ABSTRACT: Conservation implications of the population decline of the Colorado Delta bivalve mollusk Mulinia coloradoensis

Flessa, K.W., Rodriguez, C.A. Department of Geosciences, University of Arizona

Submitted to Southwest Center for Biological Diversity
February 12, 1999

SUMMARY The bivalve mollusk Mulinia coloradoensis was once the most abundant species of clam inhabiting the marine intertidal and shallow subtidal zones of the Colorado Delta. Today, however, only a small relict population survives near the river's mouth. Geochemical and faunal evidence suggest that the decline in this species abundance is a consequence of the decline in the delivery of fresh Colorado River water to the Delta since the middle of this century. Increased salinity in the delta and former estuary appears to have had an adverse effect on the population size of Mulinia coloradoensis.

Because river water is relatively enriched (compared to sea water) in 16O and mollusks secrete their shells in equilibrium with the surrounding water, the isotopic composition of shell calcium carbonate reflects the relative proportion of river water and sea water in the mixing zone of the Delta. Empty shells of Mulinia coloradoensis frequently have a greater proportion of 16O in their shells than do the live-collected shells of Chione fluctifraga, the most common bivalve living on the Delta today. This indicates that Mulinia coloradoensis commonly lived in water lower in salinity than is now typical of the Delta.

The faunal composition of the Deltas shelly deposits also indicates that the abundance of Mulinia coloradoensis is controlled by salinity. Shelly deposits near the mouth of the river are dominated by Mulinia coloradoensis, and the relative abundance of the species decreases to the south. Evidence of the species is absent at Punta Faro, 75 km south of the river's mouth.

Preliminary analysis of shell characteristics indicates that Mulinia coloradoensis is somewhat distinct from Mulinia pallida, a closely related species that lives farther to the south, along the coast of Sonora and Sinaloa. Mulinia coloradoensis is characterized by a sharper posterior ridge, though its shell dimensions are similar.

Survival and recovery of Mulinia coloradoensis probably depends on increased flow of Colorado River water to the marine part of its Delta. Estimating the minimum amount and the timing of such flow will require additional geochemical and paleoecological analyses.

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Resumen: Implicaciones de la disminucion de colonias de organismos Mulinia coloradoensis del delta del Rio Colorado para su conservacion

Flessa, K.W., Rodriguez, C.A. Department of Geosciences, University of Arizona

Submitted to Southwest Center for Biological Diversity
February 12, 1999

RESUMEN El molusco bivalvo Mulinia coloradoensis era la especie de almeja mas abundante en las zonas poco profundas y intermareales del Delta del rio Colorado. Sin embargo, ahora esta especie solamente sobrevive en una pequena población relictual cerca de la boca del rio. Datos geoquímicos y faunistios sugieren que la disminución de la especie es consecuencia de la disminución de la corriente de agua fresca al delta del rio Colorado desde mediados de siglo. El aumento de salinidad en el delta y en el antiguo estuario parece tener efecto negativo en el tamano de la población de Mulinia coloradoensis.

Ya que el agua del rio es relativamente rica (comparada con el agua de mar) en 16O y los moluscos secretan sus conchas en equilibrio isotopico con el agua, la composición isotopica de la concha reflecta la proporción relativa en la zona del Delta donde el agua de rio y de mar se mezclan. Las conchas vacías de Mulinia coloradoensis frecuentemente tienen una proporción mas grande de 16O en sus conchas que las conchas de Chione fluctifraga, el bivalvo mas común del Delta. Esto indica que Mulinia coloradoensis vivia comunmente en agua menos salada que el agua tipica del Delta actualmente.

La composición faunal de los depositos de conchas en el Delta indica que la abundancia de Mulinia coloradoensis es controlada por la cantidad de sal. Depositos de conchas cerca de la boca del rio son dominados por Mulinia coloradoensis, y la abundancia relativa de especie disminuye al sur. La especie esta ausente en Punta Faro, el cual esta a 75 km sur de la boca del rio.

Analisis preliminar de las características de las conchas indica que Mulinia coloradoensis es parecida a Mulinia pallida, una especie relacionada que vive mas al sur, en la costa Sonora y Sinaloa. Mulinia coloradoensis es caracterizada por su rugosidad puntiaguda aunque las dimensiones de las conchas son similares.

La sobreviviencia y recuperación de Mulinia coloradoensis probablemente depende del aumento de la corriente de agua del rio Colorado a la parte maritima de su Delta. Son necesarios analisis geoquímicos y paleoecologicos para estimar la cantidad y el tiempo de la corriente.



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