct, extrinsic eye muscles and other orbital structures, the tentacles are connected to the vomeronasal organs and presumably allow the animals to test their environment for sensory data.

The caecilian skull is generally bullet-shaped, robust, thick-boned and with strongly adhering skin. Their recurved teeth are sometimes sharp-keeled and bicusped. The eyes of caecilians are sometimes visible beneath the skin and set within bony sockets (e.g., Ichthyophis), are sometimes hidden beneath the bones of the skull (e.g., Scolecomorphus), and are sometimes completely absent (e.g., Boulengerula) [UPDATE: not correct. See comments].

While the tentacle is often located close to the nostril and some distance from the eye, the eye and tentacle are close in position in some species. Scolecomorphids, containing only Scolecomorphus and Crotaphatrema, are unique to equatorial Africa and have a particularly large tentacular opening located near the tip of the snout, ahead of the under-slung mouth. The close position of the eye and tentacle mean that they’ve become connected: in its resting position, the eye is located beneath the lateral surface of the skull, but full extrusion of the tentacle causes the eye to move out of the skull and down the tentacle (O’Reilly et al. 1996). An area of the tentacle lacking in pigmentation presumably allows light to reach the retina. Scolecomorphids are the only tetrapods that can deliberately move their eyes out of their skulls. Scolecomorphids are also unusual in lacking a stapes.

It’s mostly assumed that caecilians are generalist predators of soil-dwelling invertebrates such as earthworms, ants and termites – animals termed ‘soil ecosystem engineers’ by ecologists. However, it’s also been argued that some caecilians are specialists (preying specifically on termites, earthworms or beetle pupae), or even detritivores, ingesting leaf humus and other plant fragments (Hebrard et al. 1992).

That last proposal would be radical given that amphibians are essentially all carnivorous as adults*. Alas, a later study showed that soil and detritus recovered from the gut contents of the species concerned (the east African caeciliid Boulengerula taitana) actually represented the gut contents of its earthworm prey (Gaborieau & Measey 2004). Small vertebrates including frogs, lizards, burrowing snakes (Presswell et al. 2002) and possibly rodents sometimes fall prey to the larger species. Caecilians themselves are preyed upon by burrowing snakes, fish (Gazola Da Silva et al. 2007), and by introduced animals like chickens, pigs and the tenrecs that have been introduced to the Seychelles.

* Yes yes, I am aware of the exceptions (and have written about them on Tet Zoo: herbivory in sirens, frugivory and even leaf-eating in frogs etc.).

Caecilians occur throughout the humid tropical regions of the world: in South America, equatorial eastern and western Africa, and tropical Asia including the Philippines and the western Indo-Australian archipelago. They also inhabit the Seychelles. The absence of caecilians from central equatorial Africa and Madagascar is odd and we should consider the possibility that they might await discovery in these areas. Generally assumed to be rare and highly elusive, some studies have shown that at least some caecilian species can be abundant if searched for (Measey 2004), and in fact present in sufficient numbers to be (presumably) ecologically significant, especially so given their predation on soil ecosystem engineers (Jones et al. 2006).

There is, of course, loads and loads and loads more than needs to be said about caecilians, some of which has been covered here on Tet Zoo before. Don’t worry, this additional data will be covered here (again) in time, in updated form – there is lots of exciting news from the world of caecilians!

For previous Tet Zoo coverage of caecilians, see...

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Gaborieau, O. & Measey, G. J. 2004. Termitivore or detritivore? A quantitative investigation into the diet of the east African caecilian Boulengerula taitanus (Amphibia: Gymnophiona: Caeciliidae). Animal Biology 54, 45-56.

Gazola Da Silva, F. F., Mott, T., Garey, M. V., & Vutule, J. R. S. 2007. Chthonerpeton viviparum Parker & Wettstein, 1929 (Amphibia, Gymnophiona, Typhlonectidae) in Paraná state, Brazil and the first record of predation of this species by Hoplias malabaricus (Bloch, 1794) (Actinopterygii, Erythrinidae). Pan-American Journal of Aquatic Science 2, 261-262.

Hebrard, J., Maloiy, G. & Alliangana, D. 1992. Notes on the habitat and diet of Afrocaecilia taitana (Amphibia, Gymnophiona). Journal of Herpetology 26, 513-515.

Jones, D. T., Loader, S. P. & Gower, D. J. 2006. Trophic ecology of east African caecilians (Amphibia: Gymnophiona), and their impact on forest soil invertebrates. Journal of Zoology 268, 117-126.

Measey, G. J. 2004. Are caecilians rare? An east African perspective. Journal of East African Natural History 93, 1-21.

Nussbaum, R. A. 2000. Caecilians. In Cogger, H. G., Gould, E., Forshaw, J., McKay, G. & Zweifel, R. G. (consultant eds) Encyclopedia of Animals: Mammals, Birds, Reptiles, Amphibians. Fog City Press (San Francisco), pp. 492-499.

O’Reilly, J. C., Nussbaum, R. A. & Boone, D. 1996. Vertebrate with protrusible eyes. Nature 382, 33.

Presswell, B., Gower, D. J., Oommen, O. V., Measey, G. J. & Wilkinson, M. 2002. Scolecophidian snakes in the diets of south Asian caecilian amphibians. Herpetological Journal 12, 123-126.