Zoologist, media consultant, and science writer, Dr Karl Shuker is also one of the best known cryptozoologists in the world. He is the author of such seminal works as Mystery Cats of the World (1989), The Lost Ark: New and Rediscovered Animals of the 20th Century (1993; greatly expanded in 2012 as The Encyclopaedia of New and Rediscovered Animals), Dragons: A Natural History (1995), In Search of Prehistoric Survivors (1995), The Unexplained (1996), From Flying Toads To Snakes With Wings (1997), Mysteries of Planet Earth (1999), The Hidden Powers of Animals (2001), The Beasts That Hide From Man (2003), Extraordinary Animals Revisited (2007), Dr Shuker's Casebook (2008), Karl Shuker's Alien Zoo: From the Pages of Fortean Times (2010), Cats of Magic, Mythology, and Mystery (2012), Mirabilis: A Carnival of Cryptozoology and Unnatural History (2013), Dragons in Zoology, Cryptozoology, and Culture (2013), The Menagerie of Marvels (2014), A Manifestation of Monsters (2015), Here's Nessie! (2016), and what is widely considered to be his cryptozoological magnum opus, Still In Search Of Prehistoric Survivors (2016) - plus, very excitingly, his first two long-awaited, much-requested ShukerNature blog books (2019, 2020).

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Saturday 3 June 2017


Artistic representation of the Trinidad luminous lizard's possible appearance when glowing, as based upon Ivan T. Sanderson's claim (© Philippa Foster)

Among the fishes and several different taxonomic groups of invertebrate (including comb jellies, cnidarians, molluscs, insects, centipedes, millipedes, crustaceans, and annelid worms) are many bioluminescent species. That is, living creatures which actively carry out chemical processes to produce and emit light.

Officially, however, there are no bioluminescent species among the terrestrial vertebrates - but claims have been made that there may in fact be a notable exception of the reptilian kind.

I first learnt about, and then duly investigated, this fascinating yet surprisingly little-known case back in the mid-1990s, and here is what I uncovered at that time, followed by the extraordinary revelations that have occurred since then – yielding in this present ShukerNature blog article of mine the most comprehensive account ever published online.

A selection of fully-confirmed bioluminescent creatures depicted in a vintage illustration from 1890 (public domain)

In March 1937, during an animal collecting trip to the West Indies, American zoologist and cryptozoologist Ivan T. Sanderson visited Mount Aripo (aka El Cerro del Aripo), at 3,084 ft high the loftiest peak in Trinidad and part of this island's Northern Range. He had been capturing some freshwater crabs in a series of dark subterranean pools there when he suddenly spied a faint light in a crevice beneath a ledge. The light promptly went out, but Sanderson was curious to discover its source, so he flashed his torch into the crevice - and was most surprised to find a small lizard.

Attempting to coax it into his net, Sanderson gently tickled the lizard, but instead of running out it turned its head away - and as it did so, Sanderson was very startled to see both of its flanks momentarily lighting up "...like the portholes on a ship". When he finally succeeded in capturing it, this remarkable reptile lit up again, glowing brightly in his hand with a pale greenish hue that Sanderson subsequently likened to the glow produced by the hands and figures of a luminous watch.

As zoologists were previously unaware of any bioluminescent lizards, Sanderson was very thrilled by his discovery, which he documented in his book Caribbean Treasure (1939). Ironically, however, apart from its unique glowing ability the lizard, which was a male, seemed relatively nondescript in general appearance - with a long tail but short legs, a sharply-pointed muzzle, dark brown upperparts, and rosy salmon-pink underparts (turning yellow under its head) surfaced with large rectangular scales of plate-like form.

Ivan T. Sanderson's book Caribbean Treasure (© Viking Press, reproduced here on a strictly non-commercial Fair Use basis only)

Its only distinctive features were its body's lateral eyespots or 'portholes', constituting a series of large circular black blots running from the neck to the groin on both flanks, because each of these blots contained a vivid white bead-like spot. And it was these spots that were the source of the lizard's apparent luminescence, as determined by Sanderson during some basic experiments:

We made it [the lizard] hot and cold, and moist and dry alternately; we blew a loud whistle in its ear, we tickled it, and we subjected it to flashes of bright light...This creature seemed to produce its light in response to sudden emotional disturbance, rather than through actual physical reactions...The loud whistle, sudden winds, and flashes of light greatly agitated our lizard, causing it to switch on its 'portholes'. We noticed that this light was much brighter the first time it was switched on after the animal had been quiescent for a period, and more especially after it had previously been subjected to intense illumination.

