Amphibians
Mexican Caecilian
Dermophis mexicanus · also called mexican caecilian, mexican burrowing caecilian
The Mexican caecilian is a limbless, nearly blind amphibian that spends its life tunnelling through the moist soil of Central American farms and forests. It is one of the least familiar animals there is — and one of the most extreme parents. Its young are not laid as eggs; they develop inside the mother for nearly a year, feeding the whole time by scraping a rich lining off her oviduct with tiny hooked teeth.
Overview
The Mexican caecilian lives in moist soil across Central America, from southern Mexico to Panama, in forest, plantations, pasture, and gardens. It is 30–60 cm long and pencil-thick, slate-grey to purplish, ringed, with a blunt head, skin-covered eyes, and a pair of small sensory tentacles. It eats earthworms and termites underground.
Fed from the inside
The female carries her young in her oviducts for about eleven months. The unborn young use hooked and spoon-shaped fetal teeth to scrape the oviduct wall, which secretes a nutrient-rich “uterine milk,” and they grow several times in length before being born as fully formed juveniles. It is a parallel to a mammal’s placenta, arrived at completely independently.
Built to dig
It burrows head-first, driving a reinforced wedge-shaped skull through the soil, with the mouth set back underneath so it does not fill with earth.
A farmland amphibian
Unlike many caecilians, it thrives on disturbed ground and is common in banana and coffee plantations, where its burrowing aerates the soil.
Status
The Mexican caecilian is Least Concern, common and widespread, with no significant threats. Because it descended from four-legged ancestors and lost its limbs, it is studied for how limb-building genes are switched off.
Mexican Caecilian: common questions
How does the Mexican caecilian feed its young before they are born?
The female does not simply carry passive embryos. Early in the pregnancy the young use up their yolk, and after that the lining of the oviduct changes, thickening and secreting a rich fluid made of fats, proteins, and shed cells. The developing young have a set of specialised temporary teeth, some hooked and some shaped like tiny spoons, which they use to scrape and stimulate this lining and to take in the secretion. Over the roughly eleven months of pregnancy they can grow to many times their length at the start, and they are born as capable miniature adults rather than as larvae.
Is that the same as a mammal's placenta?
It is a case of the same problem being solved in a similar way by very different animals. Placental mammals nourish their young through a placenta, a specialised organ that links the fetal and maternal blood supplies. The Mexican caecilian and some other live-bearing caecilians instead feed their young by secreting nutrients into the oviduct for the young to eat, a system called oviductal or histotrophic nourishment. It achieves the same end, transferring a large amount of the mother's resources to her offspring before birth, but through eating rather than through a blood connection, and it evolved completely independently.
What is a caecilian, and is it a worm or a snake?
It is neither. Caecilians are amphibians, the third major group alongside frogs and salamanders, and they are limbless burrowers of the wet tropics. A caecilian looks a bit like a large earthworm or a small snake but has the moist, glandular skin, the lidless skin-covered eyes, and the developmental biology of an amphibian, plus a unique pair of retractable sensory tentacles on the head that it uses to taste and smell its way through the soil. There are around 200 species, most of them rarely seen because they spend their lives underground.
Why is the Mexican caecilian so common when many caecilians are rare?
Mostly because it is a habitat generalist that does well in landscapes people have modified. Many caecilians are tied to undisturbed forest soil or to specific streams and decline when their habitat is cleared. The Mexican caecilian, by contrast, is found in good numbers in banana and coffee plantations, cattle pasture, roadsides, and suburban gardens, wherever the soil stays moist and holds enough earthworms and termites. Farm workers turning soil frequently uncover them. This tolerance, combined with a large geographic range, is why the species is listed as Least Concern.