In the tropics, I am constantly reminded of how limited my scope is for observing wildlife. A moss-like Carpenter’s anole strolls down from the canopy and excites me by its rarity, while up in what feels like the stratosphere, it is potentially a dime a dozen. I stretch and strain my neck to see through pockets in the dense foliage to watch a laughing falcon calling or get only a glimpse of iridescent blue from a one-spotted prepona butterfly that lands high up a cecropia tree. In these moments, I often wish I was a spider monkey, maneuvering the great heights with ease… but what seems even more inaccessible, is what lies beneath the surface. Gargantuan lengths of leaf cutter ant tunnels expand upwards to the forest floor revealing the tip of an iceberg, and a reclusive bushmaster drives its ridged scales through the burrows of spiny rats to coil up in its lair.
By late June, the wet season is usually in full force for the Caribbean versant of Costa Rica. Torrential rains bring more life to the rainforest, with arthropods molting in tandem, leaf frogs engaging in explosive breeding aggregations, and howler monkeys constantly announcing the sporadic rains. This year, however, it had been unusually dry. Heliophilic ameivas expanded their terrestrial domain as ephemeral swamps remained bone dry, and many species that are normally high in abundance were completely inactive. I joked with my friend, Andrés, that the three species of snail-eating snakes at La Selva were now either mythical creatures or locally extinct. I was looking for some sort of sign that different times were soon to be upon us, and finally one night, the stars had aligned. As I gazed beneath the understory palms, I saw a thick ringed body flailing clumsily to disappear under the leaf litter. Although I had never seen this creature before in life, its appearance was unmistakable, and I ran to retrieve it as my heart rate skyrocketed. One grab at the tail and its slimy body squirmed out of view… a few seconds passed… I lunged a second time to grasp its lower body as it resurfaced, and the animal contracted and extended its muscular body to slip away once again. By this time, I thought I had lost the animal for good, and I stood there staring at the ground in disbelief at my poor catching skills. Finally, I saw movement again, and though no part of the animal was visible, I furiously dug in the earth to uncover my first specimen of a unique and secretive group of amphibians— a caecilian (Dermophiidae: Gymnopis multiplicata).
With a purple-gray color and few definable features across its long limbless body, caecilians have the gestalt of an earthworm. They, too, are incredibly slimy with uniform ring-like segmentation (annuli) across the entire body— that is, until you examine the head a bit more closely. Two nostrils sit at the snout, and two extremely faint dots are present in lieu of eyes. Most noticeably, a long slit becomes indicative of a large mouth. What we have here is indeed a vertebrate!
If you are lucky enough to see the mouth agape, you will find two rows of sharp recurved teeth on the upper jaw and a single row on the bottom. Much more formidable than what they seem like from the exterior. Caecilians are capable of delivering a powerful bite, secreting enzymes (e.g., phospholipase) into their saliva from dental glands to incapacitate prey. But there is little to fear. Caecilians generally do not open the mouth in defense and will solely do so when in pursuit of their prey, which primarily includes earthworms, snails, termites, ants, and other leaf litter invertebrates. The presence of oral toxic compounds may seem surprising for an amphibian, but this strategy is useful for worm or mollusk-eaters that must immobilize prey quickly to prevent escape and for mucus control during ingestion (such is the case in neotropical snail-eating snakes). As if this wasn’t enough to conquer the slimy folk, caecilians will perform a “death roll” like a crocodile to further subdue prey. This corkscrew motion tears prey to pieces, allowing caecilians to hunt and consume prey items that exceed their maximum gape size.
Even without limbs, caecilians are adept at burrowing in the substrate, arching the neck downwards and flexing upwards to use their snout like a shovel. The tail end, at times, anchors against the ground for stability. Unsurprisingly, they are exclusively fossorial animals and rarely make their way to the surface. Small orifices in the caecilian skull provide attachment sites for connective tissues which increases rigidity and minimizes the risk of injury from repetitive abrasion when digging (see photo below). Secretions from mucous glands likewise enable their subterranean habits; mucus acts as a lubricant that helps caecilians move through burrows with little friction. As in other amphibians, the mucous secretions additionally function as a hydrophilic layer that retains moisture and facilitates cutaneous respiration, as well as providing antibacterial and antifungal effects to fight infections. Being slippery also makes caecilians difficult for predators to grab, exactly as I have experienced. Closer examination of their skin secretions has revealed that mucous glands tend to be located near the head, which makes sense for a creature that burrows headfirst. A second type, the poison (or granular) glands, are located more posteriorly in honeycomb arrangements of collagen fibers— visible from the skin’s surface as tiny pores. Although the chemical composition of the poison glands of most caecilians remains unknown, cardiotoxic and hemolytic activity has been documented in Siphonops, supporting the idea that these secretions may function as a predator deterrent. Scientists who research and handle caecilians have reported that the skin secretions can cause irritation in the eyes and during respiration if inhaled. Somehow, I always end up with red-eyed tree frog peptides in my eyes after interacting with them, but unfortunately, it did not occur to me to test the caecilian in this way!