Eventually, Sanderson shipped off his amazing little lizard to the British Museum (Natural History) in London, where it was studied in detail by fellow zoologist H.W. Parker. It was found to belong to a species already known to science (indeed, Parker himself had formally named and described it in 1935), but only just. An exceedingly rare member of the tejid (aka tegu) family Teiidae, and normally measuring 11-15 cm long, it was called Proctoporus (=Oreosaurus) shrevei (in honour of the very gifted American amateur herpetologist Benjamin Shreve), and had hitherto been represented in scientific collections only by a single preserved juvenile and one preserved adult female. Sanderson's specimen was therefore the first male of this species to have been brought to scientific attention, and until now no-one had suspected that it may be bioluminescent when alive.

Proctoporus shrevei (copyright holder presently unknown to me despite my having made considerable efforts to discover this; reproduced here on a strictly non-commercial Fair Use basis only)

During his visit to Trinidad, Sanderson collected seven more individuals of this species, and as preserved specimens these too were examined by Parker. In a paper published by the zoological journal of London's Linnaean Society in 1939, Parker revealed that it was sexually dimorphic, with only the males sporting the distinctive 'porthole' markings (a further reason why no bioluminescence had been reported from the specimens procured prior to Sanderson's), and that in every porthole the epidermis of the white bead at the centre was less than half the thickness of the epidermis of the black ring surrounding it. In addition, the white bead's epidermis was transparent, lacking any form of pigment. In other words, each porthole literally constituted a black-edged circular window.

How the portholes functioned, however, remained a mystery, because Parker found no associated nerve endings or an increased blood supply, thereby eliminating any likelihood that they were directly connected with the sensory or circulatory systems. Nor did he find any ducts connecting them with the exterior, or any complex lenses or reflecting structures.

Whatever they were, therefore, these portholes were clearly very simple in structure, and Parker offered three possible explanations for their luminosity. In life, the portholes may contain some substance that either glows when it breaks down (the principle of bioluminescence in various fishes), or glows when exposed to light (as with the paint used in luminous watches). The third option is that the transparent central beads of the portholes are underlain with reflective tissue. (A fourth possibility, that the portholes contain glowing bacteria which create their luminosity, can be rejected, because Parker did not report the presence of any bacteria within them.)

Ivan T. Sanderson as a young man (copyright owner presently unknown to me despite considerable searches made; reproduced here on a strictly non-commercial Fair Use basis only)

Inevitably, the prospect of a luminous lizard duly attracted attention from several other zoologists, who studied specimens of P. shrevei and various related tejids to find out whether any of them really did glow - but none of them did! And so in 1960, reporting at some length in the journal Breviora their own negative findings with P. achlyens from Venezuela and Neusticurus [now Potamites] ecpleopus ocellatus from Peru (both of which possess porthole markings resembling those of P. shrevei), American biologists Drs Willard Roth and Carl Gans rejected Sanderson's claims regarding P. shrevei's bioluminescence.

Yet Sanderson was an extremely experienced field zoologist, and Parker's histological studies convinced him that the portholes were genuine luminous organs. So who was correct? If P. shrevei were the only bioluminescent species, this would of course render worthless any comparative studies with related species. Moreover, at the time of my own initial examination of this case, only one zoologist other than Sanderson had actually investigated luminosity with living P. shrevei specimens, and he may simply not have stimulated them sufficiently for them to light up. (I subsequently found out that this latter zoologist was Prof. Julian S. Kenny – see later.)

My above account presents the situation concerning Trinidad's intriguing 'glowing lizard' that I had uncovered during my mid-1990s investigations. Since then, however, much additional information has come to light (pun intended!), and, as I discovered after unearthing it, this extra data includes some very significant new insights into P. shrevei and its alleged bioluminescent capabilities.

