Hi, (sorry it's long)
This is an interesting and somewhat complex subject that requires multidisciplinary
intervention...
During former El Niņo events along the coasts of Peru and northern Chile, mollusks' response
have been either positive or negative and sometimes not related to temperature. As in Ecuador,
most northern peruvian beaches (north of latitude 4šS) are related to mangrove environments and
large river discharge areas, thus during El Niņo, there is an increased sediment discharge and
most populations of estuarine/mangrove mollusk species are negatively affected (i.e: high
mortality rate of Anadara, Arca, Ostrea spp); there is an oceanographic "boundary" between 4-
6šS, or the confluence of Equatorial Countercurrent ("warm-waters") and Humboldt Current
("cold-waters"). It defines the limit between Panamic and Peruvian-Chilean faunal Provinces,
and some transitional species can be found here (warm-water and cold-water mollusks). During El
Niņo events typical cold water mollusks south of 6šS dissapeared either by increase of water
temp.(bivalve banks), temporal introduction of foreign predators like crabs and some
starfish(thaids, mytilids), local high-energy wave action (mytilids), or retreat to deeper and
/or southern waters (mobile species - Concholepas). The same events caused positive influence
on some oportunistic species, like the commercial value scallop Argopecten purpuratus, whose
populations had booms specially in low-energy, shallow embayments, and a large chocolate thaid,
Thais chocolata, which also experienced an important population growth and it was well reflected
in landings at local fish markets.
Introduction of foreign species of northern origin has been a common feature of El Niņo events
along these coasts, evidencing a larval transport from the north. We have reports of the
pinnid Atrina sp. or a large cymatid, Cymatium parthenopeum, 1000 km farther south than
their normal geographic range, among a local cold-water association of Mesodesma donacium-Donax
obesulus-Protothaca thaca-Crassilabrum crassilabrum. Species typical of the Panamic Province
have been found as far as northern Chile within cold-water mollusk communities, but only as
isolated specimens, growing to adult stages without reproductive potential. In the fossil
record, by contrast, evidence have been found as far as Middle Pleistocene that foreign species
had been able to form populations which were contemporaneous to the local cold-water
communities. Their presence, when compared to the overall geographic mollusk distribution from
north to south within the same timeframes, have been interpreted as a result of larval transport
via former El Niņo episodes, but their ability to reproduce and form populations respond mostly
to local -and favorable paleogeographic conditions: i.e: shallow waters, adequate substrate,
food availability and seasonal variations.
While La Niņa effects on these mollusks have not been studied until recently, there are quite a
few field observations along these coasts that might as well complete this part of the puzzle.
Amanda Diaz
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