The population of mysid Metamysidopsi sp. nov. 4b has been studied in the sandy beach of San Pedro village in the Guayas Province (Ecuador). It's been sampled on a fortnightly basis from June 2000 to July 2001 at low water and high water. This study covers the composition of the hyperbenthic community and the population dynamics of the dominant specie Metamysidopsis sp. nov. 4b. Are involved the density, biomass and their relation with environmental parameters, population structure, size at maturity, sex ratio, fecundity and secondary production by the size-frequency method, using three CPI to estimate maximum, minimum and average secondary production.The hyperbenthos was dominated by the group Mysidacea representing 98% of the total density. Three species were found in this group; Bowmaniella sp., Mysidopsis sp. and the dominant in the group: Metamysidopsis sp. nov. 4b.Three peaks of abundance of Metamysidopsis sp. nov. 4b were found during the dry season, at June (>26000 N/m-2 at low water and >3000 N/m-2 at high water), October (> 16000 N/m-2 at low water and >7000 N/m-2 at high water) and November (>26000 N/m-2 at low water and >400 N/m-2 at high water), and were almost absent during the wet season. A dominance of females over males along the year has been found as well as a larger size for female adults than for male adults. The sex ratio fluctuated at samplings with a tendency towards female dominance (1: 1.5 at low water and 1 :2.6 at high water). The number of embryos carried by a single female goes from 1 to 7, proportionally totheir size.The estimate of secondary production yielded results for annual estimate between 3000 mg ADW.m-2.yr-l and 15000 mg ADW.m-2.yr-1 at low water and 1000 mg ADW.m-2.yr-1 and 5000 mg ADW.m-2.yr-l, and a P/B ratio between 19 and 80 at low water and between 20 and 90 at high water.
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