A Fish Eating Snake is Discovered in the Galapagos

Why write about snakes on the Galapagos Islands when there exist such curious creatures as the marine iguanas, giant tortoises, flightless cormorants, Darwin’s Finches, and penguins on the equator?   All are unique species, found nowhere else on earth and all demonstrate adaptations to the unique environmental and ecological conditions in which they found themselves upon arrival.  And what adaptations!  Blood drinking finches, tool using too! The only marine iguana ever known on the planet!  A bird that looses the power to fly in order to be able to feed more efficiently!

In 1837 Charles Darwin wrote in his first notebook on the transmutation of species that there were two places that gave him the basis for all his ideas. In second place were the fossils in Patagonia. First place went to Galapagos, for not only did he come face to face with unique living creatures clearly related to South American ancestors, but some of them, like the mocking birds and giant tortoises, were even more closely related amongst themselves, and differing in form on the different islands. Here was variation, the very stuff of adaptation through natural selection!

In this thrill of learning, snakes were pushed aside to one line in the Voyage of the Beagle:

 “There is one snake which is numerous; it is identical, as far as I am informed by Mr. Bibron, with the Psammophis Temminckii  from Chile”.  

Chile might have been correct, but today all four species that inhabit the archipelago are endemic. That would have interested Darwin.

He never visited the westernmost island of Fernandina.  Few people probably did because of the forlorn aspect of its dark, un-vegetated lava flows. Some, still smoldering from recent eruptions, gave the island a hostile appearance bereft of life.  It was hard to imagine that this stark landscape might house a new treasure of adaptation as unique as any other.

In 1995, I set out for the northwest corner of Fernandina, Cape Douglas, to verify that black rats had not arrived on this pristine island. It was necessary to set out live traps along the shoreline and recover them early in the morning.  As I was walking over the 50m wide lava ledge that creates the cape, I saw a snake traversing the ledge almost parallel to my own course.  My curiosity was awakened as I felt the snake was on a purposeful journey, neither turning left not right to pursue lava lizards or young iguanas, their known prey.  On encountering a narrow but deep crack in the lava, the snake descended out of sight.  Seeing that the fissure ran almost straight to the sea, I rapidly moved forward so that I might have a view along the shoreline, including the exit of the crack.  This was no simple matter, as the waves were breaking on the abrupt rock face meeting the sea.  However, in a short while I was rewarded by the sight of the snake’s head and about one third of its body issuing from the fissure.  After 65 seconds the head struck sideways and the snake quickly retreated from my view.

Darting landward I was amazed to see the snake rise from the depths of the crack with a fish dangling from its mouth.  Now these snakes of Galapagos have never been recorded to eat fish from the sea.  Neither have any of their ancestors nor any other snake in this large group of colubrid snakes. 

But it was not just any fish.  The four-eyed blenny (Dialommus fuscus) is remarkable in itself.  It does not have four eyes, but there is a bar of pigmented tissue arranged almost vertically across the middle of the eye, which is flattened into two sections on either side of this bar, with an angle of 110 degrees between the flattened sections.  Experimentation suggests that this helps to give a sense of depth to the vision. This, along with eye fluid containing a refractive index of salt water, are adaptations to better vision both in the water and in air. For the blenny spends about 50% of its life searching for prey in the intertidal zone, skittering over the seaweeds and wet rocks, with impulses driven from flexing a strong tail. It is a unique fish to Galapagos. 

Thus, for terrestrial snakes to feed on marine fish, they must either become marine snakes, which occur elsewhere on the planet, or the fish must come to the snake, even if it is only half way, for the snakes do not appear to appreciate a chilly bath. The snake must fish within reach of breaking waves and beat a quick retreat if covered by seawater. Although endemic four eyed vision did not originate in the Galapagos for the foureye rockskipper, another species in the same genus Dialommus, is found on the west coasts of South America. This fish adapted to an unfilled niche in the inter-tidal zone, dispersed to Galapagos, where it evolved into an endemic species.  Later the snakes in Galapagos took advantage of yet another unfilled niche -- that of a terrestrial reptile that feeds upon marine fish.  Thus, one adaptation in a species opens the door to another adaptation in a different species, through time and space.

The western seas off Fernandina are the most productive of all Galapagos.  As the tide recedes the black rocks give way to a wealth of greens and reds as the foliose alga reveals itself.  This is the home of the four-eyed blenny where it feeds on small crustaceans.  Nineteen years after the first sighting, in 2014, I persuaded a BBC crew to visit Cape Douglas.  In the six days we spent there I was able to discover further facts concerning the habits of the snakes.  Each has its “fishing patch” and sticks to it, day after day. But, on one “patch”, two snakes occasionally fed together, even touching one another, without apparent aggression. The snakes rested on high, dry ground amongst loose lava rocks and preceded to the sea edge each day, passing through colonies of marine iguanas with attendant lava lizards.

It seems to be good feeding, for the blennies are present throughout the year, unlike the rich, but fleeting, feast of hatching marine iguanas. One snake, Mr. Light, caught eight fish in 47 minutes.

In between these observations I located another location on Fernandina where snakes eat marine fish, and not just four eyed blennies.  At Cape Hammond, 15 snakes were investigating the wet bottom of a pool that almost dries out at low spring tides and some were successful in catching fish.  Thus, this behavioral change is not restricted to Cape Douglas, but is at least found amongst snake populations separated by six kilometers on the wild black shores of this young volcanic island.

The significance of this finding is that it is rare today to find wild places that are completely free of anthropogenic change.  Fernandina is one of those places.  What we observe there is the process of adaptation taking place in a totally natural environment.  It is the reaction of life forms that find themselves in a novel relationship in which unfilled niches are available.  It is an example reptiles’ capacity of behavioral change, since survival may depend on it.  These snakes are not as they were before.  Perhaps change will continue in directions as of yet unknown.

Surely we border on the transmutation of species.

In the broader view, it is recognized that Fernandina is the youngest volcanic island produced from the Galapagos “Hot Spot”.  One calculation predicts an age of 31,000 years.  Many islands have been formed in the past from the same “Hot Spot,” with the archipelago continually changing shape over millions of years.  As the islands move eastward borne on the Nazca tectonic plate, the islands erode and may become seamounts.  Is the relationship of the present islands the “right one” to develop the highly productive marine conditions that permit an abundant population of Four eyed blennies to exist that has allowed the snakes to adapt to this novel feeding?  The snakes that dispersed to this raw, rough island would have been excellent material for natural selection to work upon.

As a personal note I was thrilled to observe the result of behavioral change in this species of snake.  It is one thing to read books about the natural processes that change life forms, but it is quite another to find the wonderful reality of these processes on the simple black shores of Fernandina.

Godfrey Merlen
Puerto Ayora
Isla Santa Cruz, Galapagos