[Abundance and morphometry of tuberculosa Anadara and A. similis (Mollusca: Bivalvia) in the Manglar de Purruja, Dulce Gulf, Costa Rica]. / Abundancia y morfometría de Anadara tuberculosa y A. similis (Mollusca: Bivalvia) en el Manglar de Purruja, Golfo Dulce, Costa Rica.

The density, population, length, yield and sex proportion of the mollusks Anadara tuberculosa and A. similis were studied in Bahía de Golfito, Golfo Dulce, Pacific coast of Costa Rica from February 1998 to February 1999. A. tuberculosa was more abundant (0.9 units m(-2)), than A. similis (0.2 units m(-2)); the highest abundance was found at the canal mouths. The average lengths were 43.3 mm for A. tuberculosa and 42.8 mm for A. similis (both under the Costa Rican legal minimal length for exploitation: 47 mm). Maximum lengths were measured in the middle and upstream Canal Mayor, respectively: 43.0 mm and 43.4 mm. The correlation between length and fresh weight was 0.81 (Pearson). The average total weights were 26.2 g for A. tuberculosa and 19.1 g for A. similis. The condition index (a meat yield measurement) was higher in A. similis (21.2%) than in A. tuberculosa (17.2%). The maximum yield for both species lies in the 31-35 mm range. The male ratio was 43.7%. A species recovery plan is urgent because these results suggest both a decrease in density and illegal exploitation.

The use of water chemistry and benthic diatom communities for qualification of a polluted tropical river in Costa Rica.

The water quality of several sections of a tropical river subjected to severe pollution was studied through physico-chemical water analysis and benthic diatom assemblages. The methodology follows the concept of differential species groups and that of its modification for the groups of nutrient-differentiating species for rivers rich in both oxygen and inorganic nutrients. The trophic indication of the latter authors correspond clearly with the results of chemical observations made in this study. The most abundant species found in this river were Navicula goeppertiana, Gomphonema parvulum, Gomphonema sp. aff. pumilum, Nitzschia palea, Nitzschia amphibia, Nitzschia clausii Nitzschia inconspicua, Navicula seminulum, Navicula sp. aff. cryptocephala, Navicula schroeterii var. escambia, Cymbella sinuata and Surirella sp. aff. roba. These species are known to be tolerant to organic pollution and eutrophication. Therefore we may conclude that diatoms are useful for biological monitoring of disturbed tropical rivers.