Map of Trinidad, highlighting its Northern Range, where Mt Aripo is (public domain)

First of all, it is nowadays deemed not to be a true tejid, so it is housed within a separate taxonomic family, Gymnophthalmidae, which contains many species. These are sometimes referred to as microtejids, because they are smaller than true tejids. Also, they tend to be quite skink-like in appearance, with certain species possessing reduced limbs.

I was pleased to learn that following further field studies, P. shrevei is no longer considered to be as rare as previously claimed. Indeed, the IUCN officially categorises it as being of Least Concern, and the IUCN Red List website states: "...although the distribution [of this species] is limited (with an extent of occurrence of 210 km2), the population trend appears to be stable, there are no current threats, and it occurs in at least two protected areas".

In addition, the IUCN assigns this species to the genus Riama, although quite a few other authoritative sources checked by me retain it within Proctoporus (so I shall do the same here for text consistency purposes), and refers to it via a very memorable common name that ties in with its supposed abilities – Shreve's lightbulb lizard. But is this name warranted?

Mark O'Shea lecturing at West Midlands Safari Park, England (© Ghaly-Wikipedia – CC BY-SA 3.0 licence)

During my original investigations of Shreve's lightbulb lizard in the mid-1990s, I communicated with the West Midlands Safari Park's internationally-renowned herpetological expert Mark O'Shea, famed not only for his numerous scholarly publications but also for his fascinating TV show O'Shea's Big Adventure in which he travelled the world seeking rare or unusual reptiles and amphibians. Mark was very interested in this mystifying lizard species, and I was delighted when he subsequently visited Trinidad to look for it. His search featured in 'Exotic Island', the tenth episode of his show's first series, screened in 1999.

After arriving in Trinidad, Mark and his camera crew teamed up with Caesar, a local guide, and with Dr Victor Quesnel (named as Quinnel in some reports), a retired Trinidad-based economic botanist who was also a very knowledgeable all-round naturalist (he died in 2014). But before they set off on their arduous trek in the hope of emulating Sanderson's original success in encountering this island's luminous enigma in 1937, they were able to chat with Javrien Capriata (aka Capriata Dickson), who had been Sanderson's guide back then, and was now over 80 years old (Sanderson himself had died in 1973). Happily, Mark's search proved successful too, as the team found two specimens, a male and a female. (Moreover, during a much later expedition in 2008, Dr Quesnel actually rediscovered the specific cave where Sanderson had captured his lizard in 1937 but which had not been found since then; it is now known as Sanderson's Cave.)

These two lizards were duly videoed in a dark room by Mark's cameraman while they were being illuminated artificially and for a time after the artificial illumination had been turned off. The video was then viewed closely to see whether there were any signs of luminescence from them. Not surprisingly, the female lizard did not glow, as it lacked the all-important porthole markings. Conversely, the male did indeed appear to glow for a short time after the illumination had been turned off. Unfortunately, however, it was not possible to determine whether this constituted bona fide glowing from the lizard, or whether it was merely a trick of the light caused by filming and the camera adjusting to the darkness after the illumination had been extinguished.

Ivan T. Sanderson in later years (© Dr Bernard Heuvelmans)

During the first half of the year 2000, I exchanged a series of letters with herpetological specialist Hans E.A. Boos from Port of Spain, Trinidad's capital, who is extremely knowledgeable concerning the reptilian fauna of this island. Needless to say, therefore, one of the subjects that we discussed was P. shrevei and its alleged bioluminescence. Hans was very sceptical about this, and even more so concerning the reliability of Sanderson's eyewitness testimony (it has to be said here that Sanderson was well known for exaggerating claims at times, although this behaviour may have been caused by a brain tumour that developed over time and apparently contributed to his relatively early death, aged just 62).

In one of his letters to me, dated 22 January 2000, Hans revealed that he had kept specimens of this lizard species in captivity for a considerable time but had never seen them light up. He also noted that both Dr Quesnel and Trinidad-based zoologist/newspaper columnist Prof. Julian S. Kenny had attempted to repeat the conditions reported by Sanderson but again had failed to achieve any success in stimulating the lizards to illuminate. In a subsequent letter, dated 14 April 2000, Hans mentioned to me that during the previous evening he'd had dinner with Dr Quesnel and had discussed fully with him the subject of P. shrevei. Quesnel had announced that he planned to try to collect a couple more specimens and this time arrange for high-quality histological sections to be prepared, with the tissues of the portholes properly fixed, in the hope of deducing something new regarding their supposed luminosity.

