16 June 2017

The Mystery of the Tawilis’ Origin and Its Transformation from a Marine to Freshwater Fish

Image credit:  by Shrumster at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=41733900.
If you are Batangueño, then it goes without saying that you are familiar with your home province’s iconic fish, the freshwater sardine found nowhere else on earth but in Taal Lake. This small fish with silvery scales is known by its scientific name sardinella tawilis, or just tawilis in the vernacular. What you probably do not know is that the origins of this iconic fish remains a mystery even to biologists up to the present day.

For the benefit of those who may not know, Taal Lake was not always the freshwater lake that we know it to be today. An archaic map drawn by the Jesuit priest Fr. Pedro Murillo Velarde in 1734 showed that what is now Taal Lake was once connected to Balayan Bay by a channel wide enough to be navigated by ocean-going vessels1.

The Murillo Velarde map shows a channel connecting Taal Lake to Balayan Bay.
What this meant was that Taal Lake’s waters were once marine or saltwater; and that the fish species that lived in it were those evolved to live in the salty waters of the open sea. However, prolonged volcanic activity in 1754 spilled unimaginable quantity of tephra2 into the channel, closing off Taal Lake’s access to Balayan Bay and – over time – transforming it into the freshwater lake we know it to be in the present day3.

Inevitably, there were marine fish species trapped inside the lake when the channel was blocked and reduced to what we know today as the Pansipit River. The tawilis was one of these. Rather than die out because of the changing properties of the waters it had to swim in, the tawilis instead adapted to the increasingly – again, over time – desalinated waters.

Augustus C. Mamaril, in a paper written in 2001, described what happened:
“The violent eruption of Taal Volcano located in the middle of the bay in 1754 might have reduced this channel to what is presently the narrow Pansipit River and in the process trapped founder stocks of biota of marine origin. These land-locked forms, which included a (species of) Sardinella, adapted to the changing hydro-biological conditions of the erstwhile marine habitat4.”
The tawilis was first described as a new species in 1927 and identified as harengula5 tawilis. However, in the eighties, it was “re-described and transferred to the genus sardinella” or sardines. Mamaril, however, expressed reservations about the tawilis being a distinct species. At the time his paper was written, it had been a “mere” 247 years since the stocks of marine sardines were trapped in the lake. Mamaril wrote, “…speciation6 would have to be unusually rapid in an evolutionary time scale.”

At this point, the obvious question to ask is “what was the tawilis’ marine ancestor before the channel was blocked and the bay became a freshwater lake.” Because Mamaril had doubts that the tawilis is a distinct species, he suggested in his 2001 paper that the tawilis’ “status as distinct species and affinities of 'tawilis' may have to be resolved using genetic and molecular techniques.”

More than a decade later and a group of scientists tried to do exactly as Mamaril suggested. In a paper7 dated January 2013, Demian A. Willette, Kent E. Carpenter and Mudjekeewis D. Santos presented the results of their genetic study of the sardinella tawilis, particularly in relation to a species of sardines more commonly found off Taiwan and the northern waters of the Philippines.

The three researchers found that the tawilis’ closest genetic relation is sardinella hualiensis, with which it shares the same morphology or physical structure. The latter is a marine species which was erstwhile thought of as restricted to Taiwanese and Chinese waters but has been found to range as far south as the northern waters of the Philippines.

Moreover, the researchers are of the opinion that their findings “support recognition of S. tawilis as a distinct freshwater species in Taal Lake.” However, although they acknowledge that sardinella tawilis and sardinella hualiensis are “sister-species,” they also state that each is a species distinct from the other.

If at all the tawilis evolved from the hualiensis, then there are still mysteries to solve. For one, oceanographic currents would have made the hualiensis’ southward migration to the south of Luzon unlikely. For another, there had been no reports of either the tawilis or the hualiensis being found outside of its known range.

Again assuming that the tawilis at some time in the past evolved from the hualiensis, the researchers’ genetic findings appear to indicate that the divergence or separation of the species must have occurred in the late Pleistocene8.

In other words, the tawilis must have separated as a species in itself from the hualiensis thousands of years before the 1754 Taal Volcano eruption and must have had established populations as a marine fish in the bay before it was closed off by tephra to become what we now know as Taal Lake.

As the waters of the lake gradually changed from marine to freshwater, the tawilis also evolved the necessary physiological mechanisms that allowed it to survive in the changing environment. Unless, as Quilang et al speculated in 2011, the tawilis has a marine ancestor that may “still be roaming in the South China Sea waiting to be discovered9.”

Notes and references:
1 “A Hydrographical and Chorographical Chart of the Philippine Islands,” online at World Digital Library.
2 Tephra is material expunged into the atmosphere by a volcanic eruption and fallen onto the ground. Wikipedia.
3Taal Lake,” Wikipedia.
4Translocation of the clupeid Sardinella tawilis to another lake in the Philippines: A proposal and ecological considerations,” by Augusto C. Mamaril, published 2001.
5 Harengula “is a genus of herrings that occur mostly in the western Atlantic, Gulf of Mexico and the Caribbean Sea, with one species in the eastern Pacific Ocean.” Wikipedia.
6 Speciation is the process by which biological populations evolve to become distinct species. Wikipedia.
7 Along with other bits of information in this article, from “Evolution of the freshwater sardinella, Sardinella tawilis (Clupeiformes: Clupeidae), in Taal Lake, Philippines and identification of its marine sister-species, Sardinella hualiensis,” by Demian A. Willette, Kent E. Carpenter and Mudjekeewis D. Santos, January 2013.
8 The Pleistocene is a “geological epoch which lasted from about 2,588,000 to 11,700 years ago.” Wikipedia.
9 “DNA Barcoding of the Philippine endemic freshwater sardine Sardinella tawilis (Clupeiformes: Clupeidae) and its marine relatives,” by Quilang JP, Santos BS , Ong PS, Basiao ZU, Fontanilla IKC, Cao EP, 2011 and cited by Willette, Carpenter and Santos.

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