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Harris
mud crab, Rhithropanopeus harrisii
photo by
Arthur Anker
STRI and
Scripps Institute of Oceanography
Invasive crabs have settled in the
Pacific side Third Cut
by Eric Jackson
We don't yet know how serious it is or what might be done about it, but
Panama, The Crossroads of the World, is faced with yet another invasion by
an exotic species. This past February and March, Smithsonian Tropical
Research Institute biologist Mark Torchin and McGill University graduate
student Dominique
Roche discovered the Harris mud crab (Rhithropanopeus
harrisii), including males, females with eggs and juveniles, living in a
brackish Pacific side lake left over from the abandoned late 1930s US
attempt to build a third set of Panama Canal locks.
This species of crab is native to estuaries on the Atlantic seaboard of
the United States, but by hitching rides on the sides of or in the ballast
water of ships, or as the result of careless intentional transplantations
of other species it spread to the West Coast of the USA and at least 21
other countries.
The earliest Harris mud crab invasion was in California in the 1880s, when
somebody created oyster beds by bringing in East Coast shellfish, among
which the crabs mingled. There is anectodatal evidence that the crabs have
fouled pipes, invaded fresh water bodies, feasted on fish caught in gill
nets and affected other species in ecosystems that they have invaded. The
problem, Torchin pointed out, is that the effects of these invasions have
not been systematically studied or quantified.
Torchin and Roche reported their findings in the September edition of
Aquatic Invasions, a
scholarly journal. Their research is in conjunction with the The
Smithsonian Institution Marine Science Network, an array of marine labs
extending from the eastern United States down the Meso-American isthmus
and crossing over to the Pacific here in Panama, which was organized to
increase scientific knowledge about the marine species invasions that have
become increasingly common with the expansion of global commerce and
shipping.
Because these crabs have spread to so many other places in the world, it
may not be possible to determine how and from whence they got here. The
possibilities of figuring out that puzzle lay mainly in DNA studies, in
which one crab population might be compared with another. What is known
about this from the DNA evidence now in is characterized by Torchin as "not
definitive."
Dominique Roche sets up a crab trap under the Miraflores
Spillway
photo by Beth King, STRI
This is not the first time that the Harris mud crab has been found in
Panamanian waters. In 1969 the species was discovered in canal waters, but
was not thought to have established a breeding population here. How did it
get into the Third Cut? The lakes left over from that discontinued project
were once connected to the canal's south entrance by a culvert, but it is
believe that the duct has been blocked for many years. Torchin and Roche
suspect that a flooding event may have brought the crabs into the Third
Cut lake, but that question is intricately linked with a much more general
and important one: how widespread has this invasion of Panama become?
The Third Cut lakes on both sides of the isthmus will become part of the
channels for the Panama Canal expansion, and if these invaders are
confined to the ones near Miraflores, then it may be worthwhile to
eradicate the crabs before they spread. But that issue raises a number of
other questions:
-
How does one eradicate Harris mud crabs?
-
Could these crabs be eliminated without
massive destruction of other species?
-
Are there predators or other factors present
in Panama from which the crabs have found refuge in the protected waters
of the Third Cut?
-
If these brackish water crabs are confined
to the Miraflores Third Cut waters and those lakes are then connected to
Panama Canal, can they survive the fresh water of Culebra Cut and Gatun
Lake to make it to the Caribbean Sea via the canal?
According to Torchin, inquiries into such questions would depend on what
is found by ongoing studies of the crabs distribution in the canal.
"Potential for eradication only becomes worth talking about if they are
restricted to a particular area," he pointed out. Further studies are
being conducted to answer that threshold question, with funding from the
Panama Canal Authority. In addition to the current canal waters, Torchin
said that a fauna survey in the Third Cut lake on the Atlantic side,
between the former Fort Davis and Gatun, is "on our list."