On 3 October 2004, the Trinidad Express newspaper published a short article written by Prof. Kenny that expanded upon Hans's comment concerning his investigations of Shreve's perplexing little lightbulb lizard. After referring to Sanderson's capture and claims regarding this species, Kenny revealed that American zoologist Prof. E. Newton Harvey (died 1959), a leading authority on bioluminescence, had once asked him to conduct an experiment to confirm his belief that Sanderson's claims were unfounded. The Harvey/Kenny experiment involved injecting some living specimens of P. shrevei with 1:10000, and 1:1000 doses of adrenalin. This treatment had already been shown to trigger light production in bioluminescent fishes, but it did not induce any reaction in the lizards.

Prof. E. Newton Harvey (public domain)

Also in 2004, what is acknowledged to be the defining scientific paper dealing with this contentious species' reputed glowing behaviour was published in the Caribbean Journal of Science. One of its three authors was Dr Quesnel, who revealed that, in fulfilment of his hopes expressed to Hans Boos in 2000, he had indeed succeeded in conducting further field investigations of P. shrevei, in May 2001 and again in May 2002.

Two male specimens were captured in rock crevices near to a cave entrance at the summit of Mount Aripo – i.e. the same general locality as Sanderson's own discovery. After examining them in the field, Quesnel took them to a field station for further investigation, where they were studied under light and dark conditions at different times of the day. Yet no observations, either in the field or at the field station, revealed any light emission from the portholes. The same was true with a third specimen that had been captured and studied previously by Quesnel. Clearly, therefore, they did not appear capable of bona fide bioluminescence, i.e. the active generation of light by living organisms via chemical means.

But what about the prospect that the portholes were highly reflective, or perhaps even phosphorescent? (That is, reflecting incident invisible light as visible light but over a longer time period than in fluorescence and without heat.)

Vintage illustration from 1904 depicting a further selection of known bioluminescent creatures (public domain)

To test this possibility, Quesnel directed high-intensity light from a xenon lamp at the lizards from varying angles. No light was emitted by the lizards, thereby demonstrating that they were not phosphorescent. However, light was readily reflected by their porthole (ocellar) scales. As Quesnel et al. explained in their paper:

...if P. shrevei is observed along the same plane from which light is directed, the normally obvious white ocelli cannot be seen against the reflection from all other scales. But, when viewed from an angle oblique to the light source, the ocelli appear brighter, while surrounding scales show no reflection. By varying the angle of reflected light, an illusion is created that the ocellar scales are intermittently emitting light, thus providing an explanation of Sanderson's original account of the lizard "switch[ing] on its portholes." The illusion produced by the reflective scales also explains recent accounts, as well as Sanderson's description of the white ocelli "remain[ing] plainly discernable in a darkened box when the rest of the animal was invisible." The ocellar scales reflect and intensify ambient light while the darker ground coloration renders the rest of the lizard invisible in a dimly lit environment.

It was also noted that the illusory effect of the reflective porthole scales was enhanced by a varying in intensity of the black pigment surrounding these scales, and that this varying of the black pigment's intensity appeared in turn to be dependent upon the lizards' stress levels - because it became darker when the lizards were first handled, but faded somewhat after several minutes. The black pigment surrounding the porthole scales heightened their reflective effect, making them look a brighter white:

When viewed immediately after handling the lizards, the ocelli appear to pulse or fluctuate in brightness as the surrounding pigment changes intensity. After a quiescent period, the ocelli are still reflective but do not appear as bright as when the surrounding skin pigmentation is darker. Again, this could explain Sanderson's description that light from the lizard "was much brighter the first time it was switched on after the animal had been quiescent for a period of time," and "after one brilliant display... it refused to shine with full brightness." The darker dermal pigmentation, presumably associated with higher stress levels during handling, heightens the reflective appearance of the white ocellar scales. Decreased pigmentation during inactive periods gives the illusion that the lizard is not producing light at full intensity.

