Temporal abundance and population parameters of the invasive medusa blackfordia virginica mayer, 1910 (Hydroidomedusae: Blackfordiidae) in pueblo viejo lagoon, mexico

The sex ratio, size at first maturity, morphological variations and abundance of the invasive species Blackfordia virginica, were analysed in the lagoon of Pueblo Viejo. Zooplankton were collected with a 505 µm net throughout the lagoon during October and December 1992, March, April, June and August 1993, and March 2009. Data were recorded simultaneously for salinity and temperature. A female/male sex ratio of 0.95:1.00 and a minimum size of gonadic maturity of = 5.25 ± 0.17 mm umbrella diameter were recorded. Blackfordia virginica presented 33 types of morphological variations. The number of marginal tentacles (53–104) increased with size (2.5–13.0 mm). Medusae were collected in this lagoon only in March and April 1993, with average densities of 1,946.75 and 2,508.83 ind/100 m³ respectively. These medusae were recorded at temperatures of 19.0–28.0 °C and salinities of 11.0– 21.0 PSU. The density of B. virginica was highly correlated with salinity (r_s = 0.796, n = 114, P < 0.001). The high density of specimens, the wide range of sizes, and the presence of both mature females and males as juveniles indicate that this species can reproduce and develop in this lagoon system. The absence of medusae in March 2009 indicates that the species is not a resident in the system in response to the oligohaline conditions (0.0–7.0 PSU) that are characteristic during June to October.