If I haven’t already convinced you that caecilians are fascinating animals, wait until you hear about their sensory ecology. The most unusual characteristic of caecilians is the presence of two eversible short tentacles that lie just anterior to the vestigial eyes. Modern day caecilians are almost entirely blind, capable of only rudimentary light-dark perception, and instead rely on chemosensation through these two tentacles. In fact, in a developmental trade-off, the musculature from the eyes has been diverted to the tentacles, emphasizing the shift in reliance on an entirely different sensorial structure. During underground locomotion, caecilians will close their nostrils, perceiving only the chemical composition of their surroundings that come into physical contact with the tentacular tips. Each tentacle has a duct that relays sensory information directly to the vomeronasal organ (VNO), totally bypassing the olfactory (nasal) system. Caecilians, like snakes, rely extensively on perception of chemical stimuli within the VNO to navigate in the environment, track prey, and locate conspecifics. It’s remarkable that as opposed to using the tongue to directly deposit chemical information in the VNO, caecilians have evolved a completely novel sensory organ. Only one species of snake (Erpeton tentaculatum) is known to have tentacles on the snout, but this aquatic species purportedly uses its tentacles solely for tactile perception of vibrations in the water. For caecilians, chemosensory tentacles coupled with an inner ear sensitive to substrate-borne vibrations make them perfectly suited to navigating through confined spaces in total darkness.
Caecilians are most commonly found during very heavy rains when they get flooded out of their burrows or at construction sites, where large volumes of removed dirt unearths them from the depths. On the contrary, when I found this caecilian it was still relatively dry (as dry as a humid lowland tropical rainforest can be), and it hadn’t rained strongly for weeks. So, was this a completely serendipitous find? Several hours later that night it finally began to rain, and as we were wrapping up our hike, Jarreth speculated that the caecilian may have sensed the pressure change as the storm was approaching. As a matter of fact, just a few minutes before finding the caecilian, we had run into another water-affiliated animal— a coral snake (Micrurus alleni).
The snake was positioned about 10 meters away from the caecilian, peeking its head into divots and crevices searching for prey. Coral snakes are known to track down and feed on caecilians, so it’s possible that this semi-fossorial snake was hot in pursuit for a midnight meal. I have encountered this species of coral snake only a handful of times near slowly moving streams, but to recount my most fortuitous time with M. alleni, I’ll bring you 200 km southeast towards the coastline to Gandoca. Here, the forest understory was flooded by half a meter of water, and everything terrestrial fled upwards to the leaves and tree trunks to avoid drowning. Coral snakes, too, were forced out of their underground refugia, and together with Andrés, we found over a dozen gorgeous adult specimens of M. alleni in a span of a few hours… and not only that, but one individual feeding on a lungfish, to boot.
Apart from being striking in coloration (and possessing neurotoxic venom, of course!), coral snakes have an amazing stereotyped defensive display. They will recoil the head, flatten the body dorsoventrally, spiral their tail tip, and perform whip-like movements to dissuade their aggressor. Often times, they will even tuck the head underneath their body and raise their conspicuous dichromatic tail tip, diverting attention away from the head to the more expendable tail. I’ve found that once the display is initiated, simply moving around the animal can stimulate motion, likely due a combination of the vibrations made from my footsteps as well as my looming figure from above. The mouth is rarely opened during the display, and bites are usually only attempted when grabbed directly. I’ve observed similar jerky display behaviors in coral snake mimics, including the red coffee snake (Ninia sebae) and Cope’s false coral snake (Pliocercus euryzonus). These two species seem more hesitant to initiate their displays and tend to flee even when restrained. Their hesitance to stay put may be a result of the lack of venom defense. Perhaps, once an encounter becomes prolonged, predators can successfully discriminate model vs. mimic, and the mimics have little chance of fending off the predators by displaying alone.