Comparative Parasitology 68(2) 2001 - Peru State College

216 COMPARATIVE PARASITOLOGY, 68(2), JULY 2001
their situation along the annual migratory routes
of Nearctic birds. Additionally, the low number
of helminth species in the basin may have readily
allowed invasion of these communities by
allogenic species.
Three helminth species on the list are recent,
anthropogenically introduced colonizers. The
first is the cestode B. acheilognathi, which is the
most widely distributed species in the basin and
is found in 22 host species. The second is the
heterophyid trematode C. formosanus. The actual
distribution of this helminth within the basin
has not been evaluated, because the intermediate
host, the thiarid snail Melanoides tuberculata
Miiller, 1774, has established along riverbanks
and in riverbeds, where few fish have been sampled.
Both these species were introduced recently
into Mexico; the cestode together with
Asian carp (Salgado-Maldonado et al., 1986),
and the trematode most probably with the intermediate
snail host (Scholz and Salgado-Maldonado,
2000). The third species is the capillariid
nematode P. tomentosa, reported from atherinids
and goodeids, as well as from cultured carp, C.
carpio, from Mexico, where it was probably introduced
along with its fish host from Europe
(Moravec, 1998; Moravec et al., 2001).
The proportions among the helminth groups
that constitute the communities in the fish of the
Lerma-Santiago river basin are also distinctive.
The dominance in species numbers of nematodes
and trematodes (principally metacercariae)
is a pattern characteristic of the fish helminth
communities of southeastern Mexico (Scholz et
al., 1995; Salgado-Maldonado and Kennedy,
1997; Scholz and Vargas-Vazquez, 1998) and
the Balsas River basin in central Mexico (Salgado-Maldonado
et al., 2001). However, data in
the present study show that cestodes, both adults
and metacestodes, are almost as important in the
Lerma-Santiago river basin in terms of numbers
as the nematodes and trematodes. Most cestodes
found, such as V. campylancristrota, occur
throughout the American continent or are cosmopolitan
(see Scholz and Salgado-Maldonado,
2001). The presence of 5 monogenean species
in the basin is also notable, as it is a higher
number than recorded in other drainages in central
Mexico. However, the monogenean fauna of
freshwater fishes in southeastern Mexico is even
richer (Kritsky et al., 1994, 2000; Mendoza-
Franco et al., 1997, 1999, 2000).
It is still not possible to form conclusions
Copyright © 2011, The Helminthological Society of Washington
about the zoogeographic characteristics of the
fish helminth parasite communities in the Lerma-Santiago
river basin, as very few studies
have been done. However, the data that do exist
suggest that the proportion of endemic parasites
is high, and thus very distinctive, as compared
for example to the lack of endemic species
among the helminth parasites of fishes from the
Balsas River drainage (Salgado-Maldonado et
al., 2001). The helminth communities were
probably initially poor, and have been invaded
by allogenic, Nearctic species transported by
birds that have enriched these multispecific assemblages.
Research into fish helminth parasites in the
Lerma-Santiago river basin has been restricted to
descriptions of some species, and more detailed
studies have been carried out only in Patzcuaro
Lake. Obviously, more complete inventories of
the fish parasites in this basin are urgently required.
Almost 7% of the fish species that originally
inhabited the basin are extinct, and an additional
23% are classified as endangered or vulnerable
because of population decline associated
with continuous habitat degradation and introduction
of competing and predatory species that are
added to the natural predation pressures in these
ecosystems (Soto-Galera et al., 1998).
Acknowledgments
This study was supported by project no.
27668N from the Consejo Nacional de Ciencia
y Tecnologia (CONACyT), Mexico, and by project
no. H007 from the Comision Nacional para
el Estudio y Uso de la Biodiversidad (CONA-
BIO), Mexico. We are indebted to Dr. Frantisek
Moravec for confirmation of identification of
nematodes and Dr. Tomas Scholz for identification
of cestodes. We also thank Nancy Minerva
Lopez-Flores, Isabel Jimenez-Garcia, Cris Caneda-Guzman,
Rafael Baez-Vale, Norman Mercado-Silva,
and Felipe Villegas-Marquez for
their assistance in the field and laboratory.
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