Population structure and mating encounter rates in a marine pelagic invertebrate, Firoloida desmarestia (Mollusca)

Vertical distribution, sex ratio, size structure and mate encounters of Firoloida desmarestia in the southern Gulf of Mexico were analyzed during 4 seasons (in February, May, August and November). A total of 395 individuals were collected with an opening-closing net at 5 levels of the 105 m upper water column. Results indicated that the species reproduces throughout the year, with a reproductive peak in May and a low reproductive period from August to February. Length measurements indicated that females (as long as 30 mm) are larger than males (18 mm). Estimations of a 'spatial overlap' index showed a high overlap degree between sexes, suggesting that patterns in their vertical distribution are nota reproductive barrier. Sex ratio (F:M) at birth seemed to be 1:1, but during most of the year the adult sex ratio was dominated by females. We propose that larger dimensions and faster swimming speeds of females provide competitive advantages over males in searching for food and avoiding predators. Simulations of mate encounters indicated that values during the low reproductive season were low under turbulent and non-turbulent conditions (