In short, these studies appear to have comprehensively refuted Sanderson's claims that Shreve's lightbulb lizard is bioluminescent. Instead:

...the lizard's unique scales act like small parabolic mirrors, reflecting light at oblique angles. The intensity of this reflectivity is, in turn, influenced by the intensity of surrounding dermal pigmentation and by the angle at which a lizard is oriented relative to a light source. Thus, ocellar reflection produces an illusion that light is emitted by P. shrevei at varying intensities, a phenomenon which obviously has confused a number of persons.

Partial view of the Northern Range, Trinidad, whose Mount Aripo is home to Shreve's still-mystifying microtejid (image cropped) (© Sanjiva Persad/Wikipedia – CC BY 2.0 licence)

Even so, one major light-related mystery concerning Shreve's very surprising microtejid still remains unsolved. Namely, why has so remarkable a morphological feature as this lizard's parabolic mirror scales evolved in the first place, and why only in male specimens?

These are questions that the study of Quesnel and his co-workers did not seek to answer, although, as they did point out, P. shrevei is a reclusive nocturnal species that inhabits dark localities and whose behaviour in the wild is unknown – all of which make any attempt at speculation fraught with difficulty. Nevertheless, it occurs to me that in view of their sex-specific and also age-specific occurrence, perhaps these scales' light-reflecting abilities function as a means of visual communication by which adult males attract adult females for mating purposes. An alternative option is that this light-reflection ability is used as a defence mechanism, to startle or ward off potential predators, but if this were true, why do only males possess the necessary scales?

Clearly it is high time that some comprehensive field studies were conducted in relation to this small yet very thought-provoking lizard, neglected by science for far too long, in the hope of finally shedding some much-needed light (in every sense!) upon the currently cryptic purpose(s) of its unique parabolic portholes.

The present ShukerNature blog article is a greatly expanded and fully-updated version of a short account that appears in my book Mysteries of Planet Earth (1999).


  1. Excellent post Dr. Shuker. Not only interesting but totally scientific and professional. This is exactly the reason I have been a fan of your blog for several years.

    1. Thank you very sincerely for your kind words, Miguel, which I greatly appreciate, and I'm delighted that you enjoy my blog so much!

  2. Very interesting species, I would only suggest that perhaps the lizard IS capable of bioluminescence, however as the portholes only occur on the males perhaps the light is only activated as a sexual attractant? Further, the fact that the 'lights switching on' were triggered by loud SOUNDS, not just reflected light, should be fair proof that it is indeed producing light and not simply reflecting it. Without knowing the breeding season (I would suggest to find out the exact time of year of Sanderson's visit and discovery, which may coincide with the breeding season?) for further research of this intriguing species. Thank you for sharing this, and for your efforts.

    1. Yes, I suggested in my article that the reflection of light by males may be a means of attracting females. However, the problem with this lizard being capable of bioluminescence is that the above-reported anatomical studies of it by Parker etc have shown that it does not possess any bioluminescent organs, merely some highly-reflective scales acting as parabolic mirrors. So yes, it can certainly reflect light via its porthole markings, and may orient its body in a manner to optimise this, but its portholes lack nerve endings and blood flow that would be needed to stimulate and feed actual active bioluminescent organs.

  3. Pondering on uses for such a light reflector, I came up with a small hypothesis, possibly of little worth but picturing it makes me smile a little. In small dark crevices, the male stands guard at the entrance, his parabolic scales reflecting light inwards where the female is searching for food. or perhaps preparing a nest or caring for young. When the male is alarmed, the scales around the reflectors darken, somewhat changing the light reflected. This change alerts the female without the male making any sound or movement. Perhaps she responds by ceasing activity, so that predators can no longer hear her.

    A slight alternative: The nest is dark. Hatchlings remain in the nest for a while. The male and female take turns tending to the young. Perhaps the female forages by day and the male by night. When the male is present, he stands in the entrance as above, and it's the youngsters who go quiet when the quality of light changes.

    I can imagine other scenarios of the male standing guard, his reflectors lighting up a feeding area for female(s) or young. They wouldn't have to provide much light, just enough to distinguish the presence of shapes more quickly than touch or taste test. I wonder what this concept, if true, would imply for what the lizard eats? Or, perhaps the light shows the presence of possibly dangerous holes inside the dark area.